Gear transmission speed change devices are capable of high torque but are complex to manufacture, exhibit backlash, gear noise, and typically require multiple stages to achieve high reduction ratios.
Traction drive speed change systems offer certain advantages but typically produce less torque then a geared speed change device of the same size.
Generally speaking, with traction drive systems it is desirable, for many applications, to provide pure rolling contact between torque transmitting members for low friction, high efficiency, long service life, and increased traction. Furthermore, is desirable for the contact forces between the torque transmitting members to increase automatically as torque increases, so high loads are only generated between traction components when necessary to allow increased torque output. This would provide the benefit of increased service life and efficiency by reducing wear surface loading when the actuator is not transmitting high torque.
Other desirable characteristics of a rotary actuator are zero backlash, backdriveability, low vibration, non-cogging output, high rigidity, and quiet operation. High torque capacity for size and weight are also desirable, as are a wide range of speed change ratio possibilities (including very high speed change ratios) high input speed capability (to allow the use of low torque input drive systems) low input inertia, and a relatively large center thru-hole for interned wiring (or possibly to allow an integrated electric w other type of rotary drive motor within (or partially within) the inner diameter (ID) of the actuator.