Hollow Shaft Actuators with Harmonic Drive Gearing
- Created: Tuesday, 01 December 1998
They can be used in demanding applications such as industrial robots and servo systems.
Harmonic drive gearing is recognized by designers for its zero backlash, high gear ratios, and compact design features. A recent development by HD Systems incorporates a large hollow shaft through the actuator, offering many benefits to the machine designer. The FHA hollow shaft actuator series allows cables, shafts, or tubing to be passed concentrically through the center of the actuator. Through this innovation, the series provides precision motion control and high torque capacity in very compact packages.
Another technical stride incorporated in this series is the patented "S" tooth profile, yielding higher torque capacity and torsional stiffness as well as twice the rated life of conventional harmonic drive gearing. Using this tooth profile, the CSF series delivers twice the performance of conventional harmonic drive gearing in roughly half the axial length. The SHF series offers the performance of the CSF with the additional benefit of a hollow shaft through the center of the gear.
Harmonic drive gears are made up of three basic parts (Figure 1). The circular spline (right) is a rigid ring with internal teeth, engaging the teeth of the flexspline (center) across the major axis of the wave generator. The flexspline is a nonrigid, thin cylindrical steel cup with external teeth on a slightly smaller pitch diameter than the circular spline. It fits over and is held in an elliptical shape by the wave generator, a thin raced ball bearing fitting onto an elliptical plug serving as a high-efficiency torque converter.
The three basic parts of the CSF series gears function in the following way (Figure 2). The flexspline is slightly smaller in diameter than the circular spline and usually has two fewer teeth. The wave generator's elliptical shape causes the teeth of the flexspline to engage the circular spline at two opposite regions across the major axis of the ellipse. As the wave generator rotates, the zone where the teeth of the flexspline engage those of the circular spline travels with the major elliptical axis. For each 180° clockwise movement of the wave generator, the flexspline moves counterclockwise by one tooth relative to the circular spline. Each complete clockwise rotation of the wave generator results in the flexspline moving counterclockwise by two teeth from its original position relative to the circular spline.
The reduction in the axial length of the CSF—depending on frame size, it can be almost 50 percent shorter than a conventional harmonic drive gear—is made possible by the "S" tooth profile. The wave generator imparts its elliptical shape onto the flexspline, which provides tooth engagement between it and the circular spline. The greater the ellipticity of the wave generator, the greater the radial deflection experienced by the flexspline. This deflection must not produce stresses above the fatigue limit of the material.