Low Cost Planetary Gearbox Source...

[Joe Johnson]

Originally Posted by The following is a redacted version of a PM I just received. FYI

i'd like to use a pair of the 16:1 gear boxes mated with 2 FPs to run a <secrect mechanism> but after several posts (namely, the ones that said they can't handle continous use), i'm worried about using them. do you think that they would survive in this application?
<more secrets removed>

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 4 m/s (12ft/sec)* 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 made a mistake in my calc. my initial posting mistakenly claimed 25m/s which was just whacked. Sorry. Even so, 4m/s is not exactly turtle speed.

Here is my calculation for review and perhaps further corrections:

16000 RPM @75% (i.e. 1/4 stall torque) = 12,000 RPM (@ motor)

12,000 RPM (@ motor) / 16:1 = 750 RPM (@ Gearbox)

750RPM = 79 Radian / Sec = w

V=w X R
79 Radians / Sec X .05 m = 4 m / s