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What is a Strain Wave Gear Harmonic Drive?

1. What is a strain wave gear?

Strain wave gearing is a unique mechanical gearing system that enables very high reduction ratios in a compact and lightweight package. Compared with traditional gear transmission systems such as helical or planetary gears, it can achieve a reduction ratio of up to 30 times in the same space.

In addition, it has zero backlash characteristics, high torque, high precision and reliability. Therefore, the gear transmission system is widely used in robotics, aerospace, medical equipment, milling machines, manufacturing equipment and other fields.

Strain wave gear was invented by C.Walton Musser in 1957, and its other common name "harmonic drive" is actually the brand name of the strain wave gear registered by the harmonic drive company.

2. The working principle of strain wave gear harmonic drive

A harmonic drive has three key components: a wave generator, a flex wheel and a circular spline.

The wave generator is oval in shape and consists of an oval hub and a special thin-wall bearing that follows the shape of the oval hub. This is the input to the gear set, which is connected to the motor shaft.

When the wave generator rotates, it creates wave motion.

The flexspline has a cylindrical cup shape and is made of a flexible but torsionally rigid alloy steel material. The sides of the cup are thin, but the bottom is thick and hard.

This makes the open end of the cup flexible, but the closed end is very rigid so we can use that as the output and attach the output flange to it. The end of the external spline has open teeth.

A circular spline, on the other hand, is a rigid ring with teeth inside. The arc spline has two more teeth than the flex spline, which is actually a key design of the strain wave gear system.

So, when we insert the harmonic drive wave generator into the flex spline, the flex spline becomes the shape of the wave generator.

As the wave generator rotates, it radially deforms the open end of the soft body spline. The wave generator and flexible splines are placed within the circular splines, meshing the teeth together.

Due to the elliptical shape of the flexspline, the gear teeth mesh only in two areas on opposite sides of the flexspline that pass through the major axis of the wave generator ellipse.

Now, as the wave generator rotates, the flex spline teeth that mesh with the circular spline will slowly change position. Due to the different number of teeth of the flex wheel and the circular spline, every 180 degrees of rotation of the wave generator, the meshing of the gear teeth will cause the flex wheel to rotate a small amount backward relative to the wave generator. In other words, every 180 degrees of rotation of the wave generator, the flex spline teeth and the circular spline mesh only advance one tooth.

Therefore, for a full 360 degree rotation of the wave generator, the flex wheel will change position or advance two teeth.

For example, if the soft body spline has 200 teeth, the wave generator must make 100 revolutions to advance the soft body spline 200 teeth, or this is just one rotation of the soft body spline. This is a 100:1 ratio. In this case, the circular spline will have 202 teeth, because the number of teeth of a circular spline is always twice as large as that of a flex spline.

We can easily calculate the reduction ratio with the following formula. This ratio is equal to the flex gear teeth-circular spline teeth divided by the flex gear teeth.

So, taking the example of 200 teeth on a flex wheel and 202 teeth on a circular spline, the reduction ratio is -0.01. This is 1/100 of the speed of the wave generator, and a negative signal indicates the opposite direction of the output.

We can get different reduction ratios by changing the number of teeth or the number of teeth.

We can do this by changing the diameter of the mechanism while having the same size teeth, or by changing the size of the teeth that maintain the size and weight of the gear set.

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