Geometrical aspects of double enveloping worm gear drive

Double enveloping worm gearing is expected to have contact over larger number of teeth and higher load carrying capacity compared to single enveloping worm gearing. In this paper, contact in this gearing is analysed by geometrical simulation of worm gear tooth generation using intersection profiles of different axial sections of worm representing the hob tooth profile with transverse plane of worm gear. The analysis reveals that in the engaging zone a straight line contact always exists in the median plane and intermittent contact exists at the extreme end sections of worm. This has lead to the idea of using two fly cutters positioned at the location identical to the extreme end sections of worm to generate full worm gear tooth thereby eliminating the need for hobs of complex geometry. For a given worm, a mating worm gear is machined in a gear hobbing machine using fly tool in two settings and nature of contact with the worm is checked by a blue test.