I also had a similar in my idea about the 3lb Razer design. I was going to make the chassis of mainly two 1/8 lexan middle pieces. [Brandon, the guy with all the craziest beetle or ant weight designs]

A: Albert Einstein is reported to have said that crazy is "doing the same thing over and over again and expecting different results." By that definition, expecting to win with an electric hammer or electric crusher is certainly crazy. Best luck.

A: Mark J. here: I'd really like to know who the two builders were.

The theory works well for drumbots. When two drumbots go 'head-to-head' they are (viewed from the sid I'm not so sure that you can count on the same effect with full body spinners that don't get support against newtonian reaction. I think having a blade that can cut from both sides is a good idea, but I think it will cut just as well if it's spinning faster or slower than the other shell.

Q: Looking at your archive gave me a interesting idea. In one of them you said it was a good idea to keep a wedge and a thwack separated, and would not work well together in one bot. That gave me the idea to literary keep them separate, and have one bot, a wedge and opponent in tow, smash into its teammate, a madly spinning thwack. I bet theres some bugs in my idea, but what do you think about it? A: I like the idea, but let's take it one step further - have the thawack madly spin on top of the wedge as it drives around so the undersized wedge doesn't have to fight to shove the twice-as-heavy opponent over to the thwack. It'd look a lot like 'Hazard'. Oh wait, it'd be 'Hazard'.

Please don't worry about us trying to create a robot that is "all weapon", because we still have a sturdy drivetrain and chassis in case all else fails. Please let me know if you think me, my teammate, (or both of us) are being absurd with our weapon plans.

A: Mark J. here: so, you're thinking about spinning a 20" long, 1.5" wide, .25" bar of steel at 10,000 RPM, storing 10,245 joules of energy in it, and slamming it into a 15 pound object - or worse, the arena wall? Have you considered what happens to the blade in an impact at that energy level? I think you get one good hit and the blade turns into a bow tie.

The laws of physics haven't changed since Hazard and Brutality were designed, and if higher RPM was the way to get better weapon performance you can bet that their designers would have taken full advantage of it. There is a balance to be struck between speed, energy storage, spin-up time, durability, and bite -- and the only way to find that balance is by trial and error. Look to robots that are successful and learn from their designs.

My recommendation is to not get greedy. I'd pick a smaller motor, put the weight savings into tip weights on the blade, and spin it at 3K RPM for about 1600 joules.

A: Mark J. here: possible - yes. Simple - no. Reliable - unlikely. Successful - I think not.

We've discussed flywheel-powered flippers in the archives. Their mechanical weak point is the dog clutch that transfers the power from the flywheel to the weapon. The concept is like revving up a car engine and trying to jam it into gear without pushing the clutch in first. The components are not 'off the shelf', they are difficult to design and machine, and the mechanism is subject to huge stress.

Pneumatics are much better at this type of work, so why make it so hard on yourself?

A: Mark J. here: sorry, but no. What you describe is a type of dead blow hammer. The passive internal mass strikes slightly later than the main hammer body and spreads out the impact while absorbing some of the rebound. The total energy of the impact is the same as a conventional hammer of equal mass, but the energy is released over a longer time period.

A dead blow hammer is used in situations where you wish to reduce damage to the surface you are striking -- just the opposite of what a combat robot is trying to do.

A: Follow the diagram at the right. Use the 'SC' and 'S2' connections on the BattleSwitch and connect the output from the BattleSwitch to a small 'coil' connection on the solenoid. The receiver energizes the BattleSwitch relay, the BattleSwitch sends a small current to the solenoid coil, and the solenoid switches the large current to the weapon motor.

If your battery has sufficient capacity, you can use it for both solenoid and motor. If the solenoid won't stay 'on' you need more battery.

A: I understand the crank and bar idea, Anthony. There was a version of the German robot 'Ansgar' that tried the design. If you watch the red speed bar in the animation, you can see the problem with the mechanical linkage: the output link reaches maximum speed and energy in the middle of each stroke, then slows and comes to a complete stop (zero energy) before reversing direction. That makes for very poor transmission of the energy from the flywheel to an impact on your opponent. Ansgar's weapon wasn't successful and there is little to be gained from repeating their design error.

A: Timing belts have limits on the power they can transmit. Some builders either don't know those limits or pretend that the limits don't exist. See the RoyMech timing belt website for guidance.

Two belts may help, but it's difficult to get twin belts to share load equally. Proper design calls for a single belt with enough load capacity to survive the abuse.

A: The BaneBots 28mm gearboxes aren't well suited to the high lateral loading you get from a weapon impact, and a slip clutch to absorb the radial impact on a spinner is not as easy to implement as it appears - we learned this from the weapon on 'Run Away'. After your experiments with the electric hammer, I'd suggest that you pick a more conventional and proven design for your next 'bot.

Q: I've been thinking about drumbot weapon designs lately. Why aren't there any asymmetrical drums (such as a disk with one tooth but balanced)? If counter weighted correctly, wouldn't it increase the "bite" of the drum / spinner since there are fewer teeth? I understand that there are gyroscopic forces to take into effect, but I'm just a little curious as to what would actually happen. Have you ever seen / tried to implement this kind of weapon?

A: Asymmetrical spinners are difficult to design and construct, but a good number of them do exist. The simplest form is a bar spinner with one end shortened and weighted for balance, but much more complicated forms are possible -- like the spiral rotor on middleweight 'Professor Chaos'.

Reducing the number of teeth does increase 'bite' as the time between passage of the rotor teeth is increased, allowing a bigger chunk of your opponent to enter the weapon's 'impact radius' before it takes a hit. The gyroscopic forces acting on the weapon are no different than those acting on a similar weapon with a symmetrical tooth distribution.

Q: Building on the idea of an asymmetrical drum such as Professor Chaos, would it be possible to make an asymmetrical "lawnmower" type horizontal bar spinner that is also asymmetrical and counterweighted? Is there any advantage to this? Has it been implemented before?

A: Mark J. here: read section 6.3 of the RioBotz Combat Tutorial for an explanation of 'bite' and the advantage of an asymmetric spinner. The tutorial includes an illustration of an asymmetric spinner bar.

I've seen counterweighted bar spinners, but I can't name a specific example.

A: Mark J. here: the answers to your questions might best be provided by a 4 semester-hour class at a good university, but I'll do my best to boil that down to a few paragraphs. It's gonna be a little sketchy.

Let's start by sorting out the relationship between motor torque, RPM, and power. Power is the product of torque and RPM:

There is a special case that does call for really high RPM: when two drum-spinners go 'weapon-to-weapon' the slower drum looses. I've seen drum spinners that cruise at a moderate weapon speed for most attacks, but have spare motor speed that their driver uses against other drums. Something to think about.

Q: You said in an earlier post that electric hammer weapons weren't effective in current competition. Does that mean that robots like frenZy are outdated by now? Because frenZy's hammer still looks like it has some oomph.

A: I really like 'frenZy', but it wasn't a very effective combat robot even back in the BattleBots days. Overall record: 7 wins and 10 losses, with zero wins in the last three events it entered.

FrenZy looked as if it was delivering powerful blows only because its small chassis would react wildly to each impact. It could make itself dance, but not its opponent.

A: Mark J. here: sure -- but there isn't any advantage to it. A rack and pinion drive for a hammer weapon is useful because it can translate linear force from a pneumatic cylinder into uniform radial acceleration of a hammer across a large arc. A flipper acts across a much smaller arc and uniform force is generally not an issue. You're better off to avoid the weight, complexity, and power loss from the rack and pinion.

Search for "Jacha Little" in this archive for an animated diagram of a rack and pinion hammer mechanism.

A: You are mistaken. According to Grant Imahara's Team Deadblow website, Deadblow's weapon system at the 1999 Long Beach event was pneumatic:

In 2000 the rotary pneumatic actuator was replaced by a conventional linear pneumatic actuator. The hammer never had electric power.

A: 'Deadblow' did not rely on high weapon power, but on speedy and repetitive attacks. Even so, the '1999' version of the robot did not do well with a relatively slow and weak (but better than electric) pneumatic hammer. Deadblow's record in 1999 was an anemic 1 win, 2 losses, and 1 no-show bye. Success came only after the weapon power and speed were increased with a linear pneumatic actuator. An electric hammer would have continued to perform poorly.

Repeat after me: electric hammer weapons do not have enough power to be effective in current open competition.

A: I think you're refering to a post where I say,

Team Hurtz has certainly had success with their pneumatic axe robots, but their electric hammer robot 'Beta' was not at all successful. 'The Judge' was also pneumatic. You can get an overhead hammer to work, but powering it with an electric motor is generally a poor idea.

A: Both have problems:

• Vertical spinners in any weight class are difficult to control because their gyroscopic effect resists turning motion. A peizo gyro in the radio system can't compensate for this this problem - you can't cheat physics.

• The axis of rotation on a horizontal spinner does not resist turning motions, but the counter-reaction to impact can send it careening away at high speed.

Q: As a follow-up to the (as of writing) most recent question, what about a disc mounted at a 45-degree angle? Would that give the benefits of both a horizontal & vertical spinner, or just the problems?

A: Mark J. here: a quick check of my trigonometry tables [cosine of 45 degrees = 0.7071] says that you'd still have 71% of the turning resistence problem with a 45 degree diagonal disc, and 71% of the lateral force vector on impact. You aren't gaining much on either problem, and you're adding a big dollop of new trouble.

Turning in one direction would be a little easier as the gyroscopic forces lift one wheel and 'flatten' the disc angle a bit, but turning in the other direction would tilt the disc more upright and add resistence.

I think you're better off shooting a big hole in one foot rather than a smaller hole in both feet; stay either flat or upright.

A: Where did you find pneumatic components small enough for an ant?

A simple single-pivot flipper will require a hinge at one end that attaches to the robot chassis, and additional hinges at each end of the pneumatic actuator where it attaches to the chassis and flipper arm. The design of the hinge is less important than making sure it is strong enough to survive the forces that will act upon it.

A: Mark J. here: the advantage of driving a pivoting arm with a rack and pinion is that a source of linear force (pneumatic actuator, linear actuator) can apply a constant rotational torque to the arm over a large range of motion. This is very useful in powering a single-pivot axe or hammer, but less useful in actuating an arm that has limited range - such as the front bar on a 4-bar lifter.

In a 4-bar lifter, the lifting force applied will be non-constant even if the force applied to either the front or back arm is constant. Whether or not you will get more force from a rack and pinion or by direct application of linear force to an arm will depend on the details of your 4-bar design. The T.i. Combat Robotics 4-Bar Simulator will let you experiment with torque requirements and lifting force with different 4-bar layouts. I don't have a tool to provide similar calculations for direct application of linear force.

A: There may not be such a thing as a single 'best'. Different opponents will have different strengths and weaknesses. A sharp 'cutting' tooth that slices thru one opponent may jam and stick in another. Any design will be a compromise. Take a look at what's working against the types of robots you expect to face and be prepared to punt.

A: A wedge takes a lot of abuse. If your wedge is detachable you can make easy repairs/replacements/modifications. If your whole robot structure is the wedge it becomes more difficult to maintain. Let your opponent beat up something you can replace quickly in the pits -- not the basic structure of your machine.

A: Mark J. here: lots of unspecified variables here! 'Power' is defined as the amount of work done in a specified time period, so:

• The first variable is how much work is involved. That depends on how much weight is actually on the lifting plate (the whole 30 pounds, I'm guessing) and how high the plate will lift the weight when the bladder is filled.

• The second variable is how fast you want the bladder to fill. With proper gearing, a very small motor could fill your bladder and lift the required weight to the required height, but it would happen very slowly.

• A 'hidden' variable is the power lost to friction in the gear train and to the piston seals.

If I were going to do this, I think I'd ditch the piston and cylinder design and use a small high-volume electric air inflator like you use to inflate a raft. I've seen versions that plug into car cigarette lighters - more compact and no heavy cylinder and gear train required.

A: You can orient a 4-bar mechanism any way you like: up, down, sideways, or someplace in-between. I don't like the idea of trying to 'push away' an opponent - you are just as likely to push yourself away from them as you are to push them away from you.

What will you have gained by increasing the distance between your opponent and the center of your robot? I don't think that counts toward aggression points, it's unlikely to do any damage, and your opponent gets just as many damage points for beating up your 'bumper' at the new location as they would if it had never moved.

The 4-bar lifter is a proven effective design. Lifting your opponent disrupts their attack, keeps their weapon away from you, and controls the match strategy. I think I'd stick with that.

A: There are several featherweight 4-bar lifters in the UK, and 'Defiant' won the 1997 US Robot Wars 'lightweight' class at a weight someplace close to 30 pounds. There are also successful examples in both lighter and heavier classes so I can't think of any design reason why it would be a bad idea.

A: Mark J. here: by 'hole in the front of the cylinder' I'm assuming you're referring to the gas port that would be used to retract the actuator in a double-acting system. I'll also assume you want to enlarge the port to speed up the actuator by allowing the 'exhaust' air to exit more quickly.

You can certainly enlarge the port, or add an additional port if you like. Consider some measures to protect against debris entering the cylinder, and be careful to retain enough strength in the end cap to keep things together. From a practical standpoint, don't expect a big increase in actuation speed. You didn't mention the size of your cylinder or the existing port but your speed gain is apt to be small.

Q: Aaron, I'm trying to build a beetleweight hammer. For the hammer, would it be suitable to use a brushless motor and pulleys or gears to effectively drive the hammer?

A: In spite of builder claims, I can't find an electric hammer with a winning record in any weight class - they just can't generate enough power. Search this archive for 'electric hammer' to find a recent discussion on the topic and a link to our electric hammer calculator spreadsheet.

Q: Mangi has been a dominate force in the 30lbs Sportsman class for the past few years.

A: Fair enough. 'Mangi' has a winning record, but I must point out:

• The 'sportsman' class isn't the same as open class combat. A lot of things fly there that wouldn't work anywhere else;

• A record of 11 wins and 9 losses isn't what I'd call 'dominant'; and

• Mangi's builder has been in combat robotics for a very long time and could win matches with a half-empty tub of margarine.

A: In general, any weapon that is controlled by its own radio channel is considered active. 'Rat Amok' has a spring-loaded bar that is held and released by a servo controlled clasp. Since the weapon servo has its own channel control, it is considered to be an 'active' weapon.

A: The kickback would be cancelled only if both blades dug into your opponent equally and at the same time. That's a really big 'if'. I'd be willing to bet that almost all the time you'd have only one blade dig in and toss you off to one side or the other. With a single blade you at least know which way you'll be kicked! Having one blade dig in would also be really hard on whatever type of drive you had that spun the blades in opposite directions.

One thing that counter-rotating blades will cancel out is gyroscopic forces. Not a big deal on horizontal weapons, but with vertical blades it would improve mobility a great deal.

Final verdict: thumbs down. We like simple designs at Team Run Amok, and counter-rotating blades add more complexity than they would be worth.

A: Yes - this has been answered previously. Search this archive for "eggbeater weapon".

Q: hi again i found the answer to my question before but what is a good speed for teeth to get a good bite in a ant weight
would 100 times a second be ok?

A: For the benefit of others, the formula is:

Example: two teeth on opposite sides of a disk spinning at 3000 RPM = 1 ÷ ((2*3000)/60) = 1/100 second between teeth passing.

I can't give you a specific time interval that's 'OK'. More time means better 'bite' and the ability to effectively impact a 'smoother' target, but more time means slower rotational speed and less energy. About 1/100th of a second is a fairly typical antweight tooth timing - you'll have some bite against edges and corners, but very little against smooth surfaces. You can always add a speed controller to your weapon motor and choose your speed to match your opponent.

Thanks so much

A: Mark J. here: we're always happy to hear from builders making good use of our design aids.

At what point is the amount of kinetic energy overkill? Many current spinners have energy numbers similar to those you calculate for your own design, but those big numbers invariably come from spinning the weapon at very high speed.

Consider: at 15,000 RPM the longer bar on your beater makes one rotation every 0.004 second! You've got to stuff some part of your opponent's 'bot inside the swept radius of the weapon after the tooth goes past but before it comes back around. Unless your opponent has a very sharp protrusion that is well anchored to the chassis, and unless the closing speed of your 'bots is very great, you're going to have a very hard time doing that. A very high speed weapon will just 'skitter' across the opponent without damage, unless the opponent did a very poor job of smoothing exposed edges.

So 3000 joules isn't itself a problem, but the rotational speed at which you are storing that energy renders it ineffective. I'd say your weapon has too little rotational mass and you're spinning it too fast. Double the mass and slow it down to about 4000 RPM to store around 370 joules - you'll have a much more effective weapon.

Note that there is one situation where high weapon speed is needed: going weapon-to-weapon with another vertical spinner. The slower weapon loses in that matchup. Consider a speed controller for your weapon so you can 'crank it up' when there's a need of speed.

A: A melty brain spinner is a very complex and expensive design to implement. It requires electronics expertise, careful construction, and a great deal of development and testing time. A drop in price for the drive motors and ESCs is unlikely to greatly increase their popularity since those components are a very small part of the total robot budget.

Melty brain spinners are able to use brushless drive motors because they do no pushing and the drive spends most of its time just maintaining spin speed. I would still recommend use of premium quality motors, as they come with reliable specifications that are critical in obtaining optimum performance from the weapon design.

In spite of their status as a considerable tecnological achievement, I'm not much of a fan of the melty spinners. They aren't much fun to drive, they're very susceptable to 'spinner killer' scoop countermeasures, and they tend to 'hockey puck' themselves around the arena after a good hit. Exciting, but likely to do as much damage to themselves as their opponent.

Thanks so much! You are so helpful.

A: I'm not sure how helpful I can be here. You haven't told me near enough for me to guess at how much load is going to be placed on that shaft. What is the mass of the spinner? How fast will it spin? How many joules of energy will it have? How many bearings will support the spinner on the dead shaft? How far apart will the bearings be? How far apart are the supports for the dead shaft? A competent engineer would require all that information at least, and as we point out in the #17: we aren't an engineering service.

I can say that in general I would not recommend an aluminum tube for a spinner dead shaft. Newton pointed out that for every action there is an equal and opposite reaction, and the point at which that violent reaction is transferred to your robot is the dead shaft. It has to absorb energy equivalent to the energy that will smack your opponent. Is your hollow shaft design up to that punishment? I'd recommend a well secured solid rod, at a minimum.

I also cannot recommend use of needle or ball bearings without an inner race for an aluminum shaft. The hard anodizing would help, but hardened steel rollers on aluminum is not a good match. I'd use bronze bushings for an aluminum shaft -- they stand up well to hard impact.

Q: If I want my [weapon] motor to be above the blade similar to 'Greenwave', what is the most reliable way of attaching the motor? I am thinking of having the motor mount clamping around the 1/2 inch aluminum [dead] shaft. Do you think it would be strong enough to stop the horizontal blade from moving upward from hits from drums and such?

A: Mark J. here: there are good reasons why you don't see many 'Green Wave' style spinners. The design places enormous stress on the dead shaft since only one end of the shaft is supported. Mounting the shaft to the chassis securely enough to withstand the impact force is a real problem. See the previous post on 'Green Wave' in this archive.

The motor mount will pretty much have to clamp onto the dead shaft since nothing else extends above the spinner! The design of the clamping mount will determine its strength but, since the whole shebang is likely to tear itself free from the chassis on the first good hit, you probably don't need to worry about the motor mount strength.

A: There are three reasons why specific types of weapons are banned:

1. Safety: weapons must not be a risk to the drivers, crew, spectators, or the arena (explosives, projectiles...)

2. Delay: weapons must not cause an unreasonable delay in the tournament to clean up after their use (liquids, adhesives...)

3. Lack of entertainment: the audience came to see robots fight. Weapons that have no visable impact (radio jamming, electrical discharge...) or that interfere with audience visibility (smoke screen, high-intensity light...) defeat the whole purpose of having a robot fight.

Problems could include protecting the delicate shaft. What do you think of the idea?

A: Mark J. here: your explanation is quite clear, but you can't gain energy by moving mass inward or outward within a rotating system. Yes, the hammer will be easier to start spinning with the mass closer to the shaft, but as the head mass moves outward the speed of the hammer as a whole will slow down to conserve angular momentum -- you don't gain any impact power beyond what the motor is providing. See the Hyperphysics notes on Angular Momentum.

However, if you get the point and rate at which the head moves just right you can gain just a little efficiency (less than 10%) by keeping the motor RPM in the optimum power zone. For multiple reasons it is better to simply pick a little more powerful weapon motor to acomplish the same effect for your hammer.

A: I agree that there are a great many problems with the 'drilling virusbot' idea, but I don't agree with all of your points.

• I don't think the virus would count as a multibot. A combat robot has to have mobility under the rules; since the virus doesn't I think I can argue that it doesn't qualify as a robot and therefore isn't part of a multibot. Call it a 'detachable weapon pod'.

• Most current multibot rules require that 50% or more of a multibot by weight must be immobilised for a K.O. If the virus does count as a multibot but weighs less than the primary 'bot, you could lose it and still continue.

• I can't recommend magnetic attraction for a virusbot. Too many common robot armor materials (aluminum, titanium, polycarbonate, stainless steel, garolite...) are non-magnetic.

Depending on the virusbot's location, the enemy could take severe internal damage. [Oakland, CA]

A: I think it has too many 'dependings' and an unfortunate 'slowly'.

• Many robots aren't going to provide any good spot for suction cups to stick;

• If you do stick down you're unlikely to be in position to drill thru anything critical;

• The drilling mechanism would be fairly heavy and complex (we like simple); and

• While slowly being drilled, your opponent is rapidly beating the lockwashers out of your main robot.

Q: Are Diesector's jaws modified ''Jaws of Life''?

A: No. Builder Donald Hutson writes on the 'Diesector' webpage:

"At first, I wanted to use a "Phoenix" Jaws of Life but they were very heavy, slow, and require a 5000psi hydraulic pump... I ended up fabricating my own jaws using two heavy-duty MOTION (1000lb) actuators and some 3/8" steel plate... The idea of the jaws wasn't exactly to crush other robots, it was to create some thing that would be fast, reliable and gain control."

Personally, I believe that the reason flamethrowers are ineffective is that they are overregulated. (In addition, the overregulation dashes my hopes of one day seeing a flamethrower bot explode.) My friend's homemade propane flamethrower is more effective than what I have seen in the robots.

A: There are a number of obstacles beyond rule infraction that would effectively prohibit use of a plasma torch in a combat robot. I don't know of any plasma cutter that will run off 48 volt battery power, electrical interference from a portable unit would fry your own electronics, and the need to 'strike an arc' and ground to the opponent would render the weapon useless against any non-conductive surface.

Topping the list for rules infractions would be section 12.4 of the RFL rules: "Heat and fire are forbidden as weapons." Granted, heat weapons may be allowed at the discretion of the event organizer, but no EO is going to allow a plasma torch loose in their arena.

Also likely to put the boot into a plasma torch is section 12.1.2 of the RFL rules, which prohibits "RF jamming equipment". Portable plasma torches typically throw enough RF interference to blot out the whole radio spectrum for a fair distance.

Terribly sorry that you find it difficult to enjoy robot combat without arena-filling explosions and balls of flame. If it were your arena, you'd think differently.

A: Mark J. here: several comments:

• Electric hammers are not effective. The difference between a correctly and incorrectly geared electric hammer is going to me minimal - neither will be impressive.

• As pointed out in the [next post down], calculating optimum gearing for an electric flipper or electric hammer weapon is similar to calculating the gearing for a spinner weapon but greatly complicated by the need to include the rotational mass of the armature [and geartrain].

• Data on the rotational mass of specific motor armatures is not available. A 'best guess' is about all you can reasonably do.

• I have a modified version of the Run Amok Spinner spreadsheet that examines the early stages of weapon spin-up and approximates the action of an electric hammer. It is difficult to use, time consuming, and provides only approximate results.

A: Mark J. here: it's tempting to think that a bigger and more powerful electric motor would result in a big lift speed increase, but that isn't always the case. A lot of the low-range torque in a large motor can be used up accelerating their own substantial rotating armature mass. This is inconsequential in the hundreds of revolutions a spinner weapon makes to get up to speed, but for an axe or lifter it's another matter. Your proposed lifter motor would rotate only half a revolution to raise a simple lifter arm 45 degrees, so a good portion of the force that you would hope to go into lifting your opponent will be absorbed by the rotational mass of the motor itself.

You'd get a good lift with a reasonable length lifter arm, but I think you can forget about the weapon being a 'flipper'.

A: Mark J here: I'm more than a little concerned about driving a hobbyweight spinner from a 1/8" shaft. The small hub diameter timing pulleys that would fit that shaft won't handle enough torque for a decent hobbyweight weapon. Consider a different motor and/or a machined hub to connect the shaft to a properly sized pulley and belt - see the RoyMech website for guidance.

For general reference, timing belts can slip if set up very loose. I've seen several insect-class spinners running loose timing belt drives. Alternately, you can turn the belt inside-out and run the smooth side against the pulleys. Align the pulleys carefully and use a wide enough belt to handle the torque or you'll have real trouble keeping the belt on the pulleys!

A: See #4. I think the probability of anyone building a successful hobbyweight crusher is very small.

The probability of someone who has to ask me 'how to do it' building a successful hobbyweight crusher is indistinguishable from zero.

A: Both possible and fairly common. Section 7.2 of the 2010 Robot Fighting League rules says:

CO₂ is stored in compressed, liquefied form and gives more 'shots' for the size of the storage tank, but the entire pneumatic system gets very cold from the conversion of the CO₂ from liquid to gas. High Pressure Air (HPA) or nitrogen can both be stored at higher pressures than CO₂ and they avoid the 'chill' problem.

A: It depends on how much 'slippage' is in the belt drive. Formula:

Example for a Small Johnson motor and a belt allowing the motor to run at 30% of max RPM with the weapon stalled:

That's peak amps. Hopefully you'll shut down the weapon quickly when stalled, so the 'continuous' amps can be considerably less.

Q: How do I figure out belt slippage, then?

A: Trial and error. Start too loose and tighten 'til you get close.

Q: Sorry to bug you on the belt slippage issue again, but I still don't get how exactly you can tighten the belt once you have mounted the motor and the two pulleys. Drilling mounting holes is hard enough as it is when it comes to motors, and you can only make so many holes in a piece of metal...

A: Off the top of my head: eccentric bearing mounts, an idler pulley, and elongate slots with adjustment shims (don't rely on just the mounting bolts to hold tension) all address your adjustment issue. As the belt wears, further adjustment will be needed.

A: Yes - more power (from two motors or a single more powerful motor) will allow for a faster lift, but selecting the optimum gear ratio is critical. A lifter that appears faster when operating under no-load may well bog down to a crawl or even stall completely when loaded down.

The trick in getting the correct gear reduction for best lifting speed is in understanding that a permanent magnet direct current (PMDC) electric motor generates its peak horsepower when loaded to produce half its stall torque:

• If you load the motor more heavily (too little gear reduction) the motor bogs, pulls excessive amperage, and the lift speed decreases.

• If you load the motor more lightly (too much gear reduction) the motor spins more freely, pulls fewer amps, but again the lift speed decreases.

To calculate for two RS-550 motors, just double the torque.

Q: Thanks so much for the help with the RS-550 lifter info Aaron and Mark.

A: Happy to help.

A: It's kinda overkill, but the PDX256 would certainly get the job done. That gearmotor will deliver enough power to lift 90 pounds at the end of a simple 8" arm using only half of its rated stall torque. I don't think I'd stress the gearbox with that much weight, but it should certainly be tough enough for a featherweight lifter.

The IFI VEX Pro Victor HV-36V is also overkill. Lifting 30 pounds on the end of an 8" arm, the PDX256 gearmotor will pull only about 25 amps at 12 volts, well within the capacity of the less expensive IFI VEX Pro Victor 884.

Q: Could you tell me the formula for calculating the amount a motor will lift? If I could get away with a smaller motor, that would be great!

A: We've given a formula previously, but I think I can make it a little clearer. For a simple lever arm lifter:

What is 'TAF' in the equations? The more heavilly a motor is loaded, the slower it runs. If loaded to its full stall torque rating, it will not move at all. The 'Torque Allowance Factor' at the end of the formula de-rates the max load to reduce current draw and allow a reasonable lift speed. I use a very conservative allowance of 2 for my robots. Some builders leave the allowance off entirely (= 1). You'll be OK with a 'TAF' around 1.5.

Examples for your PDX256 gearmotor:

A: Proper design fundamentals: powerful motor, correct drivetrain, well-designed cutting head. It didn't hurt to have a lot of opponents with weak side armor.

If nessicary, I can use two motors to power hammer.

A: Mark J here: I'm curious - if you aren't sure what a joule is, how did you decide that 40 was a good number?

Scroll down the page to the next question -- it's from a builder of a sportsman class electric hammer 'bot that uses the RS-550 motor you're interested in. Calculating weapon power from a specific motor for an electric hammer is not simple, but my solution for his weapon generates someplace close to 60 joules. I think you'll need more gear reduction than the PDX26 gearbox provides -- add some in your belt drive stage?

A: Hi, Anthony! I haven't heard from you for a while. Are you asking about powering the robot drivetrain or the weapon with the P60 gearbox?

For a drivetrain, the BaneBots P60 gearbox attached to the RS-550 motor would provide ample drive power and speed for a 30-pound robot. The RS-550 will operate quite well on 12 volts. The gear ratio will depend on the wheel size and the size of the arena. The 25.92:1 gearbox with 4" wheels would be about right for a 16 foot arena.

