Scanning for Ideas: Novel Recirculating Belt Powers Linear Actuator

June 19, 2008
Designers at Animatics Corp., Santa Clara, Calif. (, reengineered the concept of a belt-driven actuator and came up with the Harmonic Linear Drive (HLD).

Edited by Stephen Mraz

Instead of moving loads with a fixed belt, it uses a recirculating belt that folds back upon itself and over pulleys with slightly different diameters. The design uses fewer parts than fixed-belt alternatives. This is said to improve reliability and cut costs. And unlike ball-screw drives which are limited in length by the ball screw’s critical speed, the HLD has no such limitation and comes in lengths greater than 3 m.

In the HLD, a pair of pulleys are locked together at each end of the actuator. A closed-loop brushless servomotor pulls the belt through the pulleys. The different-sized pulleys give the belt different travel speeds in different areas. With the belt in tension, the difference between rates makes the traveler (which carries the load) move one way or the other. This eliminates the need for gear reduction. And the resistance to back drive means brakes aren’t needed either, even in most vertical applications. (For an animated demonstration of the principle, go to

The drive comes in ratios from 2.5 to 12.5 mm/rev and standard stroke lengths of 100 to 3,200 mm. Standard thrust is 450 N and the actuator can handle average moment loads of 1,500 Nm. Bidirectional repeatability is about 60 μm and unidirectional repeatability is less than 20 μm.

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