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I want to make a shaft rotate in 21 discrete steps, from a motor that is turning smoothly.

I am thinking of some kind of simplified clock escapement type of mechanism. Or non circular gears.

I know it could be done by programming a stepper motor. But it will be easier and more fun for me to do it mechanically with 3D printed nylon parts.

This is for a low torque low speed application. About 1 rpm and less than 0.1 Nm torque.

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    $\begingroup$ How about a Geneva mechanism ? $\endgroup$ Apr 4, 2017 at 16:10

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Geneva drive
3D Printed Geneva Drive

Geneva drive enter image description here



Anchor Escapement (Clock Mechanism)
3D Printed Anchor Escapement

enter image description here

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  • $\begingroup$ Nice visualizations. $\endgroup$
    – MrYouMath
    Apr 5, 2017 at 8:52
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    $\begingroup$ Regulating the radii ratio between the wheels of the Geneva mechanism you can tune the number of steps to a degree; at a high number of steps (like 21) you'd have a lot of play though, as the curve of the rounded part of the "star" wheel would be very shallow. But if you make the Geneva mechanism for 7 steps and then reduce this further by normal gears in 1:3 ratio, you'll have your 21 steps on output while keeping play to minimum. $\endgroup$
    – SF.
    Apr 5, 2017 at 9:00
  • $\begingroup$ How is a Geneva Drive implemented in real machinery? It appears to have a large contact area between static and moving items, leading to significant friction & wear. $\endgroup$ Apr 5, 2017 at 13:32
  • $\begingroup$ @CarlWitthoft The faces of the two wheels are not in contact. The only contact is between the pin and the slots on the indexing wheel. This creates a line contact. Depending on the loads and the size of the mechanism, this could be addressed in a number of ways or combinations of ways. The pin and the slot would likely be hardened. There would likely be lubrication. The pin might really be a roller bushing. $\endgroup$
    – DLS3141
    Apr 6, 2017 at 17:01
  • $\begingroup$ @DLS3141 ok, I see. I thought the semicircular part was there to inhibit backlash in the red gear. So why not just a regular toothed wheel for the red and a single-tooth gear for the driver? $\endgroup$ Apr 6, 2017 at 17:14

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