# Lecture Slides. Chapter 8. Screws, Fasteners, and the Design of Nonpermanent Joints

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1 Lecture Slides Chapter 8 Screws, Fasteners, and the Design of Nonpermanent Joints The McGraw-Hill Companies 2012

2 Chapter Outline

3 Reasons for Non-permanent Fasteners Field assembly Disassembly Maintenance Adjustment

4 Thread Standards and Definitions Pitch distance between adjacent threads. Reciprocal of threads per inch Major diameter largest diameter of thread Minor diameter smallest diameter of thread Pitch diameter theoretical diameter between major and minor diameters, where tooth and gap are same width Fig. 8 1

6 Standardization Coarse series UNC General assembly Frequent disassembly Not good for vibrations The normal thread to specify Fine series UNF Good for vibrations Good for adjustments Automotive and aircraft Extra Fine series UNEF Good for shock and large vibrations High grade alloy Instrumentation Aircraft

7 Standardization Basic profile for metric M and MJ threads shown in Fig. 8 2 Tables 8 1 and 8 2 define basic dimensions for standard threads Fig. 8 2

8 Diameters and Areas for Metric Threads

9 Table 8 2 Diameters and Areas for Unified Screw Threads

10 Tensile Stress Area The tensile stress area, A t, is the area of an unthreaded rod with the same tensile strength as a threaded rod. It is the effective area of a threaded rod to be used for stress calculations. The diameter of this unthreaded rod is the average of the pitch diameter and the minor diameter of the threaded rod.

11 Square and Acme Threads Square and Acme threads are used when the threads are intended to transmit power Fig. 8 3 Table 8-3 Preferred Pitches for Acme Threads

12 Power screw Mechanics of Power Screws Used to change angular motion into linear motion Usually transmits power Examples include vises, presses, jacks, lead screw on lathe Fig. 8 4

13 Mechanics of Power Screws Find expression for torque required to raise or lower a load Unroll one turn of a thread Treat thread as inclined plane Do force analysis Fig. 8 5 Fig. 8 6

14 For raising the load Mechanics of Power Screws For lowering the load Fig. 8 6

15 Mechanics of Power Screws Eliminate N and solve for P to raise and lower the load Divide numerator and denominator by cosl and use relation tanl = l /p d m

16 Raising and Lowering Torque Noting that the torque is the product of the force and the mean radius,

17 Self-locking Condition If the lowering torque is negative, the load will lower itself by causing the screw to spin without any external effort. If the lowering torque is positive, the screw is self-locking. Self-locking condition is p f d m > l Noting that l / p d m = tan l, the self-locking condition can be seen to only involve the coefficient of friction and the lead angle.

18 Power Screw Efficiency The torque needed to raise the load with no friction losses can be found from Eq. (8 1) with f = 0. The efficiency of the power screw is therefore

19 Power Screws with Acme Threads If Acme threads are used instead of square threads, the thread angle creates a wedging action. The friction components are increased. The torque necessary to raise a load (or tighten a screw) is found by dividing the friction terms in Eq. (8 1) by cosa. Fig. 8 7

20 An additional component of torque is often needed to account for the friction between a collar and the load. Assuming the load is concentrated at the mean collar diameter d c Collar Friction Fig. 8 7

22 Example 8-1 Fig. 8 4

23 Example 8-1 Fig. 8 3a

24 Example 8-1

25 Example 8-1

26 Example 8-1

27 Example 8-1

28 Example 8-1

29 Example 8-1

30 Power Screw Safe Bearing Pressure

31 Power Screw Friction Coefficients

32 Head Type of Bolts Hexagon head bolt Usually uses nut Heavy duty Hexagon head cap screw Thinner head Often used as screw (in threaded hole, without nut) Socket head cap screw Usually more precision applications Access from the top Machine screws Usually smaller sizes Slot or philips head common Threaded all the way Fig. 8 9 Fig. 8 10

33 Machine Screws Fig. 8 11

34 Hexagon-Head Bolt Hexagon-head bolts are one of the most common for engineering applications Standard dimensions are included in Table A 29 W is usually about 1.5 times nominal diameter Bolt length L is measured from below the head

36 Nuts See Appendix A 31 for typical specifications First three threads of nut carry majority of load Localized plastic strain in the first thread is likely, so nuts should not be re-used in critical applications. End view Washer-faced, regular Chamfered both sides, regular Fig Washer-faced, jam nut Chamfered both sides, jam nut

