CH-8 LEC 38 Slide 1

1
Chapter 8
Screws, Fasteners, and the Design of Nonpermanent
Joints
2
Chapter Outline
8-1 Thread Standards and Definitions8-2 The
Mechanics of Power Screws8-3 Strength
Constraints 8-4 Joints-Fasteners
Stiffness 8-5 Joints-Member Stiffness 8-6 Bolt
Strength 8-7 Tension Joints-The External
Load8-8 Relating Bolt Torque to Bolt
Tension 8-9 Statically Loaded Tension Joint with
Preload 8-10 Gasketed Joints 8-11 Fatigue Loading
of Tension Joints 8-12 Shear Joints 8-13 Setscrew
s 8-14 Keys and Pins 8-15 Stochastic
Considerations
3
LECTURE 38
8-11 Fatigue Loading of Tension Joints 8-12 Shear
Joints 8-13 Setscrews 8-14 Keys and Pins
4
Example 5 (Example 8-4 Textbook)

• The grip is l 1.50 in. From Table A-31, the nut
  thickness is 35/64 in.
• Adding two threads beyond the nut of 2/11 in
  gives a bolt length of

5

1. From Table A-17, the next fraction size bolt is 2
   1/4 in.

6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
8-7 Tension Joints-The External Load
Fatigue Analysis a) External Load
11
8-11 Fatigue Loading of Tension Joints
Fatigue Analysis

• In general, bolted joints are subject to 0-Pmax,
  e.g pressure vessels, flanges, pipes,

12
8-11 Fatigue Loading of Tension Joints

• Table 8-16 lists average fatigue
  stress-concentration factors for the fillet under
  the bolt head and also at the beginning of the
  threads on the bolt shank.
• These are already corrected for notch sensitivity
  and for surface finish.
• Use of rolled threads is the predominant method
  of thread-forming in screw fasteners, where Table
  8-16 applies.

13
8-11 Fatigue Loading of Tension Joints

• In thread-rolling the amount of cold-work and
  strain strengthening is unknown to the designer
  therefore, fully corrected (including Kf) axial
  endurance strength is reported in Table 8-17.

14
8-11 Fatigue Loading of Tension Joints

• Fatigue-loaded bolted joints subjected to Fatigue
  action can be analyzed directly by the methods of
  Chapter 7.
• Fatigue loading is the one in which the
  externally applied load fluctuates between zero
  and some maximum force P.
• Fmax Fb and Fmin Fi
• Fa (Fmax Fmin)/2 (Fb Fi)/2
• sa Fa/At

15
8-11 Fatigue Loading of Tension Joints

• Fmax Fb and Fmin Fi
• Fa (Fmax Fmin)/2 (Fb Fi)/2
• Fm (Fmax Fmin)/2 (Fb Fi)/2
• sa Fa/At

(8-34)
(8-35)
(8-36)
16

• On the designers fatigue diagram, shown in
  Figure 8-20, the load line is .
• High Preload is especially important in fatigue.
  si is a constant the load line at Fi/At has a
  unit slope, r 1.0

Figure 8-20 Designers fatigue diagram showing a
Goodman failure locus and how a load line is used
to define failure and safety in preloaded bolted
joints in fatigue.
17
8-11 Fatigue Loading of Tension Joints

• Next, find the strength components Sa and Sm of
  the fatigue failure locus. These depend on the
  failure Criteria.

• Goodman

(8-37)

• Gerber

(8-38)

• ASME-Elliptic

(8-39)
18

• For simultaneous solution between Eq. (8-36), as
  Sa Sm si and each of Eqs. (8-37) to (8-39)
  gives

• Goodman

(8-40)
(8-41)

• Gerber

(8-42)

• ASME-Elliptic

(8-43)
19

• When using relations of this section, be sure to
  use Kf for both sa and sm. Otherwise, the slope
  of the load line will not remain 1 to 1.
• The factor of safety guarding against fatigue is
  given by
• Applying this to the Goodman criterion, for
  example, with Eqs. (8-34) and (8-40) and
  gives

(8-44)
(8-45)
20
(No Transcript)
21
Example 6 (Example 8-5 Textbook)
Figure 8-21 Pressure-cone frustum member model
for a cap screw. For this model the significant
sizes are Where l effective grip. The
solutions are for ?30o and dw1.5d.
22
(No Transcript)
23
(No Transcript)
24
Figure 8-22 Designers fatigue diagram for
preloaded bolts, dr showing the modified Goodman
locus, the Gerber locus and the Langer proof
strength locus, with an exploded view of the area
of interest, The strengths used are Sp 85 kpsi,
Se 18.6 kpsi, and Sut120 kpsi. The coordinates
are A si63.72 kpsi B sa3.1 kpsi, sm66.82
kpsi C Sa7.55 kpsi, Sm71.29 kpsi D Sa10.64
kpsi, Sm74.35 kpsi E Sa11.32 kpsi, Sm75.04
kpsi
25
(No Transcript)
26
(No Transcript)
27
(No Transcript)