上海交通大学：《Design and Manufacturing》课程教学资源（参考文献）Shigley's Mechanical Engineering Design ch8 screws, fasteners and non permenent joints.pdf_大学文库

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 a

Budynas−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 invention. 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 fasteners, an important element in nonpermanent joints. This book presupposes a knowledge of the elementary methods of fastening. Typical 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 various joining methods, and the curiosity of any person interested in mechanical engineering naturally results in the acquisition of a good background knowledge of fastening 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 disassembly 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 performance 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). Standardized 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 country 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

400 Budynas-Nisbett:Shigley's Ill.Design of Mechanical 8.Screws,Fasteners,and ©The McGraw-Hil Mechanical Engineering Elements the Design of Companies,2008 Design,Eighth Edition Nonpermanent Joints Screws,Fasteners,and the Design of Nonpermanent Joints 397 Figure 8-1 Major diameter Pitch diameter Terminology of screw threads. -Minor diameter Sharp vee threads shown for -Pitchp clarity;the crests and roots are actually flattened or rounded during the forming operation t4s°chamfer (res Thread angle 20 Figure 8-2 Basic profile for metric M Internal threads and MJ threads. d=major diameter d,minor diameter d。=pitch diameter p=pitch 60 H=p 60 30 External threads larger minor diameter of both the internal and external threads.This profile is espe cially useful where high fatigue strength is required. Tables 8-1 and 8-2 will be useful in specifying and designing threaded parts. Note that the thread size is specified by giving the pitch p for metric sizes and by giving the number of threads per inch N for the Unified sizes.The screw sizes in Table 8-2 with diameter under in are numbered or gauge sizes.The second column in Table 8-2 shows that a No.8 screw has a nominal major diameter of 0.1640 in. A great many tensile tests of threaded rods have shown that an unthreaded rod having a diameter equal to the mean of the pitch diameter and minor diameter will have the same tensile strength as the threaded rod.The area of this unthreaded rod is called the tensile-stress area A,of the threaded rod;values of A,are listed in both tables. Two major Unified thread series are in common use:UN and UNR.The differ- ence between these is simply that a root radius must be used in the UNR series Because of reduced thread stress-concentration factors,UNR series threads have improved fatigue strengths.Unified threads are specified by stating the nominal major diameter,the number of threads per inch,and the thread series,for example,in-18 UNRF or 0.625 in-18 UNRF. Metric threads are specified by writing the diameter and pitch in millimeters,in that order.Thus,M12 x 1.75 is a thread having a nominal major diameter of 12 mm and a pitch of 1.75 mm.Note that the letter M,which precedes the diameter,is the clue to the metric designation

Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition III. Design of Mechanical Elements 8. Screws, Fasteners, and the Design of Nonpermanent Joints 400 © The McGraw−Hill Companies, 2008 Screws, Fasteners, and the Design of Nonpermanent Joints 397 Major diameter Pitch diameter Minor diameter Pitch p 45° chamfer Thread angle 2α Root Crest Figure 8–1 Terminology of screw threads. Sharp vee threads shown for clarity; the crests and roots are actually flattened or rounded during the forming operation. Internal threads External threads H 4 H 4 5H 8 3H 8 H 8 H p 4 p 2 p p 2 p 8 30° 60° 60° dr dp d Figure 8–2 Basic profile for metric M and MJ threads. d major diameter dr minor diameter dp pitch diameter p pitch H √ 3 2 p larger minor diameter of both the internal and external threads. This profile is especially useful where high fatigue strength is required. Tables 8–1 and 8–2 will be useful in specifying and designing threaded parts. Note that the thread size is specified by giving the pitch p for metric sizes and by giving the number of threads per inch N for the Unified sizes. The screw sizes in Table 8–2 with diameter under 1 4 in are numbered or gauge sizes. The second column in Table 8–2 shows that a No. 8 screw has a nominal major diameter of 0.1640 in. A great many tensile tests of threaded rods have shown that an unthreaded rod having a diameter equal to the mean of the pitch diameter and minor diameter will have the same tensile strength as the threaded rod. The area of this unthreaded rod is called the tensile-stress area At of the threaded rod; values of At are listed in both tables. Two major Unified thread series are in common use: UN and UNR. The difference between these is simply that a root radius must be used in the UNR series. Because of reduced thread stress-concentration factors, UNR series threads have improved fatigue strengths. Unified threads are specified by stating the nominal major diameter, the number of threads per inch, and the thread series, for example, 5 8 in-18 UNRF or 0.625 in-18 UNRF. Metric threads are specified by writing the diameter and pitch in millimeters, in that order. Thus, M12 × 1.75 is a thread having a nominal major diameter of 12 mm and a pitch of 1.75 mm. Note that the letter M, which precedes the diameter, is the clue to the metric designation.