37 Grip length l includes everything being compressed by bolt preload, including washers Washer under head prevents burrs at the hole from gouging into the fillet under the bolt head Tension Loaded Bolted Joint Fig. 8 13

38 Hex-head cap screw in tapped hole used to fasten cylinder head to cylinder body Note O-ring seal, not affecting the stiffness of the members within the grip Only part of the threaded length of the bolt contributes to the effective grip l Pressure Vessel Head Fig. 8 14

39 Effective Grip Length for Tapped Holes For screw in tapped hole, effective grip length is

40 Bolted Joint Stiffnesses During bolt preload bolt is stretched members in grip are compressed When external load P is applied Bolt stretches further Members in grip uncompress some Joint can be modeled as a soft bolt spring in parallel with a stiff member spring Fig. 8 13

41 Axially loaded rod, partly threaded and partly unthreaded Consider each portion as a spring Combine as two springs in series Bolt Stiffness

42 Procedure to Find Bolt Stiffness

43 Procedure to Find Bolt Stiffness

44 Procedure to Find Bolt Stiffness

45 Member Stiffness Stress distribution spreads from face of bolt head and nut Model as a cone with top cut off Called a frustum

46 Member Stiffness Model compressed members as if they are frusta spreading from the bolt head and nut to the midpoint of the grip Each frustum has a half-apex angle of a Find stiffness for frustum in compression Fig. 8 15

47 Member Stiffness

48 With typical value of a = 30º, Member Stiffness Use Eq. (8 20) to find stiffness for each frustum Combine all frusta as springs in series Fig. 8 15b

49 Member Stiffness for Common Material in Grip If the grip consists of any number of members all of the same material, two identical frusta can be added in series. The entire joint can be handled with one equation, d w is the washer face diameter Using standard washer face diameter of 1.5d, and with a = 30º,

50 Finite Element Approach to Member Stiffness For the special case of common material within the grip, a finite element model agrees with the frustum model Fig. 8 16

51 Finite Element Approach to Member Stiffness Exponential curve-fit of finite element results can be used for case of common material within the grip

52 Bolt Materials Grades specify material, heat treatment, strengths Table 8 9 for SAE grades Table 8 10 for ASTM designations Table 8 11 for metric property class Grades should be marked on head of bolt

53 Bolt Materials Proof load is the maximum load that a bolt can withstand without acquiring a permanent set Proof strength is the quotient of proof load and tensile-stress area Corresponds to proportional limit Slightly lower than yield strength Typically used for static strength of bolt Good bolt materials have stress-strain curve that continues to rise to fracture Fig. 8 18

54 Table 8 9 SAE Specifications for Steel Bolts

55 Table 8 10 ASTM Specification for Steel Bolts

56 Metric Mechanical-Property Classes for Steel Bolts Table 8 11

57 Threads per inch Bolt Specification Thread series Material grade ¼-20 x ¾ in UNC-2 Grade 5 Hex head bolt Nominal diameter length Class fit Head type Metric Pitch M12 x 1.75 ISO 4.8 Hex head bolt Nominal diameter Material class

59 Tension Loaded Bolted Joints During bolt preload bolt is stretched members in grip are compressed When external load P is applied Bolt stretches an additional amount d Members in grip uncompress same amount d Fig. 8 13

60 Since P = P b + P m, Stiffness Constant C is defined as the stiffness constant of the joint C indicates the proportion of external load P that the bolt will carry. A good design target is around 0.2.

61 The resultant bolt load is Bolt and Member Loads The resultant load on the members is These results are only valid if the load on the members remains negative, indicating the members stay in compression.

62 Relating Bolt Torque to Bolt Tension Best way to measure bolt preload is by relating measured bolt elongation and calculated stiffness Usually, measuring bolt elongation is not practical Measuring applied torque is common, using a torque wrench Need to find relation between applied torque and bolt preload

63 Relating Bolt Torque to Bolt Tension From the power screw equations, Eqs. (8 5) and (8 6), we get Applying tanl = l/pd m, Assuming a washer face diameter of 1.5d, the collar diameter is d c = (d + 1.5d)/2 = 1.25d, giving

64 Relating Bolt Torque to Bolt Tension Define term in brackets as torque coefficient K

65 Typical Values for Torque Coefficient K Some recommended values for K for various bolt finishes is given in Table 8 15 Use K = 0.2 for other cases

66 Distribution of Preload vs Torque Measured preloads for 20 tests at same torque have considerable variation Mean value of 34.3 kn Standard deviation of 4.91 Table 8 13

