Andymark Planetary
 

I'm working on a design for a gear box for the CIMs that would use the gears out of the am planetary (the one for the Fischer price) and I have a couple of questions concerning it.

1. Perhaps the more important is how much torque can the gears in the am planetary take. If I were the put two CIMs worth of torque into my gear box and mount a 6 inch wheel to the output shaft of the gear box would I have wonderful grinding noises? The reduction through the gearbox will be around 15:1 and the planetary will be the last stage on the reduction.

2. The other question revolves around a disparity on the andymark site and determines a fairly significant part of my design. the site says that the reduction through the planetary is 3.67:1 and the the sun gear has 15 teeth. This would mean the ring gear has 55 teeth and that isn't acceptable geometry for a planetary. The CAD model does has a sun gear of 15 teeth but the ring gear only has 40 teeth giving a reduction of 2.67 which makes it harder for me to get the reduction I want. So if somebody who knows could tell me which one that would be very helpful.

Thanks.

Re: Andymark Planetary
 

Not sure on #2 but on question #1, I can say with fair certainty that would not be a good idea. That is a far higher load than the intended application.

Re: Andymark Planetary
 

There are lots of issues here:

2 CIMs as your input
15:1 reduction
putting a wheel on the output shaft

If you simply put the torque of 2 CIMs through the gearing of the AM Gearbox, that would not be a problem, as the teeth can handle the load of 2 CIMs. However, if you are reducing the speed before the gearbox another 4x or 5x, and therefore increasing the torque, I would suggest going with 20 dp gears instead of the 32 dp gears in the AM Planetary gearbox. At that torque level, gears may start stripping.

Probably the biggest concern for what you are doing would be the side loads applied from the wheel to the gearbox. The bearings on the AM Planetary would not handle the side loads that wheels see in a FRC competition. You will need bigger shafts (1/2") and bigger bearings. Dynamic loading from a wheel is significant. Imagine wheels hitting ramps, pipes, and curbs like in the 2004, 2005, and 2006 FRC games.

Planetary, or epicyclic gearing ratio calculation is not very straightforward. The gearbox does have a reduction of 3.67:1. The sun gear is 15 teeth, while the ring gear is 40 teeth and the planet gears are 12 teeth. If the output shaft was connected to a rotating ring gear, then your calculation of 2.67:1 would be correct. However, in this case, the ring gear is stationary and the planets ride on a rotating carrier plate. Figuring the speed of the carrier plate is as such:

carrier plate speed = (# of ring gear teeth + # of sun gear teeth) / # of sun gear teeth

= (40+15)/15 = 55/15 = 3.67

Wikipedia has a good page on this, and this page on the University of Denver website is also good.

Good luck with your design.

Andy Baker

Re: Andymark Planetary
 

Thank You very much. I guess I was confused about how to calculate the reduction on the gear box and now that you tell me it does make much more sense for me.

To be more specific I would replace the output shaft and carrier plate with my own aluminum half inch shaft because I wouldn't trust the 8mm shaft to be strong enough. The torque would enter the gearbox through the ring gear with the sun gear held stationary by a bolt and some teeth milled into the aluminum housing.

In any case it sounds like I would have to use bigger gears if I wanted this to work. Thanks for the imput.