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Unit 5 threaded joint

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Basic types of screw fasteners, Bolts of uniform
strength, I.S.O. Metric screw threads, Bolts under
tension, eccentrically loaded bolted joint in shear,
Eccentric load perpendicular and parallel to axis of
bolt, Eccentric load on circular base, design of Turn
Buckle.

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Unit 5 threaded joint

  1. 1. Dr. Somnath Kolgiri (Associate Professor) S B Patil College of Engineering, Indapur, Pune.
  2. 2. Contents Basic types of screw fasteners, Bolts of uniform strength, I.S.O. Metric screw threads, Bolts under tension, eccentrically loaded bolted joint in shear, Eccentric load perpendicular and parallel to axis of bolt, Eccentric load on circular base, design of Turn Buckle.
  3. 3. DEFINITION Threaded joint is defined as a separable joint of two or more machine parts that are held together by means of a threaded fastening such as a bolt and a nut BASIC CONCEPTS Screws have been used as fasteners for a long time. Screw or thread joints are separable joints held together by screw fastenings, such as screws, bolts, studs and nuts, or by thread cut on the parts to be joined.
  4. 4. Screws engage the threads of nuts or of other parts. A nut is a threaded fastening with internal thread. It is screwed on the bolt and is of a shape designed to be gripped by a wrench or by hand. Fig.5.1 Screw (bolt) and nut
  5. 5. TERMINOLOGY OF SCREW THREADS Fig. 5.2 General screw-thread symbols
  6. 6. Where d — the largest diameter of a screw thread; dc — minor diameter, i.e. the smallest diameter of a screw thread; dp — pitch diameter; p — pitch; l — lead; — Thread angle; h — height of thread engagement; — lead (or helix) angle. 
  7. 7. According to their purpose, screw threads are classified as: (1) Fastening threads; (2) Fastening and sealing threads; (3) Power threads Fig. 5.3 General screw-thread
  8. 8. Fig.5.4 Principal type of screw threads
  9. 9. SCREW FASTENINGS Depending upon the type of screw joint involved, screw fastenings are classed as: (1) Screws with nuts, generally called bolts; (2) Cap screws inserted into tapped holes in the parts being fastened; (3) Studs, or stud-bolts, used with nuts and having threads on both ends.
  10. 10. Fig.5.5 Principal types of screw joints
  11. 11. With respect to the shape of their heads, screw fastenings are divided into: (1) Those in which the head is engaged externally by a tool (wrench , etc.); (2) Those in which the head is engaged internally and from the end face; (3) Those that prevent the screw fastening from turing.
  12. 12. Fig.5.6 Heads of cap screws
  13. 13. Setscrews are another form of fastener; the usual use for them is to prevent relative circular motion between two parts such as shafts and pulleys. They may be used only where the torque requirements are low. Fig.5.7 Applications of setscrews
  14. 14. The types of points of setscrews are follows: Flat Point , Dog Point, Cone Point, Hanger Point and Cut Point Fig.5.8 Setscrews with various points
  15. 15. The main types of nuts are as follows: Fig.5.9 Principal types of nuts
  16. 16. SREWING-UP TORQUE, EFFICIENCY AND SELF-LOCKING CONDITIONS )( vtg tg      v  21 TTT  FdT 2.0 Fig. 5.10
  17. 17. BOLT TIGHTENING AND INTIAL TENSION F----initial force Fig.5.11 Torque-wrenches
  18. 18. PREVENTING UNINTENTIONAL UNSCREWING OF SCREW JOINTS Locking can be accomplished by the following measures: (1) By supplementary friction; (2) by using special locking devices; (3) by plastic deformation or welding on.