For the weapon, I don't think you're going to be happy with an electric drive. I'd need more information about the length and weight of the hammer (and a fair amount of time) to do the calculations, but he electric hammers I've looked at for other people's designs have all shown poor performance.

Q: hi aaron its anthony. the hammer arm on my robot will be 18 inches long and the hammer head will be 1 pound. the hammer arm will be made out of 3mm thick T-6 aluminum square tubing. thank you for your help i realy appreciate it.

A: Mark J here: an electrically powered hammer is difficult to model mathematically. The inertia of the motor armature and associated gearbox elements plays a large roll in the acceleration of the hammer. A model which fails to account for drivetrain inertia provides deceptively high performance for high gear reduction ratios.

I made some modifications to my copy of the Run Amok Spinner spreadsheet to better model the very early stages of weapon acceleration for your hammer weapon. The best numbers for the RS-550 motor and P-60 gearbox came with the 64:1 gear ratio. I'm not entirely confident of the numbers, but I calculate an actuation time of about 0.25 second and around 60 joules of energy at impact. That's very poor energy delivery compared to a featherweight spinner that could conservatively pack 600 joules, so don't expect to do much damage.

Q: dear aaron thanks for your help! im now registered and preparing to go to the franklin institute 2010. now im thinking of future upgrades for my bot. the upgrade i was thinking about was istead of using a speed 540 size motor i was thinking about using an ampflow A28 400 motor geard to 2.1 and having a one pound hammer head with the same 18 inch long 3mm thick aluminum square tubing atached with strong ball bearings and on the main shaft how much power or jouls will this have compared to a feather weight spinner?

A: You'll have a much better idea of what type of upgrades your bot will benefit from after the tournament, Anthony. You might be very surprised to find out that your weapon isn't your top priority.

Calculating energy for an electric motor powered hammer weapon is not easy -- those of you who may think it is are overlooking several factors. My adaptation of the spinner spreadsheet works fairly well for hammers powered thru large gear reductions, but with big motors and small gear reductions it works very poorly. I'd need to know the rotational inertia of the AmpFlow A28-400 armature to even get a start on the calculations and I just don't have that info.

Write back after the tournament and we'll figure out what upgrades your robot really needs.

Q: dear aron i just got back franklin institute 2010 and my robot came in last place. some how my weapon motor burnt out when it was tested. this acident was my fualt for not getting upgraded spectrum transmitter equipment. [The RS-550 motor with a] 64:1 gear ratio was not very powerful and so i took apart a 12 volt cordless drill and hooked the hammer up to it and it still was not impressive. you said that electric power sucks. do you have any ideas on how to better accellerate the hammer arm. should i go with a low pressure phumatics system mabe 300psi i now have more team members pitching in so i can spend close to another grand on it. i know that i'm new to phumatics but i still want to give it a try. if you agree on this can you rcommend a sysytem for me with listed compatible component's thanks for your help. and your site is awsome!.........from anthony..builder of warpz

A: Sorry to hear that things went badly for you at the tournament. I'm puzzled about how radio problems caused your weapon motor to fail, but you seem to have a working theory. I'm more interested in how the chassis and drivetrain performed. It would be a waste of time, money, and effort to concentrate on a more powerful weapon if the other systems aren't sorted out yet.

As we said in our earlier response, a small electric motor like the RS-550 is not going to deliver enough power for a hammer weapon to do much damage to a reasonably armored opponent. Your weapon has about as much kinetic energy as a good beetleweight spinner. But since you have chosen to enter the featherweight 'sportsman' class where high-powered weaponry is not allowed, it may not be a bad match.

I suggest that you continue development of your electric hammer and the other basic systems on your robot. If you can't get the kinks worked out of a relatively simple electric weapon you really shouldn't dive into the added complexity and danger of pneumatic valving systems, actuators, regulators, and pressure tanks. Get your robot to work before you make it more complicated.

A: Yes. Read thru this archive for reasons and alternatives.

Q: I am trying to make a 30lb lifter robot similar to Sewer Snake. Would an old style Dewalt 18 volt motor and gearbox work for the lifter motor?

A: It wouldn't be my first choice; it has way too much speed and only marginal torque for the purpose. Locked in 'low' gear the 18v DeWalt @ 24 volts spins up near 500 RPM, which would make a directly connected lifter difficult to control/position. The torque would be barely adequate for a short (maybe 6") 'Sewer Snake' style lifter wedge in a featherweight -- but it would pull a huge bucket of amps when bogged down or stalled.

I'd go with less motor and a higher reduction gearbox to slow the actuation speed and keep the amperage draw reasonable.

Q: Do you know how THz's axe work? I'm asking because I'm building my third 'bot, one with a functional weapon.

A: 'TerrorHurtz' has a pneumatic axe actuated thru a rack and pinion, much like the hammer on 'The Judge'. Search this archive for 'Jacha Little' to see an animation of the layout and for 'TerrorHurtz' for additional notes.

A: A common solution is to thread bolts into the drum and use the exposed heads as impactors. You can start with the bolts holding down your shift keys.

A: If you read thru the archives you'll find many posts where we recommend that first-time builders concentrate on the basics of chassis, radio, and drivetrain and save active weapons for a later robot. Build a nice wedge for your first robot - you'll do much better with that than trying to get an active weapon right. Follow the link in #26 for more info.

Also, my brother also built a combat robot, but his is a VS (veritcle spinner) and even though mines more simple, his has already broken down after one fight, and has lots of repairs to do. I entered mine last year without a weapon, and it one quite a few matchs. Just goes to show that no weapon can [sometimes] be better than a big weapon, eh?

A: You neglected to tell me your weight class and how large a 'pick' you seek. I think you're searching with the wrong terminology. Try a search for "steel spike" and see if that turns up something suitable. Alternately, you can order some tool steel rod in the diameter you want and grind a point on one end. Another good option would be a cold steel chisel, which you should be able to find locally.

Yes, passive weapons are often better than more complex weaponry.

A: There were many versions of 'Mauler' -- the first being 'South Bay Mauler' which appeared at the 1994 Robot Wars event. It was 'Mauler 5150' that performed the famous instability flip at BattleBots 4.0. The 'standard' explanation of the cause of the flip is that the shell was not properly balanced -- but that's not correct!

'Mauler 5150' violated a key law of rotational stability: of the three natural axis around which a rigid symmetrical body may rotate, the axis with the greatest and the least rotational inertia are stable, but the third axis with intermediate inertia is not! The spinner shell had two heavy impact hammers on opposite sides of the shell that made spinning on edge with the hammers to the outside the axis with the greatest rotational momentum! Distributing the impact mass more evenly around the perimeter of the spinner shell would have prevented this problem. A web search for 'rotational stability' will provide more detail on the physics involved if you're interested.

A: No, but don't confuse power and force. Power is the product of force and distance over time. Moving the rear bar will take less force, but the greater angular distance involved in moving the rear bar offsets the saving in force and ends up requiring the same amount of actual power as moving the longer front bar.

If you are using a linear actuator, it's usually more mechanically efficient to power the longer front bar because of the smaller change in angle it undergoes as the lifter rises. If you are using a gearmotor attached to the lower pivot point, the reduced rotational force (torque) required to move the shorter rear bar may be the attractive option.

A: At BattleBots 4.0 and 5.0 middleweight 'Zion' had a single pivot pneumatic lifter with the actuator attached to a lever descending from the point of the high mounted pivot. There is a photo of Zion and a brief discussion of its design in the archive.

A: 'Son of Whyachi' has evolved thru several different rotor power configurations. The most recent version used two 15 hp Yamaha gasoline kart engines. The previous version used three Eteks. The BattleBots champion 'shufflebot' version used two Eteks.

A: We're talking about torque limiting slip clutches that decouple the motor from high loads near stall. Combat robots generally don't use them. This isn't the science fair, it's combat and you want the motor to deliver all the torque available even at the risk of melting. A fast finger on the shut-off switch and faith in the design is all the protection many weapon systems have.

The exception is for spinner weapons. Spinners often use a belt drive that can slip under high torque loading on impact. This is particularly useful with high performance brushless motors that just won't tolerate stalling.

On a related topic: combat robots don't fight with fuses in the power system. If you blow a fuse it's an instant loss and the robot becomes a chew toy for your opponent. Get every last gram of fight out of your machine. If you have to lose, go down fighting and smoking!

A: So - a horizontal, electric, full-circle hammer? The disadvantages I can think of are as follows:

• Weak impact. Even given that 180 degree plus arc, an electric hammer weapon just won't build up much energy compared to a pneumatic hammer or an electric spinner gaining energy over hundreds of revolutions.

• The lack of 'bounce away' would largely be due to that weak impact. If you did manage a high-energy impact you'd get a reaction similar to a horizontal spinner.

• Judging criteria are different now than they were in the BattleBots days. You no longer get credit for impacts that don't damage, and judges unfortunately give zero points for 'new' ideas.

A: You'll want to download the Run Amok Spinner Excel spreadsheet. The spreadsheet can take the dimensions of your spinner weapon and the materials it is made from and evaluate the speed, energy, and spin-up time of that weapon with any motor for which you can get specifications at any reduction ratio you choose. There are many posts about use of the spreadsheet in this archive. You'll also find help here with weapon drivetrains.

A: Event organizers take safety very seriously. A flamethower design with any real chance of blowing up would not be allowed to compete. It hasn't happened and it isn't going to happen.

A: Possible sure, but I see a few of problems:

• I don't know what you expect to grab onto with those hooks. Seems like a low-probability attack.

• Hammers are designed for speed to give the greatest energy on impact. A lifter is usually designed for power, particularly if you've got something 'hooked' onto the end so that you're not going to be able to 'flip' it. Those two design requirements don't mix well.

• The 'hammer' end needs a good bit of hangover on one end of the 'bot to get in a good hit. Your 'hook' end would need a good hangover on the other end to stand a chance of grabbing something on your opponent. With overhang on both ends, the wheelbase won't be able to well support either the lifting or the hammering -- the robot will be 'tippy'.

Q: i meant the hook as a crushing one not a lifter

A: A crushing hook? To 'grab hold of robots behind it' and crush them??? Nah... stick with a single function weapon.

A: Drums aren't found, they're made. Read thru this archive for info on the construction of spinning cylinders. A well-balanced cylinder is not easy to construct -- you may want to consider an eggbeater-style spinner.

A: I've said this before but builders don't want to believe me:

Drivetrain, radio set-up, general construction practice, and weapon/chassis balance are all much more important than the type of weapon you choose. There are plenty of examples of winning robots with ineffective weapons, and there are many more examples of losing robots with awesome weaponry. If you get the basics right you're going to have an above average robot no matter what weapon it carries.

Pick whatever weapon will make you happy and go have a good time!

Can u point out any good links where from I can get more information regarding this matter and the sport?
Hoping for some quick replies.

A: Mark J here: your 'hoping for some quick replies' forces me to remind you of the well-known 'project triangle': fast, good, cheap - pick any two. Since the advice you get here is as cheap as it comes and you want it quick, that means it's not gonna be very good

You've obviously read our archives, as I recognize some of our phrases in your questions. The truth of combat robot design is that the forces are wildly unpredictable. We can calculate the energy available in a weapon, but the actual force delivered is dependent on a great many factors in a complex environment. Trying to calculate the forces and vectors involved will both drive you mad and result in values that are no better than the multitude of assumptions you had to make in order to even start the process.

It is true that one cannot use 'mere judgement' and that trial and error is wasteful of time and money. Fortunately, the efforts of hundreds of builders expending their time and money on thousands of designs that have come before us can be used to good effect. We very often advise builders to 'examine successful robots of design similar to your own' when considering materials, dimensions, and design details. In fairness, examining unsuccessful robots to learn from their mistakes can be equally educational. Learn from the successes and errors of other builders.

A critical factor in designing a spinning weapon is the ability to transfer the stored energy effectively to the opponent. A huge reserve of stored energy is without value if it cannot be effectively applied. This can result in opposing design requirements: high spin rate to store maximum energy and a long period between impactor passage at the weapon circumference to allow time to insert a large piece of your opponent into the 'danger zone'. A very small number of impactors (like 'one') can help here, as can a large diameter weapon with a high moment of rotational inertia to store large amounts of energy at relatively low RPM -- but the large diameter creates other problems. Combat robot design is FULL of compromises.

I'm going to lump your last two questions together and give you a couple of links that address new developments and an analytical approach to robot/weapon design:

So I guess my question is "What am I doing wrong on this spreadsheet?". Maybe I'm using too big of a motor, but it fits well within my weight (well, not after building a chassis to withstand that energy!) Thanks.

A: Mark J here: making a few assumptions about your gearing (direct drive) and the dimensions of your steel tips, I get 2042 joules at 11340 RPM in 0.68 seconds for your proposed weapon. The steel tips should be calculated as a 'ring' equal to their actual mass with outer radius and 'thickness' corresponding to the outside radius of the bar and their length. That probably accounts for the differences in our calculations. You have, however, made a different and very common mistake:

You've got too much motor, too much tip speed and not enough tip weight. A quick calculation with double the tip weight, a 3:1 belt speed reduction and the same motor give you a weapon that spins to 395 Joules at 3780 RPM in 0.13 second -- much more workable. Try a smaller motor and add the saved weight back into the steel blade tips.

Q: I did realize that this many RPM would be too high, and as you have done I geared it down and messed with different weights, I did not assume your spreadsheet giving me high energy storage would give me a good weapon.

I originally had .7 lbs into my spinning weapon, but even lowering to a more usable 3000 rpm with gearing down gave me scary results, similar to what you got. I estimated between 50-100 joules would be more than enough, I'm not sure if I could build a chassis with 1 lb that would survive that.

In fact the higher RPMs could be usable, if in a different way--depending on the final design.

I was thinking about gearing a vertical spinner with about 6-8 inch diameter weapon at about .5 lb. I'd have the weapon mounted close to the floor with spring suspension on my axles so that when a big hit happened the force would be transferred through the mounts to the floor -- is this viable?

I was also wondering if anyone has tried using a second, smaller flywheel next to their main weapon spinning in the opposite direction to counteract the gyro-forces? I realize this would take out some weight, however it could be made to be a true cylinder with little supporting it (since it wouldn't take large hits) and therefore weigh less...

If this would work, would it happen that when I hit someone the weapon would, for a moment, slow down enough to change they gyro forces to flip me?

A second opinion would be nice! Thanks

A: Aha! A reasonable and thoughtful approach to weapon design. Please consider my previous warnings to be for the benefit of the less experienced designers who may read this discussion.

I agree that 50 to 100 joules is plenty of energy for a beetleweight spinner. Careful design can provide a light chassis that can handle the resulting impact force quite well, but you are entirely correct to be wary of high energy levels.

There is a special case where spinners can make use of really high RPM. When two drum-spinners go 'weapon-to-weapon' the slower drum looses. I have seen drum spinners that cruise at a moderate weapon speed for most attacks, but have spare motor speed that the driver uses against other drums. This is probably not useful for bar spinners.

Spring suspension: I think not. Read thru section 6.6.5 of the RioBotz Combat Tutorial. They emphasize stiffness in the 'force path' between the weapon and the floor and make good points about this approach. I don't agree with RioBotz on all topics, but they have put a lot of thought into a wide range of design issues.

Counter-rotating gyroscopic masses: I haven't seen this used on a combat robot, but your physics thinking is correct -- two coaxial masses spinning in opposite directions can cancel each other's gyroscopic forces (see: discusion on coaxial helicopter rotors). This will make turning much less of a problem for a vertical spinner as there will be no appreciable gyroscopic precession attepting to push the robot over onto its side. Unfortunately the stabilizing effect of the spinning weapon is also removed, rendering your robot more succeptable to tipping from outside forces. Calculating the relative speeds of two unequally sized counter rotating masses to precisely cancel each other is not trivial, but there are equations in section 6.15 of the RioBotz Tutorial that you may find useful.

Don't worry about the gyroscopic effects from slowing down the weapon with a hit. Slowing one of the two counter-rotating spinning masses would return some gyroscopic stability, not create instability.

Q: I read through the Riobotz tutorial (which has a ton of information) and I understand that the path needs to be as stiff as possible to transfer the most energy to my opponent.

Back to the suspension idea - The springs holding the wheels in place would provide just enough force to keep the robot ~1/16" off the ground. This being said, when being compressed further hardly any energy would be lost and the second the chassis hit the ground it would provide all the stiffness needed, as per Riobotz.

Having this set-up would possibly increase mobility (over designs such as Altitude which just slides on the ground) but would probably add too much complexity when obviously a steel ball with a rounded end gets the job done!

A: Thanks for the more detailed explanation of your suspension. You may need quite a bit more spring pressure than you think to keep the chassis from 'digging in' during maneuvers -- particularly since the weapon is generating gyroscopic precessional side-forces! I'm a big fan of keeping things simple and would probably stick with ball casters or teflon skids.

It seems like if you got that spinning you'd throw anything pretty far, but I'd rather not throw half my bar with the other 'bot. (I'm not sure if it's THAT impact resistant) Maybe I'll hook it up and test it...

A: No, I don't like the idea at all. UHMW is resistant to damage from impact because it's very flexible. You want the bar to be extremely stiff to impart as much of the stored energy as possible to your opponent rather than flex on impact to dissipate that energy. A rod spinner is quite inefficient at energy storage to start with, so you can't afford to waste energy with a squishy impact. It'd be like hitting a ball with a foam rubber bat: it wouldn't break but you're not gonna hit a home run with it.

I've seen UHMW used for the body of a disk spinner where there was less impact flexing, but even there I'd suggest something stiffer!

Q: Some more thinking has made me realize that a spinner bearing 1/3 of my weight is probably not the best route, so the rod may be shortened some (1 ft or so, with the steel impact ends), but more importantly I had another question, do current spinners incorporate any sort of slip-clutch mechanism ?

It seems like it would make sense to put one between the drive gear and the actual bar in order to put less shock on the motor (if left in full throttle during an impact) as well as providing a way to lessen the force that would fling my robot across the arena (in the case of a horizontal bar).

Any input?

A: Mark J here: the preferred drive for a spinner is a belt and pulley system which incorporates a natural 'slip' capacity. The need for weapon driveline slip decreases with the weight of the weapon, but it isn't a bad idea even in the insect weight classes. The shock on the high-reving motor will be large whether you have the throttle on or off at impact - just don't leave the motor on if the weapon gets jammed.

A slip clutch isn't going to lessen the newtonian action/reaction 'kickback' on impact -- that's applied directly to the fulcrum point of the spinner axle supports with a vector opposite to that imparted to your opponent. Your proposed flexible spinner rod would reduce the kickback, but would also (as Aaron pointed out) reduce the impact on your opponent.

I suggest you read thru this archive for spinner weapon design tips.

A: The '2010 RFL rules' define 'entanglement devices' as:

Strips of tape that could wrap around rotating parts are out, but a firm sticky surface is technically allowable. A specific event organizer may or may not agree. I'm not sure what you're going to do with your opponent stuck to the top of your 'bot that's going to score any points. A mechanical 'clampbot' might be a better choice, and is certainly legal.

Q: Back hinge flipper guy again, the reason I thought of that was so there was no way an oponent could come off before I flip it, and using something quick and cheaply replaced.

A: Your first question said you were building a lifter. Has it turned into a flipper now? I don't think you want a sticky flipper -- you need to get a flipper as far 'under' your opponent as you can, not stop them before they are in position.

If I can get the power transmission more efficient then I could possibly purchase the same actuators but with a higher speed and less force.

A: Mark J here: it sounds like you made a decision about the height of your robot and now you're trying to design a lifter assembly with specific performance requirements to fit that height. It's not good design process to compromise the performance of a critical system by forcing it into an arbitrary dimensional envelope. I don't believe it's possible to fit those actuators into an efficient mechanical linkage in a 2" space. Let's look at some options.

A rack and pinion drive would not be desireable even if it did meet your dimensional requirements. The torque requirement for a powered bar on a 4-bar lifter varies with its position -- high near the 'flat' collapsed position and typically decreasing as the lifter rises. A rack and pinion would provide equal torque across the entire range of motion, which would not be efficient.

The geometry of your current solution is even worse. For the reason you point out, the actuators would provide very little rotational force with the lifter in the 'flat' position where the requirement is greatest. That's simply not going to work.

We (very) often advise builders to examine successful robots of similar design to gleen ideas. If only there was an extremely successful, very low profile robot with a 4-bar lifter powered by electric actuators... Hey, what about the most successful combat robot of all time: 'BioHazard'? Did you happen to wonder how Carlo beat this problem?

About half way down the BioHazard mechanical design page is a section called 'How does BioHazard's arm work?' Very briefly, the solution is to use a bellcrank that offsets the lever arms against which the actuators press by 90 degrees relative to the bar. This allows maximum rotational torque exactly where the lifter needs it and then transitions to greater speed advantage as the lifter rises. There are multiple pictures of the mechanism on the mechanical design page, and more photos on other pages at the site. I recommend that every robot builder read every word on every page at Carlo's site.

Do note that it takes a great deal of force to actuate a compact 4-bar lifter even with an efficient design. The actuators in 'BioHazard' produce more than 2800 pounds of force over a 3.5" stroke to lift a 220 pound robot. Scaling that down to a 30-pound featherweight takes you close to 400 pound of force for similar performance. Check your design calculations carefully or you may find yourself with an underpowered lifter.

Q: Thank you for your help.

I had read through Carlo's website a few times, and studied his pictures (including that one) but I couldn't make out exactly what he did until now, when you pointed it out. I had thought about doing the very same, although I haven't quite worked out how to make it fit.

As for the scaling down, I'm powering the rear bar which is much shorter than the front, which means the torque reduction will be significantly less than on Biohazard where the bar looks to be about 5-7 times as long as the one the actuators push on.

I will go back to my perpendicular bar design and figure out how to make it fit -- thanks.

A: Glad to help. The photos on Carlo's site are not great, so I'm releived that you were able to figure it out. I've made a drawing for those not so familliar with the issue.

About powering the 'short' bar: yes, the rear bar will require less torque, but it also has to move farther. To get it to move farther you'll have to reduce the length of the bellcrank. Reducing the length of the bellcrank increases the 'push' required to move the bar. If you run the numbers you'll find that it makes no difference which bar you power with your linear actuators -- either one will require the same amount of force to fully extend using the full actuator stroke. You may also get into trouble with actuator angles if the short bar needs to move thru more than 90 degrees.

A: Mark J here: from both Aaron and myself, you're welcome. We enjoy answering your questions.

In general I can say that I don't recommend any tool steel alloy for an entire spinning weapon. Tool steels are hard and inelastic, which makes them a fine choice for impact teeth or inserts at the ends of bar weapons. The problem is that tool steels are not tough and resilient; they tend to shatter when placed in tensile stress. A bar spinner weapon experiences a large tensile load on impact. I'd suggest you make the bar out of a tough aluminum alloy (or titanium if you have the budget) and save the tool steel for bolt-on impact surfaces. Don't forget to have them hardened!

Now before somebody writes to mention that Ray Billings uses a tool steel bar on superheavyweight 'Tombstone' without any trouble, I'd like to point out that a very large chunk of the entire robot's weight is in that massive bar. I think you might be able to build a bar that thick out of glass and not have it break.

A: That's a huge drum for an antweight! Recheck your calculations. I show the drum itself weighing more than 5.5 ounces without the impactor or end plates. I'd recommend full circle endplates to better support the ends of the drum and keep it from distorting.

I really can't tell you if 1.5 mm is 'thick enough'. Strength depends on design details as well as quality of construction, plus it's impractical to predict the loads the drum might need to endure. I will say that 1.5 mm sounds thin given the large size. If I wanted to build an ant weapon that large I think I'd go with an 'eggbeater' design for greater strength and simplicity of construction.

A: Mark J here: that's not an unreasonable question and I'm not picking on you, but I've had a rant building up for a while and today is the day I pop the cork. Many of the design discussions we get into go like this:

Q: Has there ever been a robot with fruit as a weapon?

A: I don't recall any fruit -- but lightweight 'SPS3' carried a carrot on a stick at the 1996 US Robot Wars.

A: Go with the EFL-370-1080. It's less powerfull than the 1360, but more reliable (less likely to melt) and still overkill for an ant spinner. Geared down 2:1 at 11.1 volts and assuming a solid disk, the spin-up time is well under half a second with peak stored energy around 33 Joules at 6000 RPM. The spin rate is a little high, but with a single impact tooth I think you'll be fine. It sounds like a good match of motor and weapon design to me.

Q: Thanks for the help with the Park question. It will come in handy! What is the [stall torque] on the Park EFL-370-1080? I know is 0.73 [N-m] on the [EFL-370-1360] but I could not find the Ri [internal resistance] for the 1080. Thanks.

A: I calculate an estimated 0.78 N-m stall torque for the EFL-370-1360 at 11.1 volts (100 mOhm Ri) and 0.52 N-m for the EFL-370-1080 at the same voltage (190 mOhm Ri).

A: The 'safety requirements' section of the 1997 US Robot Wars rules states:

A: I can't recommend pneumatics for a new builder -- it's best to keep your first robot very simple. You'll have enough things to worry about with battery maintenance, R/C system set-up, traction issues, ESC mixing, driving, radio interference, wireing, tournament procedures, and repair problems. You really don't need to add to that with pneumatic valving systems, pressure tanks, weapons channels, digital switches, complex attack maneuvers, and additional pit checklist items.

There is plenty of information on pneumatic systems in this archive, and I have pointed many builders to the Team Da Vinci Understanding Pneumatics page for a good explanation of combat robot pneumatic systems.

A: We don't compete in sumo, but the last time I checked the rules a sumo 'bot has to be 'harmless'. I don't think that any flipper weapon is going to qualify as harmless. Check with your event organizer -- some don't even allow slow lifters.

A: Conveyor belts?? No. Why would there be?

Q: Conveyor belts are a dynamic weapon, even though they are ineffective. What was the most sucessful Battlebots robot that had one, if there were any?

A: I don't know of any combat robots that used anything that I could describe as a 'conveyor belt' weapon.

A: Scan down a couple questions to the post on non-pneumatic power for a 15 pound flipper robot.

A: I'm not sure what you're talking about. 'Sewer Snake' does have interchangeable devices that can be fitted to the front bracket mount depending on their opponent. You can contact TPC thru their website and get a more specific answer as to when and why they change them.

A: Mark J. here: I like builders who are willing to think outside the box. Whether you come up with something useful or not, it's a good habit to get into. Your ideas all have merit, but each has a serious design drawback as well.

Flywheel based flippers have been on the minds of designers for quite some time. The main problem is a lightweight and reliable clutch mechanism capable of dealing with that large amount of torque. It is possible -- Team Whyachi built the only successful implementation of a kinetic flipper I know of: 'Warrior SKF'. See the previous post in this archive, and seach YouTube for a video.

Spring based weapons have a similar problem. A lightweight and reliable release mechanism is difficult to design. Again, there have been examples of robots using spring powered weapons: the overhead axes of 'Son of Smashy' and 'No Apologies' come to mind. The design challenge here is less of a problem than the spinning disk clutch and would be my choice of your three designs to actually implement.

Your electromagnet solution runs afoul of the inverse square law. As the magnets move farther apart the force decreases with the square of the distance. You just aren't going to get enough power out of the design to justify the weight. No examples of such a design exist -- for good reason.

Don't give up entirely on pneumatics. A clever design can do a lot with 150 psi. Volume can make up for pressure!

Q: Alternative flipper design guy again. I thought a bit more, and figured that the neodymium magnet would stick to electromagnet's iron core, so replacing it with a second electromagnet would fix that problem and make for a more powerful actuation. What do you think?

A: The iron core is not a problem. Once energized, the magnetic dipoles in the iron re-align and the attraction of the magnet is replaced by repulsion (assuming correct polarity). You actually don't even need a core -- the electromagnetic force is produced by the electrons traveling thru the wire. The core is only present to direct the field and can be removed if the coil is properly designed. 'Coreless' electric motors have no iron armature and work just fine. Regardless, the inverse square law still applies and still defeats this design idea.

Q: Alternative flipper design guy, yet again. I developed the spring powered actuator idea a little more... check out the pdf. What do you think?

A: Double points for clever! I have not seen this design previously. Your design problem considerations are good. Note that the solenoid will have a large side-loading when the spring is under compression -- pulling it in against that side loading may be a problem!

I still think that pneumatics could be effective in this competition. Have you considered a on-board air pump to top off that 150 psi storage tank between shots?

A: Hard to define exactly who the first clampbot was -- but it certainly wasn't 'SOB' who first fought in early 2003. Middleweight clampbot 'Complete Control' fought at BattleBots 2.0 in November of 2000 and is a good candidate for 'first', although there were earlier robots that might argue for primative 'clampbot' status.

Q: who was the first flipper? was it flip?

A: You're going to get us into an arguement about the first flipper -- it's hard to draw the line between a fast lifter and a slow flipper. 'Vlad the Impaler' had a functional pneumatic lifter/flipper weapon at the 1996 Robot Wars that succeded in flipping 'Punjar' for a match win. It gets my vote.

Q: ...and first spinner? (any kind)

A: Several spinners appeared at the first Robot Wars in 1994, including 'South Bay Mauler', 'Pain Mower', and 'The Master'. I don't think there is any clear indication of who was 'first'.

