A worm drive is a gear arrangement in which a worm (which is a gear in the form of a screw) meshes with a worm wheel (similar to a spur gear). The two elements are also called the worm screw and worm gear. The terminology is often confused by the imprecise use of worm gear to refer to the worm, the worm wheel, or the worm drive as a unit.
The worm drive or “endless screw” was invented by either Archytas of Tarentum, Apollonius of Perga, or Archimedes, the last one being the most probable author. The worm drive later appeared in the Indian subcontinent, for use in roller cotton gins, during the Delhi Sultanate in the thirteenth or fourteenth centuries.
Like other gear arrangements, a worm drive can reduce rotational speed or transmit higher torque. A worm is an example of a screw, one of the six simple machines. One of the significant advantages of worm drive units is that they can transfer motion in 90 degrees. The worm in the worm drive may have single or multiple starts. Each complete 360-degree turn of a single-start worm advances the wheel by one tooth. For a multi-start worm, the gear reduction equals the number of teeth on the wheel divided by the number of stars on the worm. Both the sliding and the rolling actions of the worm and the wheel come into play during the meshing of the gears. The sliding contact dominates at high reduction ratios. Much heat is produced due to friction while sliding, which limits the efficiency of worm drives to 30 to 90 percent. The worm and the wheel are made of dissimilar metals to minimize friction and loss inefficiency.