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.

Figure 2. Operating Principle of the harmonic drive gear.

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;