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 Is 40k rpm's too fast for a drumbot???

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rjw
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PostSubject: Is 40k rpm's too fast for a drumbot???   Fri Jul 17, 2009 9:37 pm

You hear a lot of talk about grinding as opposed to getting a good bite when a drum or spinner is spun too fast.

So.... how fast is too fast??

Formulas including joules of energy, tip speed, speed of the attacking bot, blade length from drum,etc. all play a factor in determining what will happen when a weapon hits an opponent.

Well, here is an interesting short video of some testing of Sting's (our 3 lbs. drumbot) segmented single tooth drum.

The rectangular shaped block weighs 3/4 lbs. and is hit at a drum speed of 30k with the bot not really charging the block. You can see how far the block went, as it lands in the upper right corner of the video. It travelled about 20 feet up and 50 feet over.

The round block is hit at 40k rpm's and even slower bot speed. It flew around 15 feet up and 40 feet away.

Finally, notice the gouges on the round alu block. There are actually 4 of them, but this is from 2 hits. The segmented blades hit right near the intersection between 2 blades, both times!!!

Here's the video...



So, what do we get from all this?? Is 40k too fast?

Any comments are welcome..just thought I'd share this with everyone.

Stay tuned, as the bigger blocks are coming soon.

Note: drum speeds were calibrated beforehand and only 1 small belt is loosely driving the drum. Top speed of the drum is around 48k.
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SenaiERI
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PostSubject: Re: Is 40k rpm's too fast for a drumbot???   Sat Jul 18, 2009 3:33 am

AH MAN! That is way cool.

I don't know why but the robots on this forum are just getting bigger and better every time.
Time for us to step up our game.

Oh, and 40k is nothing. Let that sucker fly man.
(just make sure the vibration doesn't kill it, but I think there's a point where you would cause more damage to yourself than you would your opponent)
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rjw
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PostSubject: Re: Is 40k rpm's too fast for a drumbot???   Sat Jul 18, 2009 10:05 am

That point hasn't been reached YET with this bot, except for blade retention....which has been beefedup, but may need more tweaking.

Extreme motor mods are employed to hopefully keep the weapon drive together....So far, the almost 100 screws holding the bot together haven't come loose.

The drive motors and boxes, have as expected, but a solution to that is in the near future.
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Dan Curhan
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PostSubject: Re: Is 40k rpm's too fast for a drumbot???   Sat Jul 18, 2009 9:51 pm

I typed out a long reply showing work and numbers and stuff but i hit the freaking back button accidentally and lost it all. SO. the jist of it was:

In any collision, KE is not usually conserved, but momentum always is. If you have your aluminum cylinder spinning at 40,000 RPM (2" OD, 1.5" ID, 4" long, mass = .24 kg) you have a momentum of .6486 Nms and a KE of 1063 J. If you make the cylinder out of steel, mass = .7 kg, and you spin it at 23,418.8 RPM, you get the same KE, but a momentum almost double (1.1075 Nms). If the two drums magically had teeth, and they each hit a solid block of something, the steel drum would throw it much farther.

momentum is always conserved.

That's what I think about when designing my robots. weapon MASS first, then speed second.

just my thoughts.

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Jeff L
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PostSubject: Re: Is 40k rpm's too fast for a drumbot???   Sat Jul 18, 2009 10:01 pm

That bodes well for Infernal Contraption Very Happy.
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rjw
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PostSubject: Re: Is 40k rpm's too fast for a drumbot???   Sat Jul 18, 2009 11:11 pm

With only 3 pounds to work with, a heavy drum with anywhere near the punch, wouldn't leave much weight left over for the rest of the bot. Or am i missing something?

btw: I could get as much as 3k joules out of this drum and with a structure that can handle the hit.

It's all a compromise, and a light fast spinning drum is a crap shoot at best......but it is bound to make for an exciting outcome....win or lose
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Will Bales
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PostSubject: Re: Is 40k rpm's too fast for a drumbot???   Sun Jul 19, 2009 4:10 pm

wow.. Shocked
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PostSubject: Re: Is 40k rpm's too fast for a drumbot???   Tue Jul 21, 2009 9:13 am

Dan Curhan wrote:
I typed out a long reply showing work and numbers and stuff but i hit the freaking back button accidentally and lost it all. SO. the jist of it was:

In any collision, KE is not usually conserved, but momentum always is. If you have your aluminum cylinder spinning at 40,000 RPM (2" OD, 1.5" ID, 4" long, mass = .24 kg) you have a momentum of .6486 Nms and a KE of 1063 J. If you make the cylinder out of steel, mass = .7 kg, and you spin it at 23,418.8 RPM, you get the same KE, but a momentum almost double (1.1075 Nms). If the two drums magically had teeth, and they each hit a solid block of something, the steel drum would throw it much farther.

momentum is always conserved.

That's what I think about when designing my robots. weapon MASS first, then speed second.

just my thoughts.

well, remember, kinetic energy isn't conserved, but energy is. the KE that's lost goes into things like vibration and bending of your chassis, deformation of your wheels, and - more importantly - deformation of the other robot when your weapon hits it. basically, the conserved momentum goes into throwing the other robot, while the lost KE goes into damaging it. therefore, if you make a weapon like copperhead's drum with blunt bars for teeth, it's more of a "throwing" drum and you should be concerned with momentum more than KE. if you make a weapon with a sharp edge, both matter.
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Jeff L
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PostSubject: Re: Is 40k rpm's too fast for a drumbot???   Mon Jul 27, 2009 1:59 pm

EPIC FACEPALM. m*v = momentum; (1/2)m*v*v = KE. They're linearly related, so maximizing one maximizes the other. The big thing about weight vs speed only applies to the competition rules (fps limits & weight classes), or to how easily the weapon can transfer the energy (if it spins too fast, it can't get out a solid hit).
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rjw
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PostSubject: Re: Is 40k rpm's too fast for a drumbot???   Mon Jul 27, 2009 2:18 pm

The solid hit at too fast rpm's is what this thread is all about....

Sting hit wallop with drum spinning well over 30k and still bit enough to eject

In the second fight, a 20 to 30k hit sent the other bot across the arena

as stated before, we don't have the weight for a heavier drum/beater and so this is our hopes

1) slower speeds for in close action ( up to 1k joule)
2) higher speeds for charging (bertween 1 and 2k joules)
3) really high speeds for weapon to weapon (spinning) combat ( over 2k joules)

while this is the plan, we've noticed that even with little to no bot movement, the higher rpm bite has been pretty good.

Could just be fluke hits, or ????....maybe 30 to 40k isn't too fast to still get a bite in certain situations with our staggered blades

as mentioned elsewhere, we have even gotten double hits....when contact is made at or near the overlap of the staggered blades and at >30k r's
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