gear technology - cycloid

The word Cycloid, with its adjective Cycloidal, is derived fromHypocycloid which describes the curve traced by a point on the circumference of a smaller circle rotating inside the circumference of a larger fixed circle

GEAR RATIO CALCULATION

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.

Ratio = (P - L) / L

Where P = Number of ring gear pins/rollers
             L = Number of lobes on a cycloidal disc

The method of operation is just like this :

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.

As the eccentric bearing drives the cycloidal disc, the cycloidal disc rotates in one direction relative to its own center. However the cycloidal disc advances in the opposite direction relative to the center of the speed reducer. This planetary motion looks almost like the wobbling movement of hula hoops.

In order to convert the wobbling motion of a cycloidal disc into the smooth concentric movement of output shaft, several output shaft rollers are placed inside the small circles of a cycloidal disc. These rollers are also attached to the output shaft pins. The difference (2C) between the diameter of output shaft roller and the small circle is exactly twice the eccentricity (C) of eccentric bearing. This distance (2C) is also the radial difference between the valley and crest of a cycloidal disc lobe

With the arrangement above, the mechanism is capable of converting the rocking motion of an eccentric bearing into the wobbling planetary motion of a cycloidal disc. This motion is then transformed to the smooth concentric movement of output shaft through the output shaft rollers. The speed reduction is achieved, and torque transmission is accomplished.

ADVANTAGES & DISADVANTAGES

The best thing with this mechanism is that it is able to give high-reduction ratio in a compact space

Therefore it's commonly used as reducer where the space is confined and limited.
One more thing is the efficiency that is pretty high, Single stage said to have the efficiency approaches 93% and double stage approaches 86%

The bad thing is unlike other drive mechanism, its rotation cannot be reversed
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.