Engineering FAQs - Cycloidal
Q: What is a Cycloidal?
A cycloidal gear reducer consists of four major components: an input shaft, a cycloidal disc, a housing with internal ring pins, and an output shaft. The input shaft has an eccentric circular lobe which fits into a bearing in the cycloidal disc. The eccentric lobe causes the center of the cycloidal disc to rotate around the center of the housing. The cycloidal disc is geared to the housing but is slightly smaller and has fewer teeth than the housing has pins. This causes the cycloidal disc 
to rotate about its center but in the opposite direction of its orbit within the housing. The net effect is that the cycloidal disc rolls within the housing (thus a point fixed on the circumference of the cycloidal disc would trace a hypocycloid).
To convert the cycloidal motion into rotary motion, the cycloidal disc has holes which accommodate pins projecting from the output shaft. The holes are larger than the pins so that the motion of the disc center relative to the housing does not interfere with the tangential motion 
of the output pins. The center of each hole must be allowed to move around the center of its matching output pin with the same radius of orbit as the center of the cycloidal disc around the center of the housing.
The reduction ratio is dictated by the relative sizing of the cycloidal disc and the housing. The larger the size difference between the disc and housing, the more the disc will rotate about its center as its center orbits around the housing center. With each rotation of the disc center around the housing center, a given ring pin will mesh 
with a different gear tooth on the disc. The number of gear teeth which move past that ring pin before it meshes again is equal to the difference between the number of ring pins in the housing and the number of gear teeth on the cycloidal disc. The orbit of the disc within the housing is determined by the rotation of the input shaft, and the rotation of the disc about its center determines the rotation of the output shaft. Therefore, the reduction ratio is equivalent to the speed at which the cycloidal disc orbits divided by the speed at which it rotates and can be expressed as:
R is the reduction ratio, H is the number of ring pins in the housing, and D is the number of gear teeth on the cycloidal disc.
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Cycloidals offered by R.M. Hoffman Company:
