The worm gear drives are used to transmit power between two non-intersecting shafts . which are in general at right angles to each other . The worm gear drive consists of a worm and a worm wheel . The  worm is threaded screw , while the worm wheel envelope the threads on the worm and give line contact between mating parts. 
The advantages of worm gear .
1. The most important characteristic of worm gear drives is their high speed reduction .A speed reduction  as high as 100;1 can be obtained with a single pair of worm gear .

2. The worm gear drives are compact with small  overall dimensions ,compared with equivalent spur or helical gear drives having same speed reduction .

3. The operation is smooth and silent .

4. Provision can be made for self-locking operation .Where the motion is transmitted only from the worm to the worm wheel . This is advantageous in application line cranes and lifting devices .


1. Axial Pitch - The axial pitch (p)  of the worm is defined as the distance measured from a point on one thread to the corresponding point on the adjacent thread , measured along the axis of the worm .

2. Lead -  The lead (I) of the worm is defined as the distance that a point on the helical profile will move , When the worm is rotated through one revolution .It is the thread advance in one turn .For single thread the lead is equal to the axial pitch .For double -start threads ,the lead is twice the axial pitch and so on .

3. Lead angle -  The lead angle ( Γ ) is defined as the angle between a tangent to the thread at the pitch diameter and a plane normal to the worm axis .

4. Helix angle - The helix angle ( Ψ )  is defined as the angle between a tangent to the thread as the pitch diameter and the axis of worm .The worm helix angle is complement of worm lead angle .

5. Pressure angle - The tooth pressure angle (α ) is measured in a plane containing the axis of the worm and it is equal to one-half of the thread angle . The pressure angle should not be less than 20° for triple and multi-start worms .


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