What is a Worm Drive?

B. Nolta

In the world of manufacturing and machinery, a worm drive is a geared arrangement that is generally used to either change the rotational speed of a larger mechanism or modify the amount of torque applied. The arrangement gets its name from the worm gear, a cylindrical gear in the shape of a screw. When the worm gear turns, the threads of the screw rotate and apply motive force to a gear wheel, which in turn rotates and transmits power to the larger mechanism.

The tuning heads on guitars are worm gears.
The tuning heads on guitars are worm gears.

The term “worm drive” can be a confusing one, as it is common to use the term when talking about only the worm gear itself, even though the definition covers the entire gear system. To further complicate matters, “WORM drive” was an early term for drives that burned data to CD-R discs. WORM stood for Write Once, Read Many in that context. This usage of the term, however, has fallen into disuse, so most contemporary usages of the term refer to the gear system.

Worm drives are commonly used due to their relative small size and simplicity. Since the rotational force of the worm gear and the gear wheel mesh at a 90° angle, the gears can be placed in line with, or on top of, each other. This allows for a more compact arrangement of the gears. Since the gear ratio depends on the length of the screw and number of teeth along the shaft of the worm gear, the ratio can be readily modified while maintaining a small volume. In addition, a basic worm gear usually will only turn in one direction, which means the worm drive is self-locking unless specifically engineered to be otherwise, either through precision engineering of the gear meshing angles and surfaces or through the addition of additional worm gears. Finally, due to their simplicity, worm drives tend to be reliable, long-lasting and relatively easy to maintain.

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Worm drives and gears are found in a wide variety of applications. The tuning heads on guitars, violins and many other stringed instruments, for example, are worm gears. Many hose clamps are also examples of worm gears; the twisting of the head spins the worm gear, which in turn causes an aperture to tighten and thus apply pressure. Other common examples of worm gears include electric motors in toys, rudders on ships, automotive differential systems, as well as lifting mechanisms such as elevators.

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