Low Cost Planetary Gearbox Source...

[Joe Johnson]

Originally Posted by Richard

I ordered a couple of the 5:1 units to test on my lab dynamometer. Joe, you are not kidding these things are loud! The published 12 Volt 'specs' are as follows:

Free speed = 3120 RPM, Free current = 0.9 Ampere, Stall torque = 192 oz-in, Stall current = 37 Ampere

There is a disclaimer saying these are calculated values and they do not factor in loss from the gearbox.

Using my lab equipment (ISO calibrated stuff: Magtrol HD-715 brake and Sorenson DCS 20-150 power supply) I measured:

Unloaded speed = 3093 RPM, Unloaded current = 2.4 Ampere (note that my dyno drag at this speed is about 1.2 oz-in, due to friction, coupling flex, and un-energized brake drag)

When I increased the load to draw 5.0 Ampere, the speed was 2780 RPM and the indicated torque was 16.7 oz-in

When I further increased the load to draw 10.0 Ampere, the speed was 2070 RPM and the indicated torque was 39.3 oz-in

When I further increased the load to draw 15.0 Ampere, the speed was 1350 RPM and the indicated torque was 60.7 oz-in

When I further increased the load to draw 20.0 Ampere, the speed dropped below 500 RPM and the brushgear started to smoke, so I quickly de-energized the brake to minimize damage.

All the while the little gearbox was rattling and whining, I'd estimate about 100 dB at 1 meter. I would not recommend this unit for extended continuous duty! It's acoustic signature is annoying and losses in the gearbox are clearly significant. However as you point out it may be quite useful for mechanisms with intermittent duty and low endurance requirements. And the price is certainly right.

So my conclusion is that the gearboxes are at best about 80% efficient, and this drops to about 60% at heavy loads. I would not want to take them to more than about 10 Amperes motor draw for longer than a minute or so.

Wow, lots of good data, thanks.

As to the conclusions, I am not sure I entirely concur. In particular, much of what you recommend involves the particular motor the gearbox comes shipped with (which is not a legal FIRST motor).

Even so, plotting the data, they don't seem pretty non-linear to me. Any non-linearity seems easily explained by the motor heating up as you increased the load.

The FP motors are prone to heat build up. It is extremely easy to pound more heat into that little package than can be safely passed off to the surrounding air. The same can be said for the motor that the Banebots transmission ships with.

This is well documented elsewhere in other threads, but think about this: A curling iron is typically a 20Watt device. Think of how hot a curling iron gets and it is only dissipating 20W. At 15Amps & 12V, the test above was inputting 180W of electrical power (V*I) to the motor. Of that, only 60W as being converted to Mechanical Power (T*w), leaving 120W to heat up the curling iron, I mean, Motor.

Back to my comments on the data, I think that the data on effeciency is somewhat affected by the heating of the motor. If I assume that the motor data is correct (even when the motor was hot), I get 72%, 80%, 61% and 56% at Freespeed, 5Amp, 10Amp, & 15Amp data points.

I am not sure how to intrepret this data. My experience say that 5:1 planetary gearboxes should be about 80-90% eff. per stage (we have only 1 stage in this case). One way of looking at it is to say that the first data point is off due to drag, etc, and that the last two reflect that the motor was getting hot. But, I could be trying to fit the data to my view of the world, so I don't know what to say.

Based on the data, I would perhaps estimate the gearboxes to be say 70% per stage and (using given that relatively low effeciency) I would try to keep the 12V load point of the FP to below 1/4 of the 12V stall (i.e. keep the motor spinning above 75% of its free speed). My experience with the FP is that if you do this, they'll be able to survive a FIRST season.

My final comment is that the data generally support that these gearboxes are not jewelry. They are cheap and dirty. But... ...I would just say that the price and effort are right... ...for the right application.

Joe J.