Fastener types Fastener application

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1 1 6. Fastenersand 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 3Disassembly for inspection and repair 3Modular 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 5 Screw thread terminology (Fig. 6.1) 6 Screw thread ... more. less.

terminology " Major diameter: The major diameter is the largest diameter of the thread. It determines the nominal size. " 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. " 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 3use a hydraulic cylinder to stretch a bolt for imparting an initial tension 12 TorquingMethods "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: TorquingMethods " A ¾-UNC-grade 5 bolt is to be preloaded to 85 percent of its proof strength. "The length of engagement is 5 inches. "The bolt is new and non-lubricated but likely has traces of cutting oil present. "Determine the required torque: 14 Example Problem 6-1: TorquingMethods 3Using 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. 3Note again that the nut should be tightened, then turned snug, before turning this angle. 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 3residual 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 3E 3Young 9s modulus 3A 3Area of cross section 3L 3Grip 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 3Q 3margin factor (similar to safety factor) 3F e 3Applied load "New load on bolt is F t = F i + F b ? ? ? ? ? ? + = c b c e i k k k F Q F Eq6.7 ? ? ? ? ? ? + = 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 ? ? ? ? ? ? + + = c b b e i t k k k F F F ? ? ? ? ? ? + 2 = c b c e i c k k k F F F Eq6.8 Eq6.9 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 3P 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. "Screw thread is the most important part of a fastener. "Force acting on various fasteners have been analyzed.

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