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.
The "S" tooth harmonic drive gearing has a wave generator with far less ellipticity than conventional harmonic drive gearing. Thus the flexspline is subjected to less radial deflection. This allows its axial length to be shortened without increasing the stress level. As a result, HD Systems engineers have successfully delivered shortened axial length, high performance, and infinite life.
By combining increased performance with shortened axial length, the CSF achieves a fourfold increase in performance on a per-volume basis. For example, a robot's performance is determined by its payload, and the weight of the robot limits the payload capacity. The CSF decreases the robot's size, and thus its mass, and so it can increase its payload capacity.
The SHF series takes the additional step of incorporating a large through-bore capacity through the center of the gear. This is made possible by the development of the innovative "silkhat" flexspline. The conventional flexspline has a mounting boss on a smaller diameter than the toothbed. It has the shape of a cup, with the toothbed on the open end and the mounting boss on the bottom of the cup. The "silkhat" type has a mounting boss on a much larger diameter than the toothbed. It resembles a top hat, with the mounting boss on the rim. Since the mounting boss's diameter usually limits the available through-bore, the silkhat design provides a much larger through-bore capacity. This allows machine elements such as tubes, shafts, or ballscrews to be passed through the center.
The input element can be driven by a hollow-shaft brushless motor. Another configuration involves using a pulley to drive the wave generator from a motor mounted on a parallel shaft, yielding the advantage that the motor can be mounted a short distance from the SHF gear for an optimum package size.
The "S" tooth profile, shown in Figure 3, significantly increases the region of tooth engagement. For the traditional tooth profile, about 15 percent of the total number of teeth are in contact, while for the new profile up to 30 percent are in contact. Figure 4 shows a region of tooth engagement. One end of this region is at the major axis of the wave generator ellipse where the teeth are totally engaged. The other end is where the teeth become totally disengaged. The increase in engagement results in a 100-percent increase in torsional stiffness in the low and medium torque ranges.
The new tooth profile also features an enlarged tooth root radius, which results in a higher allowable stress and a corresponding increase in torque capacity. Furthermore, the enlarged region of tooth engagement leads to a more even loading of the wave generator bearing, resulting in more than double the bearing's life expectancy.
The FHA series actuators feature a through-bore up to 45 mm in diameter. These units consist of a DC brushless pancake motor, an encoder, and a precision harmonic drive gearhead. An encoder is built integral with the motor to reduce the axial length to a minimum. Rated torques up to 1730 in.-lb. and positional accuracy better than 1 arc-minute can be achieved. The FHA series is available in five frame sizes, ranging from 116 to 248 mm in length, and 128 to 300 mm in diameter.
For more information on harmonic drive gearing systems, please contact Brian St. Denis, HD Systems, Hauppauge, NY 11788; (516) 231-6630; fax: (516) 231-6803; http://www.HDSystemsInc.com.