Flexspline

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The thin-walled, externally toothed, cup-shaped flexible gear in a harmonic drive that gets flexed into an ellipse.

The flexspline is one of the three core parts of a harmonic drive (the other two being the wave generator and the circular spline). It's a thin-walled, cup- or hat-shaped metal part with external teeth. As the elliptical wave generator rotates inside it, it flexes the flexspline into an ellipse, so its teeth mesh with the circular spline's internal teeth only at the two ends of the long axis; the flexspline has two fewer teeth than the circular spline, so for each full turn of the wave generator, the flexspline shifts by two teeth relative to the circular spline, producing a reduction ratio in the hundreds-to-one range. The flexspline has to flex elastically over and over, so its material, heat treatment, and machining precision directly determine a harmonic drive's lifespan and accuracy, making it a key focus of China's push to manufacture harmonic drives domestically.

Also called: Flexible Spline
Related: Strain Wave Gear (Harmonic Drive) · Wave Generator · Circular Spline · Gear Ratio · Backlash
Sources: Strain wave gearing - Wikipedia