<P> An enveloping (hourglass) worm has one or more teeth and increases in diameter from its middle portion toward both ends . </P> <P> Double - enveloping wormgearing comprises enveloping worms mated with fully enveloping wormgears . It is also known as globoidal wormgearing . </P> <P> Unlike with ordinary gear trains, the direction of transmission (input shaft vs output shaft) is not reversible when using large reduction ratios, due to the greater friction involved between the worm and worm - wheel, when usually 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) is used 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 . </P> <P> Worm gear configurations in which the gear cannot drive the worm are called self - locking . Whether a worm and gear is self - locking depends on the lead angle, the pressure angle, and the coefficient of friction . </P>

Where can a worm and wheel be found