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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

  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

  3. LECTURE 37 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. 8-7 Tension Joints-The External Load Static Analysis a) External Load • Let us consider what happens when an external tensile load P, is applied to a bolt connection. • Assuming a clamping force, (preload Fi)is applied by tightening the nut before external force, P is applied. • Fi=preload • P = external tensile load • Pb=portion of P taken by bolt • Pm= portion of P taken by members • Fb= Pb + Fi =resultant bolt load • Fm= Pm – Fi =resultant load on the members • C = fraction of external load P carried by bolt • 1-C = fraction of external load P carried by members Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  5. 8-7 Tension Joints-The External Load • External load P is shared by bolt and members. The load P is tension, and it causes the connection to stretch, or elongate, through some distance d C is the stiffness constant of the joint, For typical values of C see TABLE 8-12 Most of external Load P is taken by members Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  6. 8-7 Tension Joints-The External Load b) Resultant Bolt and Member Load : Fb& Fm (8-24) (8-25) Fiis preload; high preload is desirable in tension connections. Fi= 0.75Fpfor re-use Fi= 0.90Fpfor permanent joint Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  7. 8-8 Relating Bolt Torque to Bolt Tension c) Torque Required to give preload Fi Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  8. 8-8 Relating Bolt Torque to Bolt Tension K is torque coefficient K values are given in TABLE 8-15 Average Value for K = 0.2 for f = fc = 0.15 no matter what size bolts are employed and no matter whether the threads are coarse or fine. (8-26) (8-27) The coefficient of friction depends upon the surface smoothness, accuracy, and degree of lubrication. On the average, both f and fc are about 0.15. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  9. Example 4 (Example 8-3 Textbook) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  10. Example 4 (Cont.’d) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  11. Example 4 (Cont.’d) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  12. 8-9 Statically Loaded Tension Joint with Preload d) Statically Loaded Tension Joint with Preload • Failure of joints occurs when • i)Bolt Yields Proof strength At: Tensile stress area Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  13. 8-9 Statically Loaded Tension Joint with Preload • ii)Joint separates Let P0be external load causing separation Fm= 0 • no : factor of safety against joint separation Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  14. 8-10 Gasketed Joints g) Gasketed Joints If a full gasket is present in joint, The gasket pressure p is: No. of bolts Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  15. 8-10 Gasketed Joints IMPORTANT: To maintain uniformity of pressure adjacent bolts should not be placed more than 6 nominal diameters apart on bolt circle. To maintain wrench clearance bolts should be placed at least 3 d apart . A rough rule for bolt spacing around a bolt circle is whereDbis the diameter of the bolt circle and N is the number of bolts. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  16. Example 5 (Example 8-4 Textbook) • The grip is l = 1.50 in. From Table A-31, the nut thickness is 35/64 in. • Adding two threads beyond the nut of 2/11in gives a bolt length of Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

  17. From Table A-17, the next fraction size bolt is 2 1/4 in. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

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

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

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

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

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