Budynas-Nisbett:Shigley's lll.Design of Mechanical 8.Screws,Fasteners,and T©The McGraw-Hill 401 Mechanical Engineering Elements the Design of Companies,2008 Design,Eighth Edition Nonpermanent Joints 398 Mechanical Engineering Design Table 8-1 Nominal Coarse-Pitch Series Fine-Pitch Series Diameters and Areas of Major Tensile- Minor- Tensile- Minor- Coarse-Pitch and Fine Diameter Pitch Stress Diameter Pitch Stress Diameter d Area Ar Area Ar p Area A Area Ar Pitch Metric Threads.* mm mm mm2 mm2 mm mm2 mm2 1.6 0.35 1.27 1.07 2 0.40 2.07 1.79 2.5 0.45 3.39 2.98 0.5 5.03 4.47 3.5 0.6 6.78 6.00 4 0.7 8.78 7.75 0.8 14.2 12.7 6 20.1 17.9 8 1.25 36.6 32.8 1 39.2 36.0 10 1.5 58.0 52.3 1.25 61.2 56.3 12 1.75 84.3 76.3 1.25 92.1 86.0 14 2 115 104 1.5 125 116 16 2 157 144 1.5 167 157 20 2.5 245 225 1.5 272 259 24 3 353 324 2 384 365 30 3.5 561 519 2 621 596 36 817 759 2 915 884 42 4.5 1120 1050 2 1260 1230 48 1470 1380 2 1670 1630 56 5.5 2030 1910 2 2300 2250 64 2680 2520 2 3030 2980 72 3460 3280 2 3860 3800 80 6 4340 4140 1.5 4850 4800 90 6 5590 5360 2 6100 6020 100 6990 6740 2 7560 7470 110 2 9180 9080 *Theidat used o evthis table hve beobdfromANSIB1.1-1974B18.3.1-1978.Themino diameter was found from the equation d,=d-1.226 869p,and the pitch diameter fromd=d-0.649 519p.The mean of the pith diameter and the minor diameter was used to compute the tensilestressara Square and Acme threads,shown in Fig.8-3a and b,respectively,are used on screws when power is to be transmitted.Table 8-3 lists the preferred pitches for inch- series Acme threads.However,other pitches can be and often are used,since the need for a standard for such threads is not great. Modifications are frequently made to both Acme and square threads.For instance, the square thread is sometimes modified by cutting the space between the teeth so as to have an included thread angle of 10 to 15.This is not difficult,since these threads are usually cut with a single-point tool anyhow;the modification retains most of the high efficiency inherent in square threads and makes the cutting simpler.Acme threads

