Machine Design

Flexible gears minimize gearhead backlash for life

Timing-belt tensioning and planetary gearheads that maintain out-of-box backlash levels for life are just some of the topics covered in this mechanical reference guide.

HPG Series Harmonic Planetary gearheads from HD Systems Inc., Hauppauge, N.Y., come in three frame sizes and ratios from 5:1 to 33:1. The new Size 14 (90-mm-sq frame) has a rated torque of 107 lb-in., a 204-lb-in. repeated peak torque, and a 496-lb-in. maximum torque. Maximum input speed is 4,000 rpm. HD Systems says HPG gearheads maintain 3 arc-min (standard) or 1 arc-min (optional) backlash for life. Also available in Size 20 and 32.

Backlash in geartrains can compromise positioning accuracy and repeatability. As gears wear, backlash increases. But the use of flexible gear elements can reduce or eliminate backlash for good.

One such design called a harmonic drive contains a circular spline, a flexspline, and an elliptical-shaped wave generator. Input goes to the wave generator and output to the flexspline while the circular spline is typically fixed. Rotating the wave generator imparts a continuously moving elliptical, wavelike motion to the flexspline. This meshes flexspline external teeth adjacent to the wave generator's major axis with the circular spline internal teeth. Simultaneously, teeth fully disengage about the wave generator's minor axis.

Because the flex spline has two less teeth than the circular spline, one complete revolution of the wave generator rotates the flexspline two teeth relative to the fixed circular spline. For example, a fixed circular spline with 202 teeth and an output flexspline with 200 teeth, gives the reduction ratio:

Ratios are negative because flexspline rotation is always in a direction opposite to wave generator rotation. The flexspline member lets harmonic-gear drives maintain low to zero backlash for the life of the device. It also allows extremely high reduction ratios in a compact, lightweight package.

A variation on the design comes from HD Systems Inc., Hauppauge, N.Y. It's new Harmonic Planetary gearheads use a flexible ring gear as a "backlash buffer" to maintain backlash at out-of-box levels. The ring gear is made thinner in the radial direction than a typical ring gear so it can flex radially yet remain torsionally stiff. The ring gear preloads against the planet gears to compensate for normal gear wear.

Contrast this with two other commonly used backlash control methods. The first is a manual backlash-adjustment mechanism that splits the planet carrier into two parts. These two carrier pieces, each with a planet gear at either end, manually preload (like a scissors) against each other during final assembly then bolt tight. However, geartooth wear requires that the mechanism be adjusted periodically to maintain backlash at original levels. Another method creates a "tight fit" between all gears during assembly. Shims are used to preload bevel-type gears, for example. But again, normal wear can increase backlash several times that of the original spec.

Low and sustainable backlash is important especially for servo applications because the position-control loop is typically closed on the motor encoder. The control system assumes the output position of the gearhead from motor position and the reducer gear ratio. Positioning accuracy and precision suffer when backlash increases over time. For example, a 3 arcmin backlash specification (typical in conventional planetary gearheads) may rapidly grow with use to 10 to 15 arc-min. In contrast, the Harmonic Planetary gearheads maintain 3 arc-min (standard), or 1 arc-min (optional) backlash without adjustment for the life of the device.


1 — Arrows indicate a flexspline tooth in the 12 o'clock position relative to the circular spline prior to a clockwise rotation of the wave generator. The wave generator's elliptical shape causes teeth to fully engage along its major axis and fully disengage at the minor axis.
2 — The wave generator rotates 90°clockwise. Note the arrow pointing to the original flexspline tooth has already begun a CCW rotation or displacement. Full-tooth disengagement makes this displacement possible.
3 — As the wave generator completes one revolution clockwise, the flexspline moves CCW two teeth.

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