How a cycloidal drive works.

1. Eccentric cam. The input shaft carries an off-centre round boss. As it turns, the boss swings the disc around in a small circle (the eccentricity).
2. Lobed disc. Its edge is a cycloid-shaped wave with one fewer lobe than there are pins. It is pushed round the ring but cannot spin freely.
3. Ring pins. Fixed pins around the outside. The lobes roll against them, so many pins share the load at once.
4. Output pins. Pins on the output sit in oversized holes in the disc. The holes are bigger by twice the eccentricity, so the pins ignore the wobble and pick up only the disc's slow rotation.

With N ring pins the disc has N − 1 lobes. Each full turn of the cam rolls the disc around the ring once, and because it has one lobe fewer than there are pins, it ends up one lobe behind where it started. One lobe is 1/(N − 1) of a turn, so:

ratio = (N − 1) : 1, and the output turns the opposite way

11 pins give 10:1 (our Pocket Cycloid). 26 pins give 25:1 (our free Cycloid Drive 25:1). Drag the slider above and count: one input turn moves the output by exactly one lobe.

Why robots use them

• Big reduction in one stage. 10:1 to 100:1 without stacking gear stages.
• Many contacts at once. Several lobes touch pins at any moment, which spreads the load.
• Compact and flat. Good for robot joints.
• The catch. The disc wobbles, so real drives often use two discs 180° apart to balance it. That's what our 25:1 does. The profile also has to be accurate: any clearance shows up as backlash.

• Clearance is everything. The disc profile is offset a little inward so it can turn. Too tight and it binds; too loose and you get backlash. Our 25:1 uses 0.1 mm and a fit-test coupon to check your printer first.
• Turn off X-Y hole compensation. The clearances are already designed in.
• Steel pins where it counts. Printed pins work for a hand-crank toy. A motor-driven drive wants hardened steel pins and bearings.