An inside look at planetary gears

Planetary gearheads are high-precision, motion-control devices that generate substantial torque for their size, have high torsional stiffness, and low backlash — making them suited for wide-ranging tasks. For instance, specific types of planetary gearheads:
• Run around the clock, seven days a week, for more than 30,000 hr in cartoning applications. The lubricated-for-life gearheads require no maintenance, and high torque-to-size ratios permit compact envelopes and small machine footprints.
• Help attain accuracy within a few ten-thousandths of an inch on plasma-cutting machines, thanks to exceptionally low backlash. Helical crowned gearing provides fast positioning and smooth movement, and sealed gearboxes keep out abrasive dust generated during cutting.
• Limit noise and vibration and meet strict backlash requirements in scanning tables for cardiovascular patients.
• Let food-processing equipment slice meat, bread, and frozen foods at speeds up to four slices/sec. Also available are slim, right-angle designs to fit within the machine envelope and provides quiet, smooth operation.

Planetary basics
A planetary gearhead takes a high-speed, low-torque input, say from an electric motor, then increases torque and reduces speed at the output by the gearhead ratio. This lets motors run at higher, more-efficient rpms in equipment that operates at low speeds. It also reduces inertia reflected back to the motor, increasing stability. And using a planetary gearhead often lets machine builders reduce the size and cost of motion-control hardware.

Planetary units with helical gears, rather than spur gears, have a larger contact ratio. The contact ratio is the number of teeth in mesh at any given moment. While typical spur gearing has a 1.5 contact ratio, helical gearing more than doubles it to 3.3. Benefits of higher contact ratios include:

• 30 to 50% more torque capacity than equivalent spur-type planetary gearing.
• Better load sharing, which increases life.
• Smoother and quieter operation.
• Backlash reduced by as much as 2 arc-min.

The gearhead’s helix angle also has a significant impact on performance because the greater the angle, the more teeth in the mesh at any one time. So increasing the helix angle from the typical 12° up to 15° raises torque capacity by 17 to 20%; and by as much as 40% over straight-cut spur gears. Gears with a 15° helix angle also emit less noise.

Helical-gear teeth generate axial loads on the motor shaft. Gearhead bearings must compensate for these loads. Helical gearheads using ball bearings with little or no axial load capabilities can suffer premature motor-bearing or gear failure. A better approach uses tapered roller bearings, such as in Micron Helical gearheads, to completely compensate for axial loads.