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Mechanical Engineering Design Chapter 8

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8-7 Tension Joints-The External Load. 8-8 Relating Bolt Torque to Bolt Tension ... Assuming a clamping force, (preload Fi )is applied by tightening the nut before ... – PowerPoint PPT presentation

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Title: Mechanical Engineering Design Chapter 8


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 37
8-3 Strength Constraints 8-4 Joints-Fasteners
Stiffness 8-5 Joints-Member Stiffness 8-6 Bolt
Strength
4
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

5
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
6
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
7
8-8 Relating Bolt Torque to Bolt Tension
c) Torque Required to give preload Fi
8
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.
9
Example 4 (Example 8-3 Textbook)
10
Example 4 (Cont.d)
11
Example 4 (Cont.d)
12
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
13
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

14
8-10 Gasketed Joints
g) Gasketed Joints
If a full gasket is present in joint, The gasket
pressure p is
No. of bolts
15
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.

16
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

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

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