Worm Gears

The Operating principle of a worm gearbox

Worm gears consist of a worm wheel and a worm. The worm gear enables the transmission of high loads at a high transmission ratio.
The worm can be understood as a special design of a gear or a helical gear. Here, the tooth winds around the worm shaft like the thread of a screw. The counterpart to the worm is the worm wheel. 

The "number of teeth" on a worm is referred to as the number of gears. Accordingly, a worm is referred to as a single-flight, double-flight or multiple-flight worm. Usually, single-flight worms are produced, but in special cases the number of flights can be as high as four. If you look at the operation of a worm gear using the example of a single-start worm gear, one revolution of the worm advances the thread by one position. The worm wheel is thus advanced by one tooth. In the case of one revolution of a double-flight worm, on the other hand, two threads would each advance one tooth. In total, two teeth of the worm wheel would then have been moved further.

Due to the very low number of threads of the worm and the comparatively large number of teeth of the worm wheel, the transmission ratio of worm gear units is very high. Thus, very high transmission ratios >100 can be realized with worm gear units in a space-saving manner. 

In a worm gear, several threaded sections of the worm wheel are generally engaged at the same time, which makes the load capacity of these gears very high. They are also very quiet due to the continuous meshing of the flanks between the worm and the worm wheel and the lubrication or cooling required.

Worm types

Cylindrical worms

If the outer shape of the worm is cylindrical, it is called a cylindrical worm. If the cross-section of the worm wheel reflects this cylindrical profile on the circumference, it is referred to as a globoid worm wheel. Due to the relatively simple manufacture of a cylindrical worm, this variant is preferably used (cylindrical worm gear).

Globoid worms