Hauppauge, NY-Clearance between gear teeth in transmission components causes backlash, which compromises system accuracy and repeatability. Engineers commonly attempt to counteract backlash either mechanically during the assembly process, or electronically while the system is operating.

Mechanical means include shims or manual-adjustment mechanisms designed to pre-load geartrain components, tightening the fit of looser components, and raising the transmission's total spring constant. However, this is somewhat of a Catch-22 since the tighter the fit, the greater the rate of wear, and the more frequently adjustments must be made to offset the backlash creep associated with gear wear. Electronic backlash compensation can be complex and costly, and typically involves a dual-loop control scheme with an extra feedback device attached directly to the load.

Another approach to overcoming backlash in speed reducers, called the harmonic drive, uses a circular spline, flex spline, and a wave generator to maintain low backlash for the long haul. However, these flexing gear designs have a reputation for limited torque capacity compared to traditional planetary gearheads. To overcome torque limitations and solve the problem of backlash creep, one manufacturer of harmonic drives, HD Systems Inc., recently applied its flex-spline design know-how and manufacturing experience to develop a new class of speed reducers called Harmonic Planetary gearheads.

Cataloged as the HPG Series Planetary Gearhead, the design combines planetary gearing with a radially compliant ring gear, called a flex ring gear. While the ring gear flexes, not all of it is radially compliant, according to Brian St. Denis, senior engineer at HD Systems. "The part near the input stage that bolts to the housing is rigid," he explains, "with a thicker cross section. In contrast, the second stage of the ring gear has a thinner cross section so that it flexes to compensate for backlash creep. So engineers switching from regular planetary gears can expect higher positional accuracy initially, and consistent performance over the long term."

Available in 60-, 90-, and 120-mm square flange sizes, HPG gearheads offer 5:1, 11:1, 21:1, and 33:1 ratios, and maintain 3 arc-min (standard) or 1 arc-min (optional) backlash for life. Maximum input speed is 4,000 rpm. At 3,000 rpm, rated torque for a 20,000 hr life ranges from 52 to 955 in-lbs. "The design achieves lower out-of-box backlash than standard planetary gears, which is typically in the 4-5 arc-min range," says St Denis. "And that backlash is sustainable over the life of the gearhead."