Strange flywheel issues

Hello! I am powering a lightweight flywheel using 6 393 motors, regeared for high speed, with 2 power expanders each handling 3 of the motors. I would go to turn on the weapon drive, and it would spin up to half speed and then shut off for roughly a second then roar back up to half speed then shut off, all the while making horrible grinding sounds when it goes from half speed to zero almost instantly. After 6 cycles of it going off and on it would then twitch back and fourth with almost no force at all. Absolutely none of the members on our vex team have ever seen this before and are just as baffled as me as to why this is happening.

I don’t know what a weapon drive is BUT, check that all 6 motor do in fact have the correct gearing inside.

Sounds like an overheat. I assume “weapon drive” refers to high speeds, yes?

Yes. Geared for very high speed (upwards of +75mph). Not so much of a weapon as it is an experiment to show gyroscopic forces.

Please explain how 6 393 motors can power a drive faster than 75 mph

Geared very very high speed, both by re gearing the motor and a gear drive for speed Increasing the speed tremendously.

75 mph is actually not that fast in rpm if I did my match correctly. (Just taught myself on the fly) Flywheels typically need a baseline of 3,000 rpm. 75 mph, presuming 4" wheels, isn’t even close.

If I’m wrong, someone please teach me how to do the math.

That’s not going to work. It’s just not.

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He’s talking about his drive, not his flywheel.

This is the math I’ve used.
75 mi/hr * 5280 ft/mi * hr/60min = 6600 ft/min
4in*pi = 12.566 in/rev -> rev/ft = 12in/ft * 12.566rev/in = 0.955
6600 ft/min * 0.955 rev/ft ~= 6303rpm

I would really like some to tell me if i’m missing something b/c when you run this for a drive

200 * 12.566/12 * 60/5280 = 2.4mph
it seems like the robots are going faster than that. But maybe on scale it appears faster.

These discussions haven’t happened on the forum for a while (as in, years), but it’s more traditional to talk about the speed of a VEX robot in feet per second, as MPH is out of scale for both the robot and field. A robot going 2fps is slow, and 4fps is fast.


There’s also the coefficient of friction to take into account. It’ll be really easy to break traction and spin out.

How could a drive go 75 mph

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The OP says a flywheel for showing gyroscopic forces but doesn’t mention a drive doing 75mph. Assuming the 75mph is the perimeter speed of the flywheel?

I presume this is you referring to your “weapon drive”.
Is this “weapon drive” you speak of an actual drive your are trying to move 75 mph, or is your “weapon drive” referring to your flywheel? you speak of both.

I’m pretty sure he means flywheel, OP doesn’t mention anything about a drive in his post.

also if you are referring to your flywheel,

please both refrain from using possibly confusing terms such as weapon drive while describing it, just call it a flywheel.
and if 75 mph is your desired flywheel perimeter speed, please just use rpm it is much easier to work with and visualize for those of us trying to help.

and if you are referring to your drive,

and you attempting to drive your robot at 75 mph, that’s simply not happening. your attempting to travel at 396,000 feet per hour. which is 6,600 feet per minute. Or 110 feet per second. For a moment let’s assume somehow 6 393 motors could provide this power, and your bearings, axles, wheels, field, and entire event venue were indestructible. Cool! Your robot would travel across the field in just about 109 milliseconds. Unfortunately this isn’t happening. I’d be interesting to see how many motor’s you’d need in a theoretical indestructible and frictionless environment to pull this off… hmm…

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Frictionless environment would be bad. You’d just spin out indefinitely.

fine. frictionless shafts/bearings. perfect traction on the wheels


Sorry having to go so long without reply!
We fixed the issue but we still have no idea what caused it. We switched out all components including cortex and it worked fine. To clarify what I was stating in the original post:

Weapon drive: gear reduction assembly that reduces from 1:1 to 1:512

Flywheel: 32 tooth sprocket connected at the end of the weapon drive with screws in it for Mass

I was never referring to a drive system for movement, but that does sound quite interesting!