Wednesday, October 7, 2009

More Power with a Differential Gear

I like to build big…very big. My current mobile robot project weighs about 25 lbs. This creates a whole new set of problems. The structure has to be built rigid enough to not bend or break. The wheels have to be chosen so they won’t go flat, so pneumatic tires aren’t a good choice. An appropriate gear ratio has to be selected so you have a good speed with enough power.

A way to get a good speed and lots of power is to use a differential gear in a different way. Everybody is familiar with using a differential gear to power the both driving wheels of a vehicle so the wheels will be powered while turning a different speed. The same can be done with motors. Rather than attaching wheels to the differential, attach motors. Then the motors can be run either independently or together. When only one motor is running, there is a gear ratio of 2:1, when together there is a 1:1 ratio with twice the power.

Two Motors with a Differential Gear
(click to enlarge)

There are some drawbacks to this;

  • Power will always travel path of least resistance. This means that if you have a high load and only one motor is turned on, you will likely just turn the rotor of the other motor. This means that you may be required to run both motors at all times. Another solution is to incorporate worm gears on each motor, giving you an extremely powerful but slow gear box.
  • To maximize power, each motor should have its own power supply, so you should double up on battery boxes and controls.
  • You may end up with extremely high power, so you might want to consider beefing up some of the power transmission elements. Two XL PF motors could easily twist and break an axle under a high load condition.

I have used this technique a few times, and once I connected four 9v motors to drive a very large heavy wheel. It works great, but can be a space hog.


  1. If you want to 'robotize' (or NXTify :D) your construction you will also have to keep in mind that 2XL motors may run fine on a single battery pack, but they draw more current than the IR receiver will provide for safety reasons!

    I found this to be quite a problem when I built an IR controlled race car using 2 XL motors for driving and an M motor for steering. I'm still working on a solution which may entail using a second IR receiver.

  2. Cool, a more complex version of this is used in Toyota's Hybrid Synergy Drive to combine two electric motors and one gas motor. In the case where your high load is leaking power by turning the shaft of the non-powered motor, this motor is generating power for you! Maybe you can capture some of it by using rechargeable batteries and implement an on-board charging system and/or regenerative braking!