Strain wave gearing

Three elements produce the reduction#

A common strain wave gear has a wave generator, a flexible externally toothed cup called a flexspline, and a rigid internally toothed circular spline. The wave generator deforms the flexspline into an ellipse, making teeth mesh on two opposite regions. Because the flexspline has slightly fewer teeth, one input revolution advances its position by only the tooth-count difference.

C. Walton Musser's original patent describes the strain-wave transmission principle. Harmonic Drive is a brand associated with this mechanism, not the generic name for every manufacturer's strain wave gear.

Use in compact robot joints#

The mechanism can place a large reduction ratio in a small coaxial package. That allows a fast electric motor to drive a slower, higher-torque revolute joint, with space through the centre available in some designs for wiring or shafts. Robot arms and humanoids use such reducers where compact packaging, positioning repeatability, and low lost motion are important.

The manufacturer Harmonic Drive identifies the wave generator, flexspline, and circular spline as its three basic elements and claims compact high reduction and zero backlash for its gearheads. Those are supplier claims about its products. Actual transmission behaviour depends on model, load, assembly, temperature, wear, and control method.

Strain wave gearing is a transmission, while a series elastic actuator is an actuator architecture with an intentional compliant element for force control. A quasi-direct drive normally uses much lower reduction. Either actuator design could be compared with a high-ratio strain wave joint on torque density, reflected inertia, efficiency, impact tolerance, and backdrivability.

Elasticity is useful and limiting#

The flexspline must deform on every revolution. The transmission therefore has compliance and friction even when tooth clearance is very small. Kim and colleagues measured compliance and hysteresis in a harmonic-drive test device and reported that these behaviours changed with service time.

Elastic wind-up, hysteresis, friction, and reflected motor inertia can reduce torque-control accuracy and backdrivability. Repeated flexing and tooth contact also make load spectrum, lubrication, temperature, and life important design checks. A compact catalogue ratio does not by itself establish efficiency, stiffness, impact survival, or suitability for a human-contact robot.