Harmonic Gears

A few weeks ago I wrote about the Abacus Drive, a new rotary transmission that could find its way into vehicles very soon. The Abacus was an evolutionary step forward from harmonic gears. They do similar things in similar ways and the idea of both of them is to increase or decrease the gear ratios available and do it in the smallest, most robust package.

In 1957 Clarence Musser invented the idea of harmonic gears – sometimes called strain wave gears, when he was a researcher at the United Shoe Machinery (USM) company. Presumably he was investigating the gearing of a machine that had an input speed that needed to be changed. Although USM started out making shoe machinery, by the time Musser was working for them, they were heavily into military equipment.

The whole concept of a gearbox is to change the input speed to a different output speed, however a box of gears can be quite large and harmonic gears and the Abacus Drive help reduce the size and weight of the components.

Harmonic gears use the flexibility of thin metal to help them perform their task of taking one speed and reducing it to another. The system uses an input shaft with an elliptical end that rotates a flexible spline in a “wave” pattern – think about the rotation of a rotary engine for example. The flexible spline was developed so that the loading on the splines is well below the tolerance of the materials used and that means, in theory, an infinite life although in practice it means a longer time between failures! The output shaft has a fixed spline that takes its input from the flexible spline with each spline having a different number of teeth.

This video is from Harmonic Drive AG – a company that is owned by USM and was formed from Musser’s invention. It nicely describes the structure of the gears. As you will see in the video, one major advantage is that the splines always fit tightly together so that there are no gaps in the rotation – having a gap is called backlash. With normal gears, as the teeth wear out the gap between the teeth gets bigger causing more backlash. With harmonic gears, the flexibility of the spline means that even though the teeth can wear away, the flexibility prevents the backlash from happening by always keeping the teeth together.

Harmonic gears have been used in many machines that require small and lightweight components – the aeronautic and aerospace industries use them a lot. This is a piece of technology that the inner mechanical geek in me wants to see in person!