LECTURE # 38

1. LECTURE # 38

2. BOLT STRENGTH • The strength is specified by stating the minimum proof strength, or minimum proof load, and the minimum tensile strength • The proof load is the maximum load (force) that a bolt can withstand without acquiring a permanent set

3. The proof strength is the quotient of the proof load and the tensile-stress area • The proof strength is about 90 percent of the 0.2 percent offset yield strength • ASTM threads are shorter (deals mostly with structures) and generally loaded in shear

7. TENSION CONNECTIONS-THE EXTERNAL LOAD • A clamping force,(preload) Fi is applied by tightening the nut before external force, P is applied • Fi = preload • P = external tensile load • Pb = portion of P taken by bolt • Pm = portion of P taken by members

8. Fb = Pb + Fi = resultant bolt load • Fm = Pm – Fi = resultant load on the members • The load P is tension, and it causes the connection to stretch, or elongate, through some distance d and

9. Since, P = Pb + Pm • The resultant bolt load is Fm < 0

10. Fig. 15.9

11. Fig. 15.9

12. Fig. 15.10

13. Fig. 15.9

14. Fig. 15.10

15. Fig. 15.11

16. Fm < 0 • These results are valid only as long as some clamping load remains in the members

18. In all cases, the members take over 80 percent of the external load • Making the grip longer causes the members to take an even greater percentage of the external load

19. TORQUE REQUIREMENTS • A high preload is very desirable in important bolted connections • If the overall length of the bolt can actually be measured with a micrometer when it is assembled, the bolt elongation due to preload Fi can be computed using formula

20. The nut is simply tightened until the bolt elongates through the distance d • This ensures that the desired preload has been attained • The elongation of a screw cannot be measured, because the threaded end is often a blind hole

21. The torque wrench has a built-in dial which indicates the proper torque • With impact wrenching, the air pressure is adjusted so that the wrench stalls when the proper torque is obtained • Or in some wrenches, the air automatically shuts off at the desired torque

22. The snug-tight condition is the tightness attained by a few impacts of an impact wrench • Or the full effort of a person using an ordinary wrench • When the snug-tight condition is attained, all additional turning develops useful tension in the bolt

23. Turn-of-the-Nut Method • The turn-of-the-nut method is the easiest and least expensive method for installing fasteners with the proper bolt tension. • The procedure generally works as follows. An iron worker tightens the bolt and nut as tight as possible using a spud wrench or a pneumatic impact wrench

24. A chalk mark or paint is then made on the bolt and nut • The bolt is tightened further by either hammering on the spud wrench or using a pneumatic impact wrench until the rotating part has rotated the required amount • The paint or chalk mark shows how far the part has rotated and the rotation is always measured relative to the rotation of the bolt.

25. The turn-of-the-nut method requires that you compute the fractional number of turns necessary to develop the required preload from the snug-tight condition • For heavy hexagon structural bolts, the turn-of-th-nut specification states that the nut should be turned a minimum of 180o from the snug-tight condition under optimum conditions

26. A good estimation of the torque required to produce a given preload is

27. Th diameter of the washer face of a hexagonal nut is the same as the width across flats and equal to 1.5 times the nominal size • dc = (1+1.5d)/2 = 1.25d

28. Define a torque co-efficient K • T = K Fi d • The coefficient of friction depends upon the surface smoothness, accuracy, and degree of lubrication

29. On the average, both f and fc are about 0.15 • K = 0.26 for f = fc =0.15 no matter what size bolts are employed and no matter whether the threads are coarse or fine

31. BOLT PRELOAD-STATIC LOADING

32. The tensile stress in the bolt can be found by dividing both terms of first equation by the tensile-stress area, At • The limiting value of sb is the proof strength, Sb

33. With the introduction of a load factor n • Any value of n > 1 ensures that the bolt stress is less than the proof strength

34. JOINT SEPARATION For safe joint, external load be smaller than that needed to cause the joint to separate If separation does not occur, then the entire external load will be imposed on the bolt Let Po be the value of the external load that would cause joint separation Llllllllllllll

35. Fig. 15.15

36. At separation, Fm = 0, • (1 – C)Po – Fi =0 • Let the factor of saftey against joint separation be

37. Fig. 8.18

38. Fig. shows stress-strain diagram of a good quality bolt material • No clearly defined yield point • Fracture at the tensile strength • No matter how much preload is given to bolt, it will retain its load-carrying capacity

39. The pr-tension is the muscle of the joint and its magnitude is determined by the strength • If the full bolt strength is not used, then money is wasted • Good quality bolts can be preloaded into the plastic range to develop more strength • A bolt will either fracture during tightening or not at all

40. It is recommended for both static and fatigue loading that the following be used for preload : • Where FP is the proof load, obtained from the equation FP = At SP • SP is the proof strength obtained from tables 8-4 to 8-6

41. For other materials, an approximate value is SP = 0.85 SY

42. Fig. 8.17

43. Fig. 8.18

44. Fig. 8.19