A worm drive is a gear arrangement in which a worm (which is a gear in the form of a screw) meshes with a worm gear (which is similar in appearance to a spur gear, and is also called a worm wheel).
The terminology is often confused by imprecise use of the term worm gear to refer to the worm, the worm gear, or the worm drive as a unit.
Given a single start worm, a 20 tooth worm gear will reduce the speed by the ratio of 20:1.
There are three different types of gears that can be used in a worm drive.
The second are single-throated worm gears, in which the worm wheel is throated.
The final type are double-throated worm gears, which have both gears throated.
Small electric motors are generally high-speed and low-torque; the addition of a worm drive increases the range of applications that it may be suitable for, especially when the worm drive's compactness is considered.
The worm gear carries the differential gearing, which protects the vehicle against rollback.
A more recent exception to this is the Torsen differential, which uses worms and planetary worm gears in place of the bevel gearing of conventional open differentials.
Very heavy trucks, such as those used to carry aggregates, often use a worm gear differential for strength.
The worm drive is not as efficient as a hypoid gear, and such trucks invariably have a very large differential housing, with a correspondingly large volume of gear oil, to absorb and dissipate the heat created.
Plastic worm drives are often used on small battery-operated electric motors, to provide an output with a lower angular velocity (fewer revolutions per minute) than that of the motor, which operates best at a fairly high speed.
A worm drive is used on jubilee-type hose clamps or jubilee clamps; the tightening screw has a worm thread which engages with the slots on the clamp band.
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