Worm gear

From Freepedia

Image:Worm-WormWheel-gearbox.jpg

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 gearbox designed using a worm and worm-wheel will be considerably smaller than one made from plain spur gears, it will also will have its drive axes at 90 ° to each other. With a single start worm, for each complete turn of the worm the worm-gear advances only one tooth of the gear, so regardless of the worms size (sensible engineering limits not withstanding), 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 typically 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 the worm arrangement is considerably smaller in volume. (by comparing the physical size of the 240 tooth gear to that of the 20 tooth gear)

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 vehcile against rollback but has largely fallen from favour due to their higher than neccesary reduction ratios.

Torsen differentials use worm-gears and planetry worm-wheels in place of the bevel gearing in conventional open differentials.