A: Mark J. here: unless this un-named someone has a very strange definition of 'performance', no.

Horsepower is the product of RPM and torque. If only RPM was important then torque would be of no value, which is simply not the case. Torque determines weapon spin-up time. Imagine trying to spin-up an 80 pound bar weapon with a direct drive 60,000 RPM inrunner brushless airplane motor. You could flip the weapon motor on, go out for lunch, come back, and it would still be trying to spin up to speed. You can play with various motor torque values for a fixed weapon design in the Team Run Amok Spinner Excel spreadsheet and see what happens to spin-up time.

Perhaps the un-named someone ment to imply that once the weapon is up to speed the weapon impact is no longer related to the power of the motor that is maintaining the weapon at speed. That is more or less correct.

A: Mark J. here: I found photos of your 'bot at the GameTechMods forum.

A part failure can be attributed to either material or design -- sometimes both. Your design places a lot of stress at the pivot point. That point in the rod has been weakened by drilling a pivot hole, the machine screw looks small for the application, and the screw runs thru a hole very close to the front edge of an un-reinforced block of UHMW polyethylene. That's a lot of weakness right at the point of highest stress.

I need more information on the nature of the spike failure:

• What actually failed: the spike, the UHMW mount, or the fastening hardware?

• If the spike failed, did it fail at the pivot point? Did other parts fail as well?

• What circumstances caused the failure: a lateral impact to the spike, straight-on wall collision, bad landing from a flip, etc.

• A photo of the damaged parts would help.

Q: Shish-Kabot builder here. The steel rod failed at the piviot point. Not the screw. It failed because it was under a pnumatic fliper when it fired it's flipper when under me. YouTube video of fight where the lifter failed.

A: That all makes sense. The pivot hole drilled thru the rod removes a considerable cross-sectional portion of the steel at that highest-stress point. The small machine screw was the next weak point -- if the rod had not failed I suspect that the screw would have.

If you want to keep the 'Vladiator' look to the spike, you could gain considerable strength by welding a 3/8" thick steel tab to the rod and drilling the pivot hole thru the tab instead of the rod. I still suggest a larger bolt for the pivot point.

Comment: Shish-Kabot builder again. Thanks for the advice/input.

Q: Does [Team Blackroot heavyweight] 'SJ' still use pnumatics in the lifter arm/claw or has it been changed to a motor of some kind?

Any details would be appreciated.

A: Team Blackroot abandoned pneumatics for their current version 'SJ' design. The weapon arm is chain driven (photo at right) from an electric power source, but I have no specific information about the motor or remainder of the lifter drivetrain. Team Blackroot dropped off the internet a couple of years ago and I have no current contact information for them.

Secondly will the flipper work well in this 10 kg category? Can you give me some links that deal with flipper mechanism in detail? Thank you.

A: Chain drive vs. gear drive: I'm a fan of chains. Chains and sprockets are easy to obtain, mechanically efficient, durable, can handle large torque loading, and are tolerant of alignment variance. Their only real drawback is that they take up a fair amount of space. A gear-reduction first stage (as in 'gearmotor') would reduce the complexity of a multi-stage chain drive.

About the motor: I'll assume that the torque figure you give is 'stall torque' and the amps are a continuous rating; a 1440 RPM 12 volt motor will not produce that much torque on only 5 amps. Stall current is probably around 120 amps. I ran a very quick analysis of a typical 4-bar design for your weight class and came up with an operational torque requirement of about 300 kg-cm when powering the rear bar (more to power the front bar). A 30:1 gear reduction with one 1400 RPM, 10 kg-cm torque motor would give you the needed torque and a loaded (half RPM) lift speed of about 1 second. I think that should do just fine.

You'll need to settle on a specific 4-bar design and calculate the actual torque needed for that layout. The T.i. Combat Robotics 4-Bar Simulator is an invaluable tool in this process. Four-bar design is as much an art as a science. Keep trying designs on the simulator until you find something that works for your robot.

An electric powered lifter will be too slow to accurately be called a 'flipper', but if your lifter is correctly designed, constructed, and integrated into the other robot systems, it has the capability of being quite effective. Lifters are statistically the most effective weapons in the sub-light weight classes, but they are also usually built by experienced teams.

I have no good links that deal with the mechanical design of electric lifters. The BioHazard Mechanical Design page has detailed photos of their lifter, but their weapon is powered by linear actuators rather than a gear or chain drive. The T.i. Combat Robotics 4-Bar Simulator page gives many good design tips, but without drivetrain details. There are also several posts in this archive about 4-bar design elements.

Q: how can an axe and flipper work together?

A: Poorly. I'm not a fan of robot designs with two active weapons -- try to think of a successful example. You're better off using the weight allowance to build a single powerful weapon than splitting the weight into two weak weapons. The axe/flipper combination is particularly awkward: one weapon trying to throw the opponent upward and another that works best when the opponent is well-supported on the floor. Not recommended.

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< Q: [Chinese Forum] Some people said that The Matador's self-flipping is "by no means fortuitous" because it has flipped itself twice in Battlebots 5.0. I know Matador [was] experimental but I want to assure: does The Matador's flipping issue is "by no means fortuitous"?

A: Combat robots with extremely high-powered weapons are often nearly as dangerous to themselves as to their opponents. Horizontal spinners may careen across tha arena as a reaction to delivering a hit. Tuna can spinners may go 'tippy-top' and flip themselves over without hitting anything. 'The Matador' had such a powerful flipper that it could flip itself if it 'missed' a shot at an opponent. No way is that good!

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Q: [Chinese Forum] Looks like Wipe Out's eletric lifting arm is obviously faster and more powerful than other electric arms. Does Team Plumb Crazy use some special technology in it?

A: No secrets or special technology, just a whole lot of input power. Lightweight 'Wipe Out' has a AmpFlow motor driving the lifter thru a very robust reduction gearbox and chain drive. Lots of power in, lots of power out.

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Q: How does middleweight Dead blow's hammer work?

A: 'Deadblow' used a rotary pneumatic actuator to power its hammer. See the Team Deadblow website.

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Q: Hi Aaron in the forum below you have said that MEGABYTE's shell was 45 kg in weight and spun at 1000 RPM approximately. Can you tell me the motor ratings like torque, current etc. for such kind of motor? Also what sort of batteries are used for such huge amount of power? Can you please specify the Ampere-Hours and weight of these batteries? [Raj]

A: Megabyte's weapon is powered by a Briggs & Stratton Etek motor producing about 15 horsepower at 48 volts. Stall current is about 700 amps with motor stall torque close to 90 N-m. The motor is geared 4:1 to the shell. A pit photo of 'Megabyte' shows four 24 volt 3.6 amp-hour BattlePack batteries supplying power to both drive and weapon. Each pack weighs 4 pounds.

Q: If megabyte uses 4 [24 volt] batteries of 3.6 a-h [each], and if the stall current is above 700A, how can the battery last for 3 minutes?

A: Mark J. here: stall current in a weapon motor is encountered only instantaneously as the weapon first starts spin-up. Note that the large peak level of current will only be seen if the battery/ESC can actually provide 700 amps. As RPM rises, current decreases until the system reaches maximum weapon RPM where the Etek may draw as little as 6 amps. Each time the weapon 'hits', energy is depleated and the motor will draw higher amperage as needed to replenish that energy -- but a restart from a full stop is uncommon.

The Team Run Amok Spinner Excel spreadsheet features a battery capacity calculator that will estimate the battery capacity required for a given weapon, motor, match length, and number of spin-ups. Using this calculator, it appears that well less than half of the capacity of Magabyte's 48 volt, 7.2 amp-hour battery capacity is used by the weapon in a typical match.

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Q: would a 5" .125" thick titanium blade work good for an ant or could I go lower on the thickness?

A: Mark J. here: I can't isolate one piece of the weapon and tell you how well your whole weapon system will work. Each piece must be in balance with the other elements of the weapon system, and the weapon system itself must be in balance with the rest of the robot. I suggest you start with the Team Run Amok Spinner Excel spreadsheet to examine the interaction of the blade, drive, and motor.

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Q: Hi, I have a large vertical titanium disc for my beetleweight that spins at around 3700 rpm. The drum spinners these days spin at over 10,000 rpm, so what can I do to prevent being thrown across the arena every time we go weapon to weapon?

A: There is more than RPM in the equation:

• a 2" drum spinning at 10,000 RPM has an impactor velocity of about 90 feet per second;

• a 6" disc spinning at 3700 RPM has an impactor velocity of almost 100 feet per second, has better 'bite', and most likely much greater potential energy than the small drum.

Looks to me like you have the advantage. If you get thrown across the arena it isn't because your weapon isn't spinning fast enough.

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Q: Hi I am building an ant weight with a 8 in. By 1/2 in. by 5 mm bar made of steel I am using the park 370, u have said before that u want to have a top speed of 3000-4000 rpm but could I run my bar at higher speeds and still get a good [bite]? if so what would be my top speed of it? Thanks.

A: Mark J. here: there is no magic number for spinner RPM, but the faster it spins the less bite it's going to have. At 4000 RPM, one of your two bar tips will pass by every 7.5 milliseconds -- it's going to be difficult to insert much of your opponent's 'bot into the 'damage zone' with any higher speed!.

As mentioned below (read down three questions), it is possible to increase the time between impactor passes by decreasing the number of impactors. In the case of a bar spinner, you'd need to shorten one leg of the bar (half an inch should do) and add weight to that shortened leg to keep the bar balanced. With a single impactor you can double the RPM of the weapon and still get the same 'bite' as a weapon with two impactors.

Q: Then what about the mantisweight Chaos Theory? It has a [96 toothed] 14 inch saw blade, and apparently it can catch the other robot very easily. [Watch Chaos Theory vs Zillion] Why is this?

A: If you are fortunate enough to fight a robot with exposed sharp edges sticking out you don't need much 'bite' to catch that edge. Watch Chaos Theory vs Mystery Box and see the big multi-toothed blade skitter off the flat wedge because it has so little bite. With fewer teeth it would have done much better.

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Q: Hi Aaron. What was the amount of Kinetic Energy stored in heavyweight MEGABYTE's spinning shell? Is it more effective weapon than a spinning Horizontal bar? in which case is the 'BITE' more? By which method were the blades attached to the spinning shell?

A: Mark J. here: Robotic Death Company has not released enough specific information for me to calculate the stored kinetic energy of Megabyte's weapon. A rough guess at speed (1000 RPM), mass (45 kilos), dimensions, and mass distribution puts the energy around 50,000 joules.

A spinning cylindrical body with the same mass and diameter of a spinning bar will store more energy, which makes it a potentially more effective weapon. A simple spinning bar the same weight and diameter as Megabyte's shell would store only about 40% of the energy at the same speed. The actual weapon effectiveness is dependent on a variety of design factors.

'Bite' is independent of the shape of the energy storing body. It is primarily a function of the time interval between passage of the weapon impactors, which is dependent on the weapon RPM and the number of impactors. A weapon with a lot of 'bite' has a longer interval between impactor passage, which allows an opponent approaching at a given speed to penetrate farther into the weapon's destructive radius. You may have noticed some spinner weapon designs that are counterweighted to allow a single impactor point for greater bite.

Robotic Death Company bolts the impactor blades onto their spinners.

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Q: How do I make a bi directional limit switch? I need it to stop the lifter on my robot.

A: I assume you're using a gearmotor with an ESC to control your lifter, and not a servo? There is a good explanation of limit switches at www.techno-stuff.com/limit.htm. The third circuit example (the one with two switches and two diodes) is a bi-directional circuit -- your ESC would substitute for the battery. Position the contact switches at the minimum and maximum lift points and you're good.

Relay boards used to control linear actuators often have built-in limit switch inputs. I use the Team Delta R/C dual ended switch to control the lifter on Zpatula to keep things simple.

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Q: HI, I have built a horizontal overhead spinner which weighs nearly 7 kgs and revolves at nearly 2600 rpm. Is ther any modification available in this to deal with a wedge, because whenever i hit a wedge my bot tends to flip in the opposite direction.

A: Hard to say much without at least a photo -- I don't even know what weight class the robot is. I do understand the basic problem: the force vectors of an impact with an overhead spinner are all above the center of gravity of the robot. A weapon impact tries to flip the robot over backward. The farther above the center of gravity the impact site is, the greater the flipping force. Hitting a wedge simply aggravates the problem. Is this a bar spinner? You could try bending the ends of the bar downward a couple of inches to move the impact point lower and closer to the level of your robot's CG.

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Q: Aaron, we have built a horizontal spinner like Son Of Whyachi, but instead of three rods we have managed to build two rod system. We have taken a leaf spring and bent it across its longitudinal axis so that when rotated it sweeps a hollow cylindrical surface around the robot chassis. It is a rigid rotor which spins at about 2500 RPM driven via belt and pulley. Speed reduction is 2. Motor spins at around 5200 RPM.

The main problem we are facing is the STABILITY of our robot. It topples over at very low speed for eg 1700 RPM, not gaining our top speed of the rotor 2500 RPM. We have balanced the rotor statically and dyanmically, but still the robot is unstable. It loses contact with the floor first and then it turns,swivels etc. Are there any effects such as gyroscopic etc. which come into play? Please suggest suitable methods for increasing the stabiltiy of our robot. Our bot weighs total 39kg ,chassis weighs 15kg, weapon weighs 8-9 kg approx. Also our C.G of the robot is not in the centre of chassis but shifted towards the rear side due to weapon motor weight which is mounted on the rear side.

You can see the robot video on youtube. The bot who wasn't flipped o'er is ours. It's urgent pls. Any help will be appreciated greatly. Thanx in advance.

A: Mark J. here: we've seen this type of instability before. It pops up in full-body spinners and designs like yours. A spinning object - such as your rotor - will attempt to orient its axis in a manner which will achieve the greatest rotational stability. If you spin a hard boiled egg on its side, it will change the axis of rotation and spin on its end. See the explanation of the Tippe Top for the related physics.

In your particular case, I believe the rotational mass is unstable and is attempting to reorient its axis to correct this -- turning your robot over in the process. I've given some thought to your problem and then did a little experimenting. Of the three natural axis around which a rigid body may be rotated, only two are stable. Your weapon is spinning along the third, unstable axis! Search the web for 'rotational stability' for more detail on this phenomenon. The three-armed rotor on 'Son of Whyachi' was stable in this particular rotational axis but the model I made of your two-legged weapon is not.

Why does your rotor become unstable at relatively low RPM when others like it do not? Check for any 'flex' in the weapon rotor -- although not responsible for the instability, flexing may create trouble at a lower speed than would otherwise be seen. That leaf spring you're using may be distorting and causing the onset of the problem.

P.S. - About the video - are you insane? Standing two feet away from an operational spinner in combat, holding a control cable up out of the way??? Somebody is going to get a chunk of robot embedded in their head! Cut it out!!!

Q: hi Mark sir. i asked a question regarding stability of our robot. u suggested that it's a sort of tippe-top phenomenon. i wanted to ask whether decreasing the rotor speed from 2000RPM TO 1000RPM will increase the stability of robot?

also did Son Of whyachi had rotor speed of only 950RPM? is that sufficient to damage the other robot? our rotor has dia. of 75cm. thank you.

A: Decreasing the speed of the rotor would not remove the instability, but would reduce the magnitude of the forces generated and make it more manageable. Unfortunately, reducing the speed of the rotor by half will decrease the rotational energy of the weapon by 75%. Not good!

The rotor on 'Son of Whyachi' did spin at 950 RPM, but there is more to the energy equation than RPM. The rotor was five feet in diameter which translates to a tip velocity of 175 MPH and way more than 50,000 joules of stored energy. A very rough estimate of the energy of your weapon spinning at 1000 RPM: 2750 joules. I'd suggest correcting the rotor instability and keeping the RPM up for maximum impact.

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Q: Are there ways to make a crushing claw other than hydraulics?

A: Sure -- but hydraulics are the simplest. A reliable crushing weapon is very difficult to design and implement, and there have been very few successful examples.

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Q: In previous posts you said to gear the Speed 300 motor at 2:1 for a 4 oz drum that is 2" in diameter. I am building an ant weight drum using two(2) Speed 300 motors with a 4 oz drum that is 3.3" in diameter and 2 1/2" long. I am curious what you think I should gear it at to gain the most success.

A: Mark J. here: that previous recommendation was a ballpark estimate based on quite a few assumptions -- I didn't have all the information needed for exact calculations, so recommended a gear reduction of 'about' 2:1. I have the same lack of information about your design.

Running what I know of your design plus a few assumptions thru the Team Run Amok Spinner Spreadsheet I get a good balance of spin-up time (0.65 seconds to 10 Joules at 3125 RPM), peak kinetic energy (23 Joules at 4700 RPM in 2.0 seconds), and tip speed with a gear reduction of 2.5:1. I'd suggest starting there.

Q: I got the dimensions wrong in my last question so here are the details for my drum it is 2" long, 2.375" in diameter, the wall thickness is .085" with two .5" and four .25" holes in it thanks.

A: And it is made out of what?

Q: sorry my drum thing was wrong again it would weigh to much so I decided to use a aluminum drum 3 in. long, 2 in. in diameter with a wall thickness of 3.2 mm, and I went to my grandmas to do the calculations but they did not come out right with a gear reduction of 2.5 to 1 using the drum i just described and two speed 300 motors I got a spin up time of 2000 seconds at 4500 rpm producing 140000 joules I think I got the torque wrong i had .39 newton-meters, I am 12 and my parents are always gone working so they cant help but you are very helpful Thanks allot (:

A: I think you're using the wrong units on the spinner spreadsheet - maybe meters instead of inches - and your torque is too large. I'm also getting the feeling that you're in over your head on this. If this is your first robot I'd strongly suggest building a passive wedge or brick to learn the important primary systems of drivetrain, radio, chassis and armor. Without strong basics even the wildest weapon will be useless.

Q: I already made two bricks and fought them each once and I have already made a atocad drawing for my bot and bought all the parts except for the timing belts and pulleys. I know I can get a balenced drum because my grandpa is a shop teacher at a high school that has all the meterials and mechines, and I tried the spinner thing again and I still got the same thing what is the n-m on two speed 300 motors, thanks?

A: As you wish: two Speed 300 motors at 7.2 volts produce a combined 0.06 N-M of torque. No-load speed is 12,400 RPM. Material density is 2760 kg.m³. Three inches is 0.076 meters height. Two inch diameter is 1 inch outer radius = 0.025 meter outer radius. Thickness is 3.2 mm. You haven't told me about the end plates for the drum, so I'm leaving them out.

Pumping these numbers into the spreadsheet gives a cylinder mass of 0.10 kilos = 3.5 ounces. With a 2:1 reduction, the drum will spin to 5 Joules of energy in 0.29 seconds and reach 10 Joules at 5890 RPM. I don't think that's enough energy, and the drum RPM is getting very high. I'd suggest making the drum larger in diameter -- more like the original dimensions you provided.

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Q: Hi Aaron, my team and I are building a robot similar to tornado mer and we were wondering what kind of tip-speed your robot's weapon was doing, and the length of the bar you used.

A: 'Tornado Mer' is not one of our robots. It was built by Team Van Cleve from Prior Lake, Minnesota. I have no specific information about its weapon.

If you're asking about the bar spinner weapon on 'Run Away', it should not be used as a model for your weapon. We added the showy but largely cosmetic spinner weapon just to please the Robot Wars crowd. It wan't a serious attempt at a damaging spinner.

There is plenty of spinner design information in this archinve, including links to our kinetic energy calculator spreadsheet.

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Q: Hello, I am thinking of building a hobbyweight crusher similar to razer, is there a formula to find out how many lbs. of force is required to pierce 1/8 in AL/Lexan or 1/16 AL/lexan? I am thinking of using linear actuators. Thanks

A: Mark J. here: too many variables here for a simple formula. The force required will depend on the cross section and shape of your piercing beak, the particular aluminum alloy, any heat treatment, and how well the piece is supported. Lexan is particularly difficult to pierce as it will deflect a great deal before failing.

I suggest you experiment with a mock-up of your piercing beak and a lever that will allow you to place a measured amount of force on a piece of test material. I believe you will find that it takes a great deal of force to pierce even relatively thin armor. I've never seen an effective piercer that used linear actuators.

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Q: I am thinking of building a hobbyweight shell spinner and saw in one of your posts that it shouldn't be direct drive. How should it be then? I have seen that some builders have their weapon motors horizontal. How? Thanks

A: The standard method of driving a spinning shell is to create a new well-supported central shaft for the spinning shell with a pulley and belt drive to the motor. The drawing shows a well supported shaft for a bar spinner -- the same design will work for a shell.

I have only rarely seen horizontally oriented motor shafts with shell spinners. It is possible to lay the motor down and drive the shell with a friction-drive wheel attached to the motor shaft and rubbing against the inside top of the shell (not recommended). It is also possible to run the motor output thru a right-angle gear drive and then to a belt and pulley system, but this adds weight and mechanical losses. Keep it simple.

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Q: Should a bulky flipper with pneumatics, like The Gap and Ripper, or a large flipper powered mainly by CO2 and high pressure, like Robochicken, be used?

A: Mark J. here: I'm not sure I undertsand your question. The design of the flippers in 'The Gap' and 'Ripper' are very efficient because they place the lifting force directly in-line with the direction of flipper travel. This does cause the pneumatic cylinders protrude at an exposed angle. More compact designes like 'Robochicken' are able to conceal the pneumatic cylinders within the robot body, but the force vectors are not as favorable and flipping force is compromised.

Whichever design is used, it must be integrated into the overall design of the robot. You can't treat the various systems of a combat robot independently -- they have to work together.

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Q: i have built bots before for a 15 lb weight class for the azbbiq, i am looking to build a fly wheel bot this year and im looking for a solid weapon design. can you help? any ideas?

A: Start reading thru our archives, and take note of our Team Run Amok Spinner Spreadsheet which allows energy and spinup analysis of proposed spinner weapon designs. When you get to specific questions, write back.

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Q: Does a squat cylinder a dome or a flattened cone fbs work best?

A: Pound-for-pound, a squat cylinder shape can store more rotational energy than a dome or cone of the same dimensions. More of the mass is located farther out from the axis. Many other factors go into the design of a successfull Full-Body Spinner, but a cylinder is a good place to start.

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Q: An electic axe always doesn't have as much power as a pneumatical one,but Beta seems to be an exception for it's much more powerful than Terrorhurtz,why is that?

A: Mark J. here: electric axes generally do have less power than a good pneumatic axe. The axe on 'Beta' is an exception. The weapon is powered by a monster Etec motor running at 60 volts thru a complex geartrain that provides a variable ratio drive to better accelerate the weapon head. The reaction torque produced by the weapon is so great that electromagnets are needed to hold the chassis down to a steel arena floor.

The machining skills and expense to construct such a weapon are beyond the reach of most teams. I should also point out that the robot was a combat failure -- unable to compete because the reaction forces pulled up the arena floor. It was an interesting (and expensive) experiment, but I would suggest sticking with pneumatics.

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Q: How can you get a crushing beak to exert piercing force?

A: Hydraulics are the usual approach. Suggest you get a copy of Chris Hannold's "Combat Robot Weapons" for a overview of design issues and solutions.

Q: Should I lubricate the weapons of my robot with Silicon Spray if the weapons are a crusher?

A: What, the outside of the crushing beak part? It won't make any real difference in it's ability to pierce, but a little lubrication may help prevent it from getting stuck.

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Q: How does an overhead blade like Proppelor Head's cause damage, when there are few sharp edges?

A: The same way a hammer causes damage: blunt force impact. Sharp edges on a spinner weapon can cut in and 'stick' in the gash, which can be a problem. Blunt is more reliable.

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Q: Hi, Ive built a few bots before so I have a bit of experience. I would like to build a 12 hobbyweight shell spinner with direct drive for the weapon. I know I shouldnt do direct drive but I am going to try. My question, I cant seem to find a decent motor, something strong enough to spin up a 3 pound shell and It has to have a large shaft...? at the robot marketplace they have a motor but the shaft of the motor is only.315 so thats to thin...

A: Mark J. here: you're right -- you shouldn't direct drive a shell spinner. You've also discovered one very good reason why not -- suitably sized motors do not have large enough shafts to support the load of an FBS shell.

If the motor you're talking about is the AME blower motor, you should also know that the motor is badly underpowered for it's weight: maybe 75 watts output at 12 volts. Really pitiful.

I can go on with reasons why a direct drive shell spinner will end in tears: slow spin-up, too high a spin speed, amp-sucking start-ups... you get the idea. Don't do it.

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Q: why is mettal cord somtimes used as antweight disk belts? would'nt it slip?

A: I haven't seen metal cord used for weapon belts, but a little slippage in a weapon drive is not a bad thing -- it can keep you from stalling the motor.

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Q: Hey, what was G.O.R.T.s weapon? it looks like a flipper and a cone with a flipper seems like a good idea

A: Superheavyweight 'G.O.R.T.' had an ineffective electrically powered spike. It never won a match. Pattern your robot after something more successful.

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Q: Hey, which system can a Hobbyweight use to build lifting forks?

A: Hey, I hope we don't go thru every weight class like this. There are available pnumatic components small enough to use in a hobbyweight. See Pop Goes the Monkey for an example. Electric gearmotors are useful lifter power sources in this class as well.

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Q: Why can Pyromancer always get underneath its oppenents while several of them have a hinged wedge just like Pyromancer?

A: Playing the 'lowest wedge' game is more complicated than just hinging your wedge. 'Pyromancer' has razor sharp leading edges to its twin wedges that are honed to lay perfectly flat on the arena floor. The twin wedges also help -- most arena floors are uneven, so one side of a single hinged wedge is likely to be raised off the floor just a bit by the other side.

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Q: I was wondering if you could help me out on starting the pneumatics system for a 120 lb robot. My team was thinking of a trapezoid shaped robot that had a fork lift type arm attached at one end. Do you think you could give me some insight?

A: Start by reading the Team Da Vinci Understanding Pneumatics page. You'll also want to read thru the previous posts in this archive. Write back with specific questions.

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Q: How many joules of energy does Hazard's weapon produce at full speed? Thanks alot, he is my favorite spinner too.

A: I've never heard Tony B. claim a figure for the energy stored in Hazard's weapon. I don't have the specific information about the weapon do so an accurate calculation, but I'm going to guess around 3000 joules.

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Q: Is 'Megabyte's shell really made of Lexan? Don't you want as much weight on the shell for an FBS? I guess the blades must weigh a lot then...

A: No -- some versions of 'Megabyte' had a protective Lexan cover on the robot body under the shell, but the shell itself is titanium. From its size and weight (100+ pounds) I'm estimating the shell material is at least 1/2" thick.

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Q: would 1/8" titanium work well for a heavyweight FBS?

A: It depends entirely on your design. See the question #17. In general, I'd say 1/8" titanium is VERY thin for a heavyweight.

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Q: Hi, what type of motor does megabyte use for it's FBS, Thanks

A: Robotic Death Company reports that 'Megabyte' used the Briggs & Stratton Etek Motor at 48 volts for the weapon and four 24-volt DeWalt motors for the drive train. There is reference to a 'souped up' Etec being used in 'Rambyte', but no details are given.

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Q: Hi Aaron, do you know what weapon middleweight "Fatal Error" has? It looks like a drum and a flipper combined.

A: A small drum on one end, a weak flipper on the other. A fine example of two poor weapons not equaling one good one. Record: zero wins, two losses.

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Q: Do you think ziggo's way of converting energy, the vertical blade is beter than ways like typoon 2 and moebius, the horizontal blade?

A: I don't think I understand the question. All three of the robots you mention are full-body spinners: 'Ziggo' was a dome, 'Typhoon 2' was a cone, and 'Moebius' was a squat cylinder. They all sprouted assorted impact blades. 'Ziggo' also had the exposed ends of a square-tube beam sticking out thru the dome. I believe this is a structural issue rather than an attempt to improve on energy conversion -- I don't see any particular advantage.

Q: Sorry for not explaining my question well. What I ment was, do you think Ziggo's way of mounting his impact blade so the sharp side stands up instead of it sticking out horizontaly causes more damage?

A: I don't think it makes much difference. I suspect that it was just more convenient for the shell design to mount the impact blade as an end-cap on the structural tube. Note that spinner impact blades are generally not 'sharp' -- a sharp blade tends to cut into armor and wedge itself stuck. A blunt blade will rip without as much risk of sticking.

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Q: Would a spinning disk on an arm like the origanal dead metal work in todays competitions?

A: There's another post about 'Dead Metal's weapon a little further down this archive. The weapon was designed for cutting rather than blunt impact, and was used only after a competitor was already weakened and 'docile'. I don't believe that a similar weapon would be useable in open combat.

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Q: what does ziggy and toro use on there flippers

A: There is plenty of information on both 'Ziggy' and 'Toro' in this archive, probably including the answer to whatever question it is that you're trying to ask. If you're asking about the gas they use to power their flippers: 'Toro' uses CO₂ and 'Ziggy' uses High Pressure Air (HPA) at around 3000 PSI.

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Q: Would a spring powered spike be a good/decent weapon?

A: There are many questions and answers about spring powered weapons in this archive. Quite a few builders have tried them, but only one has been really successful. Pneumatics are a simpler, more powerful, and more popular solution.

Q: [Chinese Forum] Aaron, in your last question you said that there was only one bot using spring loaded system really succeeded - which is that one? I guess Derek Young's 'Son Of Smashy', wasn't that? Could you analyze why had SOS been that successful?

