Types Of Worm And Worm Gears

Machine Design

Gear Drives

Introduction:

Worm gear is used mostly where the power source operates at a high speed and output is at a slow speed with high torque. It is also used in some cars and trucks.

Types of Worms

The following are the two types of worms :

1. Cylindrical or straight worm, and
2. Cone or double enveloping worm.

Fig1. Types of worms

The cylindrical or straight worm, as shown in Fig.1 (a), is most commonly used.The shape of the thread is involute helicoid of pressure angle 14 ½° for single and double threaded worms and 20° for triple and quadruple threaded worms.The worm threads are cut by a straight sided milling cutter having its diameter not less than the outside diameter of worm or greater than 1.25 times the outside diameter of worm.The cone or double enveloping worm, as shown in Fig.1 (b), is used to some extent, but it requires extremely accurate alignment.

Types of Worm Gears

The following three types of worm gears are important from the subject point of view :

Fig 2. Types of worms gears

1. Straight face worm gear, as shown in Fig2 (a),

2. Hobbed straight face worm gear, as shown in Fig.2 (b), and

3. Concave face worm gear, as shown in Fig 2 (c).

The straight face worm gear is like a helical gear in which the straight teeth are cut with a form cutter.Since it has only point contact with the worm thread, therefore it is used for light service.The hobbed straight face worm gear is also used for light service but its teeth are cut with a hob, after which the outer surface is turned.The concave face worm gear is the accepted standard form and is used for all heavy service and general industrial uses.The teeth of this gear are cut with a hob of the same pitch diameter as the mating worm to increase the contact area.