Common terms used :

The following terms, in connection with the worm gearing, are important from the subject point of view :

1. Axial pitch. It is also known as linear pitch of a worm. It is the distance measured axially (i.e. parallel to the axis of worm) from a point on one thread to the corresponding point on the adjacent thread on the worm, as shown in Fig. 31.3. It may be noted that the axial pitch (pa) of a worm is equal to the circular pitch ( pc ) of the mating worm gear, when the shafts are at right angles.

2. Lead. It is the linear distance through which a point on a thread moves ahead in one revolution of the worm. For single start threads, lead is equal to the axial pitch, but for multiple start threads, lead is equal to the product of axial pitch and number of starts. Mathematically,

A little consideration will show that if one complete turn of a worm thread be imagined to be unwound from the body of the worm, it will form an inclined plane whose base is equal to the pitch circumference of the worm and altitude equal to lead of the worm.

tan λ = Lead of the worm / Pitch circumference of the worm

where m = Module, and

           DW = Pitch circle diameter of worm.

The lead angle (λ) may vary from 9° to 45°. It has been shown by F.A. Halsey that a lead angle less than 9° results in rapid wear and the safe value of λ is 12½°.

4. Tooth pressure angle. It is measured in a plane containing the axis of the worm and is equal to one-half the thread profile angle as shown in Fig. Normal pitch. It is the distance measured along the normal to the threads between two corresponding points on two adjacent threads of the worm. Mathematically

Let l = Lead of the worm, and

       DG = Pitch circle diameter of the worm gear.

We know that linear velocity of the worm,

vW = l . NW/60

where n = Number of starts of the worm.

Design Procedure For Worm Gear: