# Need ideas for a flywheel-like part

I need a flywheel-like part, and was wondering whether anyone had suggestions on what kind of thing I should be looking for. I've
looked briefly through the McMaster-Carr and Stock Drive Products websites, but being a software guy I'm not really familiar with the hardware they describe. So I'm not sure quite what to buy.
The project that I'm working on involves making very accurate measurements of the velocity and acceleration of a DC motor using an encoder and microcontroller. I plan on using the data to help me develop a model for the dynamics of a small mobile robot. There are a lot of steps before I get there, but right now I need to be able to attach a wheel with a known moment of inertia to the motor and then measure how fast it spins up. This procedure will let me compute both the stall torque for the motor plus the torque/speed gradient. I've already been able to match the manufacture's specification on a bare motor. If I can mount a wheel with a non-trivial moment of inertial on the shaft, I should be able to directly measure the effects of energy lost through friction in the gear chain. I don't need something really massive, maybe just the weight of a typical cell phone or paperback book (though I could go heavier).
My test motors have a 6mm shaft about one and a half centimeters long. I'd like to be able to attach and remove the test wheel easily. Ideally, I'd like to get a couple of different wheels with different moments of inertia. Also, if I can't get the moment of inertia from manufacture's specs, then the shape needs to be clean enough that I can take measurements and estimate a value. And, just like everyone else, I need to keep cost within limits of my meager hobbiest's budget.
Gary
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gwlucas wrote:

Just make a cross-bar with movable weights, attached to the motor shaft with a suitable collar and set-screw. Computing the rotational inertia is then reduced to a fixed quantity for the shaft and cross bar plus the variable quantity of the weights at a given radius, each of which can probably be approximated as a mass at its centroid.
see: http://en.wikipedia.org/wiki/List_of_moments_of_inertia
-- Joe Legris
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Thanks Joe. I should have mentioned that my main problem is lack of a shop and lack of experience with hardware. I just need something I can buy and install with hand tools. Doing math and physics is easy. Shopwork is hard :-)
Gary
J.A. Legris wrote:

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"gwlucas" wrote:

emachineshop to the rescue:
http://www.emachineshop.com / Roberto Waltman
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Actually, I like your idea a lot because it allows me to change the moment of inertia without changing the mass of the flywheel-like device. Following your lead, I did a quick search on set-screw and collar. What I saw looks like thin rings (inner diameter 6mm, outter 1/2 inch) with not much material for drilling. Is there a good way to attach them to a crossbar? (probably not crazy glue, I would think). Are there more substantial parts out there that I'm not finding? I suspect that my problem here is just not knowing the right things to look for.
Gary
J.A. Legris wrote:

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Something like this might work for you:
http://www.robotshop.ca/home/products/robot-parts/mechanics/mounts-hubs/lynxmotion-hub-02-universal-hub.html
It's a universal hub for attaching a wheel to a 6mm shaft.
You can attach a bar to that (or different sized disks). All you would need is either a part with holes in it (erector set parts for example), or a simple hand drill to drill holes in it (if you don't own a battery powered hand drill/driver it's a good investment - probably my signal most used power tool in my shop).
You could also consider buying different robot wheels designed to attach to a 6mm shaft. You wouldn't be able to calculate the moment of inertia easily but I suspect if you know enough physics, you could find a way to determine it experimentally (attach to a free spinning shaft and apply a known torque to it and measure the angular acceleration).
--
Curt Welch http://CurtWelch.Com /
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Curt,
Perfect! Thanks.
Gary
Curt Welch wrote:

http://www.robotshop.ca/home/products/robot-parts/mechanics/mounts-hubs/lynxmotion-hub-02-universal-hub.html
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What Joe Legris said was a very good way of doing it.
You might have a hard time with finding wheels with varying moments of inertia (MOI). But with what Joe said, you don't need to worry with that. We can trade drawings via email pictures if that will help.
Is snipped-for-privacy@sympatico.ca your actual email address? If it is I can send you a picture of what he means. This will make it easy for you to accomplish your task.
If you need help making a cross bar and moveable weights, I can help with that too. I have done a lot of scientific testing.
Good luck, Jim
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OOps! Looks like your email might be snipped-for-privacy@adelphia.net not the other one. I was reading the wrong post!
Well, anyway, if you repond to snipped-for-privacy@direcway.com I will know for sure,
Jim
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I have an old 16 millimeter firm projector flywheel that may do what you want. They were key components in the old movie projectors to keep the film moving forward without the jerking of the shutter at 17 per second. It weighs something less than a pound, the flywheel is a steel round some 6" diameter on a shaft with two ball bearings and a driver pulley on the opposite end. I could dig it out of my junk heap and take a picture and send it to you. I'd be happy to lend it to you for the experimental session if you pick up the postage.
Wayne

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I have a foundry and machine shop at my android lab. I can cast you a flywheel(or anything else you can ever imagine) and machine it all including a locking collar. You should also be aware, such things are not very expensive to make. Something like a 4" flywheel, cast machined and ready to attach would be about 20-25\$. Just dont ever send me metric specs-I only deal in inches!(or fractionals)
Mark
gwlucas wrote:

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Just a note of thanks to all the comp.robotics.misc users who sent me advice and offers of help in response to my latest posting (both on-line and via email). Once again I find myself grateful to my fellow robotics enthusiasts for thier support.
Gary
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That is an awesome price! Please send me some images or examples of your work. I'm involved in prototyping and product development and you appear to be the kind fo shop willing to go the extra mile.
Wayne http://tinyurl.com/m397x
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Wayne Lundberg wrote:

I can vouch for Mark's work and integrity. Highly recommended.
-- Gordon