Mechanical Engineering Design Chapter 8

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Chapter 8
Screws, Fasteners, and the Design of Nonpermanent
Joints
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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
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LECTURE 37
8-3 Strength Constraints 8-4 Joints-Fasteners
Stiffness 8-5 Joints-Member Stiffness 8-6 Bolt
Strength
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8-7 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

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8-7 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
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8-7 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
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8-8 Relating Bolt Torque to Bolt Tension
c) Torque Required to give preload Fi
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8-8 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.
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Example 4 (Example 8-3 Textbook)
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Example 4 (Cont.d)
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Example 4 (Cont.d)
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8-9 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
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8-9 Statically Loaded Tension Joint with Preload

• ii) Joint separates

Let P0 be external load causing separation Fm 0

• no factor of safety against joint separation

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8-10 Gasketed Joints
g) Gasketed Joints
If a full gasket is present in joint, The gasket
pressure p is
No. of bolts
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8-10 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.

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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

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

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