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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Announcements

• HW 5 Ch 18, on WebCT
• Due Date for HW 5 is Mon. DEC. 31, 2007
• Quiz on Ch. 18, Mon. DEC. 31, 2007

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LECTURE 33
8-1 Thread Standards and Definitions
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Introduction
The fundamental operation in manufacture is the
creation of shape - this includes assembly, where
a number of components are fastened or joined
together either permanently by welding (Ch9) for
example or detachably (nonpermanent) by screws,
nuts and bolts and so on. Since there is such a
variety of shapes in engineering to be assembled,
it is hardly surprising that there is more
variety in demountable fasteners than in any
other machine element. Fasteners based upon screw
threads are the most common, so it is important
that their performance is understood, and the
limitations of the fastened assemblies
appreciated.
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Introduction

• There are two distinct uses for screw threads and
  they usually demand different behavior from the
  threads
• a   power screw such as a lathe leadscrew or the
  screw in a car lifting jack which transforms
  rotary motion into substantial linear motion (or
  vice versa in certain applications), and

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Fasteners

• a   Threaded Fastener similar to a nut and bolt
  which joins a number of components together again
  by transforming rotary motion into linear motion,
  though in this case the translation is small.

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Thread Profiles
(c)
Thread profiles. a) Square (b) ACME (c) UN, ISO
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Thread Profile Parameters
Figure 8-1 Terminology of screw threads. Sharp
Vee Threads shown for clarity the crests and
roots are actually flattened or rounded during
the forming operation
LeadLn p
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Threads
(a) Single (STANDARD)-, (b) double-, and (c)
triple threaded screws.
Text Reference Figure 15.2, page 667
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Thread Systems

• A thread 'system' is a set of basic thread
  proportions which is scaled to different screw
  sizes to define the thread geometry. Whitworth,
  Sellers, British Standard Pipe (BSP) are just
  three of the many systems which proliferated
  before the adoption of the
• ISO Metric thread system.
• The American National (Unified) Thread standards
  is used mainly in the US.
• Square and ACME

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Thread geometry
The basic profile of ISO Metric threads is built
up from contiguous equiangular triangles of
height  H disposed symmetrically about a pitch
line which becomes the   pitch cylinder of
diameter   d2 when the profile is rotated about
the axis to form the thread. The distance between
adjacent triangles - the pitch - is   p 2 H
/v3. The tips of the triangles are truncated by
h/8 to form the major diameter ( size ) d of the
thread, and the bases are truncated by h/4 to
form the minor diameter  d1 . It follows that d1
 d - 5 h/4 d - 1.08 p. This leads to the rule
of thumb for suitable tapping size
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Thread Profile For Metric System (M, MJ)
major
H 0.5(3)1/2 p
pitch
US N threads/in
minor
a ISO 68 a American National (Unified) thread
standard 60
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Thread Standards

• ISO Metric thread system Table 8-1 Major
  diameter (mm),
• 2a 60 Standard thread is RH
• Specifications e.g. M12x1.75 or
• M Basic Metric, J round root 12 nominal
  major diameter (mm) 1.75 pitch (mm)
• Tensile stress area At p/4(dmdr)/2 (see
  footnote T.8.1)

MJ12x1.75
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Thread Standards

• The American National (Unified) Thread standards
  is used mainly in the US Table-8-2 (Size
  designation) use d
• UNregular thread, UNRround root (use root
  radius)
• Specifications 5/8-18 UN, UNC, UNF
• UNR, UNRC, UNRF
• 5/8d 18 N (thread size)
• UN Unified, Ffine, CCoarse, R Round
  Root

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Thread Standards

• Square (a) and The ACME Threads (b)-used mainly
  in power screws Table 8.3 gives preferred pitches
  for ACME threads

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Power screw thread forms. Note All threads
shown are external (i.e., on the screw, not on
the nut) dm is the mean diameter of the thread
contact and is approximately equal to (d dr)/2.
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ACME Thread Profile
Figure 15.5 Details of ACME thread profile.
(All dimensions are in inches.)
Text Reference Figure 15.5, page 670
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ACME Thread Properties
N
Crest diameters, threads per inch, and stresses
for Acme thread.
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Mechanics of Power Screws

• A power screw is a device that is common to tools
  or machinery that are used to change angular
  motion into translation. It is also capable of
  developing a large amount of mechanical
  advantage. Familiar applications include clamps
  or vises, presses, lathes lead screws, and jacks.

The joyce warm-gear screw jack
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Mechanics of Power Screws
Weight supported by three screw jacks. In each
screw jack, only the shaded member rotates.
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Mechanics of Power Screws
Example of catalogue http//www.roton.com/web/qui
ck.power_screws.jsp
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Vehicle Screw Jack
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Power Screw with collar
Different types of collars