A worm gear is used when a large speed reduction ratio is required between crossed axis shafts which do not intersect. A worm drive consists of a large diameter worm wheel with a worm screw meshing with teeth on the periphery of the worm wheel. The worm is similar to a screw and the worm wheel is similar to a section of a nut. As the worm is rotated the worm wheel is caused to rotate due to the screw like action of the worm. The size of the worm gear set is generally based on the centre distance between the worm and the worm wheel.
Worm gears provide a normal single reduction range of 5:1 to 100-1 and can go easily up to 127:1 or more under rare circumstances. The pitch line velocity is ideally up to 30 m/s. The efficiency of a worm gear ranges from 98% for the lowest ratios to 20% for the highest ratios. As the frictional heat generation is generally high the worm box is designed disperse heat to the surroundings and lubrication is and essential requirement. Worm gears are quiet in operation. Worm gears at the higher ratios are inherently self locking - the worm can drive the gear but the gear cannot drive the worm. A worm gear can provide a 50:1 speed reduction but not a 1:50 speed increase. In practice a worm should not be used a braking device for safety linked systems.