Worm gearing is among the oldest of mechanisms that have been found useful to mankind. Even so, when trying to comprehend the various methods of torque transmission, the interaction of the worm and wheel—because of its complexity—is the hardest to understand. The mathematics of the gear action and the various interacting forces, in combination with the available tooth forms, complicates our understanding. These features, also, complicate our ability to precisely determine the rating. The end result is that no precise analytical design method has yet evolved. Extensive trial and error testing, supported by field experience, has been and continues to be the most reliable rating method (empirical). Even with this complication, the majority of worm gears give satisfactory, dependable performance when selected by the existing methods and are correctly assembled and maintained.
Laws applied to other gearing forms do not function in worm gearing design. The precise computation of the gear set capacity by an analytical method is also not feasible. Rating calculations are made difficult by the many variables, such as tooth form, materials, accuracy, mounting, deflections, lubrication, power source, worm surface finish, variations in center distance, and input fluctuations. An example of the difficulty, are the effects of center distance tolerances which are acceptable for spur or helical gears but not worm gears.
The normal worm gear driving force can be resolved into three major or perpendicular components of a driving, separating, and thrust force. The worm driving force acts as a thrust force on the worm gear.