The operating principles of strain wave gearing are both elegant and unique.|
The high performance attributes of this gearing technology including zero backlash, high torque, compact size, and excellent positional accuracy are a direct result of the unique operating principles.
Form Follows Function...
Click to View Video Clip of Component Assembly
The mechanism is comprised of three components: Wave Generator, Flexspline, and Circular Spline. Each of these will be explained below.
The Wave Generator is typically used as the input member, usually attached to a servo motor.
The cup has a rigid boss at one end to provide a rugged mounting surface. During assembly, the Wave Generator is inserted inside the Flexspline such that the bearing is at the same axial location as the Flexspline teeth. The Flexspline wall near the brim of the cup conforms to the same elliptical shape of the bearing. This causes the teeth on the outer surface of the Flexspline to also conform to this elliptical shape. Effectively, the Flexspline now has an elliptical gear pitch diameter on its outer surface.
The Flexspline is usually the output member of the mechanism.
Important: Although the steel Flexspline flexes during normal operation, there is no concern about fatigue failure. The stresses developed are far below the endurance limit of the material. Thus the Flexspline will achieve infinite life when used according to catalog ratings. This is explained better in the figure below.
The Circular Spline is located such that its teeth mesh with those of the Flexspline. The tooth pattern of the Flexspline (which is now elliptical -as a result of conforming to the Wave Generator's elliptical shape) engages the tooth profile of the Circular Spline (circular) along the major axis of the ellipse. This engagement is like an ellipse inscribed concentrically within a circle. Mathematically, an inscribed ellipse will contact a circle at two points. However, the gear teeth have a finite height. So there are actually two regions (instead of two points) of tooth engagement.
In fact, up to roughly 30% of the teeth are engaged at all times. The load torque is distributed as shown in the following diagram.
The pressure angle of the gear teeth transforms the output torque's tangential force into a radial force acting on the Wave Generator bearing. The teeth of the Flexspline and Circular Spline are engaged near the major axis of the ellipse, and disengaged at the minor axis of the ellipse.
Here's the trick...
Kinematics and Performance
In addition, the unique kinematics allow the teeth of a gear to be engaged on both sides of the tooth flank (yes really!). Since backlash is defined as the difference between the tooth space and tooth width. This difference is zero strain wave gearing.
In conventional gearing, wear results in an increase in backlash over time.
In the gear as the gear teeth wear, this elastic radial deformation acts like a very stiff spring to compensate for space between the teeth that would otherwise cause an increase in backlash. This allows the performance to remain constant over the life of the gear.
High Positional Accuracy
Highest Torque -to- Weight Ratio