ME 307 Machine Design I ME 307 Machine Design I

ME 307 Machine Design I ME 307 Machine Design I 8-1 Thread Standards and Definitions 8-2The Mechanics of Power Screws 8-3Strength Constraints 8-4Joints-Fasteners Stiffness 8-5Joints-Member Stiffness 8-6Bolt Strength 8-7Tension Joints-The External Load 8-8Relating Bolt Torque to Bolt Tension 8-9Statically Loaded Tension Joint with Preload 8-10Gasketed Joints 8-11Fatigue Loading of Tension Joints 8-12Shear Joints 8-13Setscrews 8-14Keys and Pins 8-15Stochastic Considerations 8-1 Thread Standards and Definitions 8-2The Mechanics of Power Screws 8-3Strength Constraints 8-4Joints-Fasteners Stiffness 8-5Joints-Member Stiffness 8-6Bolt Strength 8-7Tension Joints-The External Load 8-8Relating Bolt Torque to Bolt Tension 8-9Statically Loaded Tension Joint with Preload 8-10Gasketed Joints 8-11Fatigue Loading of Tension Joints 8-12Shear Joints 8-13Setscrews 8-14Keys and Pins 8-15Stochastic Considerations

ME 307 Machine Design I ME 307 Machine Design I 8-3Strength Constraints 8-4Joints-Fasteners Stiffness 8-5Joints-Member Stiffness 8-6Bolt Strength 8-3Strength Constraints 8-4Joints-Fasteners Stiffness 8-5Joints-Member Stiffness 8-6Bolt Strength CH-8 LEC 37 Slide 3 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

ME 307 Machine Design I ME 307 Machine Design I Static Analysis a) External Load 8-7Tension Joints-The External Load CH-8 LEC 37 Slide 4 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Let us consider what happens when an external tensile load P, is applied to a bolt connection. Assuming a clamping force, (preload F i )is applied by tightening the nut before external force, P is applied. F i = preload P = external tensile load P b = portion of P taken by bolt P m = portion of P taken by members F b = P b + F i = resultant bolt load F m = P m – F i = resultant load on the members C = fraction of external load P carried by bolt 1-C = fraction of external load P carried by members

ME 307 Machine Design I ME 307 Machine Design I 8-7Tension Joints-The External Load CH-8 LEC 37 Slide 5 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints 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  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

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 6 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints 8-7Tension Joints-The External Load b) Resultant Bolt and Member Load : F b & F m (8-24) F i is preload ; high preload is desirable in tension connections. F i = 0.75 F p for re-use F i = 0.90 F p for permanent joint (8-25)

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 7 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints 8-8Relating Bolt Torque to Bolt Tension c) Torque Required to give preload F i

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 8 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints 8-8Relating Bolt Torque to Bolt Tension (8-27) (8-26) The coefficient of friction depends upon the surface smoothness, accuracy, and degree of lubrication. On the average, both f and f c are about K is torque coefficient K values are given in TABLE Average Value for K = 0.2 for f = f c = 0.15 no matter what size bolts are employed and no matter whether the threads are coarse or fine.

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 9 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Example 4 (Example 8-3 Textbook)

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 10 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Example 4 (Cont.’d)

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 11 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Example 4 (Cont.’d)

ME 307 Machine Design I ME 307 Machine Design I 8-9Statically Loaded Tension Joint with Preload CH-8 LEC 37 Slide 12 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Failure of joints occurs when i)Bolt Yields d) Statically Loaded Tension Joint with Preload Proof strength A t : Tensile stress area

ME 307 Machine Design I ME 307 Machine Design I 8-9Statically Loaded Tension Joint with Preload CH-8 LEC 37 Slide 13 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints ii)Joint separates Let P 0 be external load causing separation F m = 0 n o : factor of safety against joint separation

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 14 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints 8-10Gasketed Joints g) Gasketed Joints If a full gasket is present in joint, The gasket pressure p is: No. of bolts

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 15 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints 8-10Gasketed Joints IMPORTANT: 1.To maintain uniformity of pressure adjacent bolts should not be placed more than 6 nominal diameters apart on bolt circle. 2.To maintain wrench clearance bolts should be placed at least 3 d apart. 3.A rough rule for bolt spacing around a bolt circle is where D b is the diameter of the bolt circle and N is the number of bolts.

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 16 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Example 5 (Example 8-4 Textbook) (a)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 2/11 in gives a bolt length of

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 17 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints (a)From Table A-17, the next fraction size bolt is 2 1/4 in.

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 18 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 19 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 20 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

ME 307 Machine Design I ME 307 Machine Design I CH-8 LEC 37 Slide 21 Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints