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In my project a knob (usually rotated by hand) can be turned also by a stepper motor. On the other hand, when the motor is not energized, the knob can be rotated manually as normally.

To transmit the motion from the motor I would like to use three spur gears made in acetal (low friction material). My fear: when the motor is off will the operator receive a bad feeling if turning the knob manually? I mean: the teeth of the gears will not generate too much friction (and a "clunky" feedback) when compared to a timing belt? (See attached image.)

My intuition says that adopting a belt I should obtain a more smooth movement and low friction. Is my intuition right? Anybody has a direct experience in this kind of problems?

P.S. No high rotation speed here. The knob will be turned by the motor for max 180° at a speed comparable by a normal human wrist.

enter image description here

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  • $\begingroup$ Belts require proper tension, but with good bearings this doesn't contribute much to friction vs turning the shafts, although it adds slightly to complexity. You get the extra part count back, compared to gears, in some geometries (e.g. input and output axis offset a significant distance... look at an inkjet printer or a flatbed scanner for example). $\endgroup$
    – Pete W
    Feb 22 at 23:01
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The gears will have significantly lower friction forces compared to the pulley. However, the way I understand it, you are not really concerned about the actual friction forces developed. What you are more interested is the force feedback you will get with either system.

The problem with gears and custom made stepper motors is the alignment of the gears. That problem becomes more severe with custom gears (and from what I understand you are using acetal to cut custom gears). Even having two gears would create a problem a clunky feeing, and you are planning on using 3.

Of course the problem is mitigated because a) you are only planning on half a turn so you can choose a portion of the larger gear to work that has less problems, b) using acetal will eventually wear down enough the gears to make them operate better.

Comparably the pulley will have a much easier setting up. Like you already mentioned, if you are worried about slippage you could use timing belts.

IMHO, the timing belt would be a lot easier to set up and tune, compared to the gears. However, if there is enough expertise to properly implemented the gears then it will be a superior solution in terms of resistance and forces.

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You will note that the work required to manually turn the shaft of the stepper motor itself when it is idle is surprisingly large- more so than the friction drag in your manual adjustment system. This means you can go with the cheapest and easiest solution, which is the belt drive option.

You probably do not need a toothed pulley set and a toothed timing belt. Where the torques being transmitted are small, urethane O-rings are typically used with pulley sheaves specifically machined to fit O-rings of various diameters.

If you want no slippage, then a toothed belt will be necessary. Miniature toothed belts and the toothed pulleys used with them are available through industrial supply houses like McMaster-Carr and Grainger.

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I am not a mechanical engineer, so my response can be false. In my opinion, for concern of friction, I'll choose the gear system rather than the belt, because you cannot expect the pulley to move, if there is a lack of friction between the belt and the pulley. On the other hand, in the gear system, the movement is initiated through the normal force exerted by the teeth, for which the friction can be kept low.

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  • $\begingroup$ What do you think of a toothed pulley system? The caption says "timing belt solution" which implies toothed belt. $\endgroup$
    – fred_dot_u
    Feb 22 at 22:18
  • $\begingroup$ @fred I still lean towards the gear system, as timing belt still needs considerable amount of friction to move the pulley. The flexible nature of the belt material has precluded it to produce significant push force, and stay tight. I don't know the design/built of the modern timing belt, but have very good memory on the frequent need to adjust timing belt on my motorcycle in the old days. $\endgroup$
    – r13
    Feb 22 at 22:40

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