Worm gears are used when large gear reductions are needed. The set-up allows the user to determine rotational speed and also allows for higher torque to be transmitted.
Many worm gears have an interesting property that no other gear set has: the worm can easily turn the gear, but the gear cannot turn the worm. This is because the worm angle is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place.
The three different types of worm gear are:
- No throat – a straight worm without a groove machined around the circumference. A single moving point provides tooth contact that makes it subject to high unit load wear and tear.
- Single throat – concave helical teeth wrapped around the worm for line contact, meaning higher unit loads with less excess wear.
- Double throat – usually called a cone or hourglass, this type has concave teeth on both the worm screw and the gear to help increase the contact area and allow for increased unit loads with lower wear and tear.
How They Work
An electric motor applies rotational power to the worm, the worm rotates against the wheel, and the screw face pushes on the teeth of the wheel. The wheel is pushed against the load. The worm (screw) continuously rotates and drives the worm wheel.
In a worm gear drive, power is transmitted from screw to wheel but never transmitted from wheel to screw (worm). This is called self-locking and is useful in many applications.
Velocity ratio is determined by the number of worm gear teeth and the number of starts. Power transmission decreases with increased velocity ratio.
Note: A screw (worm) is said to have one start if it advances one groove (in linear direction), in one complete revolution. It is said to have two starts if it advances.
Choosing a worm gear over a standard gear
A worm gear can have a massive reduction ratio with little effort by adding circumference to the wheel. It can be used to either greatly increase torque or greatly reduce speed. It takes a conventional gear set multiple reductions to achieve the same reduction level of a single worm gear. This means that worm gear users have fewer moving parts and fewer places for failure.
Because of the friction between the worm and the wheel, it is difficult for a wheel with force applied to it to start the worm moving. On a standard gear, the input and output can be turned independently once enough force is applied. This necessitates adding a backstop to a standard gearbox, further increasing the complication of the gear set.
Although a worm gear will always have a few complications compared to a standard gear set, it can be an effective and reliable piece of equipment. With a little attention to setup and lubricant selection, worm gears can provide reliable service.