To head off the "why not use a bigger motor" question, I'm a high school mentor for an FTC (First Tech Challenge) robotics team, with a bit of a soft mechanical / hardware background. They're somewhat limited in what they're allowed to use, motors in particular. Up to 8x 1.5 ft-lb 12v DC motors, max.

This year they came across the problem of needing more lifting power, which gearing could help with, but to get the lift they needed cost a lot of speed, which in a very short (~2 minutes) competition, was a costly trade off when there was a need to go up and down frequently.

Possibilities I've considered (and haven't had time to mess with yet, but I'm trying to research and get input from engineers better than myself..)

  • Having two motors directly driving the same gear on the shaft seems like the first obvious answer, but error prone (slipping out of sync and grinding potentially.)
  • Two motors driving two separate pulleys on the driven shaft, which should theoretically gain power without the gear jamming issues.

I'm planning on having the kids experiment and test, but as my ME experience is extremely weak, I'm soliciting input.

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    $\begingroup$ If the motors have axis going all the way through, you can couple them in line, axis to axis (common shaft) with no gearing in between. $\endgroup$
    – SF.
    Jun 25 '18 at 1:12

A differential is a mechanical device designed to do exactly what you propose. It will allow the two motors to spin at slightly different rates while still combining the power. The most common use of a differential is in the drivetrain of an automobile in which it is used to power both wheels from one engine while still allowing the wheels to spin at different rates, i.e. acting in the reverse sense from the application you are looking for.

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  • $\begingroup$ That's fantastic, I'll look into that. :) $\endgroup$
    – trycatch
    May 11 '15 at 19:50
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    $\begingroup$ Check out this classic 1930's film short from Chevrolet titled "Around the Corner". This video is the most concise and simple explanation of the differential gear I've ever found. Not only does it explain the problem motivating the differential, but also how simple incremental adjustments result in the differential gear: youtu.be/F40ZBDAG8-o $\endgroup$
    – Paul
    Jan 15 '16 at 15:25
  • $\begingroup$ @Paul That is a nice clear explanation of how it works. Thanks for the link. $\endgroup$ Jan 15 '16 at 15:31

While a mechanical differential does what you ask, you don't really need it.

You can connect two identical electric motors together on the same shaft. There is no "slipping out of sync" because there isn't a issue of sync in the first place. Drive the two motors the same and both will develop close to the same torque. One will have slightly more torque than the other, but the two torques still add. There is no harm in a little mismatch.

In the worst case, you drive one motor at full and the other not at all. The undriven motor will just add a little frictional load to the shaft as long as its electrical connections are left open. As long as you drive each motor hard enough so that it would rotate at that shaft speed with no load, it's not going to slow anything down. You'd have to drive the two motors significantly differently for one to add torque to the shaft while the other is adding drag.

This assumes that you don't have a closed loop controller that is trying to regulate motor speed. As long as your controller is only changing the effective equivalent voltage the motor sees, directly connecting the two motors is fine.

  • $\begingroup$ Hm, really? Somehow I imagined the worst case would be something like one motor running at X rpm and the other either running at some non power of two fraction, causing the teeth to occasionally meet or something. But I suppose the higher powered motor will keep pushing things along? $\endgroup$
    – trycatch
    May 11 '15 at 19:53
  • $\begingroup$ @tryc: With both motors connected to the same shaft, both will always run at the same speed. I don't see how gear teeth have anything to do with this as you'd be using shaft couplers. There is no need for any gears between the two motors. At a given speed the only question is how much torque (positive or negative) the motor is putting onto the shaft. With the same model motors connected electrically, the torque at the same voltage and speed is going to be pretty close. Even if one is, say, 80% of the other, no real harm will be done other than less than optimum use of the motors. $\endgroup$ May 11 '15 at 19:59
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    $\begingroup$ If the motors are permanent magnet DC type and have the same nameplate but different characteristics (due to manufacturing inconsistencies) it's better to connect in series than parallel. This is because motors act as dynamos, producing a back voltage that must be overcome by the applied voltage. Example: 2 motors, one with back voltage 10V at some speed, the other with back voltage 11V. Both have resistance 1 ohm. Connect them to a 12V supply, motor A draws (12-10)/1=2 amps, motor B draws (12-11)/1=1 amp. Torque is proportional to current, so there's different current/torque in each motor. $\endgroup$ May 11 '15 at 22:05
  • $\begingroup$ You can avoid this problem by instead selecting two nominal 6V motors and connecting them in series, so they are sure to have the same current through each (and hence virtually identical torque). Though in both cases you should think about what the effect the failure of one motor will have on the other. $\endgroup$ May 11 '15 at 22:17
  • $\begingroup$ "I don't see how gear teeth have anything to do with this as you'd be using shaft couplers." Oh. OH. I didn't even think about having them both DIRECTLY DRIVING THE SAME SHAFT. In my mind I was going: Motor A gear driving -> gear on shaft <- Motor B gear driving And I saw a potential for one gear to slip and get out of sync with the gear on the shaft and cause grinding / catching. $\endgroup$
    – trycatch
    May 11 '15 at 22:28

Coupling two motors used to be a problem. They would balance the two by monitoring current draw between them and balance. Coupling two motors is not longer a problem anymore with the advent of digital drives, which can pair each up with one another. This solves the old coupling of two shafts with a jack shaft.


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