A Strain Wave Gear is a unique type of mechanical gearing system which allows very high reduction ratio in a compact and lightweight package. Compared to traditional gearing systems such as helical gears or planetary gears, it can achieve much higher reduction ratios of up to 30 times in the same space. In addition to that, it has zero-backlash characteristic, high torque, accuracy and reliability. Therefore, this gearing system is used in many applications, including robotics, aerospace, medical machines, milling machines, manufacturing equipment and so on.
The Strain Wave Gear was invented in 1957 by C. Walton Musser, and the other name which is commonly used for it, “Harmonic Drive”, is actually a brand name of strain wave gear trademarked by the Harmonic Drive company.
How It Works
All right, so let’s take a look how it works now. A harmonic drive has three key components, a wave generator, a flex spline and a circular spline.
The wave generator has an elliptical shape and consists of an elliptical hub and a special thin walled bearing which follows the elliptical shape of the hub. This is the input of the gear set and it’s connected to the motor shaft.
As the wave generator rotates it generates a wave motion.
The Flex spline has a form of a cylindrical cup and it’s made out of flexible but torsionally stiff alloy steel material. The sides of the cup are very thin but the bottom is thick and rigid.
This allows the open end of the cup to be flexible, but the closed end to be quite rigid and therefore we can use it as an output and connect the output flange to it. The flex spline has external teeth on the open end of the cup.
On the other hand, the Circular spline is a rigid ring with teeth on the inside. The circular spline has two more teeth that the flex spline, which is actually the key design of the strain wave gear system
So, when we insert the wave generator into the Flex spline, the flex spline takes the shape of the wave generator.
As the wave generator rotates, it radially deforms the open end of the flex spline. The wave generator and the flex spline are then placed inside the circular spline, meshing the teeth together.
Because of the elliptical shape of the flex spline, the teeth mesh only in two regions on the opposite sides of the flex spline, and that’s across the major axis of the Wave Generator ellipse.
Now, as the wave generator rotates, the Flex spline teeth that are meshed with those of the circular spline will slowly change position. Because of the tooth count difference between the Flex spline and the Circular spline, for each 180 degrees rotation of the wave generator, the teeth meshing will cause the flex spline will rotate a small amount backward relative to the wave generator. In other words, with each 180 degrees rotation of the wave generator, the flex spline teeth mesh with the circular spline will advance by only one tooth.
For example, if the flex spline has 200 teeth, the wave generator has to do 100 revolutions in order the flex spline to advance 200 teeth, or that’s just a single rotation for the flex spline. That’s a ratio of 100:1. In such a case the circular spline will have 202 teeth, as the circular spline number of teeth is always greater that the flex spline teeth by two.
A Strain Wave Gear is a unique type of mechanical gearing system which allows very high reduction ratio in a compact and lightweight package. Compared to traditional gearing systems such as helical gears or planetary gears, it can achieve much higher reduction ratios of up to 30 times in the same space. In addition to that, it has zero-backlash characteristic, high torque, accuracy and reliability. Therefore, this gearing system is used in many applications, including robotics, aerospace, medical machines, milling machines, manufacturing equipment and so on.
The Strain Wave Gear was invented in 1957 by C. Walton Musser, and the other name which is commonly used for it, “Harmonic Drive”, is actually a brand name of strain wave gear trademarked by the Harmonic Drive company.
How It Works
All right, so let’s take a look how it works now. A harmonic drive has three key components, a wave generator, a flex spline and a circular spline.
The wave generator has an elliptical shape and consists of an elliptical hub and a special thin walled bearing which follows the elliptical shape of the hub. This is the input of the gear set and it’s connected to the motor shaft.
As the wave generator rotates it generates a wave motion.
The Flex spline has a form of a cylindrical cup and it’s made out of flexible but torsionally stiff alloy steel material. The sides of the cup are very thin but the bottom is thick and rigid.
This allows the open end of the cup to be flexible, but the closed end to be quite rigid and therefore we can use it as an output and connect the output flange to it. The flex spline has external teeth on the open end of the cup.
On the other hand, the Circular spline is a rigid ring with teeth on the inside. The circular spline has two more teeth that the flex spline, which is actually the key design of the strain wave gear system
So, when we insert the wave generator into the Flex spline, the flex spline takes the shape of the wave generator.
As the wave generator rotates, it radially deforms the open end of the flex spline. The wave generator and the flex spline are then placed inside the circular spline, meshing the teeth together.
Because of the elliptical shape of the flex spline, the teeth mesh only in two regions on the opposite sides of the flex spline, and that’s across the major axis of the Wave Generator ellipse.
Now, as the wave generator rotates, the Flex spline teeth that are meshed with those of the circular spline will slowly change position. Because of the tooth count difference between the Flex spline and the Circular spline, for each 180 degrees rotation of the wave generator, the teeth meshing will cause the flex spline will rotate a small amount backward relative to the wave generator. In other words, with each 180 degrees rotation of the wave generator, the flex spline teeth mesh with the circular spline will advance by only one tooth.
For example, if the flex spline has 200 teeth, the wave generator has to do 100 revolutions in order the flex spline to advance 200 teeth, or that’s just a single rotation for the flex spline. That’s a ratio of 100:1. In such a case the circular spline will have 202 teeth, as the circular spline number of teeth is always greater that the flex spline teeth by two.
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