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Chapter 8

Chapter 8. Screws, Fasteners, and the Design of Nonpermanent Joints. Chapter Outline. 8-1 Thread Standards and Definitions 8-2 The Mechanics of Power Screws 8-3 Strength Constraints 8-4 Joints-Fasteners Stiffness

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Chapter 8

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  1. Chapter 8 Screws, Fasteners, and the Design of Nonpermanent Joints Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  2. Chapter Outline 8-1 Thread Standards and Definitions8-2 The Mechanics of Power Screws8-3 Strength Constraints 8-4 Joints-Fasteners Stiffness 8-5 Joints-Member Stiffness 8-6 Bolt Strength 8-7 Tension Joints-The External Load8-8 Relating Bolt Torque to Bolt Tension 8-9 Statically Loaded Tension Joint with Preload 8-10 Gasketed Joints 8-11 Fatigue Loading of Tension Joints 8-12 Shear Joints 8-13 Setscrews 8-14 Keys and Pins 8-15 Stochastic Considerations Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  3. LECTURE 36 8-3 Strength Constraints 8-4 Joints-Fasteners Stiffness 8-5 Joints-Member Stiffness 8-6 Bolt Strength Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  4. 8-4 Joints: Fastener Stiffness When a connection is desired that can be disassembled without destructive methods and that is strong enough to resist external tensile loads, moment loads, and shear loads, or a combination of these, then the simple bolted joint using hardened steel washers is a good solution. • Twisting the nut stretches the bolt to produce the clamping force. This clamping force is called the pretention or bolt preload. • This force exists in the connection after the nut has been properly tightened. Figure 8-13 A bolted connection loaded in tension by the forces P. Note the use of two washers. Note how the threads extend into the body of the connection. This is usual and is desired. LG is the grip of the connection. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  5. Joints: Fastener Stiffness Figure 8-14 shows another tension-loaded connection. This joint uses cap screws threaded into one of the members. Figure 8-14 Section of cylindrical pressure vessel. Hexagon-head caps crews are used to fasten the cylinder head to the body. Note the use of an O-ring seal. LG’ is the effective length of the connection (See Table 8-7) . Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  6. Joints: Fastener Stiffness • An alternative approach to this problem (of not using a nut) would be to use studs. • A stud is a rod threaded on both ends. The stud is screwed into the lower member first; then the top member is positioned and fastened down with hardened washers and nuts • Studs are regarded as permanent, and so the joint can be disassembled merely by removing the nut and washer. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  7. Joints: Fastener Stiffness • Spring Rate : The ratio between the force applied to the member and the deflection produced by that force. • The grip LGof a connection is the total thickness of the clamped material. • Total distance between the underside of the nut to the bearing face of the bolt head; includes washer, gasket thickness etc. The grip LG here is the sum of the thicknesses of both members and both washers. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  8. Table 8-7 Suggested Procedure for Finding fastener Stiffness To find different parameters use table 8-7 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  9. In joint under tension the members are under compression and the bolt under tension: kb= equivalent spring constant of bolt composed of threaded ktand unthreaded kdparts acting as springs in series. From Chapter 5 (Springs in series) For short bolts kb= kt At: tensile stress area (Tables 8-1, 8-2), lt: length of threaded portion of the grip; Ad: major diameter area of fastener; ld: length of unthreaded portion in grip. kb: is the estimated effective stiffness of the bolt or cap screw in the clamped zone. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  10. 8-5 Joints- Member Stiffness Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  11. 8-5 Joints- Member Stiffness • There may be more than two members included in the grip of the fastener. • All together these act like compressive springs in series. • Equivalent spring constant km • If one of the members is a soft gasket, its stiffness relative to the other members is usually so small that for all practical purposes the others can be neglected and only gasket stiffness used. • With no gasket, the stiffness of the members is difficult to obtain, except by experimentation. • Compression spreads out between the bolt head and the nut and area is not uniform. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  12. 8-5 Joints Member Stiffness Joint pressure distribution theoretical models • Ito used ultrasonic techniques to determine pressure distribution at the member interface. Results show that pressure stays high out to about 1.5 bolt radii. • Ito suggested the use of Rotscher’s pressure cone method for stiffness calculations with a variable cone angle. This method is quite complicated. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  13. 8-5 Joints Member Stiffness Figure 8-15 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  14. 8-5 Joints Member Stiffness • We choose a simpler approach using a fixed cone angle. • The contraction of an element of the cone of thickness dx is subjected to a compressive force P is, from Eq. (5-3), • The area of the element is Figure 8-15b general cone geometry using a half-apex angle . Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  15. 8-5 Joints Member Stiffness • Substituting this into the previous equation and integrating the resulting equation from 0 to t gives. • For Members made of Aluminum, hardened steel and cast iron 25o<<33o • With =30o, this becomes (8-19) (8-20) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  16. 8-5 Joints Member Stiffness • Substituting this into the previous equation and integrating the resulting equation from 0 to t gives. • With =30o, this becomes (8-19) (8-20) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  17. 8-5 Joints Member Stiffness • If members of the joint have the same E with symmetrical frusta (l=2t), then they act as two identical springs in series km = k/2. For= 30° andD = dw = 1.5 d, this can be written as, • Finite element analysis agree with  = 30orecommendation coinciding exactly at the aspect ratio d/l = 0.4. • Additionally, FEM offered an exponential curve-fit of the form • where A and B are given in Table 8-8. (8-22) (8-23) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  18. 8-5 Joints Member Stiffness Figure 8-16 The dimensionless plot of stiffness versus aspect ratio of the members of a bolted joint, showing the relative accuracy of method of Rotsher, Michke and Motosh, compared to Finite Element Analysis (FEA) conducted by Wileman, Choudury, and Green. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  19. 8-5 Joints Member Stiffness Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  20. 8-6 Bolt Strength • Bolt strength is specified by: • minimum proof strength Sp • or minimum proof load Fp, • and minimum tensile strength, Sut • The proof load is the maximum load (force) that a bolt can withstand without acquiring a permanent set. • The proof strength is the quotient of the proof load and the tensile-stress area. • The proof strength is about 90% of the 0.2%offset yield strength. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  21. 8-6 Bolt Strength • The SAE specifications are given in • Table 8-9 bolt grades are numbered • according to minimum tensile strength. • The ASTM Specs for steel bolts (structural) are in Table 8-10. • Metric Specs are in Table 8-11. If Sp not available use: Sp =0.85 Sy Fp = AtSp Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  22. 8-6 Bolt Strength Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  23. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  24. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

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