A: Yes, but no points for correct answers to your own questions Derek's success with 'Son of Smashy' had less to do with the spring power than it did with being the first powerful overhead axe middleweight to turn up at BattleBots (Long Beach, 1999). The judges were impressed by Derek's aggression and ability to do some damage. When overhead axes became less of a novelty you needed more than spring power to impress the judges and pneumatic power took over.

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Q: If one had a spinning weapon powered by a motor geared down 3:1, could they attach a flywheel to the motor shaft and make the blade lighter by the same weight as the flywheel in order to increase kinetic energy without increasing weight or blade rpm?

A: Mark J. here: you've been thinking pretty hard about this. The short answer is 'yes', but there are problems in implementing such a design.

• The kinetic energy of a rotating object depends on mass, speed, and the shape of the object. The amount of KE lost from the blade would depend on where the mass was removed as well as how much mass was removed. The kinetic energy of the added flywheel is also dependent on it's shape as well as it's mass. The Team Run Amok Spinner Spreadsheet will help you calculate the rotational inertia of the blade and the proposed flywheel.

• The energy stored in the added flywheel would need to be transmitted thru the gearing system to the blade at the instant of impact. That will place a great deal of extra stress on the gearing system. If a belt-drive is being used, the stored energy from the flywheel could be lost to slippage. If a chain or gear system is used, you risk breaking the drive. The added weight needed to reinforce the weapon drive could cost more weight than you'll save.

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Q: I have an interesting idea. I was thinking of a dustpan like the one on S.O.B., but instead of a saw, a vertical spinner, going downwards. The point is too remove the other bots contact from the floor, negating the lift caused by the spinner hitting downwards. Or will it just tear itself apart?

A: I get it -- if you have a dustpan under your opponent you can strike downward without flipping your own robot upward. Not bad, but a downward striking rotary blade will also strike 'inward', jamming the blade and your opponent into the dustpan and against your own 'bot. You are correct that there would be large forces applied to your own robot structure as well.

Q: how about a four sided wedge with a blade in the center? so if you go up any side, you get shreded. Kind of like an arena hazard on wheels.

A: Relying on an opponent's actions to cause their own damage is not a winning strategy. You're also in real trouble if your opponent can get under your wedge - how do you apply your weapon then? Better to put your weapon in a position where you have direct control of its application.

I'm not trying to shoot down your ideas, but a good number of very creative builders have been thinking and experimenting with combat robot design for 15 years now. I'm convinced that every design that you or I can come up with has already been considered and possibly built by some competitor by now. The time for big new designs in combat robots is long past. Successful combat robots are based on quality design and construction.

Q: could I do my "moving arena hazard" Idea of having a blade in the top center with wedges on the sides, but have the blade on pneumatics, so that it raises up from inside the bot when my opponent is on the top?

A: That either makes for a very tall 'bot or a very small blade, and I don't see what advantage you get by raising and lowering that blade. You could miss your opponent completely as they drive over. Better to put the blade somewhere where you can show aggression by pointing it at somebody and driving into them.

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Q: Would it work if I put a spinning blade on the end of a four bar lifting arm above a wedge, so I could shred then flip?

A: Two problems:

• When an impact spinner hits, there is enormous force placed on the weapon mount. Putting all that force out at the end of a lifting arm is going to give that force a lever to help it tear apart your lifter mounts. Robot Wars housebot 'Dead Metal' had a spinning blade mounted on a four-bar aparatus, but the circular blade had very small teeth designed for cutting -- not for impact.

• I'm not a fan of multi-weapon designs. Each of the weapons will have less weight allowance and therefore less power. If your spinning bar is strong enough, you won't need to do any flipping and vice-versa. There have been plenty of multi-weapon robots, but no champions.

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Q: Hi. I am building an ant spinning drum. I want to use a 4 oz. drum. Would the Radio Shack 9VDC Micro Super High Speed Motor work? It runs at 24,000 rpm with 36 g:cm stall torque. I would probably gear it down to 4 or 5 to 1. Thanks, your web site has helped me learn a lot.

A: That Radio Shack motor is a very small, inexpensive, general purpose, low-power unit designed to run a small cooling fan or the like. It has a lot of speed, but not nearly enough torque for an effective antweight spinner weapon. It would take a very long time (6 or 8 seconds) to spin the weapon up to speed, and you don't have much spin-up time in a small antweight arena.

Staying with an inexpensive brushed motor, I'd suggest a 'Speed 300' geared around 2:1 for a 4 ounce drum about 2" in diameter. These motors were used to great effect in the VDD antweight kits to power their spinning weapons.

Q: Would the Speed 300 motor work the best or could I get something better? I dont mind paying for it.

A: There's always 'something better', but exactly what that might be depends on your design and your skill as a builder. It takes more than power to make an effective weapon, and too much power can be worse than too little.

The Axi 2208/20 brushless motor would be a 'top end' powerplant for an antweight spinner, but it would require a brushless motor controller, a more robust weapon drive system, and a higher level of precision all around. For a first spinner, I'm going to suggest sticking with the Speed 300 - it'll be plenty.

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Q: [Chinese Forum] Does 'Code:Black's weapon really have a top speed about 7000 rpm?! I heard this from the Big Bang Robotics site. If that's true, how did Team Logicom do that?

A: No secrets to the weapon in Code:Black. With a 12 HP internal combustion weapon motor spinning a low-mass undercutter blade in a lightweight robot you can spin the weapon pretty much as fast as you like. However, weapon speed is only one part of the spinner success formula. There is plenty of information about spinner design in this archive.

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Q: Can you tell me the 'common' engine power of a spinner? Is 'Warhead's 22 horsepower too high or '[name deleted]'s 7 horsepower too low?

A: Mark J. here: horsepower is only part of the spinner equation. The idea is to store kinetic energy in the rotating mass of the weapon and release that stored kinetic energy on impact. My own rule: an effective spinner will have a minimum 20 joules of stored kinetic energy per pound of weight class and will be able to spin up to at least half that level before an opponent can attack them.

There are many trade-offs in spinner design: speed versus 'bite', kinetic energy versus maneuverability, horsepower versus rotational mass, optimum mass placement versus durability, etc. Read thru this archive for plenty of discussion on spinner design.

Opinion: Warhead's weapon was awesome to watch, but the gyroscopic effect of the spinning mass made maneuverability a real issue. Too much weapon, I think. With a different design that 22 horsepower might be more useable - or it might become a real disaster.

That other robot, who's name has been recently banned from the page, had no control issues and ample weapon power. I don't have enough info to calculate the weapon's energy storage, but it was certainly plenty. Given a choice between the two robots, I'd rather drive [name deleted] than Warhead.

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Q: [Chinese Forum] How much power does Beta's hammer actually has? What do you think of it?

A: Beta's hammer has a claimed 3000 joules of energy. The robot was an interesting experiment, but usless without a suitable arena in which to fight.

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Q: [Chinese Forum] A question about Mechavore for Mark J.: you said before that Mechavore's ICE weapon power was about 7hp (actually 6.5 hp,wasn't it?), but is this the actual power of 4.0 and 5.0 version? I know Mechavore equipped two ICEs for that blade since 4.0......

A: Mark J. here: the 7 HP I cited (I like to round up) was the version used at BB 4.0. The earlier version had about half that power. I don't know of any motor upgrade for BB season 5. Their best result came in BB 3.0 with the less-powerful engine.

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Q: Who is the smallest / most compact pneumatic flipper in the world? Is it the hobbyweight 'Hexy Macro'?

A: Not even close. I know of at least one 150 gram UK antweight with a pneumatic flipper and there are plenty of 1 pound US antweight flippers.

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Q: [Chinese Forum] How do KHZ and THZ's pneumatic axes work? Do they have the same mechanical design as the hammer on 'The Judge'? All of them have 180 degree range of motion.

A: Terrorhurtz uses a true rack and pinion weapon drive similar in concept to the drive on 'The Judge' but without the chain and sprocket bodge. The design on Killerhurtz was a much more complicated system involving lever arms, gears, and chains. It worked, but the rack and pinion is superior.

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Q: The person who has troubled you with a dozen of questions in one day has a strong belief that 'Warhead' can't improve itself by using more sharp teeth. That's why he has been asking so many questions about Warhead within these days. So what do you think of this question: can Warhead improve itself by using teeth like Mechavore's? If yes, why he didn't do so in BB 5.0? If no, what is your reason?

A: Building a combat robot is roughly equal parts engineering skill, past experience, and 'that looks about right'. Sometimes an 'improvement' in one aspect of a robot can lead to unexpected consequences in other areas. I suspect that the style and position of Warhead's impact teeth were chosen to avoid stalling the weapon. A more aggressive tooth design might have caused trouble -- Team Razer is known to do extensive testing.

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Q: [Chinese Forum] Someone in our forum said the blades for Spining Weapons MUST BE 'SHARP'. Otherwise those Spining Weapons could be stopped/disabled easily before their opponents get smashed. And he also thought this is the main cause of Warhead's loss in the BB 5.0 quarter finals. But I think Spining Hammers are quite good and they are pretty reliable, too. In my opinion the reliability of Spinners is not related to whether their blades are 'sharp' or not. Who is right between the two of us?

A: Whether the impact surfaces of a spinning weapon are sharp or blunt is a function of the intent of the weapon. If the weapon is designed to penetrate, the impact area should be sharp - but it risks becoming stuck in the gash or possibly in the arena itself. If the intent is to deliver a smashing blow, a blunt impact zone is easier to maintain, stronger, and less prone to damage.

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Q: [Chinese Forum] For Mark J.: you said Warhead "had more than three times the power of MechaVore's weapon." Have you calculated it in detail? I think a gas engines' maximum velocity is pretty hard to measure.

A: Mark J. here: My comment was based on engine power - 'Mechavore' claimed 7 horsepower and 'Warhead' claimed 22. I don't have enough information on the weapon from either robot to calculate kinetic energy storage. You are correct that the non-linearity of the torque curve for an internal combustion engine makes weapon calculations difficult - but not impossible.

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Q: is a braking system really important for those really fast spinner weapons?

A: Most tournament rulesets require a maximum 'spin-down' time for rotary weapons. The tournament director does not want to hold up the tournament while a weapon takes five minutes to stop spinning. If your weapon won't come to a full stop in the alloted time (currently 60 seconds for BattleBots and RFL) you'll be disqualified - so yes, very important!

Q: If you had a spinner with no brake system for stopping your weapon at the end of a match, could you just hit it against the arena walls to stop it? It dosen't specify in the RFL rules.

A: Nope. The RFL ruleset does specify. Section 10.2 says:

"Spinning weapons must come to a full stop within 60 seconds of the power being removed using a self-contained braking system."

Use of the arena wall does not qualify as 'self-contained'.

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Q: Are there really big difference in the damaging effect between an aluminum blade and a hardened tool steel blade?

A: Tool steel is at least 50% harder than even the best 7075-T6 aluminum. When it comes to turning energy into damage, there is no subtitute for harness. An aluminum weapon blade could do more damage to itself than to the opponent. Plus, aluminum doesn't make those pretty (judge influencing?) sparks when it hits.

Q: You say "An aluminum weapon blade could do more damage to itself than to the opponent", but why does 'Mechavore' keep using aluminum blades instead of hardened tool steel blades? Weight issues, or just Robert L.'s mistake?

A: I think we have a difference in terminology. 'MechaVore' did not have an aluminum blade, it had an aluminum disk with large hardened steel impact teeth. This is entirely standard for rotating disk weapons. The aluminum alloy provides a high strengh to weight ratio for the body of the weapon and and the steel teeth provide hardness at the impact site to do the damage.

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Q: Why did 'Diesector' change its hammers' shape in BattleBots 5.0?

A: Diesector had several upgrades for BB 5.0: new jaws, new batteries, new tires, and new hammers. I suspect that the hammer change was largely cosmetic -- the new hammers looked much more aggressive than the previous design and better matched the look of the whole robot. Diesector's hammers never did a great deal of damage, but they helped in the 'aggression' scoring.

Q: And what is Warhead's spinning dome's height-adjustability used for?

A: The adjustable weapon height allowed the team to 'aim' the weapon at critical parts of the opponent. Low target, low spinner. High target, high spinner.

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Q: How come no one talks about 'Subzero'? It had a pretty powerful flipper!

A: Middleweight 'SubZero' did indeed have a powerful flipper and a very good 26 win, 10 loss record with three tournament wins. I suspect that little is said about the 'bot because the flipper design is entirely standard. However, there have been comments about the legality of the components used in the flipper. The pneumatic cylinder was rated 250 psi by the manufacturer, but was being operated at 850 psi with only minimal modification. I'm puzzled about how they got this grenade thru tech inspection.

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Q: Aaron, is there a commercially avialable joint or bar that is extremly flexible that I could put on my gas engine powered weapon to prevent internal engine damage or weapon damage? Thanks.

A: Flexible how? There are slip clutches available that will prevent a large torque load from being transmitted along the shaft. There are universal joints that will allow axial flex, but I don't think you want your weapon flopping around at the end of the shaft like that. The best option to isolate the weapon from the motor is to use a v-belt drive that will also allow a suitable speed reduction/torque increase. You really don't want to direct drive a weapon from an internal combustion engine.

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Q: When being compared with each other, what is the advantages and disadvantages in these three weapons: spinning bar, spinning disc, and spining dome? [Chinese Forum]

A: Mark J. here: for a given weight and diameter the three shapes will store different amounts of kinetic energy at a given speed. Examples, all weighing 10 kilos and spinning at 1200 RPM:

• a bar 1 meter long and by 0.1 meter wide stores 6882 Joules of kinetic energy;
• a disk 1 meter in diameter stores 9927 Joules of kinetic energy;
• a cylinder 1 meter in diameter by 0.3 meter tall stores 19,728 Joules of kinetic energy.

A spinning dome would have an energy capacity in between the disk and cylinder, but my design software doesn't calculate spherical sections.

Of these designs, the bar is the simplest to construct and is extremely durable. The cylinder is difficult to construct, tricky to balance, and much less durable. The disk falls in between.

Other trade-offs go into designing a spinning weapon - I could fill a lengthy chapter in a robot design book - but the essential consideration is a balance between efficiency, durability, and simplicity.

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Q: In my opinion, there are three top axes in Battlebots SHW class: 'Hammertime', 'The Judge', and 'No Apologies'. Do you think so? What do you think of them? Are they successful? Are they all powered by CO2? [Chinese Forum]

A: Again, I can't get excited about ranking combat robots by weapon/weight/tournament.

• 'Hammertime' used high-pressure nitrogen regulated to about 200 PSI to power it's weapon.

• 'No Apologies' had a spring powered axe/spike that was reset by an electric winch. It also had a pair of lifting arms.

• 'The Judge' was CO₂ powered.

You can judge for yourself if they are 'successful'.

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Q: What do you think of those monster flippers like 'The Matador' and 'Gravity'?

A: In arenas where it's possible to win by throwing an opponent out of the arena, a monster flipper is a viable design choice. In a fully enclosed arena, they don't carry much benefit - they are more of a novelty.

Q: But what caused 'Toro' and 'T-Minus' got Giant Nuts? They're really powerful and I think they are belonged to 'Monster Flippers',too. Aren't they? [Chinese Forum]

A: Mark J. here: Inertia Labs was one of the premiere teams in Combat robotics. They were experienced, skilled, well funded, and loved innovation. Importantly, they knew how to learn from their combat experience and improve a good robot over time to make it a great robot (they also knew when to give up on a poor idea). I'm sure they could succeed in building a winning robot with any type of weapon they might choose.

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Q: How does the spinning ring weapon on the robot 'Spinzone' work?

A: See the description of 'Ringmaster' further down in this archive.

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Q: In our Chinese Forum there is a common idea that 'SOW', 'Warhead', 'Mechavore', 'MOE', and 'Surgeon General' are the top 5 spinners ever seen in BattleBots. Do you agree with us or have other choice?

A: I guess you're giving a lot of points for destruction. In my book, top spinners win championships.

• How can you not include 'Ziggo' (13-3, 2 championships)?

• How about 'Hazard' (17-1, 3 championships)?

• 'Minion' qualifies as a spinner (12-4, 2 championships).

• and the often overlooked 'Backlash' (13-4, 1 championship) probably belongs in the mix.

Son of Whyachi has a championship, but his win over 'Shaka' in the first round of that tournament (BB 3.0) was controversial. Then there's the whole 'shufflebot' issue: after being reclassified as a SHW, SOW could only manage a 1-2 record.

I think my 5th robot would be 'Shovelhead'. His record at BattleBots was only 3-3, but he has since gone on to win 5 tournaments and amass an impressive 39-15 record.

Q: I asked you a question about top 5 spinner but I made a mistake: not in all Battlebots, just in the heavyweight of Battlebots.

A: That becomes a very narrow category. I'm not a big enough fan of spinners to try to sort out which amongst a lackluster group of robots are the 'top heavyweight spinners at BattleBots'. Outside of 'SOW' and 'Warhead' the heavyweight spinners just weren't all that impressive - IMHO.

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Q: What caused 'Hexy D's success? It doesn't has so much power in that flipper.

A: There is a great deal more to a robot than the power of its weapon. 'Hexadecimator' (21-9) was a well constructed robot that did everything well: quick, controllable, powerful, and well driven. When you have that combination going you can use a pointy stick for a weapon and still win matches.

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Q: How does 'The Butcher's CO₂ powered spining blade work?

A: I've never seen inside. I could only speculate.

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Q: Is Voltronic's weapon hydraulic? How much power does it has? [Chinese Forum]

A: Some builders share a lot of information and some don't. The only details I have on Voltarc/Voltronic are that the lifter is electric and the main drive motors are made by Leeson.

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Q: Which is better between Gas Powered Spinner and Electric Powered Spinner? It seems that Gas Powered Spinners have less reliability. [Chinese Forum]

A: Reliability is an issue with internal combustion engine (ICE) spinners. A few teams have that licked.

Electric weapons are simpler and more reliable, while ICE gives the potential for greater power at lower weight (and higher noise level).

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Q: Why are drum weapons on heavyweights not as successful as those on smaller robots? [Chinese Forum]

A: Some designs work better in a specific size range. That's why there are no eagle-sized mosquitoes or mouse-sized elephants.

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Q: Is there any robots using a electric drill as their weapons?

A: It's been tried, with very poor results. It's way too slow and does way too little damage. Think about the pressure and time needed to drill a hole in armor material. If your opponent is sitting still long enough for you to do that, he's already lost the match.

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Q: Do you think 'Warhead' is strong enough to be a BB champion? Why are 'MechaVore', 'Surgeon General' and 'MOE's blades made of other materials but 'Warhead' does not? [Chinese Forum]

A: I was quite surprised when 'Warhead' was beaten by 'Overkill' at BattleBots 5.0. I thought it was going all the way.

There are many approaches to building a spinner weapon. Some builders like the simplicity of a single-piece bar, while others attempt to gain an advantage through complex design with several different materials. Warhead's aluminum dome covers their powerful internal combustion engine and acts as armor as well as adding to the rotational mass of the weapon. It was easiest to machine the dome from a single chunk of aluminum and add on the impact teeth.

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Q: Is 'Warhead's blade made of hardened steel? [Chinese Forum]

A: Team Razor wasn't very keen on sharing construction details about their 'bots. I do know that the weapon dome on Warhead was aluminum, and it would certainly make sense to use a tool-steel alloy for the blades.

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Q: In the Combat Robot Hall of Fame, why do you say that Warhead had "the most frightening internal combustion powered heavyweight weapon ever seen at BattleBots"? I think Mechavore is more awesome.

A: Mark J. here: you're entitled to your opinion on 'awesome', but 'Warhead' is an 'Honorable Mention' in the Hall of Fame and 'MechaVore' is still waiting outside.

Warhead had more than three times the power of MechaVore's weapon, and the display of gyroscopic forces put on when Warhead fully spun-up has never been matched. I'm frightened just thinking about it! Suggest that you go watch a few videos of Warhead and see if your opinion changes.

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Q: Can Warhead's tail be used as a lifter?

A: No, it's too weak. It was only intended as a brace in the self-righting sequence.

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Q: Hey, Aaron. What makes a wedge-bot so effective?

A: Simplicity. No moving parts, low weapon weight requirement, and an attack that develops quickly and works well against many types of robot.

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Q: How is Razer's wedge so low? Everytime I see an opponent with a wedge Razer's is lower.

A: Razer has an adjustable 'torsion bar' suspension that allows very precise positioning of the front wedge 'snout'. The information I have is unclear about whether the suspension is adjustable remotely.

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Q: How does Complete Control's Weapon Work?

A: Complete Control was the original 'clampbot'. Starting with a conventional pneumatic lifter, Derek added a pneumatic arm on the lifter platform itself that clamped downward to hold the opponent in place. Once clamped, the entire platform lifted upward with the opponent in a firm grip. Awesome!

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Q: Hey Aaron! Love the site, it has helped me a lot!

I built my first 12 pound wedge bot, yet to compete, and I am wondering about adding a weapon. With the 1-1/2 pound steel wedge my robot only weighs 10 pounds 13.5 oz. I was thinking of making another lighter wedge and putting some type of lifting mechanism using a servo or linear actuator. I know both of those are slow but it does not need to be that fast. Any advice? Thanks, Daniel.

A: Glad to hear that you found the advice here helpful, Daniel.

My advice is to enter a competition with the robot as it is. You may discover a critical use for that extra 18 ounces of weight allowance. If you're pleased with the performance of your chassis and drive train you can think seriously about modifications to give you some additional offense.

Electric lifters are effective in the sub-light classes, and judges seem to like them. You are right to say that they do not need to be fast -- they do not need to lift very high either. Even a simple servo-powered hinged wedge can be very useful and can add to the versatility of the robot, but give the robot a shake-down in combat before you start modifications.

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Q: Where can I find the RPM of 'Mechavore', 'SOW', 'Warhead', 'MOE' and 'Surgeon General'?

A: What... the weapon RPM? I'd start by looking at their websites. See #30 for help on finding archived websites for teams that aren't on the web anymore. Many teams consider this type of data 'top secret' and do not make it available.

There is much more to a rotary weapon than just RPM. Serch this archive for other design criteria.

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Q: I hear that Team Whyachi's 'Warrior SKF' stores it's flipping power in the rear spining shell. How does it work?

A: Builders have been working on flywheel powered flippers for quite some time. There is a discussion thread at the RoboWars Australia Forum. The problem with such a design is building a clutch mechanism that can withstand the brutal force instantaneously transferred from the flywheel to the flipper without tearing itself apart.

I have not seen Warrior SKF up close, but unless you can match the budget and resources available at Team Whyachi I would suggest you try a more conventional approach.

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Q: How come 'The Matador' from Inertia Labs didn't do well? It was the most powerful flipper out there.

A: It takes more than a powerful weapon to make a champion robot. I say this all the time: a successful combat robot has all of it's components and systems working together in balance.

A super-weapon on a chassis that doesn't work well to use that weapon is not going to perform well. A well designed chassis with a poorly set-up radio system will have a very rough time. A robot that's hard to repair and maintain won't get far in a tough tournament. The weapon is probably the least important system on a combat robot.

By the way, I think a 4 win 2 loss record is pretty successful. They were stopped at BattleBots 5.0 only by the barbaric 'Warhead'.

Q: Why did you say The Matador had a "next generation" flipper? What was so different about it?

A: Inertia Labs refered to it as a new generation. Here's what they had to say on their website: "The Matador is the third generation of our flipping robots. Optimizing everything from our previous designs and squeezing three times the flipping force of Toro into only two thirds the weight. The Matador exerts over 20,000 pounds of flipping force and can throw a robot in its weight class over 12 feet in the air."

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Q: What weapon motor and ESC would you recommend for a beetleweight horizontal spinner?

A: Mark J. here: a well designed combat robot is not just a set of components bolted together. The design of each system must be made with consideration to the rest of the machine. Recommending a weapon drive without knowing anything about the design for the rest of the robot would be folly.

If I'm given a weapon design I can comment on potential flaws, but I have to assume the designer has an overall plan that will incorporate the proposed weapon into a well balanced robot.

Read down thru this archive a little to find an example of integrating a weapon into a beetleweight design.

See also #29.

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Q: Is there any way to detect horizontal bar contact electrically, and shut off the blade?

A: Why would you want to? On impact your opponent and your 'bot will be thrown clear of each other and your blade can start to spin back up.

I can't think of any simple circuitry to accomplish an electrically detected contact shut-down.

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Q: How do you use a dead shaft for a insect class horizontal bar spinner?

A: A dead shaft is fixed in place and does not rotate with the weapon. The weapon and the sprocket/pulley are fixed together and have bearings that allow them to rotate on the dead shaft as a unit. This eliminates the problems associated with trying to secure the weapon to a small diameter shaft. Search this archive for more tips on dead shafts.

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Q: Could you coat the edge of a spinning blade with magnesium flakes or something else so that it would spark on contact? Would it help you with the judges?

A: A steel striker bar will make very pretty sparks when hitting titanium armor. I suppose you could add a titanium or magnesium insert to your steel blade to make sparks when attacking steel armor. Aluminum and plastic armor are too soft to make sparks no matter what you hit them with, and nobody would believe sparks off plastic anyhow.

Experienced judges will not be impressed by sparks, but they could help get the audience behind you.

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Q: Hey guys, it's Dave B. again - formerly with the questions about greasing my robot's armor and building a "sliding mass" spinner. Thanks for the great info on my previous questions. Still speaking (for the moment) in terms of antweight design limits, here's my latest idea, I'd like to know what you think:

My plan is to (eventually) build a vertical disc spinner, and mount two rotating teeth on the outer edge of the disc. The teeth will be vaguely in the shape of a capital letter 'L'. My idea is to swivel mount the teeth so that the short leg of the 'L' is normally pointed out Rather than grabbing on impact, the teeth would rotate 90 degrees or so to expose the long arm of the tooth. The teeth would be spring-tensioned to return to original position when the disc slows significantly, such as after a big hit. The idea is to get the opponent well within the range of the long part of the tooth, then flip it outward by means of the smaller arm of the tooth in order to get a really deep impact. I figure this would allow me to run my spinner at a bit higher speed than I normally would, since I wouldn't need to worry about the weapon skittering over the armor of the other 'bot.

Do you envision any problems with implementing or using this design? Do you know if anyone has used a similar idea? Do you have suggestions on materials to build the disc and teeth out of? I don't plan to go too crazy with the weapon RPM as I know my robot's weapon mount won't be indestructable. Thanks again for all the help, guys!

A: Mark J. here: I don't like the idea of putting the entire stress of impact on a hinged tooth. Even an antweight spinner will transmit a big slug of energy thru the impact tooth and I think it's best to have that solidly anchored.

Suppose only one of the teeth gets rotated - you have a serious imbalance problem that would require shutting down the weapon to reset. Worse, one of the hinges gets tweeked and the tooth is stuck either in or out.

A 4" diameter disk rotating at 3000 RPM has more than 500 g's of centrifugal acceleration acting at the edge (calculator). The 'long arm' of your tooth would need to be held in place very firmly to keep from 'rotating out' prematurely. I can't think of a simple design to hold it in place, let it swing out when triggered by an impact on the short arm, and return it to position at lower RPM.

I have seen spinners with 'free swinging' impact bars that are held outward only by centrifugal action. The idea was that the bar could deliver a good blow, then rotate out of the way to prevent the weapon from stopping. This produced a series of rapid impacts that was impressive to the judges, if not as damaging as a single big impact.

Insect class rotating weapons are typically single piece steel to take great abuse. Larger disks are often aluminum with tool-steel teeth, but I've seen large disks made of steel, titanium, end even UHMW Polyethylene.

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Next Mystery Post »

Q: I'm considering building a spinner once I have some more build experience. It will likely be an antweight or beetle, vertical disc or bar design. Would there be an advantage to building the weapon with sliding weights that would be kept close to the center when the weapon is stationary or slow, but would slide out to the outer circumference once the weapon attains sufficient speed?

I'd likely use springs or rubber bands to return the weights to center after a big hit. I should specify that the weights will not be the actual teeth on the weapon, those will be solidly mounted in place. Intuitively, this seems like it would allow a quicker spin-up time, but give the weapon some advantages in terms of stored energy once the weapon was going fast enough to cause the weights to shift.

I have to believe someone would have thought of this before, and the fact that I don't see this kind of thing in common use makes me think it's somehow a bad idea. What do you think?

A: Mark J. here: first, I like your plan to put off building a spinner until you have more experience. Many builders aim to dominate the sport with their first project. That usually ends in frustration. Compliments on your patience.

Some builders reading this answer will scoff, but your idea has theoretical merit. To understand why requires a short lesson in the characteristics of Permanent Magnet Direct Current (PMDC) motors.

Take a look at the chart. A PMDC motor generates maximum torque at stall, and that torque falls off linearly to zero at maximum RPM. Power is the product of RPM and torque, and that product is greatest at 50% of the maximum RPM.

In a conventional spinner weapon, the motor strains against the large rotational mass of the weapon to slowly reach the 'fat' part of the power curve, where it really pumps energy into the weapon. Quickly, the RPMs move beyond the peak power zone and the motor again struggles to add more energy with decreasing torque and power.

