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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, 2018 at 1:12
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    $\begingroup$ You might be overthinking the grinding. It is probably no problem if one motor slightly back-drives the other, causing that one to speed up slightly and the former to slow down slightly, until they match. $\endgroup$
    – user253751
    Feb 9, 2023 at 10:14

7 Answers 7

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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.

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  • $\begingroup$ 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, 2015 at 19:53
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    $\begingroup$ @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, 2015 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, 2015 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, 2015 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, 2015 at 22:28
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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.

enter image description here

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  • $\begingroup$ That's fantastic, I'll look into that. :) $\endgroup$
    – trycatch
    May 11, 2015 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, 2016 at 15:25
  • $\begingroup$ @Paul That is a nice clear explanation of how it works. Thanks for the link. $\endgroup$ Jan 15, 2016 at 15:31
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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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Electric (DC) motors when used in trolley cars were not on a single shaft, but still had to work together and stay "in sync". Usually they were connected in series to get best starting torque and once the trolley was moving re-connected in parallel. It follows that whatever solution you select for driving single-shaft --- I prefer the both motors on the one shaft -- you should follow a similar plan to provide the most useful operating characteristic.

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    $\begingroup$ Welcome to E.SE. I think you may be mixing up some concepts here. Series connected motors would have a lower starting torque than parallel as the peak current would be halved. It would, however, reduce the surge current and may be done for that reason. In some systems the rotor and field windings could be switched from series to parallel to change the torque / speed characteristics as well. $\endgroup$
    – Transistor
    Aug 26, 2022 at 21:30
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If you use the same motor and then a belt to drive them there should be no problem or even two belts would be even less of a problem

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I'm conceptualizing a better electric boat design and this thread is on topic with my idea. A powerboat needs enormous torque when throttling up from a dead stop until you get the boat up on plane when 80% of the hull rises above the water with only the stern and prop still in the water. Once on plane, you need high RPM but a small fraction of the pre-plane torque. Electric motors' torque curves are ideal for this, but a single 200 HP electric motor is a challenge - heat, current, and concentrated weight and size. Two or three smaller motors on the same shaft would solve a lot of those problems. All three motors energized for 30 seconds to get on plane, two energized to get up to cruising speed, then just one energized for efficiency at top speed. Does that sound reasonable/logical from an engineering perspective? I'm taking batteries out of the equation for the moment and thinking gasoline or diesel generator. Lithium batteries on a boat are much more of a challenge than on land. So AC or DC motors are both possibilities. EE here with knowledge of motors, but you guys seem to have deeper knowledge than I do.

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LC-EV project

I see this is an old question but the only one that matches my requirements.

I’m also looking at driving a shaft using 2 motors at 90 degrees but concealed within a twin 2-stroke engine.

The motors would replace the pistons and con rods and the crankshaft would be replaced by a twin differential (somehow) Bear in mind this engine will still have a clutch and 6 gears. For cooling if the motors aren’t to wide the liquid cooling system will still work and the air-box and carbs just for suppling the motors with filtered cool air.

I’m unsure of; Fitting 2 diffs in a small area. The diff gear ratios required. If I could get anything like the same torque as the petrol version when moving up through the gears. Size of motors required and their heat tolerances within confined space

Also would a constant mesh type gearbox be able to handle going in reverse?

Thanks for any advice on my project.

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