[Paul Copioli]

topgun,

Actually, the RS775-18V when being used at 12V only has about a 13,000 RPM free speed. The free speed is proportional to driving voltage so 12/18*19,600 is ~ 13,000. Comparing that to the 5310 RPM of the CIM motor, you get 2.44:1. Matching at free speed is not necessary (as a matter of fact, I never match at free speed) as the robot never really operates at free speed. For general drive train use, 3:1 is the proper ratio to use for the RS-775. However, I have used as high as 4:1 before to try to match speeds at operating conditions.

Think of it this way, at free speed the faster motor will try to drive the slower motor so the slower motor will act as a generator (a load) on the smaller motor. Don't worry, though, as soon as some load gets on the system, the lower power motor will slow down much faster and the speeds will be matched. Dr. Joe Johnson posted about this several years ago. I printed out that post and still have it for reference as he explains it much better than I explain it.

So the short answer is use 3:1.

Mk.32,

I have conducted all of the testing on the VersaPlanetary Gearboxes and have made my final load rating conclusions. I am currently putting it in a format more useable for FRC teams. Included in the ratings will be rated continuous torque, Max Torque, overhung load equations (aka force on the shaft), and recommended motor / gear ratio combination ratings.

However, I will share some of the data here. Please note that something will break at the Maximum tourque rating:

All ratings are based on the output torque and the gear stage at the output.

3:1, 4:1, 5:1 gear stage maximum output load rating with 1/2" Hex Shaft = 160 N-m [~120ft-lb]
The failure mode is the actual Hex Ultimate stress failure.

10:1 gear stage maximum output load rating with 1/2" Hex Shaft = 100 N-m [~74ft-lb]
The failure mode is the carrier plate stress crack originating from the mating internal spline connection. The 10:1 stage carrier pins are very close to the internal spline. This could not be avoided, but the 10:1 is above our estimated maximum Torque.

So if you are using a 50:1 double reduction gearbox, then I suggest you put the 5:1 stage on the output.

For those of you doing the math yourselves, the 100:1 gearbox with a RS775 has an output stall torque of around 58 ft-lb so it is below the max torque rating of the gearbox.

We will have a ratings table for our recommended combinations and potential failure modes for motor / ratio combinations up to 1000:1.

Paul