Definition
A small, semicircular metal key that fits into a matching half-moon slot cut into a shaft, used to lock a mating part such as a gear, pulley, or magneto drive coupling onto the shaft so the two rotate together without slipping.
Plain English
A little half-moon piece of metal that drops into a matching slot on a shaft. It stops a wheel or gear from spinning loose on the shaft by keying the two pieces together.
Context Anchor
Seen during aircraft maintenance where gears, pulleys, or other rotating parts are mounted on shafts in engines or accessories.
Derivation
Named after the Woodruff Manufacturing Company, which standardized this half-moon key design in the late 1800s. The shape is what makes it distinctive — flat on top, curved on the bottom — so the term itself doesn't tell you that, which is why the origin is worth knowing.
Why Pilots Care
Secures critical rotating parts such as propeller hubs and engine gears, preventing slippage that could lead to loss of thrust or engine damage.
Analogy
It is like a small metal tab that fits into two matching notches, forcing both pieces to move together.
Intuition Check
Do not think of “key” as an ignition key or a door key here. In this context, a key is a small metal locking piece used between rotating machine parts.
Example Sentence 1
The mechanic replaced the sheared Woodruff key on the magneto drive shaft before re-timing the engine.
Example Sentence 2
A worn Woodruff key allowed slight movement between the shaft and hub during engine run-up.