Your 'sliding weight' design achieves an effect similar to a continuously variable transmission. The weapon initially has a smaller Moment of Inertia (MOI), allowing it to accelerate more quickly into the peak power zone of the motor output curve. If the sliding weights are designed to start moving outward as the motor approached its output peak and completed their transit shortly after the motor power peak, the motor will spend a greater percentage of the spin-up time near peak power output and would average greater power output which would give a shorter spin-up time.

So, on paper the idea works. How much benefit there is to be gained depends on the amount of mass you are able to shift and how accurately your spring loading system controls the weight progression. The drawback: increased complexity on a component that will take a lot of abuse. It might be simpler to just pop for a more powerful weapon motor; simplicity has merit.

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Q: What is the max speed I could take a 25cm x 3cm x 4mm steel bar to before it starts to get too dragged?

A: Mark J. here: aerodynamic and mechanical drag are the limiting factors on absolute top speed of a rotating weapon for a given amount of power; add more power and you can get more speed. However, if you spin your weapon too fast it won't be able to catch and 'dig in' to your opponent. It will just 'skitter' over the surface of their armor. Also extremely important in an insect class robot is spin-up time. The best weapon takes all these factors into consideration. Play around with the parameters in the Team Run Amok Spinning Weapon Excel Spreadsheet to put everything in balance.

Q: How many joules can a beetleweight bar spinner realistically dish out?

A: See the above question. Joules alone are not the answer. You see builders claiming astronomical numbers of joules, but a well-balanced design is going to win more matches.

Q: How much weight is usually alloted to a robot's weapon system - something like 'Totally Offensive'?

A: 'Totally Offensive' is an extreme example -- blade and motor alone make up more than 1/3 of the entire robot weight. Add in the weapon supports, battery, geabox, and controller and I'd guess at around 60% of the robot weight devoted to the weapon. A more typtical weight allotment might be 35%.

Q: I played around with the spreadsheet a bit and came up with two beetleweight configurations:

1. A 0.4x0.04x0.004m (The obscene length represents weight concentrated towards the ends of a 25cm blade) steel bar, running on a Axi 2820/10 at 4:1. Giving a 0.41 second spin-up. With max RPM and joules of 3600 and 500 respectively weighing in around 24 ounces.

2. A 0.2x0.04x0.004m steel blade on an Axi 2808/20 at 3:1, giving 0.17 second spin-up, and max stats of 4900 RPM and 85 joules.

Which of these is more realistic? What is that perfect balance you just talked about?

A: I get very different numbers for your weapon configurations, so I started over.

First, don't extend the length of a spinning bar to reflect extra mass out at the ends. That will overestimate the moment of inertia. The spreadsheet gets kinda clunky when trying to get this right, but here's how you do it:

• Enter the actual length, width, and thickness of the main bar -- 0.25 meter by 0.04 meter by 4mm = 0.31 kilo.

• Calculate the mass of the additional material -- say two chunks of steel each 0.025 meter by 0.04 meter by 12mm = 0.19 kilo.

• Distribute the additional mass as if it were a ring out at the end of the bar, playing with the height to get the mass correct -- outer radius 0.125 meter (the outer edge of the bar), thickness 25mm (the length of each piece of additional mass), a height of 0.0014 meter gives you 0.19 kilo.

With the Axi 2820/10 at 12 volts and a 4:1 reduction, this correct mass distribution gives a spin-up of 0.16 second to 117 joules and 2268 RPM. The brute power of this motor will likely top out near 3200 RPM and over 220 joules. Calculating for the 0.4 meter bar gives an incorrect estimate of .26 second to 191 joules at 2268 RPM.

Most brushless motor controllers have a 'soft start' function the feeds power to the motor more slowly than just switching it on. This adds considerably to spin-up time. I'd add half a second to any calculated spin-up time for a brushless motor to allow for this function.

The 2808/20 motor powering the 20 centimeter blade at 12 volts and a 3:1 reduction gives a calculated spin-up of 0.25 second to 67 joules at 3755 RPM. Again, I'd add half a second to the spin-up time for the real world.

Bars have poor aerodynamics compared to disks or drums, so you aren't likely to get anywhere near the 95% maximum speed the spreadsheet provides. That's just a 'brag number' at best. I'd guess closer to 80% of maximum RPM with a bar and a reasonably sized motor.

I'd go with the Axi 2808/20 for a beetle. Think about adding more mass to the blade and increasing the reduction ratio. An 8mm thick blade with a 4:1 reduction gives a calculated 0.28 second spin-up to 76 joules at 2816 RPM. It will realisticly top-out around 125 joules at a reasonable 3600 RPM.

One thing to consider if you do build a mega-powered spinner with a huge motor: what happens when the blade hits the arena wall? The enoromous kickback might very well rip your weapon free from the chassis and send the remains of your 'bot flying across the arena. Keep it real. Much more than 100 joules for a beetle is way out there.

As for that 'perfect balance', I wish I had a formula for it. Many of the design factors are in conflict with one another:

• A slower spinning blade does a better job of transmitting the stored energy to the opponent, but a faster spinning blade stores more energy.
• A heavier blade stores more energy, but takes weight allowance from other systems.
• A smaller reduction ratio gives more peak energy, but slows spin-up and taxes the motor.

Everything needs to be in balance, but that balance is hard to find. As I often say, look at what other builders are doing that works. If nobody is using an Axi 2820 for a beetle weapon motor, there's probably a good reason.

Q: What are you using for stats on the Axi 2808/20, because when I run it a 4:1 it gives a spin-up RPM of 2426, not 2816

A: Per the Axi website the Axi 2808/20 puts out 1490 RPM per volt and has a terminal resistance of 105 mohm. At 12 volts that's 17,880 RPM. Divided by 4 gives 4470 RPM. Take 63% for the spin-up calculation and that gives 2816 RPM.

Q: Do you need a motor controller to power an Axi, or can you just use a relay?

A: Axi motors are brushless. Brushless motors require a motor controller to operate - they won't run without one.

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Q: Does anyone run hydraulics? If so where can I find a small DC electric hydraulic pump?

A: Hydraulics are rare but they are used, usually in heavyweight robots because industrial products are too large and heavy for lighter robot classes. Pumps and actuators sized for smaller robots are available, but they tend to be low-power and expensive -- see recent posts in this archive for a source.

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Q: Hello Aaron, I looked at the site and saw that I could build a four bar lifter with a linear actuator. Where would you put it on the lifter?, Could I have a diagram? Thanks.

A: Suggest you take a look at the BioHazard Mechanical Design page. There are several photos and explanatory text covering their use of linear actuators to power a 4-bar lifter. Briefly, the actuators are attached to the front lifter bars via a bellcrank. The bellcrank allows the actuators to generate maximum force when the mechanism has the least mechanical advantage in the lift. Great design!

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Q: Dear Aaron, I want to build a 30 pound flipper robot. What would be the basic parts? Could you recommend parts or part suppliers? I looked thru your website and you mention linear actuators - which would be easier because I am a beginner in flippers. I looked at a website that sells some linear actuators but it would be better to have a fast one. Could you recommend a website? Thanks, Dan.

A: Electric linear actuators work fine for a lifter, but are way too slow for a flipper. You need pneumatics for true flipper speed. There are several posts about pneumatic system parts and links to design help in this archive.

You have quite a bit of design work to do before I can recommend a specific components. Will the liting mechanism be a 4-bar mechanism like BioHazard, a single-pivot unit like Silverback, or some alternative design? How high do you want to lift?

If you do decide to go with a linear actuator I'd suggest a supplier who knows the needs of robot builders. Have a look at the actuators offered by Team Delta and Trossen Robotics.

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Q: You said 1/2 inch titanium is one preferred armor for heavyweights. How much energy is required to pierce it with a pickaxe or a pointed ram?

A: Mark J. here: very hard to calculate because of so many variables: differing armor mounting systems, alloy type, heat treatments, pick cross-sectional area, attack angle, etc. I've never seen even 1/4" titanium pierced by such weapons. I'm told that a 30.06 armor-piercing round will penetrate 7/16" 'hardened titanium alloy'. Such a round carries close to 4000 Joules of energy.

Newtonian physics (action/reaction) effectively prohibits rams or overhead axes from developing enough power to do the job.

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Q: Are internal combustion rams or hammers allowed (diesel/gasoline/propane)?

A: Linear internal combustion actuators are not allowed under section 7.2 of the current 'RFL Standard Extensible Rule Set'. Rotary internal combustion engines (ICE) are allowed for weapon power at the discretion of the event organizer.

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Q: Were there any successful crusher robots besides 'Razer'?

A: That depends on what you consider to be a 'crusher' and what you call 'successful'. There were plenty of self-defined 'crushers' at Robot Wars -- off the top of my head:

• 'Big Nipper'
• 'Cerberus'
• 'Chompalot'
• 'Crusher'
• 'Crushtacean'
• 'Kan Opener'
• 'Mantis'
• 'Ming 3'
• 'Pincer'
• 'R.O.C.S.'
• 'Snake Bite'
• 'Tiberius'
• 'Tough as Nails'
• and even one version of 'Suicidal Tendencies'.

Not many of them could do any effective piercing or crushing -- most were simply clampers. The best tournament results I can recall from a crusher other than 'Razer' came from horizontal crushers. 'Tough as Nails' won 'Heat M' at the Seventh Wars, and 'Kan Opener' won the 'Annihilator' competition at both Extreme Series 2 and the Seventh Wars.

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Q: Hey Aaron, I just finished my first 30 pound wedge bot. It's proabably not very competitive and I want to build a robot with a weapon. I'm not sure what design: a spinner like 'Terminal Velocity' or a spinner like 'Nightmare'. Either way the design should not require welding, and not a drum spinner because I can not make the drum for it. Any advice for a 30 pound robot design?

A: Take a read thru this archive and you'll find that I'm not a big fan of spinners. The truth is that wedges are more sucessful than spinners -- see my dad's analysis of tournament results. My advice is to refine your wedge and get it competitive before you move on to an active weapon.

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Q: I'd like to build a spinning drum weapon with screws as teeth. What's the best and most accurate way to drill holes in the aluminum cylinder that's being used for the drum? I have access to a drill press.

A: Mark J. here: holes for the screw teeth? Use that drill press. Set-up a cradle for the drum that will position the perpendicular axis of the drum directly below the axis of the drill bit and still allows it to rotate and slide left/right to line up the position of the hole. Mark off the hole positions and drill away.

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Q: Hi! I am only 10 and I've loved robotic combat all my life. Could you tell me some advantages and disadvantages for weapons, taking affordability into account?

A: I was only 9 at 'Robotica' and I remember how much I loved Robot Combat back then. Good to have you here!

The arguement over which weapon is 'best' has been going on since the first robot fight. The topic has been addressed in books and in the on-line forums.

This really is too big a topic to discuss fully here, but you'd probably be interested in the results of my dad's study of results from combat robot tournaments. The study shows that, on average, robots with active weapons (spinning blades, drums, lifters) actually do less well than robots with passive weapons (wedges, spikes, rammers). Particulary for your first robot, I'd recommend keeping it simple and avoiding any type of active weapon.

If you are determined to use an active weapon, the data from the weapon study indicates that lifters are the most successful active weapon style -- at least in small robots. A servo-powered lifter is very easy and inexpensive to implement in an antweight robot.

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Q: Hi Aaron, great site. In your archives you mention that the main difference between a drum and a beater is the inertia each can wield. Does that that mean there are no advantages in the structure or shape when it comes to a hit?

A: With a weapon rotating at operational speed there is no advantage to the eggbeater structure on impact. The weapon is rotating so fast that penetration of the opponent into the weapon before impact is a small fraction of an inch. A shallow impact bar on a drum is every bit as effective as the eggbeater bar.

There are several posts about eggbeaters and drums in this archive that deal with construction, rotational speed, and impact bars. The question you reference came in after the other posts and asked if the two designs didn't basically do the same thing, and my answer generated a good deal of off-line discussion.

To clarify my original statement, a drum weapon the same weight and diameter as an eggbeater will have greater rotational inertia. Someone pointed out that an eggbeater can be built to a larger diameter and have the same mass as a smaller drum and therefore have greater rotational inertia. This is true, but it just points out the trade-offs and complexity of weapon design. Generally speaking, an eggbeater is simpler to build but a drum is more effective.

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Q: What would be some pros and cons for an axe weapon and a spinner weapon?

A: Suggest you find a copy of Combat Robot Weapons by Chris Hannold for a good discussion on weapon pros and cons. Very briefly:

• Overhead axe weapons can attack the usually weaker top armor of an opponent but must accelerate the weapon to high energy levels in very little time.

• Spinners can more slowly build up kinetic energy in the rotational mass of their weapon for a devastating impact, but must deal with gyroscopic forces and the large 'kickback' from their own weapon.

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Q: Dear Aaron, I've found a mini hydraulic sytem at gardentrucking.com. The system has almost 40 pounds of force. Would this be a good system for builders who want to make a crushing weapon, or would a cordless drill and gearbox hooked up to a worm gear or screw and conected to a claw be better. Please help. [Anthony]

A: That mini hydraulic system is expensive! A pump, cylinder, valve and assorted lines and fittings looks like about $700, and they don't give you the weight of the system or the volume output of the pump.

Crushing weapons require a great deal of force and very strong chassis, The crushing pincer on Robot Wars heavyweight champion 'Razer' had 18,000 pounds of force -- 100 times it's own weight! So, 40 to 60 pounds of hydraulic force is not going to make much of a crushing weapon and neither is a geared-down drill motor.

Q: Would it be an effective crushing system on an antweight? Would I be able to multiply that 60 pounds of force with a lever like 'Razer' did when there cylinder force was 3 tons and now the tip of the claw has 9 tons? Sorry for bombarding you with questions -- I just really like this sport. I can't help it!!!

A: If I didn't like questions, I wouldn't have this site, Anthony!

There are a couple problems I see with using this system in an antweight:

• Although the component weights are not given, the dimensions are. The pump alone is 6" long and 1.5" in diameter. That looks like a RS-385 motor powering it. I'd guess the pump alone might weigh half a pound. You wouldn't have enough weight allowance left to build a robot strong enough to withstand the force of it's own weapon.

• Gardentrucking supplies components for builders of scale trucks and construction equipment. These builders are after realistic lifelike performance from the hydraulic system, and in this case lifelike means 'slow'. Adding a force-multiplying lever system to would slow this down even more: three times the force means one-third the speed.

In short, the gardentrucking mini hydraulic system is too heavy and too slow to be the power source for an effective antweight crushing weapon.

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Q: Should I buy a motor having high torque or high RPM for a spinner weapon??

A: Either will have to be geared down to perform well with most spinner designs, but it's generally easier to work with a high torque motor. This is why the high-torque 'outrunner' style brushless motors are popular for spinner power. High RPM means high gear reduction and potential problems keeping drive belts in place on small drive sprockets.

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Q: What is the best motor rating for a spinner weapon? My weapon weighs 3 kg and bot weight is 10 kg.

A: See #29 A good place to start with a weapon around that size would be the brushless Axi 4130/16 or the DeWalt 18 volt drillmotor.

Q: I've built a spinner weapon similar to 'Son of Whyachi'. It weighs 7 lbs. The total bot weight is 22 lbs. What is the ideal motor rating for my weapon? The maximum voltage I can use is 24 volts.

A: More power is better for a spinner motor, so 'ideal' is as much power as you can reasonably get. The Axi motor I referenced above would be a good choice. Geared down about 3:1 and running at 24 volts it would give excellent performance. Use the Team Run Amok Spinning Weapon Excel Spreadsheet to check the performance of your specific weapon with various motors.

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Q: Hi. My beetle horizontal mid-cutter had its bar mounted to a 1/4" live shaft by an M Type Shaftloc. At a recent event I had a weapon to weapon collision with a drum. The nut on the Shaftloc was ripped off, stripping the threads and making the entire thing useless.

At Motorama I'm going to run it as an undercutter. What other similar components could I use for mounting the weapon to the shaft? I could use the Shaftloc again since an undercutter won't be taking the same kind of vertical hits, but I'd rather use something more proven and reliable. Thanks.

A: Mark J. here: you don't have a lot of options for securing a hub to a live shaft that small. Hobbyweight mid-cutter 'Fiasco' uses a custom hub that clamps the blade in place and is held to the shaft with four set screws. I'm not a fan of set screws! Given the options, your current solution may be the best choice; it sounds like that hit you took would have broken something else if the Shaftloc hadn't failed.

The most reliable method of securing a hub or other item to a round live shaft is to broach a keyway into the shaft and the hub and inserting a key to prevent rotation. This isn't practical for a 1/4" shaft, and neither is cross-drilling a hole thru the shaft and hub and inserting a hardened pin. I recommend using a dead shaft in insect class spinners for just this reason.

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Q: Hey Aaron, is 0.07" thick 6Al-4V titainium thick enough for a beetle wedge? The wedge is securly mounted with 6 bolts.

A: The thickness needed for a wedge depends on the unsupported length of the material, the angle of the wedge, the type of the support it receives, and the punishment to which it will be exposed. There's no way I can even make a guess about the minimum thickness you'll need for your specific design. Follow my rule for armor: make it as thick as you can and still make weight.

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Q: Are wedges cool? I have been making ants and beetles for 3 years. I want to make a beetle wedge but it seems that people say wedges are boring and lame.

A: Cool is building what you want to build. If other builders don't like what you build, beat the bolts out of their robots and smile while you're doing it.

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Q: I want to use a 12 ounce spinning blade on a beetle weight spinner. What sort of motor or motor/gearhead would be good?

A: Selection of a motor and drivetrain for a spinning weapon needs more input than just the weight of the blade. I'd suggest reading thru the many posts about spinning weapons in this archive and the archive for a start. If you have access to Excel, you can download our Team Run Amok Spinning Weapon Excel Spreadsheet to help you calculate the performance of your weapon design with various motor options. Something like an Axi 2808 brushless motor with a belt drive reduction around 4:1 would be a good starting point.

Q: I want the blade to be a 15cm by 3cm by 1cm 10 ounce steel bar for my beetle weight robot. Is this going to be too powerful and what sort of motor/gearhead should I use? I looked at the Axi 2808 but I can't find gears or pulley small enough to fit on the shaft, can you suggest a place?

A: A steel blade that size weighs closer to 12 ounces than 10, so I'll assume a thickness of 8mm to cut the weight down to 10 ounces. With a 4:1 reduction and an Axi 2808/20 running on a 3-cell LiPo battery, the weapon will spin up very quickly to about 4000 RPM and pack nearly 50 Joules of energy -- plenty for a beetle, but certainly not 'too powerful'.

4mm shaft diameter is just a little over 5/32" You can drill out 1/8" bore timing belt pulleys to 4mm. Robot Marketplace sells 1/8" bore MXL timing pulleys that would work well for this purpose. Belt drive is prefered over gears for a spinning weapon.

Q: Thanks Aaron. One more thing, I keep trying to calculate stall torque of a brushless motor using your spread sheet and I keep coming up with numbers like 1.41N-m which I know is obscene. What am I doing wrong?

A: Maybe nothing. Brushless motors can pump out short bursts of really huge power. Just make sure you get the correct numbers for RPM/volt and internal resistance. Plugging numbers for the Axi 2808/20 into the Run Amok Spinner Spreadsheet (1490 RPM/volt, an internal resistance of 105 mOhm, and 11.1 volts) I get an estimated 0.68 N-m stall torque.

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Q: Hey Aaron, I have 7 ounces left on my beetle. What is the most effective weapon I can put on it? I don't want a spinner. The beetle is a fast and powerfull wedge but I want some type of weapon. Thanks.

A: An effective weapon is integrated into the structure of the robot. The time to think about a weapon is at the start of the design process, not after the 'bot is built and you discover that have a little weight allowance left.

I'd spend the extra weight on armor, but if you're determined to tack on a weapon you'll need to take a look at your design and component layout and think about what type of weapon would make sense. Consider if there is room for a servo-powered lifter and if that would make sense in your design.

Q: Hi again -- I had the 7oz beetle weapon question. Is it possable to build an effective beetle spinner with 7 oz? Thank you!

A: I'm gonna say no. You'd want at least 5 ounces of rotating mass for an effective beetle spinning weapon. That doesn't leave much for motor, drivetrain, and support structure.

You really need to decide what your robot is going to be in the design phase and build the robot to meet that goal. If you just keep adding things on 'til you top out on weight you're going to have a robot that tries to do too many things and ends up being good at nothing. Use that extra seven ounces of weight allowance to make the robot better at what it already does well.

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Q: Hey Aaron, your site is cool. I'm making a beetle and I was wondering if a weapon like on Messin with Sasquatch would be effective? What motor does it use for the weapon? Thanks.

A: Glad you like the site!

'Messin with Sasquatch' used a hole saw spun by (I'm guessing) a drill motor. This type of weapon has been tried quite a few times in various weight classes and has never proven effective.

Think about how you use a hole saw, how much pressure you have to apply, and how long it takes to cut thru a stationary object. Then think about trying to cut into another moving robot with the pressure a beetleweight could apply. The best you could do is to scratch up a little Lexan.

Pick a robot with a better record than 0-1 to emulate.

Q: So if a powered hole saw like on 'Messin with Sasquatch' isn't effective, then what's the coolest most effective weapon I can make on a beetle? I don't have enough weight to make an effective spinner.

A: 'Cool' and 'effective' are often two very different things in robot combat. Lifters win a higher percentage of matches than anything else in the beetle weightclass, but most people don't consider them to be 'cool'. Spinners do very poorly overall, but builders think they are 'cool' so they keep building lots of them. Read thru this archive and take a look at our analysis of weapon success.

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Date marker: January 2009

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Q: I plan on making a beetleweight robot using a VDD1 blade directly powered by a GWS brushless motor (2205/15T GWBLM001A) on a 3s LiPo battery. These were my results using your spinning weapons Excel spreadsheet:

Total Moment of Inertia: 0.00012 KgM2 Mass 0.08 Kilos Weapon will spin to: 14,336 RPM [ 63% Max ] with: 140 Joules energy in: 0.23 seconds. At: 21,617 RPM [ 95% Max ] the weapon has: 319 Joules energy.

I know you recommend against direct drive weapon motors, but I will be using a Robot Marketplace blade hub, and the brushless motor will be directly mounted to .9 aluminum. There will be another piece of aluminum on the other side of the motor to support the shaft to prevent bending.

A: Mark J. here: did you wonder why other builders don't do this? Several problems:

• I think you've miscalculated the stall torque of the GWS brushless motor. I can't find a quoted figure for internal resistance, but from size and performance parameters I estimate 170 mΩ. This gives a stall-torque estimate of 0.29 N-m, not the 1.30 N-m you were apparently using. This stretches the calculated spin-up time to 1.02 seconds.

• The spinner spreadsheet was designed for weapons spinning much slower than what you propose. It does not take into account aerodynamic drag, and a 110 mm diameter saw blade with big teeth spinning that fast is gonna have a whole lot of drag. That will stretch spin-up time and reduce top RPM.

• The ESC you pick to use with the motor may limit amperage to the motor via a 'soft start' feature. This will reduce the maximum torque and even further stretch spin-up time.

• Driving the 'bot is gonna ba a real problem. A blade spinning that fast will generate sufficient gyroscopic force to make turning the robot very difficult.

• At speed, you'll have less than 0.00025 second between passing blade tips. That's not going to 'dig in' to your opponent, it'll just skitter over the surface.

I've saved the best for last:

• That blade has a maximum speed rating of 13,890 RPM. An impact at 20,000 RPM is likely to disintegrate the blade, throwing sharp chunks of steel in a deadly arc at over 300 feet per second. That'll destroy your 'bot, mess up the arena, and potentially cause serious injury to anyone unfortunate enough to be standing nearby. No sane event organizer will let you compete.

Seriously -- don't direct drive the saw blade. Spinners are geared down for a variety of reasons. The weapon will be much more effective at about 1/4 the speed you propose, and it probably won't kill anyone.

Q: I asked about connecting a GWS motor directly to a VDD1 blade, and you suggested gearing it down. I was wondering if the timing pullies from Robot Marketplace are a good choice? I am having trouble understanding what all of the things like pitch and flange diameter mean. I also don't know how to connect the the blade to the outer peices of aluminum. Thanks for your help!

A: The timing belt and pulley solution will work well, but the simplest and possibly least expensive solution would be to to use the 'VDD Polycarbonate Gearbox Kit' [no longer available].

Your GWS motor is small enough to fit right in place, the weapon shaft is attached with widely spaced bearings to support the load, and a set of gears are included for the reduction drive. The pinion gear will have to be drilled out to fit onto the 3 mm shaft on your GWS motor. It will save you from building a weapon shaft support and from figuring out the correct belt size and sprocket diameters.

Q: Hi Aaron, I have another question about my gws brushless motor, and VDD1 blade setup. Would an alternative solution to using a gearbox be leaving it direct drive, but not passing 1/3 throttle? I left all of the numbers for my motor the same, except I changed the rpm to 1/3 of what I had originally. I got these numbers, and they seemed okay:

Total Moment of Inertia: 0.00012 KgM2 Mass 0.08 Kilos Weapon will spin to: 4,779 RPM [ 63% Max ] with: 16 Joules energy in: 0.34 seconds. At: 7206 RPM [ 95% Max ] the weapon has: 35 Joules energy.

What do you think?

A: You're still using the wrong numbers to calculate torque and spin-up time. On the 'Instructions' page of the spreadsheet enter '170' as the Ri (internal resistance) value, and '2160' as the Kv (rpm/volt) value. A drop in throttle will effect both RPM and torque -- if you're going to only use 1/3 throttle, reduce the Voltage to '3.7'. The calculated stall torque value now drops to 0.10 N-m.

With these changes, I get a 63% spin-up time of 1.03 seconds, with the weapon hitting 35 Joules in about 2.5 seconds.

I know that setting up a gear reduction drive for a spinning weapon is somewhat difficult, but if you look around at your competitors you'll notice that they all do it even though it adds weight and complexity. There is more to a good spinning weapon than just Joules of stored energy. The best approach has proven to be a gear reduction. This reduces load on the motor, helps to isolate it from impact shock, increases available torque to carry the weapon past initial impact, and reduces peak amperage draw from the battery/ESC.

Trust the other builders that have done the experimental work on this. Use a gear or belt reduction.

Extra thought: for a beetle, you might consider mounting two blades on the hub. A few drops of epoxy between the blades would hold them in alignment, and the doubled mass would double the weapon energy. Spin-up time would still be good if you use a gear reduction.

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Q: How are the angled sides on some Full Body Spinners made?

A: Mark J. here: different builders use different techniques. Hall of fame member 'Ziggo' used a pre-made 20" diameter wok from a kitchen supply store for the spinner body. You can read a write-up of Ziggo's construction at the Team Ziggo website.

Other techniques include use of a sheet metal roller to create a cone that can be clipped to the proper height, and machining the entire shell from a single block of metal.

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Q: I saw a bot on a video that had a vertical spinner that had no cutting tools on the disc. It was used to flip the other bot. How would I set up a bot like this?

A: That's the standard use of vertical spinners like 'Nightmare' -- a couple of impact 'knockers' are bolted opposite each other on the edge of the spinning disk to strike the opponent. When a circular saw blade is used for the disk, a blade with large teeth is used to catch and throw the other 'bot. This design is particularly popular in antweight 'bots that fight in arenas where an instant win can be scored by tossing your opponent off the arena platform.

Browse this archive for tips on spinning weapons.

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Q: How did the dome spinner bot 'Ziggo' have so much power? Is this possible in a beetle weight?

A: 'Ziggo' was the product of a lot of hard work and experimentation. Builder Jonathan Ridder started building combat robots in 1995. His first attempts were not terribly successful, but he kept at it and learned from his experiences.

There isn't any real secret to the power of Ziggo's spinner. Quoting from the Team Ziggy website:

"Team Ziggy's robots all have one thing in common - maximum power in minimum volume. Combat robots require large amounts of power to accelerate quickly and have truly spectacular weapons. By making the robot as small as possible, you can use stronger armor and give less of a target to your opponents." Ziggo's weapon is powered by two Aveox 1415-4Y motors running at 36 volts. That's a huge amount of power, and power in means power out. There is a lot more to building a successful spinner than just power, but power is a good start.

Insect class full-body spinners have a serious problem. As I have pointed out often, insect arenas are small and there is very little time to spin the weapon up to effective speed before your opponent is on top of you. Build for the situation in which you will be fighting.

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Q: Which sportsman electric hammer bot has more weapon Joules: 'Mangi' or 'Helios Sport'? Which one do you like better?

A: Mark J. here: I don't have enough information about 'Mangi' to make even a rough calculation on weapon energy. I've written to Mangi's builder for additional info. As a pure guess I'd say 'Helios Sport' hits a little harder, but 'Mangi' is quicker. They did have one head-to-head fight won by 'Helios Sport'.

UPDATE: I gathered enough info to estimate the performance of both wearpons. According to my calculations, 'Mangi' with 78 Joules is harder hitting than 'Helios Sport' at 50 Joules.

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Q: Mark, you said you had an Excel spreadsheet for electric hammers. Would it be possible for you to e-mail me this? I'm working on a sportsman class hammer for Robot Battles and NERC. Thanks.

A: Mark J. here: the spreadsheet I have is just a few basic kinematic formulas strung together with a couple 'best guess' fudge factors. I put it together to get some order of magnitude comparisons between electric and pneumatic hammers. It has no documentation, is useable only over a small range of values without recalibration, and it relies on some questionable physics shortcuts. I don't think it would be of any use at all as a design aid.

