How a worm gear makes a huge reduction

A worm is a screw that meshes with a toothed wheel at a right angle. Each full turn of a single-thread worm pushes the wheel forward by just one tooth, so a 40 tooth wheel needs 40 worm turns for one revolution. Because the thread meets the teeth at a shallow angle, friction can stop the wheel from driving the worm backward, which lets winches and gates hold a load without a brake.

Controls

Labels

Animation speed

Worm thread starts

Number of separate threads wound around the worm.

Wheel teeth

Motor speed

Step through it

The wormThe worm is a screw on the motor shaft. It spins fast and its thread slides across the wheel teeth.
The wheelThe wheel's teeth are angled to match the thread. A single thread pushes it along one tooth per worm revolution.
More startsA worm with two or more interleaved threads moves the wheel that many teeth per turn. The ratio drops and the thread angle steepens.
Holding the loadWith one start the thread is so shallow that the load on the drum cannot spin the worm backward. Engineers still add a brake where safety matters.

Ratio and lead angle

The reduction ratio of a worm set is simply wheel teeth divided by worm starts. A 40 tooth wheel with a single start worm gives 40:1 in one compact stage, where ordinary spur gears would need two or three stages.

The lead angle is how steeply the thread climbs. Here it is estimated as atan(starts / 10), assuming a worm whose pitch diameter is ten times the gear module, a common proportion. Small lead angles mean more sliding, more friction, and lower efficiency.

Try it: start with one thread and 40 teeth, then step the starts up to 4. The wheel speeds up four times and the lead angle grows from about 6 to about 22 degrees, so the readout warns the set will back drive.

Self locking is a tendency, not a promise

When the friction angle between worm and wheel is larger than the lead angle, torque on the wheel cannot turn the worm. Engineering references caution that vibration and lubrication can break this hold, so a set with a lead angle under about 5 degrees is only likely to lock. The drawing keeps one wheel size and thread pitch for clarity.

Sources

The Design of Worm Gear Sets (N87-29871)NASA Technical Reports Server
EML2322L: Gears and gearingUniversity of Florida, Mechanical and Aerospace Engineering Design Lab
Gearing (NASA Reference Publication 1152)NASA Technical Reports Server

How to cite this model

Avid Solutions International. (2026, November 11). How a worm gear makes a huge reduction [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/worm-gear

Last updated November 11, 2026

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