Three gear motors to drive a single continuous shaft

Thread Starter


Joined Jul 19, 2021
Hi, I am designing a folding curtain of about 76M in width & 20M in height.
The main shaft will be 76M long having pulleys at 3M distance, polyester straps will be connected to the pulleys & to the bottom load bar,
on rotation the pulleys will wind the strap thus lifting the bottom bar & the curtain,
If I use one motor in centre then the 38M long free shaft has to transfer the torque!
The hollow shaft dia increases enormously, so along with the shaft the bearing block & all arrangement cost & size increases.
due to transfer of torque to such length the resulting deflection in twist is about 20mm, that will mean that the pulley closer to drive will take more load then the farther most pulley, also the twist is about 20MM.
To use three gear motors connected parallelly in principle this will work as they have the same out put rpm (12rpm),
My dark area:
If one of the motor does not work then the other two motors will be additionally loaded and fail,
I am looking out for an electronic solution where all three motors are monitored for equal voltage, shuts down the system in case of any problem, indiacte problem motor, This circuit has to be economical & dependable.



Joined Jul 10, 2017
The simplest solution is to connect them all in series and monitor the current. They will automatically share the load. If one goes open circuit, all will stop. If one goes short circuit or seizes up, the current will increase and be detected. Monitoring each motor individually can be done by connecting a LED, zener diode and a resistor across each motor. If the motors are AC then a series diode will also be needed. If one motor begins to fail, the others will take the extra load and the voltage a cross the failing motor will drop, causing its LED to extinguish.
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Joined Nov 6, 2012
A Spec-Sheet is needed for your Gear-Motors.
I suggest that they should be over-rated by at least 100% each.
Are they AC or DC ?
Voltage and Current Ratings ?
Locked-Rotor-Amps ?
Duty-Cycle ?
I assume they are Reversible, how are they controlled ?
Do You want to keep the existing Controls ?, or is this a completely new installation ?
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Joined Apr 11, 2010
Or if you can, design your folding curtain in multiple segments. Coordinating three motors whose load is interconnected will be a challenge. Question your decision to use three motors. If you divide the load, you might get away with two.


Joined Jan 30, 2016
I'd second the splitting into three or more. With the right control system it should be relatively easy to keep them in step. Putting motors in series will theoretically generate similar torque in each, subject to motor variation (typically +/- 3 - 5% on expensive motors, 10% on cheap ones), but you'll have no real control of individual speed, plus you'll need three times the voltage which could become problematic. Those ideas are fine for toys but not for this problem.

If you use multiple motors I'd advise individual motor controls slaved together, the same way its done on large machines, like rolling mills.

Just running some numbers in my head I'm reckoning you need around a 2HP motor (using a reasonably opaque fabric and assuming a 60 second lift) so this isn't (probably) a DC motor but, if you split it - for the reasons stated - each will be around 3/4HP. 3 x AC servo motors with positional feedback, the three controllers slaved together to ensure rate of lift (rather than motor speed) is the controlling factor.

Now the trick is to find an economical solution. It would be good to see your working, as pulley sizes, rate of lift, etc, will have significant impact on costs.

There may be some benefit in increasing the number of motors to get a smaller form factor, putting them into the realms of a large stepper motor, which could simplify the control side. But I'd need to see your numbers...

Oh, and what's your budget?
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John P

Joined Oct 14, 2008
I'm coming in late with this, but I wonder if multiple motors is a good idea. It just seems to be accepting a difficult design problem that doesn't have to exist. What I'd try to do first would be to lift the curtain via multiple cables which would all be fed to a central location via pulleys--put them in trunking if they're likely to be a trip hazard or anything. Then pull all the cables with winding drums mounted close together on a single shaft with only one motor. It has some mechanical complications, but electrically it is much simpler!


Joined Jan 30, 2016
Simpler electrically, but massively lossy with all those pulleys - 76m is a long run to have, and the angles are tricky. But your idea kicks off another thought process...

If you turn the pulleys through 90deg so their axis is perpendicular to the curtain then you could possibly link them all together with a long loop of HTD belting on both sides. Put the motor in the middle so you end up with 4 belts, the motor driving a right-angle dual-output shaft gear box driving both belts on each side. The torque in the output shaft is balanced and the torque in each belt is only 1/4 of the total. Properly tensioned between pulleys the toothing ensures all pulleys turn at the same speed.