Definition
A gear set consisting of a screw-like shaft (the worm) that meshes with a toothed wheel (the worm wheel) at a 90-degree angle. The worm rotates to drive the wheel, producing a large speed reduction and high torque output in a compact arrangement. In most configurations the worm can drive the wheel, but the wheel cannot drive the worm — making the assembly self-locking.
Plain English
A pair of gears where a spiral-grooved shaft turns a round gear sitting at right angles to it. Each full turn of the shaft moves the round gear by only one tooth, so the output spins much slower but with much more turning force.
Context Anchor
Seen in aircraft maintenance when inspecting or describing gear drives in engine accessories, control drives, or other mechanical systems that need slow, controlled movement.
Derivation
The name comes from the appearance of the driving shaft — its continuous helical thread looks like a worm wrapped around a rod. The term has been used in mechanical engineering since the 1700s.
Why Pilots Care
Worm gears deliver precise, high-torque motion with resistance to back-driving, supporting reliable control of variable-pitch propellers and flight-control actuators.
Analogy
Think of a screw turning into a nut. Each full turn of the screw advances the nut by only one thread. A worm gear works the same way, except the 'nut' is a toothed wheel that rotates instead of moving sideways.
Intuition Check
A worm gear is not a soft or flexible part shaped like an animal worm. In aviation maintenance, the “worm” is a hard screw-shaped gear that drives another gear.
Example Sentence 1
The elevator trim system uses a worm gear so the trim setting stays put when the pilot releases the trim wheel.
Example Sentence 2
The flap actuator uses a worm gear to produce the torque needed to extend the flaps at low airspeeds.