CIM Motor Burn Out Question

We at team 253 are working on our frisbee shooter. If we direct drive two 8" pneumatic tires with two CIM motors to launch frisbees, will the CIMs burn out? We’re a relatively inexperienced team with no mentors so apologies for technical ignorance.

Well assuming the Frisbee don’t jam up in your shooter and stall the motors. The short answer is no…

Another question: how would you go about directly attaching a CIM to an 8 inch pneumatic tire?

Depends on what type of pneumatic wheel you have. If they are these you could attach couple of shaft bearings on it, throw a shaft on it, then attach a sprocket on the mounting holes on the wheel.

I’m a programmer so sorry if I’m not too much help…

Search Mcmaster carr for shaft couplers.

8mm Key Hub

Unless you stall a CIM, and continue to try to run it for a minute or two, they are pretty hardy and shouldn’t burn out.

The wheels can cause some excessive vibration in the motor and that puts some intermittent loads on the motor. The output bearing is not meant for a lot of side loading so depending on how often you run this motor up to speed, it may produce some early wear in the bearings eventually leading to failure. As you prototype check the temperature of the motor every once in a while. If it doesn’t pass the 5 second touch test, you are over stressing the motor. If it is warm to the touch but you can hold your hand on it for five seconds, it is likely running normal.

Two issues cause most of the side loading on the shaft:

  1. Wobble of the wheel, because they aren’t perfectly balanced or centered.
  2. External force on the wheel, such as when the disc compresses the tire.

This thread addresses balancing the wheel. If you have a fairly well-balanced wheel, the wobble will be acceptable at CIM speeds.

This thread contains advice from Andy Baker on how well the CIM motor can handle the side load due to disc-tire compression.

The CIM does not depend on air cooling. They are pretty tough to burn out. I very much doubt if there will be a problem.

To connect to the CIM get some hubs from Andy Mark. They have some that are specifically intended to mount any of their wheels to a CIM, 8mm shaft diameter as I remember. If you want to support the shaft above the wheel also just attach another hub and attach a short shaft into a bearing.

Direct driving from the CIM is fine for a shooter, in terms of gear ratio (You will likely see many teams with this setup). Depending on your wheel size, you may even prefer the mini-CIM (same diameter, 2/3’s the length) because it has a slightly higher free speed RPM (6200 vs 5300).

However, as others have mentioned, be very aware of wheel balance and support for side loading - don’t rely on the CIM motor bearings to support the side loading very long.

We actually siezed a CIM motor bearing during prototype testing after serious overheating that was left unchecked. We had plans all along to properly support both sides of the wheel with bearings, and use a coupler to the CIM, but we were getting by with a direct-drive hub for testing.

The pneumatic tire we had from Harbor Freight was seriously out of balance and had a severe wobble (not a problem for the hand truck use it was intended for). Despite our attempts to balance with washers, the vibration still shook our test stand and the loading on the motor limited our wheel speed to about 3400 RPM. After repeated testing, the screws holding the motor body together vibrated loose, and the rear motor bearing siezed. The motor was too hot to handle without gloves for at least 20 minutes afterwards.

With a properly balanced wheel this likely would not have happened, even with only the motor shaft supporting the wheel, but it is still a good design practice to try and integrate another bearing to take the side load if possible

We killed a CIM. It was in our drive train last year, and we went to put it in the DT this year, and it moves at 10% speed and you can stop it by hand.

check out the pictures here…
This is likely what took out your motor. The heat generated internally eventually weakens the crimped wire to commutator connection and the wire fails by breaking off or pulling out.