Definition
A gear arrangement in which one or more small gears (called planet gears) rotate around a central gear (called the sun gear) while also meshing with an outer ring gear that has internal teeth. The planet gears are held in position by a carrier, and the system allows large speed reductions and torque changes within a compact, coaxial package. Planetary gear sets are commonly used in turboprop and turboshaft engine reduction gearboxes and in some piston engine propeller reduction systems.
Plain English
A set of gears arranged like a tiny solar system: a center gear, several smaller gears that circle around it, and an outer ring with teeth on the inside. This layout lets engineers slow down a fast-spinning shaft or speed up a slow one while keeping everything lined up on the same axis.
Context Anchor
Seen in powerplant maintenance discussions about gear reduction systems, accessory drives, and engine assemblies that must transfer power while changing rotational speed.
Derivation
The name comes from the way the smaller gears travel around the central gear, much like planets orbit the sun. Calling them 'planetary' helps you picture the motion: small gears circling a central one rather than sitting in a straight line.
Why Pilots Care
Allows a high-RPM engine to drive a slower-turning propeller for better efficiency and reduced noise without losing power.
Intuition Check
“Planetary” does not mean the gears are related to space or navigation. Here it describes the motion of small gears traveling around a central gear.
Example Sentence 1
The PT6 turboprop uses a planetary gear reduction box to bring the power turbine's high RPM down to a usable propeller speed.
Example Sentence 2
During cruise the planetary gears let the engine run at 2500 RPM while the propeller turns more slowly for efficiency.