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Text Book of Machine Design P31

Worm Gears | CONTENTS c H A P T E R 31 Worm Gears 1. Introduction 2. Types of Worms 3. Types of Worm Gears. 4. Terms used in Worm Gearing. 5. Proportions for Worms . 6. Proportions for Worm Gears. 7. Efficiency of Worm Gearing. 8. Strength of Worm Gear Teeth. 9. Wear Tooth Load for Worm Gear 10. Thermal Rating of Worm Gearing. 11. Forces Acting on Worm Gears. 12. Design of Worm Gearing. 31.1 Introduction The worm gears are widely used for transmitting power at high velocity ratios between non-intersecting shafts that are generally but not necessarily at right angles. It can give velocity ratios as high as 300 1 or more in a single step in a minimum of space but it has a lower efficiency. The worm gearing is mostly used as a speed reducer which consists of worm and a worm wheel or gear. The worm which is the driving member is usually of a cylindrical form having threads of the same shape as that of an involute rack. The threads of the worm may be left handed or right handed and single or multiple threads. The worm wheel or gear which is the driven member is similar to a helical gear with a face curved to conform to the shape of the worm. The worm is generally made of steel while the worm gear is made of bronze or cast iron for light service. 1101 CONTENTS 1102 A Textbook of Machine Design The worm gearing is classified as non-interchangeable because a worm wheel cut with a hob of one diameter will not operate satisfactorily with a worm of different diameter even if the thread pitch is same. 31.2 Types of Worms The following are the two types of worms 1. Cylindrical or straight worm and 2. Cone or double enveloping worm. The cylindrical or straight worm as shown in Fig. 31.1 a is most commonly used. The shape of the thread is involute helicoid of pressure angle 14 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