Mechanical
Worm Gear
Screw-shaped driver giving very high reduction ratios in compact packages
A worm gear is a pair consisting of a screw-like worm meshing with a helical gear (the worm wheel). Each rotation of the worm advances the wheel by one tooth, so a single-start worm produces a reduction ratio equal to the number of teeth on the wheel—commonly 20:1 to 100:1 in one stage. The geometry is also self-locking under most conditions: the wheel cannot back-drive the worm. This combination of high ratio, compactness, and lock-up makes worm gears ideal for hoists, conveyors, and steering systems. The downside is poor efficiency (40-90%) due to sliding contact, which becomes heat.
- Ratio per pair20:1 to 100:1 typical
- Self-lockingWhen friction angle > lead angle
- Efficiency40-90%
- ContactSliding (not rolling)
- Thrust loadHigh axial force on worm
- Wheel materialOften bronze for worm-steel
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Why worm gears matter
- Hoists and winches. High reduction with self-locking holding load.
- Conveyors. Compact gearmotors driving belts and rollers.
- Elevators. Older traction designs used worm reducers.
- Steering. Recirculating-ball boxes in trucks and older cars.
- Music boxes and clocks. High ratio in tiny spaces.
- Machine-tool indexing. Duplex worms for backlash-free positioning.
- Aircraft trim systems. Self-locking holds control surfaces.
Common misconceptions
- Always self-locking. Multi-start, low-friction designs can be back-driven.
- Lossless under load. Sliding contact dissipates heat; efficiency drops with ratio.
- One pair handles any torque. Heat and wheel-tooth wear cap continuous capacity.
- Bronze is purely cosmetic. Bronze conforms and runs cool, vital for sliding contact.
- No axial load. Worms see large thrust forces requiring stout thrust bearings.
- Replaceable like spur gears. Worm and wheel must be matched as a pair, broken in together.
Frequently asked questions
How does it work?
The worm is essentially a screw with helical threads. Its threads mesh with the curved teeth of a worm wheel mounted on the perpendicular output shaft. As the worm rotates, each thread engages successive wheel teeth, advancing the wheel one tooth per worm revolution (for a single-start worm). Multi-start worms have several parallel threads that increase output speed and decrease the ratio.
Why is it self-locking?
Below a critical lead angle, friction in the contact prevents the wheel from back-driving the worm. The wheel can't rotate the worm because the friction force opposes motion is greater than the component of force trying to rotate the worm. Self-locking holds when lead angle is less than the friction angle. Single-start worms are usually self-locking; multi-start may not be.
Where is self-locking useful?
Hoists, lifts, jacks—anywhere a load must hold position when power is removed. Stage rigging, scissor lifts, and old-style automotive recirculating-ball steering all rely on worm self-lock to prevent runaway. Not all applications want it: when back-driveability matters (efficient regenerative braking, for example), worms are the wrong choice.
Why is efficiency lower than other gears?
Sliding contact. Spur and helical gears roll teeth past each other, but worm teeth slide along worm wheel teeth. Sliding generates heat through friction. Efficiency drops with ratio: a 100:1 worm pair may be only 50% efficient; a 10:1 may reach 90%. The lost energy heats the gear oil and limits continuous power.
Why bronze for the wheel?
A bronze wheel against a hardened-steel worm runs cooler, withstands higher sliding velocity, and survives marginal lubrication better than steel-on-steel. Bronze conforms slightly under load, distributing pressure across more teeth. The trade-off is wear: bronze wheels are sacrificial and replaced periodically. Phosphor bronze and aluminum bronze are common alloys.
How is heat managed?
Splash lubrication in oil-filled gearboxes carries heat to the housing, where it dissipates to ambient air. Large worm reducers may have cooling fans or oil coolers. Synthetic gear oils (PAO, PAG) handle higher temperatures than mineral oils. Service factor in gearbox catalogs derates capacity based on duty cycle, ambient temperature, and shock loading to keep oil under safe limits.
What's a duplex worm?
A worm with slightly different lead angles on each flank, creating a tooth thickness that varies along its length. By shifting the worm axially, the gearbox manufacturer eliminates backlash without adjusting center distance. Duplex worms are used in precision indexing applications—machine-tool dividing heads, telescope mounts—where backlash must be near zero.