A full-blown hammer simulator would be kind of a big deal. The motor torque continually changes with increasing speed, there is no good way to calculate the inertia of the armature from available data, and the effect of gravity on the hammer changes non-linearly all along the arc.

My best advice, as has often been repeated on the Ask Aaron webpage, is to use the construction examples provided by other builders as a starting point. In your case I'd carefully examine Helios Sport from Team Cosmos. Best luck!

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Q: Has anyone used a C02 powered engine for a spinning weapon before?

A: Yes, 'The Butcher' from Inertia Labs had a massive pneumatic powered spinning weapon which was not very successful. Pound for pound, electric batteries store much more energy than compressed gas cylinders. Electric motors are also very efficient (80%) compared to compressed gas engines (15%). Pneumatics are great for quick bursts of power, but not efficient for continuous motion.

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Q: Would a beetle hammer bot be effective? I'm using a speed 400 motor geared 25:1. The arm is 12" and hammerhead is 4oz with a sharpened tip.

Q: How much weight will the hammer head need to be able to do damage?

Q: Would the hammer get more damage if I increased the arm to 18" and lightened the hammer head to 3oz?

A: Mark J. here: assuming a 180 degree hammer arc and taking a guess at torque losses to armature acceleration:

• the Speed 400 motor geared 25:1 will provide about 5 G's acceleration to a 4 ounce weight on a 12" arm;

• the hammer head will complete a 180 degree arc in about 0.2 second;

• the velocity of the hammer head at the end will be about 31 feet per second;

• the kinetic energy of the hammer head at impact will be around 5 Joules.

For comparison, the pneumatic axe on heavyweight 'TerrorHurtz' generates 2300 Joules, more than 10 Joules per pound of weightclass. Electric motors are not nearly as good as pneumatic systems for producing large bursts of energy.

It's important to remember that the hammer is just a storage device for power from the motor -- you can't get more energy out of the weapon than you put in!

Weapon energy is a product of the acceleration provided by the motor and the length of time that acceleration is applied. Changes to the gear ratio, length of the arm, or mass of the hammer will change the weapon acceleration, but will be offset by the change in time available before impact. As long as these design elements remain anywhere near reasonable you'll still get about the same weapon energy output. To get more energy output, you'll need more motor power input.

Q: Is 5 joules strong for a beetle hammer?

A: A strong hammer runs close to 10 Joules per pound of weight class -- about 30 joules for a beetle. Electric hammers just can't put out enough power.

Q: How many joules did 'Helios Sport's hammer have?

A: Mark J. here: 'Helios Sport' is a 30 pound 'sportsman class' robot with a 3 pound hammer on the end of an 18" arm powered by a DeWalt 14.4 volt motor at 18 volts. That works out to about 50 Joules -- comparable to a 5 Joule hammer on a beetle. You can see from the video of 'Helios Sport' vs. 'Bounty Hunter' that the hammer isn't effective.

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Q: I have the T.i. Combat Robotics 4-Bar Simulator. Is the weight on there the force of the lifter?

A: Mark J. here: not quite. The 'weight' input is the constant force pressing down on the end of your lifter. The motor torque needed to offset that force is shown as the green line in the output chart. Note that the torque needed varies with the position of the lifter, but the maximum torque available from your gearmotor is constant. This means that the maximum lift force will vary with lifter position.

Also, the motor torque shown in the graph is the amount needed to just offset the weight on the lifter. Permanent magnet electric motors produce maximum torque at stall (zero speed), so a gearmotor with the torque shown in the graph could hold position against that much weight at that position but could not lift it further. Aim for a gearmotor with about twice the maximum torque shown for the weight you want to lift.

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Q: Who has the most powerful flipper: 'Ziggy', 'Toro', 'The Matador', or 'The Gap'? How much does The Gap weigh?

A: 'The Matador' from Inertia Labs claims the most powerful flipper title with 20,000 pounds of lifting force. Team Run Amok's The Gap is a heavyweight that last weighed in at 208 pounds with an empty CO₂ tank.

Q: Is Matador's flipper more powerful than Toro's?

A: What did I just say? Matador's is the more powerful. Toro's superheavyweight CO₂ flipper had 7000 pounds of lift. The Matador's 'next generation' flipper had 20,000 pounds of lift in a heavyweight.

Q: Aaron, do you know how many pounds of force Ziggy's lifting arm has? Do you know what psi Ziggy's pneumatic system runs on? Could you give me a link to Ziggy's website if it has one?

A: CM Robotics claims 14,000 pounds of lifter force for 'Ziggy', but the 4-bar flipper mechanism makes comparison with direct-acting flippers like 'Toro' difficult.

Ziggy's flipper runs on high-pressure air, someplace in the 3000 to 5000 psi range.

Some robot teams, such as Ziggy's CM Robotics, don't have websites. It isn't that they aren't capable, it's just that they have a problem with sharing.

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Q: You said Ziggy's lifter has 14,000 pounds of force. How do you calculate that force?

A: Mark J. here: that number comes from CM Robotics -- I don't know how they are calculating. In a conventional single-pivot lifter it's fairly easy to apply a little geometry to figure the lifter force based on the force available at the pneumatic cylinder (surface area of piston times available pressure), the angle at which the force is applied, and the leverage between the force application point and the point of lift. Ziggy has a four-bar flipper mechanism, which makes the calculation more difficult. They may simply be giving the raw force available at the cylinder.

Actual flipper performance depends not only on the maximum force available, but also on the gas flow capacity of the pneumatic system. Valves and hoses must be large enough to flood the cylinder with an instantaneous burst of high-pressure gas or the 'flip' becomes a gentle 'lift'.

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Q: What do you think of 'The Matador'? What is it's fight record?

A: The Matador, like all of Inertia Labs' robots, was beautifully made and well thought out. The amount of force in it's weapon was certainly overkill. A little less weight devoted to the weapon and a little more to a more controllable chassis might have worked out better.

It's record was 4 wins and 2 losses -- the last loss to the fearsome 'Warhead' at BattleBots 5.0. I don't remember the match, but there may not have been enough of 'The Matador' left to put back together.

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Next Mystery Post »

Q: Are there any good gearmotors I can use in a simple beetleweight lifter? I want to be able to rotate the arm all the way around, something I can't do with a servo.

A: There are certainly some gearmotors that will work for your purpose, but it is also a fairly simple task to hack a servo for continuous rotation.

The power you'll need from a gearmotor (or hacked servo) will depend on the length of the lifter arm attached to it. For reasonable speed and reliability, select a gearmotor that will stall with no less than 1.67 times the weightclass on the end of the lifter. Here's the formula to calculate the desired torque:

Stall Torque (oz-in) = length of lifter arm (inches) × weight class (ounces) × 1.67

Example - a gearmotor for a 4" lifter arm on a beetleweight should have:

4 inches × 48 ounces × 1.67 = 321 oz-in stall torque

The 'B231 231:1 gearmotor' [no longer available] running at 12 volts would be a good choice. You could also use the BaneBots 64:1 24mm gearmotor [no longer available] running at 7.2 volts, but it's higher speed could be difficult to control on a lifter.

Some builders believe you can use a gearmotor with much less torque than we recommend. There is an extended discussion on gearmotors for lifting arms in this archive: search the page for 'RS-540'.

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Q: What antweight weapon would you suggest for a beginner robot builder, beside a wedge and lifter?

A: That's a fairly complete list -- you might want to remove 'lifter'. Active weapons add several layers of additional complexity onto an already complex undertaking. I recommend that first-time builders keep their robots very simple. You'll have enough new things to worry about with battery maintenance, R/C system set-up, traction issues, ESC mixing, driving, radio interference, wireing, tournament procedures, and repair problems.

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Q: How does 'Original Sin's hinged wedge work?

A: The front wedge on the current #1 ranked heavyweight is mounted on a heavy rod that pivots in very sturdy mounts attached to the chassis. This allows the front of the wedge to pivot down and scrape along the surface of the arena, or drop and lie flat if the 'bot is inverted. The pivot point is low -- near the centerline of the robot -- which helps prevent the wedge from folding back under on heavy impact.

Zero-clearance wedges have a drawback: they can get hung up on joints in the arena floor. In a smooth arena a low-pivot hinged wedge can be a plus.

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Q: What do you think about 'The Butcher' from Inertia Labs?

A: Mark J. here: it was in interesting experiment: a pneumatic powered rotary weapon with a claimed 150 horsepower. Record: 1 win, 1 loss. The problem was that it isn't possible to effectively store enough energy in the form of compressed gas to power such a weapon for more than a very few seconds. Pound for pound, electric batteries store much more energy than compressed gas cylinders.

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Q: Is 300-400 Joules decent for a beetleweight drum with the current competition?

A: That's way better than decent, assuming that you have a reasonable spin-up time.

Q: What size screws would be best for use as teeth on a 2.5" OD Beetleweight drum spinning at 18,000 RPM?

A: Mark J. here: unless that is a very narrow drum, it isn't reasonable to try to spin it at 18K RPM. Anything less than perfect dynamic balance will shake your 'bot around like jello in an earthquake. Even if it's perfectly balanced to start with, it won't be after the first hit. Worse, with two rows of teeth a tooth will pass by every 2 milliseconds -- you can't get any 'bite' into your opponent with teeth passing by that fast.

Slow down the drum by a factor of at least two. I'd suggest even slower. Speed and Joules are less important on a small drum than are bite and torque. For teeth try 3/16" hex cap screws. Anything smaller might shear off.

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Q: How well did the flamethrower antweight 'Crack Torch' do? Wouldn't flame weapons be good for the insect classes because a lot of the robots are not fully covered in armor and protection?

A: 'Crack Torch' never won a fight. Although they are crowd favorites, flame weapons are ineffective in any weight class. Your opponent would have to sit still while you positioned your flame weapon and held it long enough to do some damage. If your opponent is sitting that still, he's disabled and you've already won the match.

You can research the fight history of any robot at Botrank.com.

Q: What about Texas Heat? It is an effective lightweight flame thrower robot that is currently ranked 2nd. Wouldn't a similar design work for an antweight?

A: 'Texas Heat' is an effective wedge robot designed by the very experienced and well financed CM Robotics. It would be equally effective without the flame weapon. Flames are for show.

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Q: How do you design a drum weapon with the motor inside the drum, like Inertia Labs' 'Hummer'?

A: Small drums may have the rotating can of a brushless outrunner motor press-fit into the inner diameter of a custom machined drum. The motor mount becomes the support for one end of the drum. This design is simple and rugged, but gives you no gear reduction control for maximum drum speed and spin-up time.

I've also seen larger drum weapons with an internal motor mounted on a hollow non-rotating 'dead shaft' with the drum supported on bearings. Power wires can be run to the motor thru the center of the dead shaft. Power was transferred to the inner surface of the drum by a friction drive. In general, an external motor with a belt drive is a simpler and better choice.

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Q: Do you like Blendo? Do you know how much horsepower Blendo's Briggs and Stratton ICE had?

A: 'Blendo' was a great robot and a real benchmark in robot design. Their Briggs & Stratton internal combustion engine was rated at 5 HP -- not much by today's standards for heavyweight spinner power.

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Q: How do I calculate the length of the bars in the four bar linkage to get a scooping motion similar to Ziggy's flipper weapon? [Leo]

A: There are several tips on 4-bar mechanism design in this archive. The T.i. Combat Robotics 4-Bar Simulator is a valuable design tool and a good place to start.

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Q: Are pneumatics expensive? About how heavy are they? I'm looking to build a 60lb robot that can toss my opponents a few feet in the air, but I don't know how much weight and cash it would cost me.

A: Combat robots are generally expensive. Larger combat robots are more expensive. You'd better have an overall budget of at least a couple grand to cover a minimal lightweight flipper.

Standard pneumatic components are off-the-shelf products available thru industrial suppliers. If you can put your system together from these standard components you can keep the price reasonable. Pricing for a budget CO₂ low-pressure (150 psi actuation pressure) lightweight pneumatic system might run something like this:

• an actuator cylinder: less than $100
• a useable 5-port valve: about $150
• a small paintball regulator: about $100
• a small CO2 paintball tank: less than $30
• assorted connectors, hoses, valves, guages, electronics, and mounts: another $100.

That will get you a system that, if properly constructed, will toss an opponent over onto their back. Higher power systems to flip an opponent higher will require add-ins: buffer tank, quick-exhaust valve, higher flow solenoid valve, etc. A full blown high-pressure system will double or triple the cost.

How much will it weigh? That's like asking how much a rock weighs -- it depends on the rock. A basic lightweight system might weigh as little as 6 or 8 pounds, but adding on performance will quickly add to your weight as well as the bulk of the system. Pneumatics take up a lot of room, so your 'bot may need to be larger which makes for more weight in the armor and chassis as well.

There is also the matter of assembling a safe and reliable system. Pneumatics are not for beginners. They can generate a lot of force and move very quickly. That is a recipe for severe injury if you don't know what you're doing. A first-time builder has plenty to worry with just getting the radio, electrical, and mechanical systems of a simple combat robot working correctly. Don't add on the complexity of an active weapon 'til you get the basics down.

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Q: How do you create a flywheel powered flipper?

A: Mark J. here: Although it is an interesting theoretical design, a flywheel powered flipper has a number of engineering challenges that render it inferior to pneumatic flippers. The idea is to store energy from an electric motor in a rotating disk, just like a conventional spinning weapon, but then to engage a clutch mechanism to transfer that stored energy via a linkage to a flipping arm or platform. The clutch must be able to engage quickly, transmit enormous force, and disengage quickly to prevent excess energy from ripping the weapon apart.

I've seen a few clever designs on paper, but no one has actually built a successful flywheel flipper. The complexity, weight, and expense would be much greater than a pneumatic system of similar capacity. Stick with pneumatics.

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Q: I want to use some pneumatics in my next 'bot. Is HPA simply pressurized Oxygen? I have a compressor, could I use that to fill my tanks? Also, do I have to buy a special type of air tank for HPA?

A: Mark J. here: HPA is High Pressure Air (78% nitrogen, 20% oxygen) right out of the atmosphere. HPA systems in combat robots typically have their tanks pressurized at 2500 psi or above -- likely much greater than the capability of your compressor. The'RFL rules' require HPA pressure tanks to be rated for al least 120% of the maximum pressure they will be used at, and to have a current hydro test certifying that capacity. High pressure paintball tanks are commonly used in small 'bots, and SCUBA tanks in larger 'bots.

Before you go any farther, read the Team Da Vinci Understanding Pneumatics page. That will answer a lot of questions you didn't even know you should be asking.

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Q: How is the spinning ring on a robot like 'Ringmaster' attached?

A: Mark J. here: there have been several attempts to build a successful invertible 'full body' spinner, with wheels sticking out both top and bottom. Ringmaster fought as a heavyweight at BattleBots 5.0 and had a good run, winning 4 fights before losing to Omega-13.

A rotating ring weapon is typically supported by a number of small wheels attached around the upper and lower perimeter of the chassis. The wheels run in grooves in the inner surface of the ring, supporting the ring and allowing it to rotate.

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Q: I want to build a vert spinner that has a wedge in front so it can get at and chew up the soft underbellies of the current lightweights, destroying anything attached. How many joules should such a weapon pack?

A: Mark J. here: it sounds like you're planning a small diameter weapon with the intent to grind away at the bottom armor rather than a traditional impact spinner weapon. What you're building is more of a mobile power tool than a sledge hammer. Joules of stored kinetic energy don't apply -- you're not going to rely on stored energy to do damage, but on continuous application of power.

You'll need enough power to keep the weapon spinning while in contact with the other 'bot and their full weight bearing down on the spinning blade/bar/grinder. I don't have any simple formulas to calculate the power you'll need for that. I'd suggest keeping the RPMs relatively low and gearing for torque.

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Q: I'm building a featherweight (12 kilo) robot for entry in to comptitions in the U.K. How should I power my four bar lifter: pneumatics, linear actuators or a high torque electric gearbox? And how much weight should it be able to lift?

A: Mark J. here: each of the power options you list have strengths and weaknesses:

• Pneumatics components are widely available and can provide high thrust, speed, and design flexibility. They do require additional support equipment and add complexity to the confined space inside your robot.

• Linear actuators are self-contained, compact, and have the same design flexibility found in pneumatic systems without the need for additional support equipment. They are, however, much slower and potentially less powerful than pneumatic systems.

• An electric gearbox capable of supplying the torque needed for a 12 kilo robot would be bulky and heavy. The T.i. Combat Robotics 4-Bar Simulator can give you torque requirements for a gearbox powered 4-bar lifter. You might be surprized by the torque required.

I'd suggest linear actuators if you can locate suitable units. Calculating required thrust is a little tricky, but there are some notes in this archive that will help.

If you are using electric power for your lifter, I recommend designing to stall the lift motors at twice the weight of the weightclass (24 kilos in your case). That will load the motors down to about half speed when lifting the maximum weight you will likely face -- right at the horsepower peak of the motors. Many builders design for less lift in an effort to get more speed, but their lifters bog down under load.

A similar amount of lift should work well with a pneumatic system. You may be able to get much more, but you did say 'lifter' and not 'flipper'.

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Q: I'm thinking of using an Axi 2808 press fit into my beetleweight's drum. How would I get the press fit?

A: A true 'press fit' involves machining the inside of the drum to a diameter a couple of thousandths of an inch smaller than the outside diameter of the motor and using a hydraulic press to shove the motor into place. If your machining isn't accurate, the drum won't run true.

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Q: I was thinking of using two Magmotor E-150 motors at 36 volts to power my middleweight sminner weapon. They would give me a good amount of power at a reasonable price. How does that sound?

A: For weapon power, two E-150s would weigh nearly twice as much as an A28-150 and produce 1/3 the power. Weight is the greatest challenge in building a combat robot. You need to make the best use of every ounce. The weight you save in the motor can go into the spinner disk for more stored energy and damage potential, while the added power will improve your spin-up time and maximum RPM. Well worth the added expense.

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Q: I've been thinking about mounting a spinning disk on an arm and moving it like a chop saw. Do you think this is a good idea?

A: If you're building a large weapon I would recommend putting all available weight into the battery, motor, and weapon mass rather than on structures that do not increase the impact and energy of the weapon. The Robot Wars housebot 'Dead Metal' used a circular saw weapon that swung forward and down, but a cutting saw does not produce anywhere near the kick-back force of a rotary impact weapon.

Every action produces an equal and opposite reaction, so your weapon arm would need to absorb the same amount of force that the weapon inflicts on your opponent. That would take a massively reinforced arm assembly! Keep it simple.

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Q: How many RPM will I want for a 3 foot diameter spinning disk?

A: The energy stored in a spinning weapon is dependent on the speed of the disk, the mass of the disk, and how the mass is distributed on the disk. There is also the question of the time needed to spin the weapon up to an effective energy level. Search this archive for information on calculating weapon energy, appropriate energy levels, and spin-up time.

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Q: How were the chains attached to the weapon sprockets on 'The Judge'?

A: Careful examiniation of the photo at right shows that the last chain link is bent inward toward the shaft and riveted thru the sprocket. An animation of this weapon is in this archive -- scroll down about 40 questions.

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Q: Can I use an IFI Victor 883 ESC to control a lifter instead of a relay and solenoid?

A: Yep.

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Q: Is there a guideline or a formula for figuring out battery requirements a spinner motor needs? My spinner runs at 36 volts and needs a separate pack, but don't know how much capacity it should have.

A: There are a lot of variables in calculating weapon battery requirements, but we gave a 'rule of thumb' and approximate formulas in an earlier post. Search the archive for 'battery capacity'.

Q: I took a look at your formulas to figure out weapon battery requirements and added them into your Spinning Weapon Excel Spreadsheet. It's formatted exactly like your work, and the formulas are all correct. Would you like to add it to your spreadsheet?

A: Nice idea! Michael Maples sent me his addition to the spreadsheet and it's way cool. I've replaced the downloadable file with the new version. Thanks, Michael.

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Q: How do I decide how much 'burst current' is enough for my weapon battery?

A: Mark J. here: ideally you should be able to meet the full stall current demand of your weapon motor. Current equals torque, and if you can't supply the current the spin-up time of your weapon will suffer. From a practical standpoint, if you can supply 80% of your weapon motor stall current you'll be fine.

Note that LiPoly batteries can be damaged by current drain above their rated capacity. If your LiPoly pack is rated below stall current, you might want to use a 'servo slower' module to 'feather in' the throttle to your weapon ESC and avoid the big start-up amp rush.

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Q: I'd like to be able to use my Thor spin controller with my featherweight's 6 pound disc weapon, but I can't find any brushed motors other than the Mini-mag (a bit overkill) for my use. Do you have any suggestions?

A: How about the classic DeWalt 18 volt drill motor? At 24 volts it kicks out 1.5 horsepower and it's less than a third the weight of the A28-150 Magmotor motor. The problem with it and other small brushed motors is high RPM. You'll have to figure out a weapon drive that will stay together at 20K RPM.

Check out the performance of your weapon/motor combination with the Team Run Amok Spinning Weapon Excel Spreadsheet.

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Q: I'm running my featherweight's weapon Green Wave style: motor on top. As a result, I have a dead shaft. How should I put the bearing in my weapon bar?

A: Mark J. here: 'Green Wave' was a middleweight spinner with the weapon motor mounted above the robot body on a short, stationary (dead) shaft. The motor support shaft also functioned as the axle upon which the bar weapon spun. The design allowed the bar weapon to be mounted very low on the chassis.

'Green Wave' had serious problems with weapon bearing disintegration caused by heavy off-axis loads on impact. To prevent this problem with your 'bot, the weapon bar assembly should be supported by two bearings separated on the shaft to spread the load and prevent twisting. I'd suggest one bearing in the belt drive pulley and a second bearing in a flange mount on the bottom of the weapon bar.

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Q: How are most eggbeaters constructed? Are they simply several steel bars welded into a rectangle shape?

A: Eggbeaters take enormous punishment. Most are cut from a single plate of metal for greatest strength.

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Q: How thick should heavyweight eggbeater's shaft be?

A: Mark J. here: come on, guys -- you've got to give me more info than the general type of weapon and the weight class! It's like asking, "What should I feed my yellow pet?" Dog? Canary? Goldfish? See comment in the next question below on 'how thick'.

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Q: How thick should the wall of my lightweight's drum weapon be?

A: Mark J. here: my standard answer to a 'how thick' question is, "as thick as you can make it and still make weight." In this case, a thicker drum wall will mean more energy storage and more durability. Like most design factors in a combat robot, there is a trade-off in adding weight to one system while pulling weight out of another.

A typical drum makes up around 20% of the total weight of the 'bot. About 3/16" aluminum might be a good starting point for a lightweight drum, but the size, speed, and design of the drum can modify this.

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Q: Say I have a 6 inch diameter, 6 inch long drum. How far should the teeth extend?

A: Optimum drum tooth length depends on tooth spacing and the axial speed of the teeth, not the size of the drum. You want your target to get as far inside the arc of the rotating teeth as possible in order to get maximum 'bite'. Faster spinning drums require wider tooth spacing and run shorter teeth, while slower spinning drums can have closer and deeper teeth.

You need to calculate the amount of time between teeth when the weapon is up to speed, then calculate how far your 'bot is likely to move forward in that time period when attacking. That distance is the optimum tooth length.

Alternately, since your attack speed is so variable, you could just stick the teeth out about 3/8" and go for it!

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Q: How are bolts used as weapon teeth (such as on 'Tillah') effective? Wouldn't the dull edges keep them from catching on an opponent?

A: Bolts are not ideal drum teeth -- but they are inexpensive, easy to obtain, and simple to replace. The simple blunt impact is fairly effective, plus a little time with a hand file or Dremel tool can put a nice, sharp edge on a bolt head.

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Q: What would be the best way to mount a bar or other weapon to the TWM3R Gearbox?

A: Mark J. here: the Team Whyachi TWM3R gearbox has a nominal 1 inch titanium shaft with a 3/8-16 tapped hole in the end and a .25 inch keyway down the side. This makes it convenient to lock a keyed weapon hub onto the shaft. I'm sure that Team Whyachi would be willing to machine a suitable weapon hub for you.

Note that a keyed hub has no slip or give and will transmit all weapon shock loads back to the gearbox, potentially destroying it. I prefer a slipable weapon drive like a v-belt, but the Team Whyachi gearbox is built to take high loads. Your choice.

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Q: In most cases, would a drum's teeth have to be hardened?

A: Mark J. here: If you are using hardenable metal it's a waste not to harden them, but I see a lot of non-hardened drum teeth. For longest life and greatest damage, harden the teeth.

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Q: What is the maximum actuation pressure of a pneumatics system?

A: There is no maximum pressure specified in the RFL rules, but all components in the system must be rated for the pressure used. Above 250 psi, components must be over-rated to 120% of the pressure used. Systems operating above 2500 psi require pre-qualification by the event officials.

Consult section 7 of the 'RFL rules' for details.

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Q: How did Son of Smashy's spring powered weapon work? Would it be possible to do something similar in a lightweight?

A: Son of Smashy used an electric winch to reset the spring after each strike. The difficult part in building a set-up like this is the release mechanism that allows the weapon to fire then re-engages the winch. In theory it could be done in any weight class but pneumatic systems are more powerful, quicker to reset, and can be built from off-the-shelf components. Spring powered weapons are rare for a reason.

Search the archive for 'winch' for other posts on this topic, and scroll down about eight questions for the post on 'Red Square'.

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Q: Do builders make or buy axes?

A: Axes are neither popular nor effective weapons, but I've seen hardware store axes, picks, hammers, and chisels used. High energy weapons usually have custom made impact pieces.

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Q: I want to build a super heavyweight with a three arm caged rotor (like 'Son of Whyachi') spun by 2 Etek motors. What is the best way to connect them to the weapon -- gears, chains, belts? What kind of battery power would I need?

A: The Etek motors have very large armature intertia which will cause the rotor to shift on the shaft if stopped abruptly. This effectively destroys the motor. To prevent this you should drive a rotary weapon with a slipable system like a v-belt. You may need several parallel belts to handle the power output of twin Eteks. Gears, chains, or timing belts will lead to trouble.

Interestingly, the battery capacity needed for a rotary weapon is more dependent on the weight, geometry, and speed of the weapon than on the motor powering it. An example, with formulas, of calculating spinning weapon battery capacity requirement is given in a previous answer in the Radio and Electrical archive.

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Date marker: March 2008

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Q: Can I use a stainless steel knife for a ramming spike?

A: You can use a slice of well-frozen pepperoni pizza if you like -- the result will be much the same. Neither the pizza nor the knife are designed to take large loads at the pointy end. Either will simply shatter on good impact. You need something that you could hammer into concrete. Repeatedly.

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Q: How did KillerHurtz's weapon work? How was it actuated? What kind of linkage was used?

A: KillerHurtz is an unusual robot in nearly all design areas: all-plastic chassis, combined differential and pivot steering, computer controlled ESCs, and a pneumatic actuated overhead axe weapon with a chain drive.

The weapon is powered by a 100 mm bore pneumatic cylinder operating at 150 psi. The cylinder is attached to a lever directly connected to a large chain sprocket. A chain carries the force to a smaller sprocket attached to the axe. There is a good photo on an archive of their site: KillerHurtz Design page.

Q: Did KillerHurtz use a double-acting pneumatic cylinder?

A: Yes. The pneumatic plumbing appears to be incomplete in the photo -- see the KillerHurtz pneumatics page for details.

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Q: How would I make a 'Frostbite' like thresher weapon?

A: Frostbite's thresher is simply eight short vertical bars mounted on hubs along a common central support shaft. Mount the shaft on bearings and spin it with a belt drive.

Mark J. here: the most efficient design for a spinner weapon is a drum -- it can store more energy for size and weight than other configurations. A disc weapon is less efficient, and a rotating bar even less efficient. Multiple short bars, like the thresher, are very poor at storing energy.

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Q: How would I use your spinner spreadsheet with an eggbeater weapon?

A: Mark J. here: it takes just a little extra work.

1. Input a bar the correct length and width to represent one side bar on your eggbeater. Double the thickness to set the mass correct for two bars.

2. Manually calculate the mass of your two crossbars.

3. Represent the mass of the crossbars with a cylinder the diameter of your weapon - tinker with the thickness to get the mass equal to the calculated crossbar mass.

That will get you very close to correct.

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Q: How do you make a full-body drum?

A: There is plenty of information on drum weapons in this archive. You may also want to get a copy of Combat Robot Weapons by Chris Hannold -- it has specific information on many types of weapons in detail that I cannot afford to provide here.

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Q: What's a leaf-spring flipper?

A: A leaf spring is a curved metal plate that has been tempered to 'spring back' to it's curved shape after being flattened. One end of the leaf spring is securely fastened to a chassis and the other end is winched down to flatten the spring, This stores a large amount of energy in a thin and flat profile. When another 'bot is on top of the spring the free end is released and the spring tosses the other 'bot skyward.

The principle is simple, but designing the winch and release mechanism is tricky. Team Whyachi's middleweight 'Red Square' was a successful leaf spring flipper -- ranked 15th, with a record of 19 wins and 12 losses.

Q: Would it be possible to make a spring powered flipper in a featherweight?

A: Possible, sure -- but there are reasons that spring flippers are not a popular design. Team Whyachi was successful because they are a very experienced team with great resources.

Q: How would I make a spring powered flipper in a hobbyweight?

