Worm and worm gear
A worm gear, or worm wheel, is a type of gear that engages with a worm 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.
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 determined by the size of the worm gear:1.Given a single start worm, a 20 tooth worm gear will reduce the speed by the ratio of 20:1. A typical spur gear of 12 teeth (the smallest size permissible, if designed to good engineering practices) would require 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.
A double bass features worm gears as tuning mechanisms
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 gears are a compact, efficient 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-drives have also 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.
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.
Torsen differentials use worm-gears and planetry worm-wheels in place of the bevel gearing in conventional open differentials.