The Harmonic Drive® strain wave gear is made up of three basic components. These are first of all the so-called wave generator, an elliptical steel disc that forms the heart of the gear unit, which has a centric hub and a special thin, elliptically deformable ball bearing. The wave generator is also connected to the motor shaft. Secondly, the wave generator consists of the flexspline – a deformable, cylindrical steel bushing with teeth arranged around its outer circumference – and the circular spline. The latter surrounds both the wave generator and the flexspline as a rigid, cylindrical outer ring, with teeth along its inner circumference. What is significant is the number of teeth – the outer toothing of the flexspline has fewer teeth than the inner toothing of the circular spline. The difference in number is frequently two.