Worm and Worm Wheels
Worm gear sets are comprised of a worm and a worm wheel, or worm gear. These gear sets transmit power between two non-parallel, non-intersecting shafts, generally have an angle of 90 degrees between them. A worm is essentially a thread wrapped around a shaft while a worm gear is a gear with corresponding gear features to that thread. Most worm gears are used when a large speed reduction is necessary in a system. One major difference that sets worm sets apart from all other gear sets is that the worm turns the worm wheel but the worm wheel cannot turn the worm. A major reason that worm gear sets are used is their ability to produce high speed reduction ratios and through that high torque multiplication.
Enveloping in worm gear sets refers to the contouring of the two different components in order to create higher contact and more efficiency in the system. In industry practice if one of the components is a double meaning that the worm is contoured to fit the curve of the worm wheel, or the gear is “throated” to heighten the addendum on the outside of the gear teeth, the other generally not also double enveloping or throated. Enveloping and throating increases the number of teeth engaged in the system and can increase the life of the set. This increase in engagement does however increase the amount of heat produced by the system and creates a higher need for the set to have some sort of oil or coolant on the system. The manufacturing techniques required to produce both components when they are double enveloping can increase the cost of the system dramatically.
A worm can have one start or multiple starts to the thread. In single start worm screws, the worm wheel will only advance one tooth for every full rotation of the worm screw. The amount of starts refers to the amount of threads cut into the shaft. A worm gear is very similar to a thread with single enveloping. During the operation of a single enveloping worm, it has line contact, where the contact line sweeps across the entire width and height of the zone of action.
Meshing between the worm and the worm wheel consists of a mixture of sliding and rolling actions. The sliding contact however does dominate when applied to high reduction ratios. These sliding actions create friction and heat in the system which can reduce the efficiency of the worm and worm wheel. Many times the worm and the worm wheel will be manufactured out of two different types of material in order to reduce friction and therefore heat. The most common combination of materials for worm gear sets are a steel worm wheel and a bronze worm wheel. A benefit to the sliding action is the low noise of the set while in operation. Most applications of worm gear sets require a coolant or oil in order to draw heat away from the components to prevent wear and galling that can cause the system to fail. In some cases of worm gear set failure, the worm will get score marks in the thread and tear apart the worm wheel that it is running against. For further explanation and galling calculation, Marples Gears has an online galling calculator.
The accuracy and precision of the gear housing for worm gears and worm wheels is also important to the function and efficiency of the gear set. The 90 degree angle between the shafts and the center distance between the worm and worm gear are critical features on these gears sets, especially if one or more of the components is double enveloping in order to maintain the correct contact pattern between them and prevent excessive wear to either component or failure of the system. In worse case scenarios the mounting of the worm and gear would create point contact between the components instead of the intended line contact.
Due to the geometry of a worm, many of the same method used to manufacture threads can also be used to manufacture worms. They can be milled, ground or cut on a lathe though the varying ways that the worm is produced will form different worm teeth. Multiple start worms must be made through hobbing. Worm wheels are produced in the same way that gears are made, but must go through an extra process if they are double enveloping, or “throated”. The most common method of manufacture would be hobbing.
