Worm Gears

Worm Gears

Worm Gears - Another form of gear used to connect shafts whose centre lines do not meet is the worm gear. which differs from all the preceding types of gears in that one element is a screw (worm, W) and the other is the worm gear (WG) with enveloping teeth. Worms and worm gears are used when a large speed reduction ratio is wanted from one shaft to another.

Ratios as high as 70 to 1 can be made in with worm and worm gear, whereas the other types of gears cannot easily give more than about 6 to 1 in one pair. Nearly all worm gears work with shafts at 90°, although they can be made for any shaft angle between a bout 80° and 100°.

The teeth on the worm gear are helical and conform with the helix angle of the tooth on the worm. The helical tooth on the worm is a form of thread, similar to an acme thread. Worms may have single, double or triple threads. One revolution of a double thread worm revolves the worm gear aft amount equal to two teeth on the gear and so on.

The worm gear cannot turn the worm when a single thread worm is used. This type of gear mechanism is self locking and hence is used in steering mechanisms and in hoisting equipments. Worm gearing is used largely for speed reduction. All the gears discussed till now had teeth on their external surface. There are internal gears also in use.