Various Terms Used In Connection With Worm Gears

Worm Gears : 

The worm gears are widely used for transmitting power at high velocity ratios between two non-parallel, non-intersecting shafts that are generally, but not necessarily, at right angles. It can give velocity ratios as high as 300 : 1 or more, but it has a lower efficiency.

There are three different types of worm gear:

Non-throated :– 
This involves a straight worm without a groove machined around the circumference. A single moving point is what provides tooth contact, meaning this particular type of worm gear is subjective to high unit load wear and tear.

Single-throated :– 
Concave helical teeth are wrapped around the worm for line contact, meaning higher unit loads with less excessive wear.

Double-throated :– 
Usually called a cone of hourglass, this type has concave teeth on both the worm screw and the gear itself. Increasing the contact area in such a way allows for increased unit loads with lower wear and tear.

The various terms used in connection with Worm Gears are as follows :-

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 pont on one thread to the corresponding point on the adjacent thread on the worm.

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 axial pitch, but for multiple start threads, lead is equal to the product of axial pitch and number of starts.

3. Lead angle :- 
It is the angle between the tangent to the thread helix on the pitch cylinder and the axis of the worm. It is denoted by λ.

The lead angle (λ) may vary from 9° to 45°. It has been shown by F.A. Halsey that a lead less than 9° results in rapid wear and the safe value of λ is 12 ½ °

4. Tooth pressure angle :-   
It is measured in a place containing the axis of the worm and is equal to one-half the thread profile angle.

5. Helix angle :-    
It is the angle between the tangent to the thread helix on the pitch cylinder and the axis of the worm. The worm helix angle (αw) is the complement of worm lead angle(λ),i.e.

αw +λ =90°

It may be noted that the helix angle on the worm is generally quite large and that on the worm gear is very small.

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