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Bolted Joints Bolted Joints ME 423: Machine Design Instructor: Ramesh Singh 1 Outline • Introduction • Geometry and classification of fasteners • Types of fasteners ME 423: Machine Design Instructor: Ramesh Singh 2 Bolted Joints Bolts find application due to ease in following • Field assembly • Disassembly • Maintenance • Adjustment • Screw Jacks for lifting and power transmission • Screws are also used for linear actuation Fun Fact: Boeing’s 747 require as many as 2.5 million fasteners ME 423: Machine Design Instructor: Ramesh Singh 3 Bolted Joints • Bolted joints are connectors which affect: – Stiffness (as they are not rigid) – Vibration, Damping and Stability – Load bearing capacity • Bolted Joints are needed for disassembly – Maximum benefits are obtained when they are preloaded – Threads can plastically deform – Reusing is no recommended • Note that the assemblies using bolts can resist tensile, shear loads and moments but bolts are only designed to take tension ME 423: Machine Design Instructor: Ramesh Singh 4 Anatomy of a bolted joint l Grip Geometry of Bolt Ad At Tensile stress area tw HBolt Ad Major diameter area l t1 l d L t lt 2 lt Threaded length in grip H Nut tw LT ld Unthreaded length in grip A d t Major diameter (unthreaded) ©Martin Culpepper, All rights reserved 5 ME 423: Machine Design Instructor: Ramesh Singh 5 ManufactureBolted joint components: of Threads Bolt Rolled Threads NEVER in shear or bending Stress concentrations at the Head root of the teeth Fatigue crack propagation! Exception: Shoulder bolts Keep threads clean & Shank lubed to minimize losses ~50% power to bolt head Cut Threads friction ~40% power to thread friction ~10% power to deforming Steel is most the bolt and flange common ©Martin Culpepper, All rights reserved 7 ME 423: Machine Design Instructor: Ramesh Singh 6 Functions ofBolted Washer joint components: Washers Purpose of Washers: • Spacer Spacer • Distribute load in clamped member Distribute load in clamped member • Reduce head-member wear Reduce head-member wear • Lower coefficient of Lower coefficient of friction/losses friction/losses • Lock bolt into the joint Lock bolt into the joint (lock washer) (lock washer) Increase preload resolution (wave washer) • Increase preload resolution (wave washer) ME 423: Machine Design Instructor: Ramesh Singh 7 ©Martin Culpepper, All rights reserved 8 bud29281_ch08_409-474.qxd 12/16/2009 7:11 pm Page 411 pinnacle 203:MHDQ196:bud29281:0073529281:bud29281_pagefiles: Definitions • Pitch–distance between adjacent threads. Reciprocal of threads per inch Screws, Fasteners, and the Design of Nonpermanent Joints 411 • Major diameter–largestFigure diameter 8–1 of Major diameter Pitch diameter thread Terminology of screw threads. Minor diameter Sharp vee threads shown for Pitch p • Minor diameter–smallestclarity; diameter the crests and roots of are thread actually flattened or rounded 45° chamfer • Pitch diameter–theoreticalduring the diameter forming operation. between major and minor diameters, where tooth and gap are same width Root • The lead, l, is the distance the nut moves Crest Thread angle 2α parallel to the screw axis when the nut is given one turn Figure 8–2 H 8 p 8 • Multiple threads are alsoBasic possibleprofile for metric M Internal threads and MJ threads. d ϭ major diameter p p 3H dr ϭ minor diameter 8 5H 2 2 d ϭ pitch diameter p ME 423: Machine DesignH 8 p ϭ pitch Instructor: Ramesh Singh p H 60° √3 4 4 8 H ϭ 2 p 60 H ° d 4 30° dp p External threads dr threads. The MJ profile has a rounded fillet at the root of the external thread and a 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 1 Table 8–2 with diameter under 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 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 5 diameter, the number of threads per inch, and the thread series, for example, 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. Standards • The American National (Unified) thread standard defines basic thread geometry for uniformity and interchangeability • American National (Unified) thread – UN normal thread – UNR greater root radius for fatigue applications • Metric thread – M series (normal thread) – MJ series (greater root radius) ME 423: Machine Design Instructor: Ramesh Singh 9 Standards • Coarse series UNC – General assembly – Frequent disassembly – Not good for vibrations – The “normal” thread to specify • Fine series UNF – Good for vibrations – Good for adjustments – Automotive and aircraft • Extra Fine series UNEF – Good for shock and large vibrations – High grade alloy – Instrumentation – Aircraft ME 423: Machine Design Instructor: Ramesh Singh 10 bud29281_ch08_409-474.qxd 12/16/2009 7:11 pm Page 411 pinnacle 203:MHDQ196:bud29281:0073529281:bud29281_pagefiles: Screws, Fasteners, and the Design of Nonpermanent Joints 411 Figure 8–1 Major diameter Pitch diameter Terminology of screw threads. Minor diameter Sharp vee threads shown for Pitch p clarity; the crests and roots are actually flattened or rounded Thread Profile45° chamfer during the forming operation. • Thread profileRoot M and MJ Crest Thread angle 2α Figure 8–2 H 8 p 8 Basic profile for metric M Internal threads and MJ threads. d ϭ major diameter p p 3H dr ϭ minor diameter 8 5H 2 2 d ϭ pitch diameter p H 8 p ϭ pitch p H 60° √3 4 4 H ϭ 2 p 60 H ° d 4 30° dp p External threads dr threads. The MJ profile has a rounded fillet at the root of the external thread and a 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 isME specified 423: Machine by giving Design the pitch p for metric sizes and by giving the number of threadsInstructor: per inch Ramesh N for Singhthe Unified sizes. The screw sizes in 1 Table 8–2 with diameter under 4 in are numbered or gauge sizes. The second column 11 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 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 5 diameter, the number of threads per inch, and the thread series, for example, 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. Thread Nomenclature Bolt Specification Threads per inch Material grade Thread series ¼-20 x ¾ in UNC-2 Grade 5 Hex head bolt Nominal diameter length Class fit Head type Metric Pitch M12 x 1.75 ISO 4.8 Hex head bolt Nominal diameter Material class Shigley’s Mechanical Engineering Design ME 423: Machine Design Instructor: Ramesh Singh 12 bud29281_ch08_409-474.qxd 12/16/2009 7:11 pm Page 412 pinnacle 203:MHDQ196:bud29281:0073529281:bud29281_pagefiles: 412 Mechanical Engineering Design Table 8–1 Nominal Coarse-Pitch Series Fine-Pitch Series Diameters and Areas of Major Tensile- Minor- Tensile- Minor- Diameter Pitch Stress Diameter Pitch Stress Diameter Coarse-Pitch and Fine- dpArea At Area Ar p Area At 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 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.
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