# Cycloidal drive

For the marine propulsion system, see Voith-Schneider.

A cycloidal drive or cycloidal speed reducer is a mechanism for reducing the speed of an input shaft by a certain ratio. Cycloidal speed reducers are capable of high ratios in compact sizes. [1]

The input shaft drives an eccentric bearing that in turn drives the cycloidal disc in an eccentric, cycloidal motion. The perimeter of this disc is geared to a stationary ring gear and has a series of output shaft pins or rollers placed through the face of the disc. These output shaft pins directly drive the output shaft as the cycloidal disc rotates. The radial motion of the disc is not translated to the output shaft.

## Theory of operation

Parts of a 10:1 cycloidal speed reducer mechanism

The input shaft is mounted eccentrically to the ball bearing, causing the cycloidal disc to move in a circle. The cycloidal disc will independently rotate around the bearing as it is pushed against the ring gear. This is similar to planetary gears, and the direction of rotation is opposite to that of the input shaft.

The number of pins on the ring gear is larger than the number of pins on the cycloidal disc. This causes the cycloidal disc to rotate around the bearing faster than the input shaft is moving it around, giving an overall rotation in the direction opposing the rotation of the input shaft.

The cycloidal disc has holes that are slightly larger than the output roller pins that go inside them. The output pins will move around in the holes to achieve steady rotation of the output shaft from the wobbling movement of the cycloidal disc.

The reduction rate of the cycloidal drive is obtained from the following formula, where P means the number of the ring gear pins and L is the number of pins on the cycloidal disc.

$r = \frac{P - L}{L}$

Single stage efficiency approaches 93% and double stage approaches 86%[2] with single stage reduction ratio up to 119:1 and double stage up to 7569:1 [3]

Unlike many other drive mechanisms the cycloidal drive is not typically backdrivable, the input and output shafts of the cycloidal drive cannot be reversed. This is because rotating the output pins will not rotate the cycloidal disc in the correct way.

However, in the event that the cycloidal disk is worn or manufactured to a loose tolerance, a large input torque on the output can propagate through the drive and back drive the input. This is not a typical operation and the efficiency of that transmission is very poor, leading to premature failure.

Also, the eccentrically mounted cycloidal disk will cause vibration in the drive which will propagate through the drive/driven shafts. This will also cause increased wear on the exterior teeth of the cycloidal disk, as well as the interface with the output roller pins due to small relative motion caused by the vibrations. A second cycloidal disc installed a half-rotation relative to the first will balance the input shaft and reduce vibration.