Budynas−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 401 Companies, 2008 398 Mechanical Engineering Design Nominal Coarse-Pitch Series Fine-Pitch Series Major Tensile- Minor- Tensile- MinorDiameter Pitch Stress Diameter Pitch Stress Diameter d p Area At Area Ar p Area At Area Ar mm mm mm2 mm2 mm mm2 mm2 1.6 0.35 1.27 1.07 2 0.40 2.07 1.79 2.5 0.45 3.39 2.98 3 0.5 5.03 4.47 3.5 0.6 6.78 6.00 4 0.7 8.78 7.75 5 0.8 14.2 12.7 6 1 20.1 17.9 8 1.25 36.6 32.8 1 39.2 36.0 10 1.5 58.0 52.3 1.25 61.2 56.3 12 1.75 84.3 76.3 1.25 92.1 86.0 14 2 115 104 1.5 125 116 16 2 157 144 1.5 167 157 20 2.5 245 225 1.5 272 259 24 3 353 324 2 384 365 30 3.5 561 519 2 621 596 36 4 817 759 2 915 884 42 4.5 1120 1050 2 1260 1230 48 5 1470 1380 2 1670 1630 56 5.5 2030 1910 2 2300 2250 64 6 2680 2520 2 3030 2980 72 6 3460 3280 2 3860 3800 80 6 4340 4140 1.5 4850 4800 90 6 5590 5360 2 6100 6020 100 6 6990 6740 2 7560 7470 110 2 9180 9080 *The equations and data used to develop this table have been obtained from ANSI B1.1-1974 and B18.3.1-1978. The minor diameter was found from the equation dr d 1.226 869p, and the pitch diameter from dp d 0.649 519p. The mean of the pitch diameter and the minor diameter was used to compute the tensile-stress area. Table 8–1 Diameters and Areas of Coarse-Pitch and FinePitch Metric Threads.* Square and Acme threads, shown in Fig. 8–3a and b, respectively, are used on screws when power is to be transmitted. Table 8–3 lists the preferred pitches for inchseries Acme threads. However, other pitches can be and often are used, since the need for a standard for such threads is not great. Modifications are frequently made to both Acme and square threads. For instance, the square thread is sometimes modified by cutting the space between the teeth so as to have an included thread angle of 10 to 15◦. This is not difficult, since these threads are usually cut with a single-point tool anyhow; the modification retains most of the high efficiency inherent in square threads and makes the cutting simpler. Acme threads

Budynas−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 405 Companies, 2008 402 Mechanical Engineering Design Since we are not interested in the normal force N, we eliminate it from each of these sets of equations and solve the result for P. For raising the load, this gives PR = F(sin λ + f cos λ) cos λ − f sin λ (c) and for lowering the load, PL = F( f cos λ − sin λ) cos λ + f sin λ (d) Next, divide the numerator and the denominator of these equations by cos λ and use the relation tan λ = l/πdm (Fig. 8–6). We then have, respectively, PR = F[(l/πdm) + f ] 1 − ( f l/πdm) (e) PL = F[ f − (l/πdm)] 1 + ( f l/πdm) (f) Finally, noting that the torque is the product of the force P and the mean radius dm/2, for raising the load we can write TR = Fdm 2 l + π f dm πdm − f l (8–1) where TR is the torque required for two purposes: to overcome thread friction and to raise the load. The torque required to lower the load, from Eq. (f), is found to be TL = Fdm 2 π f dm − l πdm + f l (8–2) This is the torque required to overcome a part of the friction in lowering the load. It may turn out, in specific instances where the lead is large or the friction is low, that the load will lower itself by causing the screw to spin without any external effort. In such cases, the torque TL from Eq. (8–2) will be negative or zero. When a positive torque is obtained from this equation, the screw is said to be self-locking. Thus the condition for self-locking is π f dm > l Now divide both sides of this inequality by πdm . Recognizing that l/πdm = tan λ, we get f > tan λ (8–3) This relation states that self-locking is obtained whenever the coefficient of thread friction is equal to or greater than the tangent of the thread lead angle. An expression for efficiency is also useful in the evaluation of power screws. If we let f = 0 in Eq. (8–1), we obtain T0 = Fl 2π (g)

