Fastening (more complex shapes = better function)

1. Fastening (more complex shapes = better function) • Non-permanent • Bolted • Permanent • Bolted • Welded • Bonded

2. Outline • General Thread Nomenclature & Types • Power Screws • Stresses in Threads • Preloading Fasteners/Joints • Fasteners in Shear

3. Threads p pitch in./thread d diameter (major) in. dp pitch diameter in. dr minor diameter in. L Lead in.

5. UNC –coarse UNF –fine UNEF –extra fine coarse fine Thread Pitch Class 1 Class 2 Class 3 several levels Tolerance d=12mm metric fine Class 2 d=0.25” p=1.75 mm/thread external threads 20 threads/in. Screw Classifications ISO (Metric) Unified National Standard ¼-20 UNF –2A M12 x 1.75 **see Tables 14-1 and 14-2 for standard sizes**

6. At also in Tables 14-1 and 14-2 Tensile Stress F F

7. Outline • General Thread Nomenclature & Types • Power Screws • Threads • Loads • Self-locking • Efficiency • Stresses in Threads • Preloading Fasteners/Joints • Fasteners in Shear

8. Power Screw Applications Where have you seen power screws? • jacks for cars • C-clamps • vises • Instron material testing machines • machine tools (for positioning of table)

9. Power Screw Types • Square • strongest • no radial load • hard to manufacture • Acme • 29° included angle • easier to manufacture • common choice for loading in both directions • Buttress (contrafuerte) • great strength • only unidirectional loading

10. P y x f F L N  dp LIFTING Load Analysis What “simple machine” does a power screw utilize?

11. More Completely… LIFTING LOWERING P y x f F L N  dp

12. LIFTING LOWERING For Acme Threads 

13. Friction Coefficients oil lubricated= collar w/ bushing=0.15 ± 0.05 collar w/ bearing=0.015 ± 0.005

14. Self-Locking / Back Driving self-locking – screw cannot turn from load P back-driving – screw can be turned from load P for self-locking: would square or Acme of same dimensions lock first?

15. Efficiency -for lifting- higher efficiency for lowering (also derive with frictionless torque/torque)

16. Ball Screw

17. Outline • General Thread Nomenclature & Types • Power Screws • Stresses in Threads • Body Stresses • Axial • Torsion • Thread Stresses • Bearing • Bending • Buckling • Preloading Fasteners/Joints • Fasteners in Shear

18. At also in Tables 14-1 and 14-2 Tensile Stress F F

19. Torsional Stress depends on friction at screw-nut interface • For screw and nut, • if totally locked (rusted together), the screw experiences all of torque • if frictionless, the screw experiences none of the torque • For power screw, • if low collar friction, the screw experiences nearly all of torque • if high collar friction, the nut experiences most of the torque

20. Thread Stresses – Bearing F p/2 p/2 Abearing=(p/2)(dpnt)

21. Thread Stresses – Bending F p/2 p/2 transverse shear is also present, but max stress will be at top of tooth For both bearing and bending, F and nt are dependent on how well load is shared among teeth, therefore use Factual=0.38F and nt=1 (derived from experiments)

22. Mohr’s Circle F p/2 p/2 z y x

23. Buckling use dr

24. Outline • General Thread Nomenclature & Types • Power Screws • Stresses in Threads • Preloading Fasteners/Joints • Proof Strength • Spring Behavior • Loading & Deflection • Separation of Joints • Fasteners in Shear

25. Preloading • static loading: preload at roughly 90% of Sp • dynamic loading: preload at roughly 75% of Sp Preloading & Proof Strength • Sp stress at which bolt begins to take a permanent set

26. Spring Behavior BOTH material being clamped and bolt behave as springs (up to yield/permanent set stresses) for the bolt, threaded vs unthreaded have different spring constants: applied load P

27. Area is hard to define… from experiments, the following is accurate: When no edges nearby and same materials, even simpler form can be used: A and b are from Table 14-9, pg 916 Affected Area of Material For material, basic model is as follows (shown for 2 materials being clamped)

28. Pb Fb P Pm Fm m1 b1 P=Pb+Pm Papplied relieves compression in material & adds tension to bolt Fm=Fi-Pm Fb=Fi+Pb Loading & Deflection F Fi m b  MATERIAL BOLT

29. Distribution of Applied Load b= m

30. Applied Load to Equal Sp How many times more would the loading on the bolt need to be to incur permanent set? (assuming no material separation)

31. Yielding Safety Factor Fm = Fi + P(C-1) Fb = Fi + CP Ny=Sy/b

32. Separation Separation occurs when Fm=0 Fm = Fi + P(C-1)

33. Strategy Reviewed See Example 14-2, p. 906 Given: joint dimensions Find: bolt set preload equal to 90% Sp find lt so that you can find kb find km calculate C, then Pb, Pm, then Fb, Fm find stress in bolt and separation load Such that: factors of safety>1

34. Dynamic Loading of Fasteners • Bolt only absorbs small % of P • Stresses • Bolt is in tension • Material is in compression • Fatigue is a tensile failure phenomenon •  Preloading helps tremendously in fatigue

35. Outline • General Thread Nomenclature & Types • Power Screws • Stresses in Threads • Preloading Fasteners/Joints • Fasteners in Shear • What is Shear? • Straight Direct Shear

36. Direct Shear

37. Doweled Joints “It is not considered good practice to use bolts or screws in shear to locate and support precision machine parts under shear loads” • Shear can be handled by friction caused by bolts… but, better practice is to use dowels • Bolts need clearances… at best 2 out of a 4 bolt pattern will bear all of load Norton dowels support shear, but not tensile loads bolts support tensile loads, but not shear

38. Direct Shear Ashear=2x(cross sxn of dowels) dowels support shear, but not tensile loads bolts support tensile loads, but not shear

39. Outline Revisited • General Thread Nomenclature & Types • Power Screws • Stresses in Threads • Preloading Fasteners/Joints • Fasteners in Shear

40. Chapter 9 Welding, Brazing, Bending, and the Design of Permanent Joints From Shigley & Mischke, Mechanical Engineering Design Part 3 Design of Mechanical Elements

41. Welding Symbols

42. Butt Welds

43. Fillet Welds

44. Welding Issues • Requires • Careful Design • Skilled Welder • Can Cause • Weakened adherends • Thermal distortion • Removal of heat treatment

45. Welding References • AWS (American Welding Society) • Lincoln Electric • ASME Codes & Standards • Pressure Vessels & Piping • Nuclear Installations • Safety Codes • Performance Test Codes

46. Bonded Joints (thin members)

47. Bonded Joint Types

48. More Types

49. Peel Stresses

50. Good Practices