What are the Basics of Worm-Gear Design?

Jeremy Laukkonen

The worm-gear design involves one helical gear, the teeth of which form segments of a helix, and another component that resembles a screw. The screw-like component is known as a worm, and its threads typically form a complete helix. This worm is typically used to drive the gear, which can provide a great deal of torque. In most cases, attempting to use the gear to turn the worm will result in the teeth locking. Both of these basics of worm gear-design can be exploited in the tuning knobs of stringed instruments, which provide the torque necessary to pull the strings taught and then automatically lock in place to keep them that way.

Helical gears resemble spur gears, but they have diagonally arranged teeth. Due to the curved nature of gears, this results in each individual tooth resembling a segment cut out of a helix. Traditional helix sets use two gears, though the worm-gear design is a variant that does not. Instead, these use one helical gear and a cylinder or rod that has a screw-like thread on it. This worm can have a single tooth that wraps around the circumference only once, two teeth that span the entire length like a normal screw, or anything in between.

The main benefit of worm-gear design is that a large amount of torque can be achieved compared to other helical gears. Gear ratios for helical sets are usually limited to about 10:1, and worm-gear sets can achieve up to a 500:1 ratio. Due to the nature of worm-gear design, each full rotation of the worm only moves the gear forward one tooth, so the gear reduction is technically limited only by how many teeth the driven gear has. A driven gear with 12 teeth would result in a 12:1 ratio, one with 120 teeth would have a 120:1 ratio, and so on.

Another basic of worm-gear design is the way that the sets tend to self-lock. Many worm-gear sets have teeth on the gear that have a very small lead angle, which means they are close to being 90 degree parallel teeth, such as can be found in on normal spar gears, instead of the sharply angled diagonal teeth used by helical gears. If the gear has a small lead angle, then the gear will tend to lock against the teeth of the worm instead of turning it. In some cases, this is a desirable outcome that can be exploited to the benefit of a device. This locking action can prevent undesired movement of the system due to external forces on the gear, such as a tensioned string or gravity.