Budynas-Nisbett:Shigley's Ill.Design of Mechanical 8.Screws,Fasteners,and T©The McGraw--Hill 399 Mechanical Engineering Elements the Design of Companies,2008 Design,Eighth Edition Nonpermanent Joints 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. The major diameter d is the largest diameter of a screw thread. The minor (or root)diameter dr is the smallest diameter of a screw thread. The pitch diameter do is a theoretical diameter between the major and minor diameters. The lead l,not shown,is the distance the nut moves parallel to the screw axis when the nut is given one turn.For a single thread,as in Fig.8-1,the lead is the same as the pitch. A multiple-threaded product is one having two or more threads cut beside each other (imagine two or more strings wound side by side around a pencil).Standard- ized products such as screws,bolts,and nuts all have single threads:a double-threaded screw has a lead equal to twice the pitch,a triple-threaded screw has a lead equal to 3 times the pitch,and so on. All threads are made according to the right-hand rule unless otherwise noted. The American National(Unified)thread standard has been approved in this coun- try and in Great Britain for use on all standard threaded products.The thread angle is 60 and the crests of the thread may be either flat or rounded. Figure 8-2 shows the thread geometry of the metric M and MJ profiles.The M profile replaces the inch class and is the basic ISO 68 profile with 60 symmetric threads.The MJ profile has a rounded fillet at the root of the external thread and aBudynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition III. Design of Mechanical Elements 8. Screws, Fasteners, and the Design of Nonpermanent Joints © The McGraw−Hill 399 Companies, 2008 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. The major diameter d is the largest diameter of a screw thread. The minor (or root) diameter dr is the smallest diameter of a screw thread. The pitch diameter dp is a theoretical diameter between the major and minor diameters. The lead l, not shown, is the distance the nut moves parallel to the screw axis when the nut is given one turn. For a single thread, as in Fig. 8–1, the lead is the same as the pitch. A multiple-threaded product is one having two or more threads cut beside each other (imagine two or more strings wound side by side around a pencil). Standard￾ized products such as screws, bolts, and nuts all have single threads; a double-threaded screw has a lead equal to twice the pitch, a triple-threaded screw has a lead equal to 3 times the pitch, and so on. All threads are made according to the right-hand rule unless otherwise noted. The American National (Unified) thread standard has been approved in this coun￾try and in Great Britain for use on all standard threaded products. The thread angle is 60◦ and the crests of the thread may be either flat or rounded. Figure 8–2 shows the thread geometry of the metric M and MJ profiles. The M profile replaces the inch class and is the basic ISO 68 profile with 60◦ symmetric threads. The MJ profile has a rounded fillet at the root of the external thread and a