A: No answer that I can give in a couple of paragraphs is going to help you much. Very few spring flippers have been built due to the difficulties in constructing a releaseable winch system to reset the spring. The mechanism from an automotive power seat would be a good start, but custom machine work would be needed for the release spool. I'd suggest building something else.

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Q: Would it be better to make my vertical spinning weapon spin 'up' or 'down'? What are the advantages of spinning 'up' versus 'down'?

A: Mark J. here: Newtonian physics states, "For every action there is an equal and opposite reaction." When your spinning weapon strikes your opponent, equal forces are applied to both robots.

If the front edge of your weapon is spinning upward: the force applied to your opponent will be upward and your robot will be subject to an equal downward force. Since a solid surface is holding your robot up, nothing much will happen to it. Since nothing except gravity is holding your opponent down, they will fly end over end into the air -- greatly impressing the judges and spectators in your favor.

If the front edge of your weapon is spinning downward: the force applied to your opponent will be downward and your robot will be subject to an equal upward force. Since a solid surface is holding your opponent up, nothing much will happen to them. Since nothing except gravity is holding your robot down, you will fly end over end into the air -- greatly impressing the judges and spectators, but not in your favor.

Upward spinning, please.

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Q: I'm considering adding a lifting arm to my lightweight. What would be the best way to implement this? If I choose to go pneumatic, does RoboGames have filling stations?

A: Mark J. here: The 'best way' depends on a balance of your budget, experience, skills, and expectations.

• Pneumatic lifters are very quick and powerful but require complex, bulky components and a fair bit of weight allowance. Pneumatics are applicable to either a simple single-pivot lifter or a more efficient 4-bar design.

• Electric actuator lifters are typically slow but are comparatively compact, safe, and simple to support and maintain. Again, they can be used with single pivot or 4-bar lifters.

• Driving one arm of a 4-bar lifter with a gearmotor requires a heavy gearbox capable of handling enormous torque, and careful design work -- but the result can be quick and effective.

RoboGames is not a 'competitor friendly' event -- don't expect more than table space and an electrical outlet for support. Also, be aware that there are a number of 'improvements' to their procedures for 2008 that include very strict match schedules and new radio requirements that prohibit standard AM / FM / FM-PCM systems. You can email RoboGames organizer Dave Calkins with specific questions.

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Q: In season five of Battlebots, 'El Diablo' and 'El Diablo Grande' both sported new thresher weapons. How would I go about building a weapon similar to the ones they used?

A: I can't find a picture of the weapon used by middleweight 'El Diablo' at BattleBots 5.0, but I did find some construction notes in a build report:

The weapon was made from four 1" thick polyethylene discs, each with a central bearing riding on a stationary steel shaft. Bridging across the four discs on opposite sides were two 1/4" thick 6160 aluminum plates with bolts threaded thru from the back side for 'teeth'. The entire assembly weighed 24 pounds and was reduction belt-driven by a Bosch GPA 750 motor to about 1300 RPM.

'El Diablo' went 0 for 1 at BB 5.0, losing their opening match to 'Ankle Biter'. Heavyweight 'El Diablo Grande' did better with a similar weapon, winning 3 matches before a loss to 'MechaVore'.

You can find pictures of El Diablo and El Diablo Grande as they fought at BB 5.0 at robotcombat.com, under 'Event Reports'.

Thanks! I'd forgotten about their extensive collection of BattleBots photos.

It looks like the team changed the polyethylene and aluminum design before the competition. Those appear to be four thick aluminum discs with steel striker hooks bolted in. I don't think I could recommend a similar design -- a drum weapon of the same size, weight, and speed would store much greater energy than the 4-disc setup.

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Q: I'm building an antweight but I'm having trouble with the drum weapon. If I make the drum out of a metal tube and weld or screw on teeth it becomes extremely unbalanced. Is there a trick to making one? Is there a way I could buy one?

A: Drum weapons are not projects for the average builder without access to and experience with precision machine tools. An error of a couple hundredths of an inch will leave you with a useless paperweight. A competent machine shop could certainly make a custom drum weapon for you, but the cost would be prohibitive (hundreds of dollars). Consider switching to an eggbeater weapon -- much easier to construct and balance. See also the previous post on balancing rotary weapons.

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Q: How does Ziggy's flipping arm work? Why is it so powerful compared to other super-heavyweight flippers?

A: Clan MacCanIKill's 'Ziggy' has a 4-bar flipper mechanism with a pneumatic actuator acting on the front bar. Click the photo for a larger image. The pneumatics operate on high-pressure nitrogen at up to 3000 psi. Most pneumatic flippers use carbon dioxide gas which gives more 'flips' per tank but which also restricts their maximum pressure to about 850 psi.

In spite of the high-pressure nitrogen pneumatics, I don't believe that Ziggy is more powerful than other top-rank flippers. The 'laydown' position of Ziggy's actuator cannot match the mechanical efficiency of the twin upright actuators in Toro.

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Q: What's the ideal material to use in a lightweight spinner killer's wedge?

A: 6AL-4V titanium, about 5/16" thick should do.

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Q: Can I have a link to Team Entropy's site? 'Redrum' is my favorite bot.

A: Team Entropy never had large or complete site, and what they had was taken down about a year ago. I can direct you to an archived version of their site that has a picture of 'Redrum'.

Q: I once heard an announcer say that the new Redrum's internal combustion engine is located INSIDE the drum! Is that true? Is that even possible?

A: Yes, and yes. It's a very big drum. If you think about a drum as being kinda like a full body spinner on it's side, you might get more comfortable with the idea.

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Q: In your opinion, what kind of I.C.E. would be best to use on a middleweight 'bot with a spinning weapon?

A: My advice: stay away from Internal Combustion Engines (ICE) in combat robots. They seem like such a good idea, but every team I've ever seen use an ICE has suffered multiple problems at tournaments:

• the engine becomes inexplicably difficult to start;
• the engine stalls at idle waiting for the match to start;
• the engine delivers one hit and quits;

ICE 'bots don't win tournaments, plus many tournaments simply do not allow ICE due to fire, fume, and noise concerns. Stay away!

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Next Mystery Post »

Q: Can you show me a diagram of how to use a pneumatic cylinder to make an axe arm rotate almost 180 degrees, like the original 'SlamJob'?

A: Mark J. here: 'SlamJob' kinda cheated. Scott Kincaid built an ingenious system that used two pneumatic cylinders that worked in sequence to accelerate the weapon arm. Power is not evenly applied over the swing, but it worked very well. See if you can figure it out from this early construction photo of SlamJob. Hint: neither of the cylinders are physically attached to the axe arm, except for a retract cable.

`The Judge' did not cheat. Jacha Little built a system that evenly powers the hammer throughout a 180 degree swing. You can find descriptions and pictures at its Build Report (archived).

Q: I looked at the pneumatic system for the hammer on 'The Judge' on their website. It's kind of confusing to me. It looks like a rack and pinion but it's backwards, and has chains. Do you think you can explain it better to me?

A: You're correct, it is a reversed rack and pinion. The pneumatic cylinder pushes and pulls the 'rack' in a straight line. The 'pinion' is the pivot point of the hammer: it swings the hammer in an arc as the rack is moved.

Their 'rack' is actually a pair of heavy power chains, and their 'pinion' is a pair of chain sprockets. One chain is wrapped clockwise around the sprocket and attached, the other chain is wrapped counter clockwise around the other sprocket and attached. One chain is always under tension and can transmit power to the hammer. There is less chance of 'jumping a tooth' under high loads with the chain system.

Q: In the illustration, which side is the front end of 'The Judge'?

A: The layout in 'The Judge' has the pneumatic cylinder at the rear of the robot, so 'front' would be to the right.

Q: So, The Judge's weapon can be compared to a spring loaded weapon, in the sense that natural physical forces fire the weapon (spring) and a special mechanism is required to reset the weapon into a ready position (winch), right?

A: No: the pneumatic actuator powers the weapon in both directions. I've modified the illustration to better show the action. Hammer motion and force is roughly equivalent in either direction.

Q: How does 'The Judge' get so much power from retracting the cylinder?

A: 'The Judge' uses a 4" bore double-acting cylinder that produces (almost) equal power when extending or retracting. Gas pressure to the actuator is under microprocessor control to quickly vent the cylinder for quick cycling. Take a look at the Team Da Vinci pneumatics page for an explanation of double acting cylinders and for a different animation of the hammer on 'The Judge'.

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Q: You have a 'bot with a pneumatic flipper. Is it worth it to be constantly changing the tanks?

A: Our heavyweight flipper has a very large CO2 tank that will power the weapon thru multiple matches. We also designed the tank mounts and connections to allow quick removal and installation of the tank without tools. With careful design tank changes aren't a problem, and if you enjoy tossing your opponent in the air there really isn't an alternative.

Note: High Pressure Air (HPA) tanks can be refilled with the tank in the robot, but CO2 tanks need to be 'pre-chilled' and filled with a specific weight of liquefied gas -- not easy to do with the tank in the robot. Check with your event organizer for specific pressure tank refill regulations at the event.

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Q: Are there any benefits in NOT direct driving a fairyweight weapon?

A: Many: spin-up time, optimum speed... the same elements that apply to larger rotary weapons apply to fairys. Read thru this section of the archive and you'll find plenty of pointers.

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Q: I was wondering what you thought a good ant spinner weapon would be? I sorta like saws, but I can't find any small enough and light enough. Can I make one?

A: Go take a look at the 'VDD Antweight Kits' at the Robot Marketplace for parts and complete saw weapon kits. [no longer available]

The idea of a spinner weapon is to store energy in a rotating mass and release a big burst of energy into a your opponent, sending them flying. if you make the weapon too 'small and light', it will just run into your opponent and stop. Browse this archive for design ideas.

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Q: How do I attach a weapon directly to a brushless motor?

A: Not a great idea. See the previous post in the Ask Aaron Archive.

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Next Mystery Post »

Q: Currently there is a robot competing in the RFL known as 'Sewer Snake'. Can you tell me how Sewer Snake's unique weapon works?

A. Sewer Snake is from the stable of Matt and Wendy Maxham's Team Plumb Crazy. It does better than just compete -- it was the 2005 RFL heavyweight champion, it is the #1 historic ranked heavyweight robot at botrank.com, and is a member of the Combat Robot Hall of Fame.

I wrote to Matt and Wendy for some details on Sewer Snake's weapon. They sent the following description and photos:

"The fork/wedge is powered by a Mini Magmotor motor on a 25:1 Apex gearbox to a 3:1 chain reduction. The chain is attached to the weapon mount so the mount can spin freely 360 deg. There are no electronic stops for this weapon system, I have to remember when to turn off the power or reverse direction so I don't burn up the motor!!! Weapons can be designed and mounted in any position [on the weapon mount]. With the red forks the receiver is in the up and angled down position. With one of the wedges for spinners we have the receiver down to keep the angle of the wedge as low as possible!

"You can also see in [the pictures] that the [red] lifter fork on top of the robot works in conjunction with the wedge/fork on the front. The lifter fork is mounted on a pin and only gravity holds it down, but when the wedge/fork is rotated forward it pushes the lifter fork up. When the wedge/fork is rotated backwards it lets the lifter fork fall back to a resting position on the top of SS."

Q: Isn't Sewer Snake currently a superheavyweight?

A. Sewer Snake fought very briefly (and unsucessfully) as a superheavyweight in 2006. It returned to the heavyweight rankings to fight at the Combots Cup in 2007.

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Q: How do I make a drum? Is it just a sheet of metal rolled into a cylinder or is there more?

A. Drum weapons are usually cut from a section of thick-walled tube, or machined from a solid block of metal. Getting a rolled sheet of thick metal perfectly round and well balanced would be very tough. There is a lot of material about drum weapons on this archive page.

Q: Where would I find the tube I would need to make my drum?

A. It would help if I knew whether you were building an antweight or a superheavyweight. Multiple on-line metal suppliers offer aluminum and steel tube in assorted sizes.

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Q: Is there a good H-bridge IC to drive a BaneBot FF-180 motor (6.7A stall current @ 4.5 volts) with a 7.4V Li-Poly battery?

A: Mark J. here: something tells me you're not building a combat robot. An H-bridge provides on/off/forward/reverse control for a motor -- too crude for proper combat robot control. Our robots run either full ESC control for drive motors or relay H-bridge control for lifter weapons. We don't have any experience with selection of H-bridge IC chips.

Q: I'm using a resistor in series with the motor to lower the voltage when the current is big. The H-Bridge would also take a voltage drop, right?

A: No. Resistors are not voltage control devices -- they are current limiters. Adding a resistor in series on the motor circuit will limit the maximum current the circuit can pull, but because of the variable back EMF the motor generates in operation the voltage the motor 'sees' will depend on the load the motor is under. At low load conditions the voltage across the motor brushes (and in the rest of the circuit) will be very close to full available voltage. At high loads, both the voltage and current at the motor will be reduced by the series resistor. A series resistor in the motor circuit beyond the H-bridge will not impact the voltage at the H-bridge.

Q: I'm not interested in speed control but in torque/current/dutycycle and direction control, with a high current output (for the same FF-180 BaneBots motor). Would an ESC do the work for this purpose? If so, which do you think is a better choice, the H-bridge, or the ESC?

A: Yes, an ESC can control torque/current (they are equivalent in a permanent magnet DC motor) and provide directional control. The H-bridge and a series power resistor can perform similar functions. If I knew what you were actually using the motor and controller for I might be able to give an opinion on which is 'better'.

Q: Remember the questions about a worm gear driven robotic gripper I posted before? I want to control the pinch force of the gripper and of course it must be able to use the full power of the BaneBots motor. Since I also need to control direction, I thought of an H-Bridge like the LMD18200T, but its operating voltage is from 12 to 55V.

A: Yes, welcome back! Go take a look at Pololu Robotics line of motor controllers. I'm sure you can find something there that will suit your application. In particular, the 'MC33887 Motor Driver Carrier' looks like a winner, but there are several other options. Add a series power resitor to the motor and you'll be in great shape. Or you could go with an ESC...

Q: If I were to use an ESC for this purpose, can you tell me how to connect the ESC to control torque/current instead of speed?

A: The term 'Electronic Speed Controller' is not really accurate: the device controls the power available to the motor rather than directly controlling speed. It effectively controls amperage, which directly controls torque and, only indirectly, speed.

Q: OK, so the signal I need to send to the ESC would be the same as the signal needed to control a servo, right? A 'high' (4 to 6 volt) pulse on a 20 millisecond period where:

• 1.0 to 1.5 millisecond pulse is proportional high to low power 'forward' rotation;
• 1.5 millisecond pulse is neutral - no rotation; and
• 1.5 to 2.0 millisecond pulse is proportional low to high power 'reverse' rotaton.

So rotation direction is included in the input signal?

A: You've got it exactly correct -- the same signal you sent to the servo you're replacing with the gearmotor. You can control torque by limiting the pulse width to a range nearer the 1.5 millisecond 'neutral' range.

Note that 'forward' and 'reverse' are relative -- you can always reverse the motor leads if the direction is not correct for your application.

Q: What ESC would you recommend - perhaps the BaneBots 3 Amp continuous / 9 Amp peak ESC?

A: The Banebots ESC should work very well for you, It is small, thermally protected, reversible, and has a fail-safe startup routine.

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Q: Can you tell me how Alpha-Gamma Raptor's arm worked? I can tell that it was similar to a four bar lifter, but I'm still confused on how it's powered. Also, can you please give me the link to the Team Raptor website?

A: Team Raptor's successful series of lightweight robots (Alpha / Beta / Gamma Raptor) all used 4-bar electric lifters. I believe the short rear arm of the lifter was powered by a high-reduction gearmotor -- possibly an Astro Flight cobalt.

Team Raptor's website is no longer on-line, but you can access an archive of their site from 2005.

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Q: Is it possible to make a proper reverse-angle flipper with an electric motor?

A: Mark J. here: electric motors can power effective lifters, but a flipper must unleash enormous power within a very short time period -- no practical electric motor can deliver that much instantaneous power. You could use an electric motor to spin up a big flywheel and then release the stored energy into a flipper, but the weight and complexity of the system would be prohibitive. Pneumatic flippers are cheaper, lighter, simpler, and more effective.

Q: I didn't necessarily mean "flipper". Is it possible to create a platform that rises up and forward (like Firestorm from the UK Robot Wars) without using a pneumatic cylinder or linear actuator of any kind?

A: Sure! It wouldn't be as fast or powerful as a pneumatic system, but it can certainly work. Driving the platform from a shaft at the hinge point requires a lot of torque and a heavy duty gearbox, but other designs -- like a modified 4-bar linkage (animation at right) -- can reduce the required torque and simplify construction and control elements.

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Q: What motor does 'BioHazard' use for it's lifter?

A: BioHazard's lifter uses a pair of highly modified 12 volt linear actuators running at 24 volts. Estimated power from each of the twin actuator motors is over 1 horsepower, with each actuator delivering 1400 pounds of thrust. The motors used are not standard robotics items. More information on the actuators can be found at the BioHazard website.

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Q: If the Perm motor is so powerful, why can't I find any 'bots that use it?

A: A price tag of nearly $1000 is certainly one reason. The Perm PMG 132 motor can crank out 34+ horsepower at 72 volts and would make a mighty weapon motor for a heavy or superheavy 'bot. However, that much power requires a big stack of battery capacity and a very serious motor controller. Overall, you might be better off with an internal combustion powerplant for a mega-weapon drive.

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Q: I have read through most of your archives -- what's with your bias for expensive electronics? An alternative to expensive weapon controllers are relays. They do in fact cause a big power draw from the batteries, but if you add a capacitor in parallel with the batteries, the weapon will put the strain on the capacitor, which is made to take it, if you buy the right one. I'm sorry if I have insulted you, but I just don't see dumping alot of money into an electronics system that is going into a vehicle that is doomed to be ripped apart.

A: I don't think you've read closely enough. Most of Team Run Amok's active weapons, from the lifter in our beetleweight champion 'Zpatula' to the Bosch-powered spinner on our heavyweight Robot Wars competitor 'Run Away', are controlled by relays. We don't have anything against them -- in the right application.

The choice of a mechanical or electronic relay versus an ESC for weapon control depends on many factors. For very large weapons, a reliable relay or contactor may cost as much as an ESC and will certainly weigh more. The high amperage surge that comes along with using a relay is hard on the batteries, and for a very large weapon the capacitor you suggest would have to be huge to be any use at all against a multi-hundred ampere spike.

As far as combat robots being 'doomed to be ripped apart', we have never lost any electronics to opponent caused damage. Maybe we're lucky, or maybe we just build 'em right.

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Q: Can any of the RS-540 gearmotors be directly mounted to a 12 inch lifting arm if the shaft is supported on both sides?

A: Yes. The BaneBots 125:1 36mm planetary RS-540 gearmotor could be used on a 12" lifting arm for a hobbyweight, if the shaft was well supported. It should provide over 20 pounds of stall force at the end of the arm at 12 volts. You might also consider the 256:1 42 mm planetary RS-550 gearmotor for it's larger diameter shaft and greater torque.

Q: You told the guy that asked about the RS540's on a lifter to consider the 256:1 RS-550 motor. I dont know about the 42mm gearbox, but the 38mm 256:1 gearbox's durability is very poor. You may want to say something to that guy before he gets the 256:1 and it busts on the first attempt. Maybe recommend the 64:1 since it has way more than enough torque, or preferably the 48:1, again way more than enough torque.

A: The BaneBots 256:1 38mm gearbox is a 4-stage unit and is not offered with the RS-540 or RS-550 motors. The 125:1 RS-540 38mm gearmotor is a stronger 3-stage design -- the same as the 64:1 and 48:1 boxes -- and should be fine in the hobbyweight application I recommended. The 256:1 42mm gearbox is much stronger than the 38mm gearbox and should also be fine for a small lifter.

I don't agree that the 64:1 and 48:1 gearboxes with an RS-540 motor would provide 'way more than enough torque'. For reliability and maximum lift speed, you want to load the system to no more than 60% of stall torque at maximum lift weight. With a 12" arm and allowing for gearing losses, the RS-540 motor with a 64:1 box stalls at about 10 pounds, and the 48:1 box stalls at less than 8 pounds.

Q: Unless you are fighting a very tall, thin bot all 12 lbs will never be on the lifting appendage at one time, unless some is higher than the point, like say a 6" wide bot is on the arm, with one edge at the tip and extending towards the shaft, and in that case it wouldn't be stall torque ÷ 12", it would be stall torque ÷ 6".

A: The needed torque is calculated by measuring from the pivot point to the location of the center of mass of the opponent -- not to the closest edge. In your example, the center of mass would be 9" from the pivot. If your arm tip is under the center of mass, you'll need full-weight lift capacity regardless of the width of the oponent.

Having the full weight of the opponent on the end of your lifter is not as unusual as you think. The end of the arm can and does lodge in 'bot openings, and large lift torque is needed when your opponent is pinned against the arena wall.

Q: No one designs their lifters with that much torque. It's a waste of weight and generally unrealistic unless you have a super huge lifting spatula or whatever at the end of the lifting arm. Biohazard is the greatest heavyweight lifter of all time, and it couldn't lift 220 pounds out at the end of it's arm. I'm not trying to criticize your answer, just trying to help another bot builder.

A: Mark J. here: we design our lifters to lift the full weight of the class they enter, and then some. Other builder do as well. For example, Team Cosmos' very successful hobbyweight 'IO' has a minimum 15 pound lift capacity. Carlo Bertocchini claims that Biohazard will lift 220 pounds, and the photo at right appears to show it doing just that.

Team Run Amok's lifters do have large surfaces that focus the lift toward the end of the arm, but the original questioner did not specify a design or even a weight class.

As for being a waste of weight, the BaneBots 125:1 36mm gearbox weighs the same as their 64:1 and 48:1 boxes. With the higher gear reduction you get greater lift capacity, less motor stress, lower amperage draw, and faster lift rate at heavy loadings. I'll back-up Aaron's recommendations.

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Q: I was trying to come up with a way to brag about my spinners energy that hasn't already been used, and I came up with this:

( Joules ÷ 32.32 ) ÷ opponent weight = max feet opponent will be thrown.

32.32 in this case represents G, the gravitational constant. This isnt the kind of question you normally answer, but is my math correct? I know that the answer probably won't be realistic, but it would be fun to have a stat like that.

A: Mark J. here: You're not quite ready for the physics final exam. Joules don't directly convert to distance, but can convert to height. You're also mixing english and metric units -- keep the height in meters, mass in kilos, and the gravitational constant in metric units (9.8, not 32.3). The equation itself is correct, but can be further simplified:

Maximum Height Opponent Will Be Tossed = Weapon Joules ÷ (Opponent Mass × 9.8)

So, a weapon with 6000 joules of energy has the potential to toss a 100 kilogram opponent: 6000 ÷ (100 × 9.8) = 6.1 meters high. You're correct in saying the number will not be realistic, since it assumes a conversion of all of the weapon energy directly into vertical motion of your opponent.

Q: Wow! That formula yields 18.7 meters of potential height for a hobbyweight with only 1 Kj of energy. That really makes me appreciate how much power that is.

To calculate distance, I figure that an object may get thrown at about a 45 degree angle from a drum hit, which allows the trajectory to be modeled with height equal to distance -- a trajectory ratio of 1:1 with the height and distance both half of the original height: ( 1000 joules ÷ ( 5.45 kilos × 9.8 ) ) ÷ 2 = 9.4 meters high and 9.4 meters far. Is that about right?

A: You're more than a little off on your calculation of trajectory. With a 45 degree launch angle, the energy will be equally divided between vertical and horizontal vectors. The veritcal speed slows, stops, and then reverses under gravitational acceleration. However, the horizontal speed of an object thrown on a ballistic path stays constant (negating drag). As a result, an object thrown with identical initial vertical and horizontal speed (45 degree angle) will travel four times further than the maximum height it reaches. The formulas are:

Maximum Altitude = Initial Vertical Speed2 ÷ 2g Horizontal Distance = ( 2 × Initial Vertical Speed × Initial Horizontal Speed ) ÷ g

Your maximum height calculation is correct, but the theoretical maximum distance thrown would be 37.4 meters. There is a very nice trajectory calculator about half-way down the page at the HyperPhysics Trajectory webpage.

Q: For every action there is an equal and opposite reaction, so won't half of the energy of the weapon hit go to throw my 'bot backward a distance equal to the distance my opponent is thrown forward?

A: The vertical component of the hit energy will act against the arena surface, resulting in no movement of your 'bot and full transfer of that energy into the height of your opponent's trajectory. The horizontal component of the hit energy will act equally on both 'bots, but your 'bot will be sliding across the arena surface with high friction resistance, while your opponent will be flying thru the air with minimal resitance. You'll get a little 'kick back' distance, but it will not be anywhere close to the distance your opponent will fly.

There are lots of factors that will reduce the actual distance the opponent will travel, but remember that this topic started out as just a 'brag number' -- a maximized ideal just for shock and awe.

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Q: How do I calculate the Moment of Inertia (MOI) of a spinning bar? What about a drum or disk?

A: Mark J. here: MOI is a measure of the resistance of an object to a change in rotation. It is dependent on the mass and shape of the object, and the axis of rotation. If you want to do it the hard way:

• Flat rectangular bar, axis thru midpoint: MOI = 1/12 Mass × ( Width² + Length²) -- yes, that is 1/12th, not 1/2.
• Disk, spinning on symmetry axis: MOI = 1/2 Mass × Radius²
• Tube, spinning on symmetry axis: MOI = 1/2 Mass × ( Inside Radius² + Outside Radius²)

Note that a typical drum weapon is a tube plus a disk on each end. Calculate the three MOI and add them together.

The easy way is to use the Team Run Amok Spinning Weapon Excel Spreadsheet and get the MOI plus the spin-up time for your weapon as a bonus.

Q: OK, I got the MOI, but how do I calculate spin up time?

A: See the previous article on kinetic energy and weapon spin-up time for a link to a Paul Hills' site with all the math. The Team Run Amok Spinning Weapon Excel Spreadsheet will calculate the spin-up time for you.

Q: How many joules of energy is average for a spinning mass weapon on a hobbyweight?

A: See the previous article on kinetic energy and weapon spin-up time.

Q: My hobbyweight has a drum with an MOI of 21 lb-in². It's being driven by a 24V DeWalt motor at 24V. How much capacity should the battery for the DeWalt have?

A: Energy required for spin-up is dependent on the gear reduction you choose for the drum. I'm guessing you'll want about a 3:1 reduction to keep the drum RPM reasonable. That'll spin it up to 1000 joules in 1.8 seconds. That's gonna be some weapon!

There are too many variables to calculate a 'hard number', but I'd estimate that a high-amp 600 mAH pack would give you enough energy for a three-minute match. Common practice is to use a single battery for weapon and drive motors. See the article on battery selection.

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Q: I'm having some problems with your Spinning Weapon Excel Spreadsheet:

1. No matter what I put in for a bar, disk, or tube, it always says 0.00 in the MOI space.

2. There is no way to put teeth on the disk or tube, and the presence of teeth obviously make a huge difference on the outcome by altering the MOI, speed, and mass.

A: Mark J. here: all of the blue input boxes (material density plus all dimensions) must be filled in for a bar, disk, or tube before mass and MOI can be calculated. If that isn't the problem, your spreadsheet may have been corrupted -- download a new copy.

The differing shapes, placement, and number of teeth on a drum spinner made it problematical to include a simple calculation box in the spreadsheet for them, but if you get a little creative you can approximate their presence. Calculate the total mass of the teeth, then increase both the outer radius and thickness of the tube to add in the extra mass. Alternately, for just a few teeth, increase the radius of the endcap(s) to bring the mass up.

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Q: How long should a spinner take to spin up in the 12lb class? Is 3 seconds OK?

A: How quickly you need to spin up depends on:

• The size of the arena;
• The ability of your 'bot to dodge the first attack; and
• How much energy you have in your weapon.

Hobbyweights often fight in small arenas -- maybe 12 or 16 feet across. Your opponent is going to be on top of you very fast! If you can turn your weapon away from the initial attack or zip to one side, you'll gain some spin-up time. For a hobbyweight spinner, I'd design to spin up to at least 200 joules in the first second if you can't count on evasive tactics.

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Q: How would I go about making a pneumatic hobbyweight lifter? How would I connect to the radio system?

A: See the previous post on pneumatics and 4-bar lifters. The solenoid valve systems are electrically operated and connect to your radio system via a R/C Switch Interface.

Q: Where can I buy parts for a pneumatic hobbyweight flipper?

A: Hobbyweight and larger combat robots use standard industrial pneumatic components.

• For actuator cylinders, valves, tubing, and connectors, look in your phone book under 'Hydraulic Equipment and Supplies'.
• On-line try www.mcmastercarr.com. -- search for 'pneumatic cylinders' to get started.
• For small pressure tanks and regulators, check with your local or on-line paintball gear stores.

You might also want to contact Team Death by Monkeys and talk with them about their Hobbyweight Pneumatic Flipper. Contact info is on their website.

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Q: How do I calculate how high a pneumatic flipper will be able to throw an opponent weighing X pounds and with Y amount of force at a given point of contact?

A: Mark J. here: The questions are getting tougher. A full answer to your question would make a good chapter in an advanced physics text.

