Strain Wave Gear is a unique mechanical gear transmission system that can achieve a very high reduction ratio in a compact and lightweight package. Compared to traditional gear transmission systems (such as helical gears or planetary gears), it can achieve up to 30 times the reduction ratio in the same space. Additionally, it features zero backlash, high torque, high precision, and reliability. Therefore, this gear transmission system is widely used in fields such as robotics, aerospace, medical devices, milling machines, manufacturing equipment, and more.
Strain wave gear was invented in 1957, and its other common name is "Harmonic Drive".
The wave generator is elliptical, consisting of an elliptical hub and a special thin-walled bearing that follows the shape of the elliptical hub. This part is the input of the gear set and is connected to the motor shaft. When the wave generator rotates, it creates a wave motion.
The flex spline is a cylindrical cup made from flexible but torsionally stiff alloy steel material. The sides of the cup are very thin, but the bottom is thick and rigid. This makes the open end of the cup flexible, but the closed end very rigid, allowing it to be used as the output end, to which the output flange can be connected. The outer spline end has open teeth.
On the other hand, the circular spline is a rigid ring with internal teeth. The circular spline has two more teeth than the flex spline, which is essentially the key design feature of the strain wave gear system.
When the Strain Wave Gear wave generator rotates, it radially deforms the open end of the flex spline. The wave generator and the flex spline are placed within the circular spline, meshing the teeth together. Due to the elliptical shape of the flex spline, the teeth only engage in two areas on opposite sides of the flex spline, which intersect with the major axis of the wave generator ellipse.
Now, as the wave generator rotates, the teeth of the flex spline that are engaged with the circular spline will change position slowly. Because the flex spline and the circular spline have different numbers of teeth, each 180-degree rotation of the wave generator results in the flex spline rotating backward a small amount relative to the wave generator. In other words, each 180-degree rotation of the wave generator will cause the flex spline teeth to advance by only one tooth with respect to the circular spline.
All these parts can be 3D printed, so we only need some bolts, nuts, and a few bearings to complete the assembly. Design the three key elements of the Strain Wave Gear: circular spline, flex spline, and wave generator. Since the 3D printer has its own limitations—regarding printing quality, accuracy, and precision—the first thing to decide is the gear module or tooth size. Of course, the flex spline needs to have 2 fewer teeth, which is 70 teeth. This will produce a 35:1 gear ratio while keeping the gear set relatively small.
As for the wave generator, we can't really use those special types of thin-walled bearings mentioned earlier, as they are not easily available. Instead, we will use ordinary ball bearings arranged around the elliptical circumference. The size of the ellipse should be determined according to the size of the inner wall of the flex spline.
