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A 'worm gear', or 'worm wheel' or 'worm drive', is a type of gear that consists of a cylinder with a spiral groove mounted on a shaft. It is used to greatly reduce rotational speed, or to allow higher torque to be transmitted. The image shows a section of a gear box with a bronze worm gear being driven by a worm. A worm gear is an example of a screw, one of the six simple machines.

Contents

Explanation

Direction of transmission

Applications

Notes

Explanation

A gearbox designed using a worm and worm-wheel will be considerably smaller than one made from plain spur gears and has its drive axes at 90° to each other. With a ''single start'' worm, for each 360° turn of the worm, the worm-gear advances only one tooth of the gear. Therefore, regardless of the worm's size (sensible engineering limits notwithstanding), the gear ratio is the ''"size of the worm gear - to - 1"''.
Given a single start worm, a 20 tooth worm gear will reduce the speed by the ratio of 20:1. With spur gears, a gear of 12 teeth (the smallest size permissible, if designed to good engineering practices) would have to be matched with a 240 tooth gear to achieve the same ratio of 20:1. Therefore, if the diametrical pitch (DP) of each gear was the same, then, in terms of the physical size of the 240 tooth gear to that of the 20 tooth gear, the worm arrangement is considerably smaller in volume.

Direction of transmission

Unlike ordinary gear trains, the direction of transmission (input shaft vs output shaft) is not reversible, due to the greater friction involved between the worm and worm-wheel, when a single start (one spiral) worm is used. This can be an advantage when it is desired to eliminate any possibility of the output driving the input. If a multistart worm (multiple spirals) then the ratio reduces accordingly and the ''braking effect'' of a worm and worm-gear may need to be discounted as the gear may be able to drive the worm.
Worm drives where the gear can not drive the worm are said to be ''self-locking''. Whether a worm and gear will be self-locking depends on a function of the lead angle, the pressure angle, and the coefficient of friction; however it is approximately correct to say that a worm and gear will be self-locking if the tangent of the lead angle is less than the coefficient of friction.
An enveloping (hourglass) worm has one or more teeth and increases in diameter from its middle portion toward both ends. ¹
Double-enveloping wormgearing comprises enveloping worms mated with fully enveloping wormgears. It is also known as globoidal wormgearing. ²

Applications

Worm gears are a compact, efficient [efficient only in terms of volume; heat friction issues cause vast inefficiencies ranging up to 50%] means of substantially decreasing speed and increasing torque. Small electric motors are generally high speed and low torque, the addition of a worm and worm-wheel increases the range of applications that it may be suitable for, especially when the worm gears compactness is considered.
Worm gears are used as power gearings in presses, in rolling mills, in the conveying engineering, in mining industry machines and rudder machines. In addition, milling heads and rotary tables are positioned, using the high precision duplex worm gears with adjustable backlash.
In the era of sailing ships the introduction of a worm gear drive to control the rudder was a significant advance. Prior to its introduction, a rope drum drive was used to control the rudder and rough seas could cause substantial force to be applied to the rudder, often requiring several men to steer the vessel, with some drives having two large diameter wheels to allow up to four crewmen to operate the rudder.

Worm-drives have been used in a few automotive differentials. The worm-wheel carries the differential gearing. This protects the vehicle against rollback. This has largely fallen from favour due to the higher than necessary reduction ratios. The exception to this is the Torsen differential, which uses worm-gears and planetary worm-wheels in place of the bevel gearing in conventional open differentials. Torsen differentials are most prominently featured in the HMMWV and some commercial Hummer vehicles, and as a center differential in some all wheel drive systems such as the Audi quattro. Very heavy trucks, such as those used to carry aggregates, often use a worm gear differential for strength. The worm gear 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.
Worm gears are used as the tuning mechanism for many musical instruments including guitars, double-basses, mandolins and bouzoukis, although not banjos which use planetary gears or friction pegs. A worm-gear tuning device is called a machine head.
Plastic worm gears 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. This motor-worm-gear drive system is often used in toys and other small electrical devices.
A worm drive is used on jubilee type hose clamps or jubilee clamps, the tightening screw has a worm thread which engages with a slots on the clamp band.

Notes

1. ''AGMA Standards'', "Gear Nomenclature, Definition of Terms with Symbols", p. 3.

2. ''AGMA Standards'', "Gear Nomenclature, Definition of Terms with Symbols", p. 4.