The primary good thing about worm gears is their ability to provide high reduction ratios and correspondingly high torque multiplication. They can also be applied as speed reducers in low- to medium-velocity applications. And, because their decrease ratio is based on the amount of gear teeth alone, they are more compact than other styles of gears. Like fine-pitch business lead screws, worm gears are usually self-locking, which makes them suitable for hoisting and lifting applications.
Although the sliding contact minimizes efficiency, it provides very quiet operation. (The use of dissimilar metals for the worm and gear also contributes to quiet procedure.) This makes worm gears well suited for use where noises should be minimized, such as in elevators. In addition, the use of a softer material for the gear means that it can absorb shock loads, like those skilled in weighty equipment or crushing equipment.
The meshing of the worm and the apparatus is a mixture of sliding and rolling actions, but sliding contact dominates at high reduction ratios. This sliding action causes friction and heat, which limits the productivity of worm gears to 30 to 50 percent. So as to minimize friction (and for that reason, temperature), the worm and equipment are made from dissimilar metals – for example, the worm may be made of hardened metal and the gear made of bronze or aluminum.
Like a ball screw, the worm in a worm gear may well have an individual start or multiple starts – and therefore there are multiple threads, or helicies, on the worm. For a single-start worm, each full turn (360 degrees) of the worm advances the equipment by one tooth. Thus a gear with 24 teeth will provide a gear reduction of 24:1. For a multi-begin worm, the apparatus reduction equals the quantity of teeth on the gear, divided by the number of starts on the worm. (That is different from almost every other types of gears, where the gear reduction is usually a function of the diameters of both components.)
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