  19. 19. Fig.5.12 Locking devices based on the application of supplementary friction
  20. 20. Fig.5.13 Locking devices using special locking elements
  21. 21. Fig.5.14 Permanent locking
  22. 22. • Bolts are subjected to shock and impact loads in certain applications. The bolts of cylinder head of an internal combustion engine • Bolts of connecting rod are the examples of such applications. In such cases, resilience of the bolt is important design consideration to prevent breakage at the threads. • Resilience is defined as the ability of the material to absorb energy when deformed elastically and to release this energy when unloaded. BOLT OF UNIFORM STRENGTH
  23. 23. When this bolt is subjected to tensile force, the stress in the shank and the stress in the threaded portion are equal. In the second method, the diameter of the hole (d1) is obtained by equating the cross-sectional area of the shank to that of the threaded part. Therefore, Fig. 5.15 Bolts of Uniform Strength
  24. 24. A bolted joint subjected to tensile force P is shown in Fig. 7.12. The cross-section at the core diameter dc is the weakest section. The maximum tensile stress in the bolt at this cross-section is given by, BOLTED JOINT—SIMPLE ANALYSIS Fig. 5.16 Bolt in Tension
  25. 25. The height of the nut h can be determined by equating the strength of the bolt in tension with the strength in shear. The procedure is based on the following assumptions: (i) Each turn of the thread in contact with the nut supports an equal amount of load. (ii) There is no stress concentration in the threads. (iii) The yield strength in shear is equal to half of the yield strength in tension (Ssy = 0.5Syt). (iv) Failure occurs in the threads of the bolt and not in the threads of the nut. Substituting,
  26. 26. • The strength of the bolt in tension is given by, The threads of the bolt in contact with the nut are sheared at the core diameter dc. The shear area is equal to (p dc h), where h is the height of the nut. The strength of the bolt in shear is given by, Therefore, for standard coarse threads, the threads are equally strong in failure by shear and failure by tension, if the height of the nut is approximately 0.4 times of the nominal diameter of the bolt. The height of the standard hexagonal nut is (0.8d). Hence, the threads of the bolt in the standard nut will not fail by shear.
  27. 27. Rewriting the height of the standard nut, The The design of the bolt consists of determination of correct size of the bolt. The size of the bolt is given by the nominal diameter d and pitch p. In design calculations, many times the core diameter dc is determined. Therefore, it is necessary to convert the core diameter dc into the nominal diameter d. The correct relationship for ISO metric screw threads is as follows Since there are two unknowns on the right hand side, it is not possible to find out the value of d by knowing the value of dc. Therefore, the following approximate relationship can be used,
  28. 28. Example 1. An electric motor weighing 10 kN is lifted by means of an eye bolt as shown in Fig. 5.17 The eye bolt is screwed into the frame of the motor. The eye bolt has coarse threads. It is made of plain carbon steel 30C8 (Syt = 400 N/mm2) and the factor of safety is 6. Determine the size of the bolt. Fig. 5.17 General screw-thread
  29. 29. Example 2. Two plates are fastened by means of two bolts as shown in Fig. 5.18. The bolts are made of plain carbon steel 30C8 (Syt = 400 N/mm2) and the factor of safety is 5. Determine the size of the bolts if, Fig. 5.18
  30. 30. In structural connections, a group of bolts is frequently employed, as shown in Fig. 5.19. Let A1, A2, ..., A5 be the cross-sectional areas of the bolts. The co- ordinates (x1, y1), (x2, y2), ..., (x5, y5) indicate the position of bolt-centres with respect to the origin. G is the centre of gravity of the group of bolts. The co- ordinates (x, y) indicate the location of the centre of gravity. ECCENTRICALLY LOADED BOLTED JOINTS IN SHEAR
  31. 31. Fig. 5.19 Fig. 5.20
  32. 32. The primary and secondary shear forces are added by vector addition method to get the resultant shear forces P1, P2, P3, and P4. In this analysis, it is assumed that the components connected by the bolts are rigid and the bolts have the same crosssectional area. Fig. 5.21
  33. 33. Example 1. The structural connection shown in Fig. 7.16 is subjected to an eccentric force P of 10 kN with an eccentricity of 500 mm from the CG of the bolts. The centre distance between bolts 1 and 2 is 200 mm, and the centre distance between bolts 1 and 3 is 150 mm. All the bolts are identical. The bolts are made from plain carbon steel 30C8 (Syt = 400 N/mm2) and the factor of safety is 2.5. Determine the size of the bolts.
  34. 34. Example 2. A steel plate subjected to a force of 5 kN and fixed to a channel by means of three identical bolts is shown in Fig. 7.19(a). The bolts are made from plain carbon steel 45C8 (Syt = 380 N/mm2) and the factor of safety is 3. Specify the size of bolts
  35. 35. A bracket, fixed to the steel structure by means of four bolts, is shown in Fig. 7.20(a). It is subjected to eccentric force P at a distance e from the structure. The force P is perpendicular to the axis of each bolt. The lower two bolts are denoted by 2, while the upper two bolts by 1. In this analysis, the following assumptions are made: (i) The bracket and the steel structure are rigid. (ii) The bolts are fitted in reamed and ground holes. (iii) The bolts are not preloaded and there are no tensile stresses due to initial tightening. ECCENTRIC LOAD PERPENDICULAR TO AXIS OF BOLT
  36. 36. The force P results in direct shear force on the bolts. Since the bolts are identical, the shear force on each bolt is given by,
  37. 37. The bolts denoted by 1 are subjected to maximum force. In general, a bolt, which is located at the farthest distance from the tilting edge C, is subjected to maximum force. Equations (7.9) and (7.10) give shear and tensile forces that act on the bolt due to eccentric load perpendicular to the axis of the bolts. The direct shear stress in the bolt is given by,
  38. 38. Example 2. A wall bracket is attached to the wall by means of four identical bolts, two at A and two at B, as shown in Fig. 7.21. Assuming that the bracket is held against the wall and prevented from tipping about the point C by all four bolts and using an allowable tensile stress in the bolts as 35 N/mm2, determine the size of the bolts on the basis of maximum principal stress theory.
