1 1 6. Fasteners and Fastening methods Objectives " Describe many types of fastening systems and their uses. " Understand principles of stress area, pitch diameters, and thread types and forms.
" Understand different types of tensioning systems and how preloaded joints are created and the analysis of appropriate tightening loads. " Describe principles of elastic analysis and how the preload affects the ability of joints to resist future loading and/or pressures. " Recognize types of fastening systems, their basic principles, and where they may be applicable.
2 Introduction " Principal purpose of fasteners are 3 Disassembly for inspection and repair 3 Modular design, where a product consists of a number of subassemblies. 3 Fastener types " Removable: This type permits the parts to be readily disconnected without damaging the fastener, e.g. nut and bolt.
" Semi-permanent: For this type, the parts can be disconnected, but some damage usually occurs to the fastener, e.g. cotter pin " Permanent: When this type of fastener is used, the parts will never be disassembled. e.g.
rivets and welding 4 Fastener application " Primary function " Appearance " Number of fasteners " Operating conditions " Frequency of disassembly " Adjustability " Types of materials " Consequences of failure ... more. less.
5 Screw thread terminology (Fig. 6.1) 6 Screw thread terminology " Major diameter: The major diameter is the largest diameter of the thread. It determines the nominal size.<br><br> " Minor diameter: It is the smallest diameter of the thread. In external thread, it is also called as root diameter. " Pitch: is the axial distance between any point of one thread and the corresponding point of an adjacent thread.<br><br> " Lead: The distance a bolt advances into a nut in one revolution is called lead. 2 7 Cross-section of a Unified thread 8 9 10 11 Tightening Methods " Using a torque wrench with a specified torque limit " Turning through a specified angle after full engagement " Hydraulic tensioning 3 use a hydraulic cylinder to stretch a bolt for imparting an initial tension 12 Torquing Methods " Relationship between torque and preload " Torque, T H C D F i " D = nominal diameter of thread " F i = desired initial preload " C = torque coefficient = 0.15 for lubricated assemblies = 0.20 for non lubricated with traces of oil = 0.34 for dry assemblies 3 13 Example Problem 6-1: Torquing Methods " A ¾-UNC-grade 5 bolt is to be preloaded to 85 percent of its proof strength. " The length of engagement is 5 inches.<br><br> " The bolt is new and non-lubricated but likely has traces of cutting oil present. " Determine the required torque: 14 Example Problem 6-1: Torquing Methods 3 Using C = 0.2 non-lubricated with traces of oil: T = C D F i T = 0.2 (¾in) 24,130 lb T = 3620 in-lb or 302 ft-lb A S = 0.334 in 2 S p = 85 ksi F = SA F i = 0.85 S p A s F i = 0.85 (85,000 lb/in 2 ) (0.334 in 2 ) F i = 24,130 lb (Table 6-1) (Table 6-3) (6-1) 15 Turn-of-the-Nut method " Find the elongation needed to produce the appropriate preload " Required torque angle = E A L F ´ , Elongation = pitch 360 16 (6-3) ´ = FL AE or SL E ´ = 24,130 lb 5 in .334 in 2 30x10 6 lb/in 2 ´ = .012 in (6-4) torque angle = ´ 360° pitch (Table 6-1) Pitch for ¾ UNC is .1 inch: torque angle = .012 in 360° .1 in torque angle = 43.4° Example Problem 6-2: Turn-of-the-Nut Method " From prior problem, determine the angle of rotation needed, using the turn-of-nut method. 3 Note again that the nut should be tightened, then turned snug, before turning this angle.<br><br> 17 Heating Method " Use the linear expansion of the material under heat " We know that Elongation, ´ = ± L T " Temperature required for the elongation T = L ± ´ 18 (Appendix 8) ± = 6.5 x 10 -6 in in °F (6-5) T = ´ ± L T = .012 in 6.5 x 10 -6 in 5 in in °F T = 370° F Example Problem 6-3: Heating Methods " In Example Problem 6-1, to obtain the same preload, determine the temperature we would need to heat this bolt above the service temperature. 4 19 Elastic Analysis of Bolted Connections " Bolted connection 3 residual tension in the bolt, residual compression in the clamped part " Applied load to the part gets compensated by this to some extent. E A L F ´ , Elongation = 20 Elastic Analysis of Bolted Connections " If k is the stiffness of the joint " The stiffness of the bolt is " The stiffness of the joint is 3 E 3 Young 9s modulus 3 A 3 Area of cross section 3 L 3 Grip length ´ F k = b b b b L E A k = c c c c L E A k = 21 Elastic Analysis of Bolted Connections " Recommended preload, F i is given by 3 Q 3 margin factor (similar to safety factor) 3 F e 3 Applied load " New load on bolt is F t = F i + F b ?<br><br> ? ? ?<br><br> ? ? + = c b c e i k k k F Q F Eq 6.7 ?<br><br> ? ? ?<br><br> ? ? + = c b b e b k k k F F 22 Elastic Analysis of Bolted Connections " Total force on the bolt " Total force on the flange ?<br><br> ? ? ?<br><br> ? ? + + = c b b e i t k k k F F F ?<br><br> ? ? ?<br><br> ? ? + 2 = c b c e i c k k k F F F Eq 6.8 Eq 6.9 23 Force analysis of fasteners 24 Bolt in shear Ä = Shear stress in the bolt P = Applied shear force A = cross- sectional area of the bolt A P Ä = 5 25 Multiple fasteners A N P Ä = 26 Eccentric loading 27 Dual loading 28 Shear force on multiple fasteners 29 Shear force on multiple fasteners 30 Shear force on multiple fasteners 6 31 Secondary shear forces " Direction is perpendicular to the line running from the center of the bolt to the centroid " Direction opposes the applied torque, T " Magnitude is proportional to the distance from the center of the bolt to the centroid 32 Secondary shear forces F 1 l 1 + F 2 l 2 + F 3 l 3 + F 4 l 4 3 P e = 0 2 1 2 1 F F l l = 2 4 2 3 2 2 2 1 1 1 e P F l l l l l + + + = 33 Bolted connector 34 Bolted connector 35 Graphical addition 36 7 37 Conclusions " A variety of mechanical fasteners are discussed.<br><br> " Screw thread is the most important part of a fastener. " Force acting on various fasteners have been analyzed. <br><br>