Worm gears have been used for many centuries. In fact, their existence was described by Archimedes around 250 BC. Worm gear drives are different from all other types of gears. They consist of a spirally grooved screw that engages with and drives a toothed wheel (the worm wheel or gear). By nature of their design worm drives can achieve large reductions in speed in a compact space. It changes the rotational movement by 90 degrees, and the plane of movement also changes due to the position of the worm on the worm wheel as shown on the right.
The input power (usually from an electric motor) is applied to the worm gear. The rotation of the spiral ‘screw’ on the worm pushes the teeth of the wheel forward and rotates it as depicted by the animation on the left. A worm gear set can have a massive reduction ratio with little effort. Worm drives normally consist of a brass or bronze wheel and a steel worm. The wheel is designed to be sacrificial because it is normally cheaper and easier to replace than the worm itself.
Worm gear drives have several advantages over other gear types. The two primary ones are:
HIGH REDUCTION RATIO. Worm gears can achieve reduction ratios ranging from 20:1 to 300:1. You can use it to reduce speed significantly and at the same time increase torque considerably. It will take multiple reductions of a conventional gearset to achieve the same reduction level of a single worm gear set. Worm gears therefore have fewer moving parts and less chances of failure.
SELF-LOCKING. With standard gears the output shaft can also turn the input shaft. This necessitates adding a backstop to a traditional gearbox (to prevent reverse rotation) which increases the complexity of the gear set. With worm drives it is practically impossible to reverse the direction of power since it is unlikely that the worm wheel will rotate the worm. The compact size and inability of worm gears to reverse the direction of power (self-locking) render them particularly suitable for many lift/elevator and escalator drive applications. The least complicated form of worm gears is those that are used to tune stringed musical instruments.
There is one particularly obvious reason why one would not choose a worm gear drive over a standard gear set and that is sliding friction. With standard gear types (spur, bevel, spiral bevel, helical and hypoid) the gear teeth slide and roll on each other when they mesh. The rolling action helps to distribute the lubricant on the teeth of the gears. With worm gears the contact is predominantly of a sliding nature. As the worm slides across the teeth of the wheel, it rubs most of the lubricant off. The result is that the worm is in contact with the wheel in a boundary lubrication regime (see OilChat 22).
The high rate of sliding in worm gears results in significant frictional heat build-up. This demands the use of high viscosity lubricants, typically ISO 460 or 680 viscosity grade and even ISO 1000 in severe applications. Traditionally compounded gear oils have been used extensively in worm gears with great success in a wide variety of applications. These are mineral oils with rust and oxidation inhibitors and tallow or synthetic fatty acid (the compounding agent), giving excellent lubricity to minimize sliding wear.
In days gone by gear oils that contain active extreme pressure (EP) additives were not recommended for worm gears. There was a concern that the sulphur-phosphorous EP additives would react with the brass or bronze gear wheels. Modern non-active sulphur EP additive technology, however, has eliminated corrosive attack of the worm wheel. EP gear oils work particularly well when shock loading occurs and also protect steel gears better than compounded gear oils.
Q8Oils offers a comprehensive range of high-quality gear lubricants for a wide variety of automotive, construction, industrial, mining, agricultural and other applications. For more information about the complete range of Q8 lubricants, phone 011 462 1829, email us at info@bcl.co.za or visit www.bcl.q8oils.co.za.