comparing backlash of different mechanisms

I have a cylinder with a shaft in its center that I need to rotate at 120 RPM on 180°, for costing reasons, I would like, if possible, to reuse the same DC motor that I already use for another system. That motor has a speed of 6000 RPM so I need a reduction ratio of 1:50.

I don't have a lot of space under the cylinder, so I decided to move that motor to make it parallel to the cylinder (and then to the cylinder shaft), this also means that I won't be able to direct drive the cylinder shaft and I will need to use at least a pair of pulleys (with belt :)) or gears.

So what would be the best way to reduce the speed of the motor adding the least backlash ?

Currently I see these 5 solutions on which I would like to get external reviews on them :

1. Only pulleys: since I'm using GT pulleys and the maximum reduction ratio with 2 pulley is 1:4, I need at least 3 pairs of pulleys to get 1:50.

2. Planetary gearbox with 1:50 ratio + a pair of pulleys or gears having the same teeth count (just because the shaft are parallels)

3. Planetary gearbox with a ratio smaller than needed + a pair of pulley or gears that reduce again the speed appropriatelly to get the 1:50 at the end.

4. A pair of gears giving me the 1:50 ratio directly (if I find a gear with enough teeth..)

5. Replacing the motor by a servo which I do not know why are larger but shorter than the motors I found, and so it could fit under the cylinder.

• Although this is really late, one might consider a strain wave drive, which can easily get those kinds of ratios with nearly or no backlash at all. Sep 8, 2017 at 14:19
• Harmonic drive seems to be the best option, but it costs too much. I also thought about cycloidal drive... also direct drive with a torque motor... Sep 8, 2017 at 15:41

It sounds like, short of replacing the motor options (2) or (3) would be the most practical solution.

Compound sets of gears or pulleys is going to end up quite complex and by the time you've packaged 3 sets of shafts and bearings you are eating into any cost savings from keeping the motor and adding a lot of complexity to your design.

In terms of option 2 vs option 3 it really just depends on what parts are easiest/cheapest to source. DC motors with integrated planetary gearboxes are very common so it really comes down to whether it is more convenient to mount your existing motor with a separate gearbox or just get a complete new unit.

• And what about option 5 ? Can't i expect quality gearing from a 60\$ RC Servo ? Feb 16, 2016 at 0:14
• @liohau arent servos position controlled? Mar 16, 2016 at 23:34
• @agentprovocateur well the goal is to position control the cylinder, right ? So it's exactly what I need. Mar 17, 2016 at 9:34
• @LiohAu u said u needed 120 RPM as well. not sure if speed control is a strict requirement or not or if its a more important requirement. position control =/= velocity control... unless you figured this part out already. Mar 17, 2016 at 20:07
• Well, I wonder what's the scale of this thing. 120RPM means the 180 degrees is done in 0.25s. It's pretty hard to position anything accurately with this kind of angular velocity unless it's a very light device.
– SF.
Apr 11, 2017 at 14:32

Pulleys have a lot of play. This option is off. You don't have a lot of space, so a single 1:50 gear, while optimal for play, would take a lot of space.

High ratios are easily obtainable with worm drive, but they turn the axis 90 degrees, so if the motor must be parallel, you'd need two sets of these (or one set + bevel gears to return to original direction). Still, that would probably create least play while keeping the motor orientation. Although if you could install the motor 90 degrees to the cylinder axis, a single worm drive, with the worm directly on the motor axis, and firmly held with in a bearing would provide your 1:50 ratio with minimum backlash quite easily.