Harmonic planetary drive

prusaprinters

In this arrangement, the planets and sun gears have the same number of teeth along their entire length, but the two outside rings of the gear have different numbers of teeth. In this example, the sun and planet gears have 15 teeth. The lower (fixed) outer ring has 45 teeth, while the upper (output) ring has 42. That combination means that as the planet gears engage with both the fixed and the output rings, they are forced into alignment, and the output ring advances one tooth per rotation of the sun gear. This mechanism has a reduction ratio of 42:1.The motor cage fits a 28BYJ-48 stepper motor, which already has an internal 64:1 step-down gear, so the motion of this particular mechanism is particularly slow. I'm using it as the drive gear for a turntable for photogrammetry.Assembly is straightforward, I hope: put the servo in the motor cage (from the top, wires first: it's a tight fit and you need to be careful not to damage the wires on the cage); the fixed ring of the planetary assembly screws into the motor cage which pushes the sun onto the motor axle, and the lock ring screws onto the fixed ring to hold it in place.The output ring is flexible enough that it goes over the planet gears like a rubber band. The only alignment you need to worry about is making sure the output ring is equally spaced around the planets. That's made easier by the features in the compliant mechanism, which are spaced at 60 degree increments.