In this paper

the gearing type of double enveloping worm is classified

and the main existing modification method is also analyzed. The calculated results show that the type II is the preferred gearing type

and the △i and △a complete modification is the best one. The contact zone of worm wheel tooth surface can be enlarged by introducing the △b>0 modification variable. At last

the optimum scope of △i

△a and △b modification parameter is also presented.