Fastening%20(more%20complex%20shapes%20=%20better%20function)

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Fastening (more complex shapes better function)

• Non-permanent
• Bolted
• Permanent
• Bolted
• Welded
• Bonded

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Outline

• General Thread Nomenclature Types
• Power Screws
• Stresses in Threads
• Preloading Fasteners/Joints
• Fasteners in Shear

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Threads
p pitch in./thread d diameter
(major) in. dp pitch diameter in. dr minor
diameter in. L Lead in.
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(No Transcript)
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Screw Classifications
Unified National Standard
ISO (Metric)
¼-20 UNF 2A
M12 x 1.75
see Tables 14-1 and 14-2 for standard sizes
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Tensile Stress
F
F
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Outline

• General Thread Nomenclature Types
• Power Screws
• Threads
• Loads
• Self-locking
• Efficiency
• Stresses in Threads
• Preloading Fasteners/Joints
• Fasteners in Shear

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Power Screw Applications
Where have you seen power screws?

• jacks for cars
• C-clamps
• vises
• Instron material testing machines
• machine tools (for positioning of table)

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Power Screw Types

• Square
• strongest
• no radial load
• hard to manufacture
• Acme
• 29 included angle
• easier to manufacture
• common choice for loading in both directions
• Buttress (contrafuerte)
• great strength
• only unidirectional loading

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Load Analysis
What simple machine does a power screw utilize?
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More Completely
LIFTING
LOWERING
P
y
x
f
F
L
N
?
?dp
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For Acme Threads
?
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Friction Coefficients
?oil lubricated ?collar w/ bushing0.15 0.05
?collar w/ bearing0.015 0.005
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Self-Locking / Back Driving
self-locking screw cannot turn from load
P back-driving screw can be turned from load P
for self-locking
would square or Acme of same dimensions lock
first?
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Efficiency
-for lifting- higher efficiency for lowering
(also derive with frictionless torque/torque)
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Ball Screw
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Outline

• General Thread Nomenclature Types
• Power Screws
• Stresses in Threads
• Body Stresses
• Axial
• Torsion
• Thread Stresses
• Bearing
• Bending
• Buckling
• Preloading Fasteners/Joints
• Fasteners in Shear

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Tensile Stress
F
F
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Torsional Stress
depends on friction at screw-nut interface

• For screw and nut,
• if totally locked (rusted together), the screw
  experiences all of torque
• if frictionless, the screw experiences none of
  the torque

• For power screw,
• if low collar friction, the screw experiences
  nearly all of torque
• if high collar friction, the nut experiences most
  of the torque

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Thread Stresses Bearing
F
p/2
p/2
Abearing(p/2)(?dpnt)
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Thread Stresses Bending
F
p/2
p/2
transverse shear is also present, but max stress
will be at top of tooth
For both bearing and bending, F and nt are
dependent on how well load is shared among teeth,
therefore use Factual0.38F and nt1 (derived
from experiments)
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Mohrs Circle
F
p/2
p/2
z
y
x
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Buckling
use dr
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Outline

• General Thread Nomenclature Types
• Power Screws
• Stresses in Threads
• Preloading Fasteners/Joints
• Proof Strength
• Spring Behavior
• Loading Deflection
• Separation of Joints
• Fasteners in Shear

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Preloading Proof Strength

• Sp ? stress at which bolt begins to take a
  permanent set

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Spring Behavior
BOTH material being clamped and bolt behave as
springs (up to yield/permanent set stresses)
for the bolt, threaded vs unthreaded have
different spring constants
applied load P
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Affected Area of Material
For material, basic model is as follows (shown
for 2 materials being clamped)
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Loading Deflection
F
Fi
?
?b
?m
BOLT
MATERIAL
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Distribution of Applied Load
??b ??m
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Applied Load to Equal Sp
How many times more would the loading on the bolt
need to be to incur permanent set? (assuming no
material separation)
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Yielding Safety Factor
Fm Fi P(C-1) Fb Fi CP
NySy/?b
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Separation
Separation occurs when Fm0
Fm Fi P(C-1)
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Strategy Reviewed
See Example 14-2, p. 906
Given joint dimensions
Find bolt
set preload equal to 90 Sp find lt so that you
can find kb find km calculate C, then Pb, Pm,
then Fb, Fm find stress in bolt and separation
load
Such that factors of safetygt1
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Dynamic Loading of Fasteners

• Bolt only absorbs small of P
• Stresses
• Bolt is in tension
• Material is in compression
• Fatigue is a tensile failure phenomenon
• ? Preloading helps tremendously in fatigue

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Outline

• General Thread Nomenclature Types
• Power Screws
• Stresses in Threads
• Preloading Fasteners/Joints
• Fasteners in Shear
• What is Shear?
• Straight Direct Shear

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Direct Shear
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Doweled Joints
It is not considered good practice to use bolts
or screws in shear to locate and support
precision machine parts under shear loads
Norton

• Shear can be handled by friction caused by bolts
  but, better practice is to use dowels
• Bolts need clearances at best 2 out of a 4 bolt
  pattern will bear all of load

dowels support shear, but not tensile loads bolts
support tensile loads, but not shear
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Direct Shear
Ashear2x(cross sxn of dowels)
dowels support shear, but not tensile loads bolts
support tensile loads, but not shear
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Outline Revisited

• General Thread Nomenclature Types
• Power Screws
• Stresses in Threads
• Preloading Fasteners/Joints
• Fasteners in Shear

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Chapter
9
Welding, Brazing, Bending, and the Design of
Permanent Joints
From Shigley Mischke, Mechanical Engineering
Design
Part 3 Design of Mechanical Elements
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Welding Symbols
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Butt Welds
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Fillet Welds
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Welding Issues

• Requires
• Careful Design
• Skilled Welder
• Can Cause
• Weakened adherends
• Thermal distortion
• Removal of heat treatment

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Welding References

• AWS (American Welding Society)
• Lincoln Electric
• ASME Codes Standards
• Pressure Vessels Piping
• Nuclear Installations
• Safety Codes
• Performance Test Codes

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Bonded Joints (thin members)
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Bonded Joint Types
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More Types
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Peel Stresses
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Good Practices
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Bonding Issues

• Can achieve
• Lighter joint
• Less costly joint
• Better sound absorption
• Beware
• Peel stresses
• Environmental effects
• Thermal mismatch

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Bonding References

• SAMPE (Society for the Advancement of Material
  Process Engineering)
• ASTM Committee D-14 on Adhesives