Under the optimum conditions for any worm gear tooth profile only line contact is present when two gears of finite diameter mesh and rotate. Depending on the flexibility, a contact area of greater or lesser size is only produced under load and by a corresponding elastic deformation of the tooth surfaces. Worm gears are, in effect, crossed helical gears. However, unlike true crossed helical gears they envelop each other. With the shaft angle of the two axes usually at 90 degrees to one another, the enveloping provides a much larger area of contact. No worm gear will mesh perfectly with its mate—however carefully they are made. If such a condition was achievable one would still have to precisely locate the gearset, both axially and on the center distance. If these assembly conditions were obtainable one would still have to contend with the deformation that takes place under load.
There is a wide choice, in tooth forms, each with its own share of merits and demerits. These advantages and disadvantages can be related to the application. The tooth form selected for the gear must be conjugate, i.e., when both the worm and wheel are rotated at a specific relative uniform motion, one generates the other. Therefore, it is advisable for both the worm and mating worm wheel be produced by the same manufacturer.
The rotated worm develops a series of rack profiles advanced along its axis as shown in Fig. 3.1. The center section has identical pressure angles on both sides, but off-center the sections lose their symmetry. The hob has an identical series of rack sections that generate the worm teeth—the conjugate action being the same as that between a rack and a pinion. The shape of the rack contours from the tip to the root have no effect on the conjugate action when the worm and worm gear are generated with hobs that have the same type of profile and pressure angles.