Comparing Cycloidal and Planetary Gearboxes

Engineers have long used precision gearboxes to let servomotors control heavy loads at high cycle rates in robotic and industrial-automation applications. Over that time, two types of gearboxes have become de facto standards: cycloidal and planetary. Let’s take a look at each.

Cycloidal gearboxes
Cycloidal gearboxes or reducers consist of four basic components: a high-speed input shaft, a single or compound cycloidal cam, cam followers or rollers, and a slow-speed output shaft. The input shaft attaches to an eccentric drive member that induces eccentric rotation of the cycloidal cam. In compound reducers, the first track of the cycloidal cam lobes engages cam followers in the housing. Cylindrical cam followers act as teeth on the internal gear, and the number of cam followers exceeds the number of cam lobes. The second track of compound cam lobes engages with cam followers on the output shaft and transforms the cam’s eccentric rotation into concentric rotation of the output shaft, thus increasing torque and reducing speed.

Compound cycloidal gearboxes offer ratios ranging from as low as 10:1 to 300:1 without stacking stages, as in standard planetary gearboxes. The gearbox’s compound reduction and can be calculated using:

where n_hsg = the number of followers or rollers in the fixed housing and n_ops = the number for followers or rollers in the slow speed output shaft (flange).

There are several commercial variations of cycloidal reducers. And unlike planetary gearboxes where variations are based on gear geometry, heat treatment, and finishing processes, cycloidal variations share basic design principles but generate cycloidal motion in different ways.