Ch08 - bud21932_ch08_395-456 09/11/2006 08:36 PM Page 395...

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Screws, Fasteners, and the Design of Nonpermanent Joints Chapter Outline 8–1 Thread Standards and Definitions 396 8–2 The Mechanics of Power Screws 400 8–3 Threaded Fasteners 408 8–4 Joints—Fastener Stiffness 410 8–5 Joints—Member Stiffness 413 8–6 Bolt Strength 417 8–7 Tension Joints—The External Load 421 8–8 Relating Bolt Torque to Bolt Tension 422 8–9 Statically Loaded Tension Joint with Preload 425 8–10 Gasketed Joints 429 8–11 Fatigue Loading of Tension Joints 429 8–12 Bolted and Riveted Joints Loaded in Shear 435 8 395
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396 Mechanical Engineering Design The helical-thread screw was undoubtably an extremely important mechanical inven- tion. It is the basis of power screws, which change angular motion to linear motion to transmit power or to develop large forces (presses, jacks, etc.), and threaded fas- teners, an important element in nonpermanent joints. This book presupposes a knowledge of the elementary methods of fastening. Typ- ical methods of fastening or joining parts use such devices as bolts, nuts, cap screws, setscrews, rivets, spring retainers, locking devices, pins, keys, welds, and adhesives. Studies in engineering graphics and in metal processes often include instruction on var- ious joining methods, and the curiosity of any person interested in mechanical engi- neering naturally results in the acquisition of a good background knowledge of fasten- ing methods. Contrary to first impressions, the subject is one of the most interesting in the entire field of mechanical design. One of the key targets of current design for manufacture is to reduce the number of fasteners. However, there will always be a need for fasteners to facilitate disas- sembly for whatever purposes. For example, jumbo jets such as Boeing’s 747 require as many as 2.5 million fasteners, some of which cost several dollars apiece. To keep costs down, aircraft manufacturers, and their subcontractors, constantly review new fastener designs, installation techniques, and tooling. The number of innovations in the fastener field over any period you might care to mention has been tremendous. An overwhelming variety of fasteners are available for the designer’s selection. Serious designers generally keep specific notebooks on fasteners alone. Methods of joining parts are extremely important in the engineering of a quality design, and it is necessary to have a thorough understanding of the per- formance of fasteners and joints under all conditions of use and design. 8–1 Thread Standards and Definitions The terminology of screw threads, illustrated in Fig. 8–1, is explained as follows: The pitch is the distance between adjacent thread forms measured parallel to the thread axis. The pitch in U.S. units is the reciprocal of the number of thread forms per inch N .
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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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Ch08 - bud21932_ch08_395-456 09/11/2006 08:36 PM Page 395...

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