  39. 39. Example 3. A bracket is fastened to the steel structure by means of six identical bolts as shown in Fig. 7.22(a). Assume the following data: l1 = 300 mm l2 = 200 mm l3 = 100 mm l = 250 mm P = 50 kN Neglecting shear stress, determine the size of the bolts, if the maximum permissible tensile stress in any bolt is limited to 100 N/mm2.
  40. 40. Example 3. A crane-runway bracket is fastened to the roof truss by means of two identical bolts as shown in Fig. 7.23. Determine the size of the bolts, if the permissible tensile stress in the bolts is limited to 75 N/mm2.
  41. 41. Example.5 Cast iron bracket, supporting the transmission shaft and the belt pulley, is fi xed to the steel structure by means of four bolts as shown in Fig. 7.25(a). There are two bolts at A and two bolts at B. The tensions in slack and tight sides of the belt are 5 kN and 10 kN respectively. The belt tensions act in a vertically downward direction. The distances are as follows, l1 = 50 mm l2 = 150 mm l = 200 mm The maximum permissible tensile stress in any bolt is 60 N/mm2. Determine the size of the bolts.
  42. 42. Many times, a machine component is made with a circular base, which is fastened to the structure by means of bolts located on the circumference of a circle. Flanged bearings of the machine tools and the A round flange bearing fastened by means of four bolts is shown in Fig. 7.26(b). It is subjected to an external force P at a distance l from the support. The following assumptions are made: (i) All bolts are identical. (ii) The bearing and the structure are rigid. (iii) The bolts are not preloaded and there is no tensile stress due to initial tightening. (iv) The stress concentration in the threads is neglected. (v) The bolts are relieved of shear stresses by using dowel pins. ECCENTRIC LOAD ON CIRCULAR BASE
  43. 43. As shown in Fig. 7.26(a), when the load tends to tilt the bearing about the point C, each bolt is stretched by an amount (d), which is proportional to its vertical distance from the point C. Or, Therefore, it can be concluded that the resisting force acting on any bolt due to the tendency of the bearing to tilt, is proportional to its distance from the tilting edge. If P1 P2 … are the resisting forces induced in the bolts,
  44. 44. where C is the constant of proportionality. Equating the moment due to the external force P about C with the moments due to resisting forces,
  45. 45. Example 1. A round flange bearing, as shown in Fig. 7.26(b), is fastened to the machine frame by means of four cap screws spaced equally on a 300 mm pitch circle diameter. The diameter of the flange is 400 mm. The external force P is 25 kN, which is located at a distance of 150 mm from the machine frame. There are two dowel pins to take shear load. The cap screws are relieved of all shear force. Determine the size of the cap screws, if the maximum permissible tensile stress in the cap screw is limited to 50 N/mm2.
  46. 46. Example 2. A pillar crane, shown in Fig. 7.27, is fastened to the foundation by means of 16 identical bolts spaced equally on 2 m pitch circle diameter. The diameter of the pillar flange is 2.25m. Determine the size of the bolts if a load of 50 kN acts at a radius of 7.5 m from the axis of the crane. The maximum permissible tensile stress in the boltis limited to 75 N/mm2.
  47. 47. Example 3. Figure 7.28 shows the bracket used in a jib crane to connect the tie rod. The maximum force in the tie rod is 5 kN, which is inclined at an angle of 30° with the horizontal. The bracket is fastened by means of four identical bolts, two at A and two at B. The bolts are made of plain carbon steel 30C8 (Syt = 400 N/mm2) and the factor of safety is 5. Assume maximum shear stress theory and determine the size of the bolts.
  48. 48. Example 4. A bracket, subjected to a force of 5 kN inclined at an angle of 60° with the vertical, is shown in Fig. 7.29. The bracket is fastened by means of four identical bolts to the structure. The bolts are made of plain carbon steel 30C8 (Syt = 400 N/mm2) and the factor of safety is 5 based on maximum shear stress. Assume maximum shear stress theory and determine the size of the bolts.
