Title: Mechanical Engineering Design Chapter 8
1Chapter 8
Screws, Fasteners, and the Design of Nonpermanent
Joints
2Chapter 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
3LECTURE 37
8-3 Strength Constraints 8-4 Joints-Fasteners
Stiffness 8-5 Joints-Member Stiffness 8-6 Bolt
Strength
48-7 Tension Joints-The External Load
Static Analysis a) External Load
- Let us consider what happens when an external
tensile load P, is applied to a bolt connection. - Assuming 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
- Fb Pb Fi resultant bolt load
- Fm Pm Fi resultant load on the members
- C fraction of external load P carried by bolt
- 1-C fraction of external load P carried by
members -
-
58-7 Tension Joints-The External Load
- External load P is shared by bolt and members.
The load P is tension, and it causes the
connection to stretch, or elongate, through some
distance d -
-
-
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
68-7 Tension Joints-The External Load
b) Resultant Bolt and Member Load Fb Fm
(8-24)
(8-25)
Fi is preload high preload is desirable in
tension connections.
Fi 0.75 Fp for re-use Fi 0.90 Fp for
permanent joint
78-8 Relating Bolt Torque to Bolt Tension
c) Torque Required to give preload Fi
88-8 Relating Bolt Torque to Bolt Tension
K is torque coefficient K values are given in
TABLE 8-15 Average Value for K 0.2 for f fc
0.15 no matter what size bolts are employed and
no matter whether the threads are coarse or fine.
(8-26)
(8-27)
The coefficient of friction depends upon the
surface smoothness, accuracy, and degree of
lubrication. On the average, both f and fc are
about 0.15.
9Example 4 (Example 8-3 Textbook)
10Example 4 (Cont.d)
11Example 4 (Cont.d)
128-9 Statically Loaded Tension Joint with Preload
d) Statically Loaded Tension Joint with Preload
- Failure of joints occurs when
- i) Bolt Yields
Proof strength
At Tensile stress area
138-9 Statically Loaded Tension Joint with Preload
Let P0 be external load causing separation Fm 0
- no factor of safety against joint separation
148-10 Gasketed Joints
g) Gasketed Joints
If a full gasket is present in joint, The gasket
pressure p is
No. of bolts
158-10 Gasketed Joints
- IMPORTANT
- 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 . - A rough rule for bolt spacing around a bolt
circle is -
- where Db is the diameter of the bolt circle
and N is the number of bolts.
16Example 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
17- From Table A-17, the next fraction size bolt is 2
1/4 in.
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