What is a Enveloping Worm Gear?
In the enveloping worm gear design, either the contour of the gear teeth or the contour of the worm threads or the contour of the both is modified in such a way that the amount of the contact surfaces increases. Thus, you will get increased accuracy from the worm gear.
When only the contour of the gear teeth is modified like below to get the increased amount of contact surface then the arrangement is called single enveloping worm gear.
If you observe the right hand side view, you can see that the profiles of the gear teeth are curved to match the circular profile of the worm thread. But still you can see from the left hand side view that, hundred percent engagements is there only for one thread (the middle one) and the two of the side threads have partial engagement and this is not desired. This problem can be solved by using double enveloping worm gear design.
In double enveloping worm gear, the teeth profile of the gear as well as the shape of the worm threads are modified to achieve better engagement.
From the right-hand side view of the picture you can see how the shapes of the worm threads have modified in line with the circular shape of the gear teeth. Now, you can see the complete engagement throughout the length and width of the worm gear and that’s what we want.