Gearmotors: Understanding Planetary Gears

Sinotech    February 4, 2013

Reversible AC Gearmotors- Heavy Duty 60 Watt 100mm

Reversible AC Gearmotors- Heavy Duty
60 Watt 100mm

Also called an epicyclic gear, a planetary gear is a system containing one or more outer (planet) gears that revolve a central (sun) gear. The outer gears are mounted onto a carrier (moveable arm) that rotates in a manner that’s relative to the sun gear. There are two types of planetary gears: simple and compound. The simple gear has one sun with a single ring, a single set of planets and one carrier – one example would be a manual pencil sharpener that uses a crank. Compound gears have a meshed-planet, stepped-planet or multi-stage structure – for example, bicycle gears.

Gear Ratios

In most planetary gear systems, the sun, planet carrier or annulus (outer ring with inward-facing teeth) is stationary while another component powers the gears. The input and output ratios depend upon the number of teeth each gear contains, as well as the component that’s stationary.

In a reversible gearmotor or a gearmotor with brake, the motor may use helical, worm or spur gears to control the speed and direction of the system. A gearmotor with encoder that’s used with other sensing mechanisms measures the movements of gears to tell you the position of a wheel or the distance it has traveled.

Compound Gears

Meshed-planet gears use two or more outer gears that are meshed with each other. In stepped-planet gears, there’s a shaft connected between two planet gears. A multi-stage structure has a gear system with two or more planet sets. The gears in compound systems must move in relation to one another so the teeth engage the annulus and sun at the opposite ends of the planet.  Consequently, compound planetary gears offer more flexible configurations, larger reduction ratios and a higher torque-to-weight ratio.

Planetary gears offer enhanced stability and load distribution. Plus, the torque density and load ability increase with the number of planets in a system. At Sinotech, we offer a vast array of gearmotors and encoders for use in a variety of industries – from medical equipment to RC airplanes to heavy machinery.



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