Shaft specs for CIM motors

[Cal578]

I'm looking for specifications for the shaft loading on CIM and mini-CIM motors. In particular, we'd like to know how much axial and radial load the motors can take.

Please reference actual, quantitative specifications.

[philso]

Did you install a shooter wheel directly on the motor shaft? Did you happen to have some of these motors fail?

We don't have manufacturer's specifications but a teardown of a failed motor (mini-CIM) showed that there was a ball bearing at the output shaft end and a sleeve bearing at the other end. The sleeve bearing was severely worn after about 20-30 shots through our shooter. This lead to the rotor dragging on the stator magnets.

I asked one of the engineers in our Motor Design group about the failure we experienced the other day. He said that motors experiencing side loads above what a belt or a chain would apply must use ball or roller bearings at both ends of the shaft and that sleeve bearings are inadequate for such applications.

[Andy Baker]

Originally Posted by Cal578

I'm looking for specifications for the shaft loading on CIM and mini-CIM motors. In particular, we'd like to know how much axial and radial load the motors can take.

Please reference actual, quantitative specifications.

Thanks for sending an email to us at AndyMark about this also. We never really looked at this, as we have not had issues with side loading the CIM motor (and hence, one of the reasons we all love the CIM Motor).

Here is my answer to Gerry in my previous email. I figured that the rest of you folks may like this:

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We don't currently have any specs for this. However, we have not heard of this being an issue with this motor, when it shot poof balls in 2006, basketballs in 2012, and driven many high-load gearboxes during the years. This sort of shock load to the CIM is significant this year, but I believe that the CIM motor can handle it. The bearings are bronze bushings, which have high load ratings. Here is some analysis and explanation on the from one of the bearing manufacturers:

http://www.buntingbearings.com/data.html

If we consider this example, we can plug in some numbers:

• PV is a means of measuring the performance capabilities of bearings.
  
• P is expressed as pressure or pounds per square inch on the projected area of the bearing. V is velocity in feet per minute of the wear surface (surface feet per minute).
  
• For sleeve (plain) bearings, the surface speed is .262 (pi ÷12) x RPM x shaft diameter in inches. P is equal to the load on the bearing in pounds divided by the projected area in square inches. For sleeve (plain)bearings, the projected area is the length x the inside diameter of the bearing.

PV is then obtained by multiplying P x V as shown in the following example:

Assuming that the contact force between a shooting wheel and the frisbee disc is 30 pounds (which is a high guess)...

5/16 (8mm)" shaft @ 4500 RPM, 30 lb. total load, bearing length 1/2".

V = .262 x RPM x shaft diameter, or .262 x 4500 x .312 = 370 sfpm.

P = total load ÷ projected area (area = .312 x 0.5 = .156 sq. in.), or 30 lbs. ÷ .156 = 192 psi.

PV = 192 psi x 370 sfpm = 71,000 PV

Looking back at the web link's PV chart, I see the weakest bearings at 75,000 for their max PV ratings.

So, with this analysis, I think that we are ok for this application. If this was something that happened to this mechanism 24/7/365, then I would recommend a motor with a larger diameter. Since this is a very infrequent use of this motor, I think that we are good.

Sincerely,
Andy Baker
AndyMark, Inc.

[Cal578]

Originally Posted by philso

Did you install a shooter wheel directly on the motor shaft? Did you happen to have some of these motors fail?

Our prototype has had no failures, but the final design is not the same. We'll keep CD posted.

Do you know how much radial load you had on your mini-CIM shaft? 20-30 shots seems like a really quick failure. Did you also have much axial load?

Originally Posted by Andy Baker

Thanks for sending an email to us at AndyMark about this also. We never really looked at this, as we have not had issues with side loading the CIM motor (and hence, one of the reasons we all love the CIM Motor).

Here is my answer to Gerry in my previous email. I figured that the rest of you folks may like this:
<snip>

Sincerely,
Andy Baker
AndyMark, Inc.

I was going to post your reply here, but you beat me to it, Andy. Anyway, this gives me a chance to thank you publicly for the great customer support you are providing. Just one more example of how the FIRST community is awesome.

[billbo911]

While Andy has provided some very valuable information here, I'm not sure that it applies to this situation or not. Yes, the CIM and Mini-CIM share the same output geometry and circumference, but they are not the same motor.

The failure described above was of a Mini-CIM, not a CIM. So, what specifically are the differences in the two motors that can account for this failure? I know many teams have been prototyping with direct drive CIM shooters, and I don't believe we have seen failures of those. Is it just the Mini-CIM that doesn't like this treatment, or just this one motor in this one event?

[philso]

Originally Posted by Cal578

Do you know how much radial load you had on your mini-CIM shaft? 20-30 shots seems like a really quick failure. Did you also have much axial load?

I don't have a quantitative measure of the side load on the motor shaft. The guys were experimenting with different amounts of compression so it could have gotten higher than most people would actually use. Matters are complicated by the fact that they mounted the motor on a 1/8 inch thick polycarb plate that was mounted to the shooter plate (1/4 inch plywood). Both the polycarb and plywood were observed flexing as the frisbees went through.

I suspect that in our application, the shock load of the frisbee being put into the shooter may be what ultimately damaged the bearing. Another possibility is that we got the one bad mini-CIM in the batch since it doesn't seem like anyone else had a similar failure.

[bradleyarg]

Originally Posted by billbo911

Is it just the Mini-CIM that doesn't like this treatment, or just this one motor in this one event?

Our mini-CIM did the same thing, it ended up cracking the top plate of the mini-cim... we are going to try it with it super bolted down and it supported from more spots...

I imagine you're using the pneumatic wheels or something like them to shoot as our team is doing... make sure you align the motors properly, center them, and balance them, plus check to make sure the wheel is actually on right (we found that one of our wheels wasnt on correctly and so we had to shave the inside (rubber) down to make it match