  49. 49. Example 5. A rigid bracket subjected to a vertical force of 10 kN is shown in Fig. 7.30(a). It is fastened to a vertical stanchion by means of four identical bolts. Determine the size of the bolts by maximum shear stress theory. The maximum permissible shear stress in any bolt is limited to 50 N/mm2.
  50. 50. Step I Problem Specification: It is required to design a turnbuckle for connecting the tie rods in the roof truss. The maximum pull in the tie rods is 50 kN. of a circle. Step II Construction : The construction of the turnbuckle is shown in Fig. 7.31. It consists of a central portion called coupler and two rods. One rod has right-hand threads while the other rod has left-hand threads. The threaded portions of the rods are screwed to the coupler at the two ends. As the central coupler rotates, the rods are either pulled together or pushed apart depending upon the direction of the rotation of the coupler. The outer portion of the coupler is given hexagonal shape so that it can be rotated by means of a spanner Sometimes a hole is provided in the coupler as indicated by a dotted circle in the figure. Instead of using a spanner, a tommy bar is inserted in this hole to rotate the coupler. The turnbuckle is used for connecting two rods which are in tension and which require slight adjustment in length during the assembly. Some of its applications are as follows: DESIGN OF TURNBUCKLE
  51. 51. (i) to tighten the members of the roof truss; (ii) to tighten the cables or the stay ropes of electric distribution poles ; and (iii) to connect the tie rod to the jib in case of jib-cranes.
  52. 52. Step III Selection of Materials The coupler has a relatively complex shape and the economic method to make the coupler is casting. Casting reduces the number of machining operations. Grey cast iron of grade FG 200 (Sut = 200 N/mm2) is selected as the material for the coupler. The rods are subjected to tensile force and torsional moment. From strength considerations, plain carbon steel of grade 30C8 (Syt = 400 N/mm2) is selected for the rods. Step IV General Considerations Many times, turnbuckle is subjected to rough handling in use. Sometimes, workers use a pipe to increase the length of the spanner and tighten the rods. Some workers even use a hammer to apply force on the spanner. To account for this ‘missuse,’ a higher factor of safety of 5 is used in the present design. The coupler and the rods are provided with ISO metric coarse threads. Coarse threads are preferred because of the following advantages: 1. Coarse threads are easier to cut than fine threads; 2. They are less likely to seize during tightening; 3. There is more even stress distribution.
  53. 53. The rods are tightened by applying force on the wrench handle and rotating the hexagonal coupler. The expression for torque required to tighten the rod with specific tension P can be derived by suitable modification of the equation derived for the trapezoidal threads. The torque required to overcome thread friction in case of trapezoidal threads is given by Eq. (6.13) of Chapter 6. where d is the nominal diameter of the threads. The coefficient of friction varies from 0.12 to 0.20 depending upon the surface fi nish and accuracy of the thread profi le and lubrication. Assuming, u = 0.15 and substituting above values in Eq. (a), The above expression is used to find out torsional moment at each end of the coupler. Step V Design of Rods : The free body diagram of forces acting on the rods and the coupler is shown in Fig. 7.32.
  54. 54. Each rod is subjected to a tensile force P and torsional moment Mt. In the initial stages, it is not possible to find out torsional moment. Considering only tensile force, where A is the tensile stress area of the threaded portion of the rod and st is the permissible tensile stress. The rod is made of steel 30C8 (Syt = 400 N/mm2) and factor of safety is 5. Therefore,
  55. 55. The factor of safety is satisfactory. Therefore, the nominal diameter and the pitch of the threaded portion of the rod should be 42 mm and 4.5 mm respectively. In Fig. 7.31, the length of the threaded portion of the rod in contact with coupler threads is denoted by l. It is determined by shearing of the threads at the minor diameter dc. Equating shear resistance of the threads to the tension in the rod,
  56. 56. HW Assig Assig Assig
  57. 57. References • Shigley, Joseph; Mechanical Engineering Design, Seventh Edition, 2003,McGraw Hil, pg 396 • VB Bandari; Design of Machine Elements, 1994, Tata McGraw Hill, pg 175 66

Basic types of screw fasteners, Bolts of uniform strength, I.S.O. Metric screw threads, Bolts under tension, eccentrically loaded bolted joint in shear, Eccentric load perpendicular and parallel to axis of bolt, Eccentric load on circular base, design of Turn Buckle.

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