What are harmonic gears? Harmonic gears are a unique mechanical gear system that allows for very high reduction ratios in a compact and lightweight package. Compared to traditional gear systems such as helical or planetary gears, it achieves up to 30 times higher reduction ratios in the same space. In addition, it features zero backlash, high torque, accuracy, and reliability. Therefore, this gear system can be used in many applications, including robotics, aerospace, medical machines, milling machines, manufacturing equipment, and more.
Harmonic drive was invented by Walton Mousset in 1957. How does it work?
Harmonic drives have three key components: a waveform generator, a flexible spline, and a circular spline.
The waveform generator is elliptical, consisting of an elliptical hub and a special thin-walled bearing that follows the elliptical shape of the hub. This is the input to the gear set, which is connected to the motor shaft.
When the waveform generator rotates, it produces wave motion.
The flexible spline has a cylindrical, cup-shaped structure made of a flexible yet torsion-resistant alloy steel. The sides of the cup are thin, but the bottom is thick and rigid.
The open end of the cup is flexible, but the closed end is quite rigid, so we can use it as the output end and connect the output flange to it. The flexible spline has external teeth at the open end of the cup. On the other hand, the circular spline is a rigid ring with internal teeth. The circular spline has two more teeth than the flexible spline, which is actually a key design feature of harmonic drive systems.
Therefore, when we insert a waveform generator into a Flex spline curve, the Flex spline curve will adopt the shape of the waveform generator.
As the waveform generator rotates, it causes the open end of the flexible spline to deform radially. The waveform generator and the flexible spline are then placed inside the circular spline, engaging the teeth together.
Due to the elliptical shape of the elastic spline, the teeth mesh only in two regions on opposite sides of the elastic spline, and these two regions span the main axis of the wave generator ellipse.
Now, as the waveform generator rotates, the flexible spline teeth engaging with the circular spline will slowly change position. Due to the difference in the number of teeth between the flexible spline and the circular spline, for every 180 degrees of rotation of the waveform generator, the tooth engagement will cause the flexible spline to rotate slightly backward relative to the waveform generator. In other words, for every 180 degrees of rotation of the waveform generator, the flexible spline teeth engaging with the circular spline will advance only one tooth.
Therefore, when the waveform generator rotates 360 degrees, the flexible spline will change position or advance two teeth.
For example, if a flexible spline has 200 teeth, the waveform generator must rotate 100 times for the flexspline to advance 200 teeth, or the flexspline may only rotate once. This is a 100:1 ratio. In this case, an arc spline would have 202 teeth because the arc spline always has two more teeth than the flexible spline.
We can easily calculate the reduction ratio using the following formula. This ratio equals the ratio of flexible spline tooth – circular spline tooth – to flexible spline tooth.
Therefore, taking 200 teeth on a flexible spline and 202 teeth on a circular spline as examples, the reduction ratio is -0.01. This is 1/100 of the wave generator speed, and the minus sign indicates that the output direction is reversed.
By changing the number of teeth, we can obtain different reduction ratios.
