Low Cost Planetary Gearbox Source...

[Richard Wallace]

Originally Posted by Joe Johnson

While looking for some gearboxes for Robotic Amusements on ebay, I came accross the company BaneBots.



As it turns out, they make some pretty nice transmissions. AND they make some that are that the Fisher-Price and the Mabuchi motors will (pretty much) just bolt to (buy a gear or pull of the one that is on the stock motor, press the pinion on the FIRST legal motor, reassemble the gearbox, and mount it on your robot).

They come in a variety of ratios (5:1, 16:1, 20:1, 25:1, 64:1, 100:1, 256:1). The gearboxes have nice mounting points and beautiful 3/8" Dia output shaft that is long, easy to access (and support) and keyed w/.125 key.




I have 2 of these jewels on my desk right next to me.

Here is my thoughts.

They are sweet! But you are going to have to take the good with the bad.

The good:

• Reasonable Price (~$40)
• Nice mount points
• Nice output shaft (but it should be supported by an added bearing if you sideload it much)
• Compact
• Light
• Easy to use for the Fisher Price or Mabuchi Motors

The bad:

• Not exactly quiet
• I am skeptical that the transmissions can take the kind of torque we may be able to generate if you use a F-P motor and a high gear ratio (256:1 for example) if you stall the motors. I calc. that you could get over 50N-m out of the shaft if you really do use the 256:1 and a F-P motor -- that is alot of torque for a 3/8 shaft. I also have concerns about the face width of the last stage of the gearbox if you actually get to these higher torque outputs -- both the Globe gearboxes and the Dewalt transmissions have much beefier final gearstages than these gearboxes (this is just for comparison, not necessarily a criticism).

The Bottom Line:

• For the right application these gearboxes are AWESOME!
• I don't recommend trying to get 50N-m of torque output.
• I don't recommend using them as drive motors on a FIRST size robot.

FYI, I just talked to them the day before kickoff, they have 1000's in stock.

Good Luck,

Joe J.

Disclaimers:

• I have no ties (financial or otherwise) to Banebots, Inc.
• I have not field tested these on robots. All I can say is that my 11 years of FIRST experience leads me to believe that these gearboxes will be great in the right application on a FIRST robot.
• If you try this and it doesn't work out, I want to hear about it, but I don't want to hear things like "Dr. Joe, you are the reason we lost the Championship." As always, you mileage may vary.

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.