06 Budynas-Nisbett:Shigley's Ill.Design of Mechanical 8.Screws,Fasteners,and T©The McGraw-Hill Mechanical Engineering Elements the Design of Companies,2008 Design,Eighth Edition Nonpermanent Joints Screws,Fasteners,and the Design of Nonpermanent Joints 403 which,since thread friction has been eliminated,is the torque required only to raise the load.The efficiency is therefore To Fl e= TR 2TR (8-4④ The preceding equations have been developed for square threads where the nor- mal thread loads are parallel to the axis of the screw.In the case of Acme or other threads,the normal thread load is inclined to the axis because of the thread angle 20 and the lead angle A.Since lead angles are small,this inclination can be neglected and only the effect of the thread angle(Fig.8-7a)considered.The effect of the angle o is to increase the frictional force by the wedging action of the threads.Therefore the frictional terms in Eq.(8-1)must be divided by cos a.For raising the load,or for tightening a screw or bolt,this yields Fdm TR=2 l+πf d sec a (8-51 πdm-fl sec In using Eq.(8-5),remember that it is an approximation because the effect of the lead angle has been neglected. For power screws,the Acme thread is not as efficient as the square thread,because of the additional friction due to the wedging action,but it is often preferred because it is easier to machine and permits the use of a split nut,which can be adjusted to take up for wear. Usually a third component of torque must be applied in power-screw applications. When the screw is loaded axially,a thrust or collar bearing must be employed between the rotating and stationary members in order to carry the axial component.Figure 8-7b shows a typical thrust collar in which the load is assumed to be concentrated at the mean collar diameter de.If fe is the coefficient of collar friction,the torque required is Ffede Te= 2 (8-6) For large collars,the torque should probably be computed in a manner similar to that employed for disk clutches. Figure 8-7 (a)Normal thread force is increased because of angle a; b)thrust collar has frictional F/2 2 diameter d -Collar Nut 2a= Thread angle (a) (b)

Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition III. Design of Mechanical Elements 8. Screws, Fasteners, and the Design of Nonpermanent Joints 406 © The McGraw−Hill Companies, 2008 Screws, Fasteners, and the Design of Nonpermanent Joints 403 Thread angle Collar Nut F cos F⁄ 2 (a) (b) F⁄ 2 F 2 = F⁄ 2 F⁄ 2 Figure 8–7 dc (a) Normal thread force is increased because of angle α; (b) thrust collar has frictional diameter dc. which, since thread friction has been eliminated, is the torque required only to raise the load. The efficiency is therefore e = T0 TR = Fl 2πTR (8–4) The preceding equations have been developed for square threads where the normal thread loads are parallel to the axis of the screw. In the case of Acme or other threads, the normal thread load is inclined to the axis because of the thread angle 2α and the lead angle λ. Since lead angles are small, this inclination can be neglected and only the effect of the thread angle (Fig. 8–7a) considered. The effect of the angle α is to increase the frictional force by the wedging action of the threads. Therefore the frictional terms in Eq. (8–1) must be divided by cos α. For raising the load, or for tightening a screw or bolt, this yields TR = Fdm 2 l + π f dm sec α πdm − f l sec α (8–5) In using Eq. (8–5), remember that it is an approximation because the effect of the lead angle has been neglected. For power screws, the Acme thread is not as efficient as the square thread, because of the additional friction due to the wedging action, but it is often preferred because it is easier to machine and permits the use of a split nut, which can be adjusted to take up for wear. Usually a third component of torque must be applied in power-screw applications. When the screw is loaded axially, a thrust or collar bearing must be employed between the rotating and stationary members in order to carry the axial component. Figure 8–7b shows a typical thrust collar in which the load is assumed to be concentrated at the mean collar diameter dc . If fc is the coefficient of collar friction, the torque required is Tc = F fcdc 2 (8–6) For large collars, the torque should probably be computed in a manner similar to that employed for disk clutches.