Low Cost Planetary Gearbox Source...

[Richard Wallace]

Originally Posted by Joe Johnson

This is my redacted reply:

This is not true. The FP motors CAN take continuous use. They CANNOT operate near 12V stall or even at 70% of 12V stall (a.k.a. below 30% of their 12V free speed when applying 12V to the motor) for long periods of time without turning into a stinky, smokey blob of copper.

You do not give enough information to know if the conditions you are talking about are reasonable.

At 12V, the FP motor puts out .4 N-m at stall. With 2 stages of 4:1 each @ 70% eff. you will get Tgearbox stall = .4 X 4 X 4 X .7 X.7 = 3.1N-m.

I recommend that you operate at about 1/4 of this number to run continuous and happy all season long (not that you can't muscle through more than that for a short time - a few seconds say - but that you should not need more than this for 10s of seconds at a time).

Can you keep the continous torque on each motor to below .75N-m?

Let's assume that you have 4" pulleys on your <secrect mechanism> This works out to a radius of about 50mm = .05m. That means that you could get 15N (3.4lbs) on the <secrect mechanism> continuously.

Is that enough? It is hard to know without my getting into the nickers of your <secrect mechanism> design.

By the way, the <secrect mechanism> would be FLYING at almost 25m/s when driven by a 4" pulley with a tangential load of 15N.

If you don't need a <secrect mechanism> that is so fast, use a smaller pulley or a higher gear ratio gearbox.

Joe J.

I never intended to discourage anyone from using the FP motor. As Joe J. points out above, there should be no problem with running it continuously at loads that are less than ~25% of its 12V stall current. When using the single-stage 5:1 BaneBots gearbox with an FP motor, I would not be too worried about running continuously at up to 15 Ampere, because the FP has about 50% higher stall current and is therefore a more capable motor than the one that comes with the gearbox. The gearbox itself will probably wear out faster at FP continuous loading than it would with the standard motor, but that is probably not an issue for a mechanism that will be used for one FRC season.

My recommendation above was just to avoid continuously loading the BaneBots gearmotor at currents greater than about 10 Ampere (about 25% of its 12V stall current). I really don't like to see smoke coming from motors, and this one started smoking at 20 Ampere.