Screws, Fasteners, and the Design of Nonpermanent Joints Chapter 8 Screws, Fasteners, and the Design of Nonpermanent Joints Dr. A. Aziz Bazoune 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 8-5 Joints-Member Stiffness 8-6 Bolt Strength 8-7 Tension Joints-The External Load 8-8 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

LECTURE 38 8-11 Fatigue Loading of Tension Joints 8-12 Shear Joints 8-13 Setscrews 8-14 Keys and Pins Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

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 2/11 in gives a bolt length of Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

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

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

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

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

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

8-7 Tension Joints-The External Load Fatigue Analysis a) External Load Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

8-11 Fatigue Loading of Tension Joints Fatigue Analysis In general, bolted joints are subject to 0-Pmax, e.g pressure vessels, flanges, pipes, … Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

8-11 Fatigue Loading of Tension Joints Table 8-16 lists average fatigue stress-concentration factors for the fillet under the bolt head and also at the beginning of the threads on the bolt shank. These are already corrected for notch sensitivity and for surface finish. Use of rolled threads is the predominant method of thread-forming in screw fasteners, where Table 8-16 applies. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

8-11 Fatigue Loading of Tension Joints In thread-rolling the amount of cold-work and strain strengthening is unknown to the designer; therefore, fully corrected (including Kf) axial endurance strength is reported in Table 8-17. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

8-11 Fatigue Loading of Tension Joints Fatigue-loaded bolted joints subjected to Fatigue action can be analyzed directly by the methods of Chapter 7. Fatigue loading is the one in which the externally applied load fluctuates between zero and some maximum force P. Fmax = Fb and Fmin = Fi Fa = (Fmax – Fmin)/2 = (Fb – Fi)/2 σa = Fa/At Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

8-11 Fatigue Loading of Tension Joints Fmax = Fb and Fmin = Fi Fa = (Fmax – Fmin)/2 = (Fb – Fi)/2 Fm = (Fmax + Fmin)/2 = (Fb + Fi)/2 σa = Fa/At (8-34) (8-35) (8-36) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

On the designer’s fatigue diagram, shown in Figure 8-20, the load line is . High Preload is especially important in fatigue. σi is a constant the load line at Fi/At has a unit slope, r = 1.0 Figure 8-20 Designer’s fatigue diagram showing a Goodman failure locus and how a load line is used to define failure and safety in preloaded bolted joints in fatigue. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

8-11 Fatigue Loading of Tension Joints Next, find the strength components Sa and Sm of the fatigue failure locus. These depend on the failure Criteria. Goodman (8-37) Gerber (8-38) ASME-Elliptic (8-39) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Goodman Gerber ASME-Elliptic For simultaneous solution between Eq. (8-36), as Sa = Sm + σi and each of Eqs. (8-37) to (8-39) gives Goodman (8-40) (8-41) Gerber (8-42) ASME-Elliptic (8-43) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

When using relations of this section, be sure to use Kf for both σa and σm. Otherwise, the slope of the load line will not remain 1 to 1. The factor of safety guarding against fatigue is given by Applying this to the Goodman criterion, for example, with Eqs. (8-34) and (8-40) and gives (8-44) (8-45) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

With no preload, C=1, Fi=0, and Eq. (8-45) becomes Preload is beneficial for resisting fatigue when is greater than unity. For Goodman, Eqs.(8-45) and (8-46) with puts an upper bound on the preload Fi of If this cannot be achieved, and nf is unsatisfactory, use the Gerber or the ASME-elliptic criterion to obtain a less conservative assessment. After solving Eq. (8-44), you should also check the possibility of yielding, using the proof strength (8-46) (8-47) (8-48) Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

Example 6 (Example 8-5 Textbook) Figure 8-21 Pressure-cone frustum member model for a cap screw. For this model the significant sizes are Where l = effective grip. The solutions are for =30o and dw=1.5d. Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

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

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

Sut=120 kpsi. The coordinates are A: σi=63.72 kpsi Figure 8-22 Designer’s fatigue diagram for preloaded bolts, dr showing the modified Goodman locus, the Gerber locus and the Langer proof strength locus, with an exploded view of the area of interest, The strengths used are Sp= 85 kpsi, Se= 18.6 kpsi, and Sut=120 kpsi. The coordinates are A: σi=63.72 kpsi B: σa=3.1 kpsi, σm=66.82 kpsi C: Sa=7.55 kpsi, Sm=71.29 kpsi D: Sa=10.64 kpsi, Sm=74.35 kpsi E: Sa=11.32 kpsi, Sm=75.04 kpsi Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints

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

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

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