Gearboxes and gear-heads are the other name for speed reducers. Speed reducers have a combination of gears of different sizes along with shafts and bearings, and are assembled in a metallic housing. These gearboxes are used in such application where the input speed is converted to a known output with reduced RPM and increased torque.
Worm gears are required in applications where drastic speed reduction is required. In this type of speed reducers, longer cylindrical worms, which look like screws, are meshed with flatter disk like worm gears, which are known as worm wheels. In such gearboxes the worm is usually the driver of the worm wheel, which is connected to the axis of the machine tool. In whichever application a worm gear is used, the worm should never be used as a braking mechanism, holdbacks, and conventional braking system. The axis of the worm wheel is at right angle to that of the worm. Both of these are hobbed in order to mate. The direction of the rotation is decided by how the teeth are cut. Usually the worm is thread-milled, where-as the teeth of the worm wheel are cut with a hob and mated with the worm.
Worm gears are commonly right-angle speed reducers. This speed reducer induces a large speed reduction between the input and output shaft, which is usually over 3, but at times the ratio is as large as 300:1. Smaller worm gears are also capable of meeting these large speed reductions, with high torque output. The gearboxes, which are designed with large shafts, tougher materials for the worm and the wheel, and one-stage gearing also provide a large speed-reducing ratio. Modern worm gear designs utilize multiple gear combinations, achieving the same ratio but with a higher efficiency output to about 97%. The single reduction speed reducer drives are less efficient, having an output up to 60%.
In some of the designs, the teeth of the worm or the worm wheel, or both, are curved. This allows the two to have increased meshing between the teeth. This type of worm gear-sets is preferred in most of the applications, and is called enveloping or globoid gears. The features of double-globoid gear-sets have a curved top wheel teeth, with hyperboloid worm teeth which provides greater yield in form of higher capacity, resulting from meshing of several gear teeth at once producing multiple transmitted torque.
The tooth formation in a worm gear does not follow the standardized design as in other gear systems. Amongst the different worm tooth shapes, the one that is called involutes is most common. This type of tooth is easily manufactured, and is widely available, with detail description available in AGMA and DIN standards.
The worm wheel is yet to be optimized, and even after three-quarters of century, research into finding out design advantages is still going on. The teeth of the worm are typically thinned to fight backlash. The biggest challenge for worm is to provide smooth operation, and that is the reason why it has sliding worm teeth but at the expense of generated heat. The higher rate of wear, with lower efficiency has been the focus of improvement in worm gear operation. The present V-shaped teeth have introduced some form of performance.