Simplest case - where the flipper is positioned directly below the opponent's center of mass, and the flipper force perfectly vertical and uniform for the entire stroke length:

Calculate the net upward acceleration force available:	net force = flipper force - weight of opponent
Calculate the net acceleration of the opponent:	acceleration = ( net force ÷ weight of opponent ) × g
Calculate the time over which the acceleration will be applied:	seconds = square root ( 2 × flipper stroke ÷ acceleration )
Calculate the speed of your opponent as it leaves the flipper:	launch speed = acceleration × seconds
Calculate the peak altitude the opponent will achieve:	height = launch speed ÷ ( 2 × g )
For English units (pounds, feet, feet/second): g = 32.15 ft/sec2 For metric units (newtons, meters, meters/second): g = 9.8 m/sec2

Complex cases - if you want to calculate the height achieved from off-center flipper hits, non-constant flipper force, and non-linear flipper vectors, break out your calculus text. You'll need to calculate - amongst other things - the moment of inertia for a specific opponent and axis of rotation. That ain't easy, and life's too short. For more information on Newtonian mechanics, visit the Hyperphysics website.

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Q: How do I use a winch to reset a spring powered flipper or hammer?

A: Search the Ask Aaron Archive for 'winch' to see previous answers. There's a reason you don't see many spring-powered weapons on combat robots: pneumatic systems are more powerful, faster, safer, and easier to build.

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Q: How do I attach a heavy, high-energy horizontal spinning weapon to my robot so it won't tear away on impact? I need something really, really strong!

A: Take a look at Team Boomer's Fright Knight for a good example. Note that their weapon shaft is supported both above and below the plane of the disc. The supports form a triangular shape and are anchored directly and firmly to the chassis. The shaft anchor point is heavily gusseted with flat metal plate to spread the load to the supports. The support arms are also no longer than they need to be.

Q: Well, that type of design works well for 'Fright Knight', but I was talking about a 48 inch bar similar to 'Hazard' in design. I needed to know: how do I keep it from simply pulling out of the bearing at the base of the robot?

A: Sorry -- my mind reading is a little weak. You'll still want to support the shaft at two locations, above and below the drive pulley. If the pulley is pinned to the weapon shaft and tubular spacers are inserted as needed, the pulley will locate the shaft.

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Q: Where can I find a more detailed explanation of how the Judge's hammer gearing system works?

A: The hammer system is actually operated by chains and sprockets that emulate a rack and pinion. You can find descriptions and pictures at the Team Mechanicus Website (archived).

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Q: Why does the four-bar lifter design only have three bars?

A: One... two... three... hey, it only has three bars! How come nobody ever counted before? The base counts as the fourth 'bar'.

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Q: Would two RS-540 motors be enough to power a 'Breaker Box' style plow lifting arm? If so, what gear ratio?

A: Read the previous post about Breaker Box. The problem isn't power, it's finding a gearbox that can handle the huge amount of torque. A single RS-540 could provide enough power for a quick lift of 200 plus pounds at a gear reduction of around 1600:1, but the gearbox would have to handle 80,000 in-oz of torque!

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Q: I'm working on a lifter and it occured to me that I might save weight by not using bearings. It's only a 12-pounder, so it doesn't have much torque to transmit, and it's a lifter so it goes slow. I was thinking of just mounting it between two pieces of metal with holes drilled for the axles. Would that work?

A: You're going to transmit more torque than you probably think, but yes you can get away without bearings. Keep the axle size fairly large and lubricate with a little grease. Better still, use oilite bushings on the axles -- cheap and light.

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Q: Could I use a banebots 36mm gearmotor to power a small saw on my anklebiter-like hobbyweight. I know it wouldn't do much, but I just don't want to have a normal wedgebot, and sparks are fun!

A: Sure, but support the weapon shaft on both sides of your saw with bearings or you'll destroy the gearbox. Try the 5:1 ratio box for a small spinning weapon.

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Q: Can any of the RS-540 motors be used for a lifting arm? How about a hammer weapon with the ability to self-right the robot?

A: Electric lifters require great gobs of torque. See the earlier articles on electric lifters and 4-bar lifters. An electric hammer has pretty much the same trouble -- it's very difficult to get enough power to make one effective. Pneumatics work much better.

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Q: I'm trying to design a sportsman-class 4-bar lifter like 'BioHazard', but the math is sticky and I can't find a good balance between all the different variables. Do you think you could tell me the lengths of Zpatula's lifter bars, the torque needed, and how much it can lift? Thanks.

A: Mark J. here: yes, designing a 4-bar lifter system is 'sticky'. I recommend giving the T.i. Combat Robotics 4-Bar Simulator a try. It gives charts of torque and lift, as well as lift height and lateral displacement.

Our beetleweight lifter Zpatula uses a gearbox to power the front arm of the 4-bar lifter. The bar lengths between pivot points) are:

• Front: 3.375"
• Rear: 2.25"
• Top: 4.5"
• Bottom: 5.625"

The top bar extends another 5" forward before dropping straight down 1.5" and then straight forward to form a 1" lifting lip. In the 'down' position, the top bar is parallel to the floor and the bottom bar is raised 0.75" at the rear. Plug these lengths into the T.i. simulator as a starting point and change the dimensions 'til you get what you want.

Q: Ok, I got my measurements. How much weight should it be able to lift? I was thinking 30lbs would be enough for the featherweight sportsman class, since it would hardly ever experience the full weight of the opponent. About how high should it be able to lift?

A: I like your reasoning on the weight the lifter should be able to lift, but remember that the 4-bar simulator gives you the torque needed to HOLD a weight at a given position -- you'll need excess torque to be able to LIFT that weight. But you're right when you say the lifter will only rarely see full opponent weight.

How high is a tricky question that depends a bit on your attack strategy. The usual lifter attack involves breaking the opponent's traction and then shoving them into a wall, rather than trying to turn them over while everybody just stands still. I think 12 or 14 inches would be plenty.

The question you aren't asking is: how fast should it lift? Speed will depend on the weight on the lifter, and faster is certainly better. If you're set-up to 'hold' 30 pounds at stall, then a 15 pound load will take your lifter motor down to about half the no-load speed. I'd aim for no slower than 2 seconds to full lift at half load.

Q: One last question then I swear I'll leave you alone about the 4-bar lifter ; - ) According to the T.i. 4-bar calculator, my design needs a gearmotor with 1000 in-lb of torque to lift 30lbs of bot. That's a lot in a small enough package for a featherweight, so how do I calculate the thrust requirements for a linear actuator actuated 4-bar lifter?

A: You've discovered why there are so few electric powered lifters. Powering the shorter rear bar of the 4-bar assembly or shortening the overall length of the lifter can reduce the torque requirement, but it's still big.

Powering the assembly with a linear actuator (like BioHazard does) trades off torque for thrust, but the total power requirement remains the same. At the point of travel that calls for peak torque, you're still gonna need 1000 pounds of thrust at a right angle to a 1 inch lever arm, or 500 pounds of thrust on a 2 inch lever arm, or ( 1000 ÷ X ) pounds of thrust on a 'X' inch lever arm..

Don't forget that your chassis has to put up with the force of all that thrust as well. The actuator will push just as hard on the chassis mount as it does on the lifter assembly. Kinda makes you appreciate Carlo's design, eh?

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Q: I have a gripping mechanism application that requires constant stalling of my HSR-5995TG servo. I've noticed that it gets hot after about 20 seconds of gripping something.

• Does the servo have any thermal shut down protection, or will it just burn up after some abuse?
• Will this stalling gradually affect the performance of the servo?
• Is there an alternative design for a gripper that doesn't require the motor to be held in stall?

A: Mark J. here: servos do not generally have a thermal shut-down. They are often used in applications where a shut-down could be more disastrous than risking damage to the servo! The HSR-5995TG pulls more than 5 amps at stall, which will quickly heat up the power controller and motor.

Failure mode will likely be abrupt rather than gradual -- a puff of smoke and zero response. You can reduce the heat build-up and stress by running the servo at lower voltage, but this will also reduce the available torque and speed of the servo.

Grippers in 'real world' robots generally use a worm drive gear train rather than the spur gears used in your servo. A worm drive does not usually require motor power input to hold against back-force on the output, so the drive motor can be powered down and the gripper will still hold.

Q: Do you know of some gearmotor with torque-size-power comparable to that of the HSR-5995TG servo with worm gear train included?

A: Nothing that powerful and that small. Tamiya makes several small motor/gearbox combinations that include a worm-drive stage, but their output is considerably less than your servo. You might investigate worm-drive mechanisms for automotive power antennas. They are larger, but have a cable output that would allow flexibility in positioning the gearmotor itself.

Q: Do you know someplace to buy a miniature worm gear train?

A: Try the Stock Drive Products/Sterling Instruments site. They have a large selection of small worms and wheels.

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Q: Do you have any ideas on where to get a good lifter motor? I planned on using the BaneBots motor 42mm 48:1 motor and gearbox, with the output shaft machined to go into the 27:1 gearbox. This would give me about 285 lbs of lift at the end of my 20 inch-long arm. That seemed good, but I heard that efficiency can decrease through extremely high gearing, so the inefficiency might reduce the force.

A: Mark J. here: the efficiency of gearboxes depends on their design, but forget about the gearing losses -- trying to put 476 foot-pounds of torque thru a BaneBots gearbox would instantly turn it into scrap. You'll need a gearbox rated for that type of torque -- search industrial suppliers like McMaster-Carr or Grainger, or look at truck/ATV winches.

Q: How much torque at the axle of that 20 inch long arm do you think I should have, since it will be a middle weight and the lifting attachment will weigh a lot?

A: Your torque calculations are about correct -- you'll want to aim for about twice as much lift as the load you expect to have on your lifter. Does your lifter arm really have to be that long? The problem is getting a gearbox heavy enough to handle that amount of torque. Why don't you write to Jim Smentowski and see if he'll tell you what gearboxes he uses on Breaker Box?

Q: OK. I talked to Jim and I think I should use an NPC motor because of their low rpm and high torque. McMaster-Carr also has worm gear reducers up to 60:1, but I still need to know how the gearbox efficiency will impact my torque output. How do I calculate the real output of my motor through a gearbox?

A: You will lose some power with any type of gear reduction. The general equation is: output power = input power × gearbox efficiency

Gearbox efficiency depends on many factors:

• the type of gears used (spur, bevel, crown, worm);
• the number of gear meshes under load;
• the number and type of internal bearings;
• the gear clearances in the design;
• the lubricant used; and
• the design and construction of the shaft seals.

Further, efficiency varies with the load that is placed on it and with individual manufacturing differences. Calculating exact gearbox efficiency is impractical -- it needs to be measured.

As a very broad rule, spur gears are more efficient than bevel, bevel are more efficient than crown, and crown are more efficient than worm. For a very rough approximation, you can figure that each stage on a spur gear reduction loses 3% of the power that goes thru it. A planetary gearbox has many meshing surfaces and is less efficient than a simple multi-stage spur gear reduction. Each right-angle bevel gear stage loses about 15%. Worm gear losses are very high and depend on the reduction ratio: a 10:1 reduction will lose about 20%, and a 60:1 reduction can lose 60%.

This is a good time to remind builders that the best way to figure out if something works is to look at similar designs built by other builders. If you're building something entirely new, you're kinda on your own. Best luck!

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Q: Can antweights have weapon designs similar to 'Overkill' or 'Toe-Crusher'? How do they work?

A: Torque reaction overhead blades/spikes are legal and easy enough to build, but they would not be effective under the current damage/aggression scoring system.

For an explanation of the operating principle, see the Toecrusher webpage.

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Next Mystery Post »

Q: How does Team Cool Robots 'Toe-Crusher' work? It's explained at the coolrobots website, but I don't understand it.

A: Mark J. here: `Toe-Crusher' uses a torque-reaction hammer weapon - a very simple weapon based on applying the torque of acceleration and braking to swing an overhead axe. Have you ever seen a motorcycle do a reverse-wheelie? The rider hits the front brake hard enough that the rear wheel comes up off the ground and the bike balances on the front wheel. If the rider braked even harder, the entire bike would flip over and crash into the ground in front of the braking wheel. If the motorcycle accelerates very hard, the opposite happens and the front wheel lifts off the ground.

A torque-reaction hammer works the same way. The robot is built with the weight balanced on the axle and a long arm with a hammer or pick on the end. When it accelerates, the hammer `pops a wheelie' and is thrown over to the rear of the `bot. When it decelerates, the hammer does a reverse-wheelie and is thrown over to the front of the `bot. Later versions of 'Toe-Crusher' had a wedge attachment stayed in contact with the floor by pivoting on bearings at the axles. Although showy, you don't get a lot of force out of this type of weapon and it is difficult to drive effectively.

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Q: How do I calculate the spin-up time and kinetic energy for my spinning weapon design?

A: Mark J. here: you'll need to know the stall torque and top speed of your electric motor, the shape and dimensions of the spinning weapon, the type of material you'll use to make the weapon, and the gear reduction between the motor and the weapon. The math gets a little sticky, but I wrote an Excel spreadsheet that will do the calculations for you:

Team Run Amok Spinning Weapon Spreadsheet

As a rule of thumb, you'll want a minimum of 20 joules of energy per pound of weight class, and you'll want to spin-up to 10 joules per pound within 2 seconds. A modern high-energy spinner might have more than ten times that energy! You might have a little more spin-up time in a big arena, or if your 'bot is nimble enough to protect the weapon from premature impact. Smaller arenas used by sub-lightweight 'bots will require quicker spin-ups.

If you want to wade thru the math yourself, go to Paul Hills' page on Spinning Disk Weapons

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Q: Is a hammer actuated by a drill motor an effective weapon for a hobbyweight?

A: Sorry, no. The hobbyweight class is dominated by passives (wedges, rammers, dustpans), spinning weapons (mostly drums/eggbeaters), and a few lifters. Spinning weapons store power delivered by their motors over several seconds of spin-up time as kinetic energy. A hammer that gains power over a fraction of a second from a similar motor just can't put together enough energy to do any damage.

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Q: I thinking about making a middleweight pneumatic 4-bar flipper. Is there a good site that explains the basics?

A: The design of 4-bar systems is very technical and I've never found a good primer on the web. There is a 'Four Bar Front Bar' calculator that could be helpful if you were building an electric lifter.

You can get a very good education in general robot pneumatic systems at the Team Da Vinci Robotics pneumatics page. Remember: high-pressure pneumatic systems are potentially VERY dangerous. Don't get in over your head.

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Q: Is there a way to put a CO2 filling jack (similar to an electrical charging jack) on a robot?

A: Yes, but it has several drawbacks and may not be considered legal under current 'RFL rules'. Not everyone interprets the rules the same way -- check with your event organizer before you build!

You'd need to add a pressure rated 'T' fitting between the tank and the system shutoff valve before the pressure regulator. A second shutoff valve on the other branch of the 'T' connects to a male quick-disconnect refill fitting.

This adds weight, bulk, and complexity to the system. I think it's easier to design the CO2 tank mounts for easy release. You can then carry the tank (instead of the whole robot) over to the refilling station.

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Q: How does 2004 RFL heavyweight champion SJ's weapon work?

A: 'SJ' (it used to be called 'Slam Job') has a classic overhead pneumatic pickaxe -- two pneumatic cylinders acting together on a pivoting arm. Getting a pneumatic system to work over a long weapon arc is tricky. SJ uses one short cylinder to give the weapon an initial 'boost' at an optimum angle until the working angle for the second longer cylinder improves.

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Q: How do I make a thwackbot and what materials do you need?

A: You'll need a narrow two-wheel high-speed drive and a long sturdy boom with a sharp pick or blunt heavy weapon on the end. The rest of the 'bot is much like any other 'bot. Search the archives for 'thwackbot' for more tips.

Thwackbots are not currently popular. The RFL Judging Guidelines demand constant aggression to score well and a simple thwackbot cannot spin and move toward an opponent at the same time.

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Q: I've seen a drum weapon that uses the exposed heads of large screws as teeth (Emsee Fry Pants). What are the things to keep in mind to make them effective? Do you think these are better than making custom teeth?

A: Drum weapons are most effective when the teeth are spaced far enough around the drum circumference to allow the other 'bot to get close enough to the drum for the teeth to dig in and get some 'bite'. Don't put on too many teeth!

I don't know if big screws are better than custom teeth, but they're inexpensive and easy to replace! Make sure you have plenty of material depth in the drum to support the threaded shaft of the screw.

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Q: What size screws should I use for drum teeth? Should I use fine or coarse threads?

A: Mark J. here: It would help if I knew what size 'bot you're building. When in doubt, go bigger.

Screws and bolts are designed to take tension stress along their axis and are not ideal for enduring the shear stress that will be imposed when you use them on a spinning drum weapon. Their advantage is that they are inexpensive and easy to replace. Use a coarse thread -- it will be easier to remove when damaged.

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Q: If you know motor torque, no-load RPM, weapon weight, and Moment of Inertia, is it possible to calculate actual weapon RPM?

A: Mark J. here: no -- weapon RPM is limited only by bearing friction, transmission loss, and aerodynamic drag. A very heavy weapon will eventually spin-up to the same RPM as a lighter weapon. However, given the information you list, you can calculate the approximate RPM of the weapon after a given number of seconds spin-up time:

Check out the Team Run Amok Spinning Weapon Excel Spreadsheet.

If you'd like to learn more about the physics of kinetic energy weapons, read thru Paul Hills' page on Spinning Disk Weapons.

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Q: How much kinetic energy should a hobbyweight Full Body Spinner weapon pack?

A: Mark J. here: the simple answer is 'all you can manage'. Successful FBS 'bots typically spin a shell representing around 20% of the total robot mass at speeds around 3000 RPM. I'll let you do the math to calculate the KE of such a weapon. The Team Run Amok Spinning Weapon Excel Spreadsheet will help.

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Q: What are the different ways of making the shell for a FBS? Is it like getting a cylinder and milling the middle out? That would waste alot of material!

A: Mark J. here: yes, milling a Full Body Spinner shell from one big chunk of metal does waste material, but I've seen it done. More commonly, somebody finds a short section of large diameter pipe at a scrap yard and welds a top onto it. I've also seen a tube formed from thick plate with a set of forming rollers, seam welded, and a top added. The key is precision to keep the finished product well balanced.

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Next Mystery Post »

Q: How do I make a flamethrower? Are flamethrowers allowed in [enter weight class here]?

A: Flame weapons are allowed at the discretion of the event organizer. Check with the specific event you plan to enter for eligibility and special rules.

Note that flamethrowers are not effective robot weapons. They are for very experienced builders who just want to show off. If you have to ask how to build one, you aren't experienced enough to attempt it. I'm not going to encourage anyone to try dangerous construction beyond their skill level -- build something else.

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Q: What is the range for the angle of incline for an attack wedge? What do you think is ideal?

A: Interesting question! I figure anything more than 45 degrees isn't a wedge, it's a sloped 'brick'. How much lower than 45 degrees you go is a function of how the wedge is going to be used. A very shallow angle is useful for defense and sneaking in under spinning weapons. A steeper angle is better for ramming attacks. A curved 'scoop' can be effective both for offense and as a 'spinner killer' -- I'd say that's ideal.

Q: What material would you use the create the "curved scoop"?

A: Titanium would be great, or steel if you have enough weight allowance.

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Next Mystery Post »

Q: I finished the rotary weapon for my 'bot, but when I turn it on the whole robot shakes and bounces. How do I balance the weapon?

A: Mark J. here: The more precisely a weapon is made, the better the balance is likely to be. If the weapon was hand drilled and cut, it isn't going to be anywhere close. Assuming the mounting bearings spin freely, you can 'static balance' the weapon:

• Remove the connection to the weapon motor (belt, chain, friction drive...) so that the weapon spins free.
• Position the robot so the weapon shaft is horizontal (parallel to the floor) and the weapon has room to spin.
• Give the weapon a spin with your hand and let it coast to a stop.
• Mark the part of the weapon that is at the bottom, closest to the floor.
• Repeat the spinning and marking about a dozen times. The part of the weapon that has the most marks on it is too heavy.
• Remove some material from the heavy part of the weapon, or add some weight to the opposite (light) side.
• Repeat the spinning, marking, and weight adjustment until there is not an obvious heavy spot.

Spinning bars and discs are fairly simple to balance, but time and care taken during manufacture to make the weapon symmetrical will make the job a lot easier. A drum is much more difficult to balance -- construction must be very precise and individual parts may need to be balanced prior to assembly.

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Q: I want to build a hammer for my 'bot. How do I do that? Have any suggestions?

A: Sure! Get hold of a copy of Robot Combat: Weapons by Chris Hannold, or Kickin' 'Bot by Grant Imahara. They both cover basic weapon construction materials, design, and technique.

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Q: What kinds of weapons are best for totally destroying someone's bot? I drew my own plans for a 'bot with a ramp up to a drill and a buzz-saw on the side that comes around and saws. Is that any good?

A: Very ambitious plans! I would suggest starting with something simpler for your first 'bot. Exotic weapons won't do a lot of good until your basic mechanical systems are up to the task.

The current top-bots have heavy, high-speed rotating 'kinetic energy' weapons that hit like a speeding truck. The preferred armor in the heavyweight class is 1/2" thick titanium. Get to a tournament and see what the competition looks like before you decide on a design.

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Q: Hi Aaron! My dad and I each want to make a robot, but we have no idea where to start. I'm was also wandering if nail guns are legal, or if robots can have huge wheels and be able to puncture the top of another robot? [Josh]

A: The first step is to carefully read the 'Robot Fighting League rules'. The RFL rules are used by most robot competitions in the US. That will answer your questions about nail guns (not legal) and huge wheels (legal, but you're not gonna get thru 'bot armor that easy). Search this archive for suggestions on books for robot construction.

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Q: I want to make an ant undercutter with a S-280 weapon motor. What's a good gear reduction for the weapon motor? Is it a good idea to use a gearbox?

A: Nice choice! Your gear reduction will depend on the size of your undercutter blade. Take a look at Johnson Junior -- a S-280 powered undercutter with a big 8.5" blade and a gearbox. They're running a 5:1 reduction which seems about right for a blade that big. A smaller and lighter blade would use less reduction -- maybe 3:1 for a 5" blade. You can use the Team Run Amok Spinning Weapon Excel Spreadsheet to pick the best gear ratio.

Most builders like belt drives over gear drives for weapons because they require less precision, can absorb impact shock loads, and won't 'jam' as easily. A well designed gearbox can still be effective and rugged.

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Q: I read a description for an outrunner brushless motor that said, "These motors can be mounted behind a firewall or, with the addition of the Radial Mounting Set, they can be mounted in front of a firewall." What's a firewall?

A: Brushless hobby motors are mostly sold to model aircraft builders. On a model airplane a 'firewall' is the flat panel at the front of the plane where the motor is attached.

Robot weapons put large side-loading forces on the motor shaft. Support the motor by mounting it as close to the weapon drive as possible -- 'behind the firewall' style.

Mark J. here: Engines in early airplanes were prone to catch fire. The 'firewall' was the solid bulkhead behind the engine that prevented flames from entering the cockpit. In that context it wouldn't make much sense to mount an engine behind the firewall, but the term survives.

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Q: Is 1/8" steel thick enough for a drum weapon?

A: For an antweight, that's probably too thick. For a heavyweight, that's probably too thin.

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Q: The antweight 'bot 'Cadaver' has the spinning can of an outrunner motor inside a drum weapon. How did they do that, and does it work well?

A: Team Inevitable Destruction's 'Cadaver' has simply hard-mounted the can of a brushless outrunner motor inside one end of a drum and put a support bearing in the other end. The force of impact is taken directly by the motor bearings, which is hard on the motor. A belt drive would isolate the motor from impact forces, give a faster spin-up, and would keep the weapon speed down enough to 'grab and toss' an opponent.

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Q: What is the easiest type of weapon to make, besides a lifter?

A: You think lifters are easy? I guess you can make an antweight lifter out of a servo without much trouble, but a bigger 4-bar linkage lifter is not easy.

A better question might be, "What is the simplest weapon that is effective at winning matches?" The answer might surprise you. My dad looked at the results from 20 recent robot tournaments to see what type of weapons did best. Take a look at the results: What Weapons Win?

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Q: I want to build a spinning drum weapon. Is there a diagram of what a spinning drum looks like inside and how to build it?

A: A drum is just a large diameter metal tube with end plates and a few teeth welded to the outside. It's empty inside except for the shaft. The weapon shaft needs strong support, like any spinning weapon. Spin it with a belt drive from your weapon motor.

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Date marker: February 2006

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Q: What are the differences between an eggbeater and a drum weapon? Don't they basically do the same thing?

A: It's like the difference between a rotating bar and a disk. An eggbeater (like Team Sawzall's 'Switchblade') can't store as much kinetic energy as a drum that weighs the same -- the weapon has less rotational inertia. They're still effective, and more durable than a thin-walled drum.

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Q: What are the keys in making a good drum weapon?

A: Same as for any other rotating weapon: solid supports, correct RPM, protected drive, good balance, and quick spin-up. Drums are particularly hard to balance, so use extra care in precision construction.

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Q: The motor pulley for a belt-driven weapon is supported on the motor shaft -- how do we support the weapon pulley?

A: The weapon/pulley shaft must be supported so that it remains aligned and does not wobble. This requires two bearing supports at separate locations along the shaft on either side of the pulley and/or the weapon. You'll need to adapt or fabricate bearing mounts strong enough to both support the correct pulley alignment and absorb the weapon loading.

Take a look at Team Basenji's antweight 'Bitsy Blade' at the Robot Riots 5 photos page for an example of a belt driven antweight spinner.

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Q: What are the differences between a belt and gear drive? What is the difference between timing belts and regular belts? Which kind should I use for a horizontal spinning bar weapon? Can you recommend websites that sell gear and pulley systems for antweights?

A: Technical question, Mark J. here: Briefly, gear drives require precision alignment and spacing to function properly. They can operate at very high RPM, but are not terribly efficient at transmitting power. Belt and pulley systems can tolerate some misalignment and are capable of absorbing the sudden shock loading a rotary weapon can produce.

Timing belts have small raised teeth along the inner surface of a flat belt that mesh with grooves in the pulley to reduce slippage and increase power capacity. They are very efficient at transmitting power and can operate at higher speed than regular belts.

I like belt drives for rotational weapon systems like bars, disks, and drums. The ratio of the number of grooves on the larger pulley to the number of grooves on the smaller pulley is your gear ratio. You'll probably want to try something around 4 to 1 for a large bar weapon.

Robotcombat.com has a selection of small timing belts and pulleys suitable for antweights in their in their Mechanical and Drive Components section. Their timing belt page has a calculator to help with correct belt length selection. Tower Hobbies has inexpensive, ready-made gearboxes designed for model airplanes that would be useful for an ant weapon. Search for `gearbox' at their site.

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Q Does torque or RPM matter more in a spinning weapon?

A: Wow - another technical question - Mark J. here: Power is calculated as the product of torque and RPM, so the real answer to your question is that both are equally important. A spinning weapon stores power in the rotating mass of the weapon and unloads it destructively onto your opponent. You need as much spinning mass as possible, enough torque to spin that mass up quickly enough for it to be effective before your opponent can get to you and stop it, and as much speed as possible to store more energy in the weapon. A spinning weapon is all about stored power!

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Q Would the gyroscopic effect of a heavy horizontal spinning bar be to much for a small two-wheeled robot?

A: Technical question - Mark J. here: Gyroscopic forces can cause odd effects for robots with a spinning mass weapon. Problems come when turning changes the orientation of a line running thru the weapon axle. With a horizontal spinning weapon, the axle points up and down, so turning the robot does not change the direction that line points -- no problem, as long as the weapon is well balanced.

With a vertical spinning weapon, turning the robot does change the orientation of the weapon axle. This results in forces that resist the turning motion and lift one side of the robot. The magnitude of the force is dependent on the speed of rotation, the mass of the weapon, and the radius of gyration. You can get full details on calculating gyroscopic forces at: www.freestudy.co.uk/dynamics/gyroscope.pdf (archived).

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Q: Please, can you tell what the best type of weapon for a lightweight robot is?

A: Different builders have different ideas about what's 'best'. It depends on your driving style and your skill as a builder. I see lots of different weapon types winning, but I notice robots with spinning drum weapons doing very well in the 12 to 60 pound classes.

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Back to Top »

Q: Aaron, what is a 4-bar lifter? (Jake)

A: Mark J. here: a 4-bar linkage is a simple arrangement of four mechanical links (like rods or beams) with pivot connectors on each end linking them together into a roughly rectangular shape. By careful selection of the relative lengths of the links used, you can create complex movement arcs and gain torque or speed without gear reduction. The Wikipedia has diagrams of 4-bar linkages in their mechanical linkages section.

Robots sometimes use 4-bar links to control and position lifter arms efficiently and allow them to scoop in a sweeping upward and outward arc rather than a simple single-pivot backward rotation. Aaron's beetleweight 'Zpatula' uses a 4-bar linkage in its electric lifter.

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Q: What is your favorite type of 'bot weapon? (Jonathan R.)

A: I like 'bots that have a lot of pushing power and good control. I think the best weapon is to be all over your opponent and don't let up -- hit 'em 'til they break!

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Due to size constraints the Ask Aaron Robot Weapons archive is divided into two parts by date of post.

This archive holds robot weapon related posts prior to 2017. Such posts from 2017 to present are found here: Ask Aaron Robot Weapons - Part 1.

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Copyright 2006, 2007, 2008, 2009, 2010 by Mark Joerger -- all rights reserved.