Planetary gear reducer

01

Set the ratio and number of stages

Neugart's explanation of epicyclic gearing describes four parts: the case, internally toothed as the ring gear ("In most cases, the case is designed to be stationary."); a sun gear in the centre ("The driving forces of the motor shaft usually act directly on the sun gear."); planet gears running between them ("These are usually three, in our example four gears"); and a planet carrier ("In most planetary gearboxes, the planet carrier is also the output shaft of the gearbox."). The tooth numbers of sun and ring gear set the ratio, the planets' tooth number does not, and the smaller the sun gear the larger the ratio. Technically sensible single-stage ratios are about 3:1 to 10:1; below and above these values, Neugart says, "the planet gears or the sun gear usually simply become too small", so further stages are added. [1]

For multi-stage gearboxes, Neugart states that the stage ratios are multiplied, that multi-stage planetary gearboxes keep the same direction of rotation, and that two stages combining single-stage ratios of 3 to 10 give 9 (3 × 3) to 100 (10 × 10). It adds that efficiency falls as stages are added, that single-stage planetary gearboxes have the best efficiency, and that a two-stage ratio of 20 is more efficient than one of 100. [2] Its PLE economy series lists ratios of 3 to 10 in one stage, 9 to 64 in two stages and 60 to 512 in three stages, with efficiency stated as 92–98%. [3]

Editorial calculation: a servo motor at 3,000 rpm and 3 Nm driving a two-stage 4 × 5 = 20:1 planetary reducer gives an output speed of 3,000 / 20 = 150 rpm and, at an assumed 94% efficiency (inside the 92–98% PLE range), an output torque of about 3 × 20 × 0.94 ≈ 56 Nm. Motor data and efficiency are assumed examples; check the reducer's rated and peak output torque against your load cycle.

Give the supplier the motor (type, rated and peak torque, speed, shaft and flange), required ratio, output torque for the duty cycle including acceleration and emergency stop, output speed, backlash and torsional stiffness required, radial and axial loads and their position on the output, mounting position, ambient temperature, protection class, lubrication and service life expectations.

02

Check backlash, stiffness and output shaft loads

Neugart's PLE data table illustrates the precision figures to compare: standard backlash from under 15–22 arcmin for the smallest size (PLE 040) down to under 6–9 arcmin for the largest (PLE 160), torsional stiffness from 0.7–1.0 to 57.5–72.0 Nm/arcmin across the same sizes, an operating range of −25 to 90 °C, IP54 protection and running noise of 58 to 70 dB(A). [3]

The same table lists output shaft limits that rise with frame size: maximum radial force from 200 N to 5,000 N, maximum axial force from 240 N to 11,000 N and maximum tilting moment from 6 Nm to 474 Nm, from PLE 040 to PLE 160. [3]

Editorial calculation: 10 arcmin of backlash at the output lets an arm of 100 mm radius move about 100 × tan(10/60°) ≈ 0.29 mm at its tip before the gears take up the load. Arm length and backlash are assumed examples; for positioning duties compare backlash and stiffness at the output radius that matters to your machine.

03

Compare what suppliers offer

Suppliers linked to this page list planetary gear reducers; each card links to the page reviewed.

Listings include industrial planetary gear reducers and modular planetary gearboxes, precision servo planetary gearboxes, micro planetary gear motors, and planetary gearboxes offered alongside harmonic drives, actuators or blowers. Ask each for ratio, rated and peak torque, backlash, stiffness, shaft loads and efficiency by stage.

04

Name the fixed member and check the tooth-count conditions

KHK's gear technical reference describes the basic 2K-H planetary system: sun gear A, planet gears B, internal gear C and carrier D, with sun, internal gear and carrier on one axis. Tooth numbers must meet three conditions: zc = za + 2zb so the centre distances match (true for standard gears; profile shifted gears allow other counts), a condition for spacing the planets evenly around the sun, and a condition that adjacent planets do not interfere. KHK names load division between planets, internal gear interference, carrier balance and jamming as inherent problems. [4]

With the carrier fixed, KHK notes, the planets are merely idlers and the train becomes an ordinary gear train. An internal and an external gear with a small difference in tooth numbers (a hypocycloid mechanism) can give a single-step reduction such as 1/100; with 30 and 31 teeth the ratio is 30. [4]

Editorial calculation: the example counts meet the first condition (16 + 2 × 16 = 48) and give a planetary-type ratio of 1 + zc/za = 1 + 48/16 = 4. Ask which member is fixed, the tooth counts and planets per stage, and how load sharing is controlled. [4]

05

Trace the reducer components

This is a functional reference built from the sources cited; it is not one maker's specification.

06

Fill the specification fields with values for your drive

A planetary gear reducer quotation needs values for your drive. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the motor, load and mounting data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Motor: [type; torque; speed; flange; shaft]
Ratio: [required; stages]
Load: [output torque cycle; peak; emergency stop]
Precision: [backlash; stiffness]
Output: [shaft or flange; radial and axial loads]
Environment: [temperature; IP; mounting position]

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