67 Distribution of Preload vs Torque Same test with lubricated bolts Mean value of kn (unlubricated 34.3 kn) Standard deviation of 2.88 kn (unlubricated 4.91 kn) Table 8 14 Lubrication made little change to average preload vs torque Lubrication significantly reduces the standard deviation of preload vs torque

68 Tension Loaded Bolted Joints: Static Factors of Safety Axial Stress: Yielding Factor of Safety: Load Factor: Joint Separation Factor:

70 Fatigue Loading of Tension Joints Fatigue methods of Ch. 6 are directly applicable Distribution of typical bolt failures is 15% under the head 20% at the end of the thread 65% in the thread at the nut face Fatigue stress-concentration factors for threads and fillet are given in Table 8 16

71 Endurance Strength for Bolts Bolts are standardized, so endurance strengths are known by experimentation, including all modifiers. See Table Fatigue stress-concentration factor K f is also included as a reducer of the endurance strength, so it should not be applied to the bolt stresses. Ch. 6 methods can be used for cut threads.

72 Fatigue Stresses With an external load on a per bolt basis fluctuating between P min and P max,

73 Typical Fatigue Load Line for Bolts Typical load line starts from constant preload, then increases with a constant slope Fig. 8 20

74 Equation of load line: Typical Fatigue Load Line for Bolts Equation of Goodman line: Solving (a) and (b) for intersection point, Fig. 8 20

75 Fatigue Factor of Safety Fatigue factor of safety based on Goodman line and constant preload load line, Other failure curves can be used, following the same approach.

76 Repeated Load Special Case Bolted joints often experience repeated load, where external load fluctuates between 0 and P max Setting P min = 0 in Eqs. (8-35) and (8-36), With constant preload load line, Load line has slope of unity for repeated load case

77 Repeated Load Special Case Intersect load line equation with failure curves to get intersection coordinate S a Divide S a by s a to get fatigue factor of safety for repeated load case for each failure curve. Load line: Goodman: Gerber: ASME-elliptic:

79 Further Reductions for Goodman For convenience, s a and s i can be substituted into any of the fatigue factor of safety equations. Doing so for the Goodman criteria in Eq. (8 45), If there is no preload, C = 1 and F i = 0, resulting in Preload is beneficial for resisting fatigue when n f / n f0 is greater than unity. This puts an upper bound on the preload,

80 Yield Check with Fatigue Stresses As always, static yielding must be checked. In fatigue loading situations, since s a and s m are already calculated, it may be convenient to check yielding with This is equivalent to the yielding factor of safety from Eq. (8 28).

81 Bolted and Riveted Joints Loaded in Shear Shear loaded joints are handled the same for rivets, bolts, and pins Several failure modes are possible (a) Joint loaded in shear (b) Bending of bolt or members (c) Shear of bolt (d) Tensile failure of members (e) Bearing stress on bolt or members (f) Shear tear-out (g) Tensile tear-out Fig. 8 23

82 Failure by Bending Bending moment is approximately M = Ft / 2, where t is the grip length, i.e. the total thickness of the connected parts. Bending stress is determined by regular mechanics of materials approach, where I/c is for the weakest member or for the bolt(s).

83 Simple direct shear Failure by Shear of Bolt Use the total cross sectional area of bolts that are carrying the load. For bolts, determine whether the shear is across the nominal area or across threaded area. Use area based on nominal diameter or minor diameter, as appropriate.

84 Simple tensile failure Failure by Tensile Rupture of Member Use the smallest net area of the member, with holes removed

85 Failure by Bearing Stress Failure by crushing known as bearing stress Bolt or member with lowest strength will crush first Load distribution on cylindrical surface is non-trivial Customary to assume uniform distribution over projected contact area, A = td t is the thickness of the thinnest plate and d is the bolt diameter

86 Failure by Shear-out or Tear-out Edge shear-out or tear-out is avoided by spacing bolts at least 1.5 diameters away from the edge

87 Shear Joints with Eccentric Loading Eccentric loading is when the load does not pass along a line of symmetry of the fasteners. Requires finding moment about centroid of bolt pattern Centroid location Fig. 8 27a

88 (a) Shear Joints with Eccentric Loading Example of eccentric loading (b) Free body diagram (c) Close up of bolt pattern Fig. 8 27

90 Example 8-7 Fig. 8 28

91 Example 8-7 Fig. 8 28

92 Example 8-7 Fig. 8 29

93 Example 8-7

94 Example 8-7

95 Example 8-7

96 Example 8-7

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