1. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 1

2. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 2

3. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 3

4. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 4

5. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 5

6. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints 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 CH-8 LEC 35 Slide 6

7. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints 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 35 Slide 7

8. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Example-3 CH-8 LEC 35 Slide 8 A single-threaded 20 mm power screw is 20 mm in diameter with a pitch of 5 mm. A vertical load on the screw reaches a maximum of 3 kN. The coefficients of friction are 0.06 for the collar and 0.09 for the threads. The frictional diameter of the collar is 45 mm. Find the overall efficiency and the torque to "raise" and "lower" the load.

9. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Example-3 CH-8 LEC 35 Slide 9 Given

10. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Example-3 (Cont.’d) CH-8 LEC 35 Slide 10 Solution

11. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 11 Example-2 (Cont.’d)

12. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints The following stresses should be checked on both the nut and the screw: 1. Shearing stress in screw body. 2.Axial stress in screw body CH-8 LEC 35 Slide 12 Power Screws Stress Analysis (8-7) (8-8)

13. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints 3. Thread bearing stress CH-8 LEC 35 Slide 13 Power Screws Stress Analysis Figure 8-8 Geometry of square thread useful in finding bending and transverse shear stresses at the thread root (8-10) where n t is the number of engaged threads.

14. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints 4. Thread bending stress The bending stress at the root of the thread is given by CH-8 LEC 35 Slide 14 Power Screws Stress Analysis (8-11)

15. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints 5.Transverse shear stress at the center of the thread root CH-8 LEC 35 Slide 15 Power Screws Stress Analysis (8-12) Notice that the transverse shear stress at the top of the root is zero

16. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints The state of stress at top of root “plane” is CH-8 LEC 35 Slide 16 Power Screws Stress Analysis  Von-Mises Stress at top of root plane is calculated using Eq. (6-14) of Sec. (6-5) and failure criteria applied (see example 8-1).

17. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 17  The engaged threads cannot share the load equally. Some experiments show that the first engaged thread carries 0.38 of the load the second engaged thread carries 0.25 of the load the third engaged thread carries 0.18 of the load the seventh engaged thread is free of load  In estimating thread stresses by the equations above, substituting n t to 1 will give the largest level of stresses in the thread-nut combination Power Screws Stress Analysis

18. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Assuming that the column (screw) is a Johnson column CH-8 LEC 35 Slide 18 Power Screws Buckling where

19. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 19 Example-3 (Example 8-1 in Textbook)

20. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 20

21. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 21 Example-3 (Example 8-1 in Textbook)

22. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 22 Example-3 (Example 8-1 in Textbook)

23. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 23 Example-3 (Example 8-1 in Textbook)

24. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 24

25. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 35 Slide 25 Types of fasteners Three types of threaded fastener: (a) Screw (b)Bolt and nut; (c) Stud and nut, (d) Threaded rod and nuts

26. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Threaded Fasteners CH-8 LEC 35 Slide 26 A- BOLTS A- BOLTS:Purpose: to clamp two or more members together.Parts: (1) Head (Square or Hexagonal) (2) Washer ( d w =1.5d ) (3) Threaded part (4) Unthreaded part Dimensions of square and hexagonal bolts are given in TABLE A-29

27. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Threaded Fasteners CH-8 LEC 35 Slide 27 The diameter of the washer face is the same as the width across the flats of the hexagon. The thread length ( L T ) is : D is the nominal diameter

28. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Threaded Fasteners CH-8 LEC 35 Slide 28 B- NUTS B- NUTS: Same material as that of a screw Table A-31 gives dimensions of Hexagonal nuts Good Practice: 1.Never re-use nuts; 2.Tightening should be done such that 1 or 2 threads come out of the nut; 3.Washers should always be used under bolt head to prevent burr stress concentration.

29. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Common Cap screws Used for clamping members same as bolt except that 1 member should be threaded. The head of a hexagon-head cup screw H cap is slightly thinner than that of a hexagonal head bot H bolt. CH-8 LEC 35 Slide 29 Threaded Fasteners Figure 8-10 Typical cap-screw heads: (a) fillister head; (b) flat head; (c) hexagonal socket head

30. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Threaded Fasteners Figure 8.11: Types of heads used in machine screws CH-8 LEC 35 Slide 30

31. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Joints: Fastener Stiffness In joint under tension the members are under compression and the bolt under tension: k b = equivalent spring constant of bolt composed of threaded (k t ) and unthreaded (k d ) parts acting as springs in series For short bolts k b = k t kmkm From Ch 5 CH-8 LEC 35 Slide 31

32. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Joints: Fastener Stiffness To find different parameters use table 8-7 CH-8 LEC 35 Slide 32

33. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Joints: Member Stiffness Members act as springs under compression Equivalent spring constant k m Compression stress distribution from experimental data Frustum Integrating from 0 to l m CH-8 LEC 35 Slide 33

34. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Joints: Member Stiffness For Members made of Aluminum, hardened steel and cast iron 25 < a < 33° For a = 30° If Members have same E with symmetrical frusta (l= 2t) they act as 2 identical springs k m = k/2 For a = 30° and D = d w = 1.5d CH-8 LEC 35 Slide 34

35. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Other equations Joints: Member Stiffness From Finite element analysis results, A and B from table 8.8 for standard washer Faces and members Of same material CH-8 LEC 35 Slide 35

36. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Bolt Strength Bolt strength is specified by minimum proof strength S p or minimum proof load, F p and minimum tensile strength, Sut proof strength The SAE specifications are given in Table 8-9 bolt grades are numbered according to minimum tensile strength The ASTM Specs for steel bolts (structural) are in Table 8-10. Metric Specs are in table 8-11. If S p not available use S p =0.85 S y F p = A t S p CH-8 LEC 35 Slide 36

37. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Static Analysis a) External Load Tension Joints Equilibrium Compatibility Relation P-  (4) 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 External Load P is shared by bolt and members CH-8 LEC 35 Slide 37

38. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Static Analysis b) Resultant Bolt & member loads: F b & F m Tension Joints c) Torque required to give preload F i F m <0 F i is preload ; high preload is desirable in tension connections K is torque coefficient K values are given in table 8-15 (Average Value = 0.2) F i = 0.75 F p For re-use F i = 0.90 F p For permanent joint CH-8 LEC 35 Slide 38

39. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Static Analysis d) Joint Safety Tension Joints or 2) Joint separates Failure of Joint occurs when: 1) Bolt yields 0 = (1-C) P 0 -F i Let P 0 be external load causing separation F m =0 CH-8 LEC 35 Slide 39

40. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Static Analysis e) Gasketed Joints Tension Joints If a full gasket is present in joint The gasket pressure p is: 0 = (1-C) P 0 -F i To maintain uniformity of pressure adjacent bolts should not be placed more than 6 nominal diameters apart on bolt circle. To maintain wrench clearance bolts should be placed at least 3 d apart 33 66 D b is the diameter of the bolt circle CH-8 LEC 35 Slide 40

41. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints PbPb FiFi FmFm FaFa F max = F b FaFa Fatigue Analysis In general, bolted joints are subject to 0-P max, e.g pressure vessels, flanges, pipes, … Tension Joints CH-8 LEC 35 Slide 41

42. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Fatigue Analysis Tension Joints High Preload is especially important in fatigue.  i is a constant the load line at F i /A t has a unit slope, r=1.0 To find s a use Goodman or Gerber Eq. 8.42 or ASME-elliptic. Eq. 8.43 Safety factor (using Goodman) Check for yielding also using proof strength: Eq. 8.48 for conservative assessment of n: CH-8 LEC 35 Slide 42

43. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Fatigue Analysis Tension Joints In case of cut threads use the method described in chapter 7 with K f values of table 8-16. The fully corrected endurance limit for rolled threads is given in table 8-17 CH-8 LEC 35 Slide 43

44. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Fig. 8.19 CH-8 LEC 35 Slide 44

45. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Fig. 8.21 CH-8 LEC 35 Slide 45

46. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Failure Modes of Riveted Fasteners Under Shear Failure modes due to shear loading of riveted fasteners. (a) Bending of member; (b) shear of rivet; (c) tensile failure of member; (e) bearing of rivet on member or bearing of member on rivet. CH-8 LEC 35 Slide 46

47. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Shear Joints Centroid of pins, rivets or bolts CH-8 LEC 35 Slide 47

48. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Shear Joints Shear of pins, rivets bolts due to eccentric loading V & M statically indeterminate problem 4 steps (assuming same diameter bolts, load shared equally) 1)Direct load F’ Primary Shear 2)Centroid CH-8 LEC 35 Slide 48

49. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Shear Joints Shear of pins, rivets bolts due to eccentric loading 3)Moment load M Secondary Load 4)Add Vectorially the direct and moment loads The force taken by each bolt is proportional to its radial distance from the centroid If bolts are not same size only bolts with max. R should be considered CH-8 LEC 35 Slide 49

50. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints See Examples 8.6 and 8.7 CH-8 LEC 35 Slide 50

51. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Fig. 8.26 CH-8 LEC 35 Slide 51

52. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Problem 8-50 ½ in-13 UNC SAE 5 CH-8 LEC 35 Slide 52

53. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Problem 8-50 1)Direct load F’ 2)Centroid is at O 3)Secondary Load 4)Add Vectorially the direct and moment loads Is As=Ad? L T =2d+1/4=1.25” H nut +2p+3/8=7/16+2(1/13)+3/8=0.97”<1.25” So As=At = 0.1419in 2 and t B =12.7 Kpsi; n=4.2 CH-8 LEC 35 Slide 53

54. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Problem 8-50 CH-8 LEC 35 Slide 54

55. ME 307 Machine Design I ME 307 Machine Design I Dr. A. Aziz BazouneChapter 8: Screws, Fasteners and the Design of Nonpermanent Joints Problem 8-50 CH-8 LEC 35 Slide 55