Planetary gears — A review of basic design criteria and new options for sizing

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.

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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 — ratios, helix angles, axial loads, crowning

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.

Single-stage planetary gearhead ratios range from 3:1 to 10:1. Gear ratios cannot exceed 10:1 because pinion gears can be made only so small. Gear ratios greater than 10:1 are possible with an additional planetary stage, although this normally increases length and cost. Planetary designs also cannot have ratios less than 3:1 because then the pinion and outer ring gear would need to be nearly the same size, leaving no room for the planet gears. Ratios between 4:1 and 8:1 provide the best combination of pinion and planet-gear size, performance, and life.

Crowning involves slightly modifying the gear-tooth profile to improve gear mesh alignment, increasing torque capacity and reducing noise. It also improves load distribution on the tooth flank, thereby minimizing high-stress regions that can cause surface pitting.

Some clearance is needed for a planetary gearhead to work effectively. Clearance prevents excessive heat and gear wear and ensures good lubrication. But the small gap between gear teeth leads to lost motion. Real-world gearheads also cannot have infinite torsional stiffness, so windup (flexing) in the gearhead generates additional lost motion.

Understanding how different manufacturers measure backlash is important when choosing a gearhead. There are no strict standards regulating how to measure backlash. This can lead to confusion and misconceptions. Some manufacturers measure and average four or more points on the output shaft to produce a backlash specification. Using this method, a unit with backlash measurements of 4, 6, 10, and 12 arc-min would have a rating of 8 arc-min. Thomson engineers believe backlash should be based on the largest measurement on the output shaft, so the above example would yield a 12-arc-min rating.

In addition, some manufacturers apply 2% of the rated torque to generate backlash ratings, while others apply less. The latter produces lower backlash measurements and doesn’t provide true backlash ratings over the life of a product.

Backlash will increase over time. A planetary gearhead might have 8 arc-min of backlash out of the box but 15 arc-min after six months of use, for example. So how well a planetary gearhead maintains accuracy over its life is an important consideration for most users.