Ch13 - bud21932_ch13_653-712 09/11/2006 10:54 PM Page 653...

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Chapter Outline 13–1 Types of Gears 654 13–2 Nomenclature 655 13–3 Conjugate Action 657 13–4 Involute Properties 658 13–5 Fundamentals 658 13–6 Contact Ratio 664 13–7 Interference 665 13–8 The Forming of Gear Teeth 667 13–9 Straight Bevel Gears 670 13–10 Parallel Helical Gears 671 13–11 Worm Gears 675 13–12 Tooth Systems 676 13–13 Gear Trains 678 13–14 Force Analysis—Spur Gearing 685 13–15 Force Analysis—Bevel Gearing 689 13–16 Force Analysis—Helical Gearing 692 13–17 Force Analysis—Worm Gearing 694 13 Gears—General 653
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654 Mechanical Engineering Design Figure 13–1 Spur gears are used to transmit rotary motion between parallel shafts. Figure 13–2 Helical gears are used to transmit motion between parallel or nonparallel shafts. This chapter addresses gear geometry, the kinematic relations, and the forces transmit- ted by the four principal types of gears: spur, helical, bevel, and worm gears. The forces transmitted between meshing gears supply torsional moments to shafts for motion and power transmission and create forces and moments that affect the shaft and its bearings. The next two chapters will address stress, strength, safety, and reliability of the four types of gears. 13–1 Types of Gears Spur gears, illustrated in Fig. 13–1, have teeth parallel to the axis of rotation and are used to transmit motion from one shaft to another, parallel, shaft. Of all types, the spur gear is the simplest and, for this reason, will be used to develop the primary kinematic relationships of the tooth form. Helical gears, shown in Fig. 13–2, have teeth inclined to the axis of rotation. Helical gears can be used for the same applications as spur gears and, when so used, are not as noisy, because of the more gradual engagement of the teeth during meshing. The inclined tooth also develops thrust loads and bending couples, which are not present with spur gearing. Sometimes helical gears are used to transmit motion between nonparallel shafts. Bevel gears, shown in Fig. 13–3, have teeth formed on conical surfaces and are used mostly for transmitting motion between intersecting shafts. The figure actually illustrates straight-tooth bevel gears. Spiral bevel gears are cut so the tooth is no longer straight, but forms a circular arc. Hypoid gears are quite similar to spiral bevel gears except that the shafts are offset and nonintersecting.
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Gears—General 655 Figure 13–3 Bevel gears are used to transmit rotary motion between intersecting shafts. Worms and worm gears, shown in Fig. 13–4, represent the fourth basic gear type. As shown, the worm resembles a screw. The direction of rotation of the worm gear, also called the worm wheel, depends upon the direction of rotation of the worm and upon whether the worm teeth are cut right-hand or left-hand. Worm-gear sets are also made so that the teeth of one or both wrap partly around the other. Such sets are called single- enveloping and double-enveloping worm-gear sets. Worm-gear sets are mostly used when the speed ratios of the two shafts are quite high, say, 3 or more.
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This note was uploaded on 03/22/2010 for the course MEEN ISEN 302 taught by Professor Kim during the Spring '10 term at Texas A&M University–Commerce.

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Ch13 - bud21932_ch13_653-712 09/11/2006 10:54 PM Page 653...

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