Most gearboxes trade size for precision. Planetary stages stack up backlash at every mesh; worm drives are compact but lossy and springy. The strain wave gear — widely known by the trade name harmonic drive, invented by C. Walton Musser in the 1950s — sidesteps the trade entirely by doing something no other gearbox does: it flexes one of its own gears.
The three parts
A strain wave gear has only three functional components:
- Wave generator. An elliptical steel cam wrapped in a thin ball bearing. This is the input; it's usually driven directly by the motor shaft.
- Flexspline. A thin-walled steel cup with fine external teeth cut at its open rim. The cup is flexible enough to be deformed elliptically by the wave generator inside it, yet torsionally very stiff. This is the output.
- Circular spline. A rigid steel ring with internal teeth, fixed to the housing. It has exactly two more teeth than the flexspline.
The wave generator presses the flexspline outward at the two ends of its ellipse, engaging the flexspline's teeth with the circular spline's teeth at two diametrically opposed zones. At the ellipse's minor axis, the teeth are fully clear of each other.
Why two teeth means 100:1
Spin the wave generator one full revolution and the engagement zones sweep around the ring once. But because the flexspline has two fewer teeth than the circular spline it walks inside, it comes up two teeth short of where it started, so the cup itself creeps backward by two teeth per input revolution.
The reduction ratio is simply the flexspline tooth count divided by the tooth difference. A flexspline with 200 teeth inside a 202-tooth circular spline gives 200 ÷ 2 = 100:1, in a single stage, coaxially, with the output rotating opposite the input. Ratios from 51:1 to 161:1 are standard across our catalogue range.
Where the zero backlash comes from
In a conventional gearbox, teeth need clearance to avoid jamming, and that clearance is backlash. In a strain wave gear, the wave generator preloads the tooth mesh elastically: roughly 30% of all teeth are engaged at any moment, split between the two zones on opposite sides of the ring. There is no designed-in clearance to take up when the load reverses.
The result: when the motor stops, the joint stops, and it holds position bidirectionally. That's the property that makes harmonic drives the default choice for robot joints, telescope mounts, and anywhere a machine has to stop exactly where it's told.
What the flexing costs you
Nothing is free. The flexspline is a spring, so a strain wave gear has finite torsional stiffness: under load it winds up slightly and springs back. It also exhibits a small ratcheting torque limit: overload it far enough and teeth can jump. Both behaviours are well-characterised, published values, and both are why honest datasheets — and honest test benches — matter more with this technology than with most. Every unit we ship is measured on the bench and logged to its serial number.
Where they run
Harmonic drives have flown since the Apollo Lunar Roving Vehicle and are now standard in collaborative robot joints, semiconductor wafer handlers, surgical robotics, optical mounts, and 5-axis machine-tool heads. If your application needs high ratio, low weight, and repeatability measured in arcseconds, this is the gear for it.
Related: How to size a harmonic drive · Harmonic, planetary, or cycloidal?.