Transcript ME 395 Introduction to Mechanical Design

ME 209 Machine Design I
Design of a C – Clamp
Asanga Ratnaweera
Dept of Mechanical Engineering
Introduction to Clamps

Some commonly used clamps
C-clamp
Hand screw clamp
Quick action clamp
Miter clamp
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Pipe clamp
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Mechanical Engineering
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C - Clamp
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Some examples
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Engineering Design Process
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Functional requirement -> Design
1. Conceptualization
2. Synthesis
3. Analysis
4. Evaluation
5. Representation
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Design of a C - Clamp

Identification of component
Collar
Screw
Nut
Handle
Screw Head
Frame
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Mechanical Engineering
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Design of a C – Clamp

Prevention of Failure


Decides the size of each component considering its safety
under the maximum permissible load conditions
Other Design Requirements

Ergonomics, Standards, Operational Parameters.
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Mechanical Engineering
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Modes of Failure

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Compressive
failure
Tensile failure
Shear failure
Bending failure
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Torsion failure
Fatigue failure
Many more…….
Note : it is important to identify the maximum stress and its
location for each element under a given load condition
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Stress analysis
Load
Direct Stress 
Nm  2
Area
Note 1 : Identification of actual load condition and the area is extremely
important
Note 2: Direction of the load applied will determine the type of the
stress
Note 3 : Bending and torsional stresses are analyzed using the following
equations
Bending
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M 

I
y
Torsion
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T 

J r
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Factor of safety

Also know as safety factor is a
multiplier applied to the calculated
maximum stress to which a component
will be subjected. Typically, for
components whose failure could result
in substantial financial loss or serious
injury or death, high safety factor is
used.
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Mechanical Engineering
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Screw Treads
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The principal uses of threads are:
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for fastening,
for adjusting, and
for transmitting power
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Mechanical Engineering
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Standard Thread Systems
SI (ISO)
Unified or American
ACME
Pipe
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Whitworth (BSW)
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Typical Screw Designation
In ACME system
1/2” - 13 UNC - 2A
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external thread
(B means internal)
Class of fit
(1 is loosest tolerance, 3 is tightest)
Thread Series
UNC (Unified Coarse)
UNF (Unified Fine)
Pitch (threads/inch)
Nominal Diameter
(also shown as decimal or screw #)
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Mechanical Engineering
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Typical Screw Designation

In Metric system a screw is designated by the
nominal size (major diameter) and pitch being
separated by the sign X. e.g. M8 X 1.
M1.6 X 0.35
M3 X 0.5
M5 X 0.8
M10 X 1.5
M16 X 2
M30 X 3.5
M48 X 5
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M2 X 0.4
M3.5 X 0.6
M6 X 1
M12 X 1.75
M20 X 2.5
M36 X 4
M56 X 5.5
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M2.5 X 0.45
M4 X 0.7
M8 X 1.25
M14 X 2
M24 X 3
M42 X 4.5
M64 X 6
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Power Screws
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Power Screws are linear actuators that
transform rotary motion into linear motion.
Power screws are generally based on ACME ,
Square, and Buttress threads.
ACME
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Power Screws
Square
Buttress
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Power Screws
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Ball screws are a type of power screw.
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Mechanical Engineering
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Power Screws
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Efficiencies of between 30% and 70% are obtained
with conventional power screws.
Ball screws have efficiencies of above 90%.
Power Screws are Used for the following three
reasons
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To obtain high mechanical advantage in order to move
large loads with minimum effort. e.g Screw Jack.
To generate large forces e.g A compactor press.
To obtain precise axial movements e.g. A machine tool
lead screw.
Familiar applications
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clamps or vises, presses, and jacks.
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Screw Threads
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Wrapping an inclined plane around a
cylinder results in a screw threaded or
power screw.
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Screw Threads
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What is the thread?
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The raised helical rib going around a screw
Lead = pitch * number of starts
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What is the tread angle (λ)?
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The tread angle is the angle of the inclined plane.
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The more threads you have the smaller the angle.
The less thread you have the bigger the angle.
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Screw Threads
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If one turn of a square
thread is unwrapped,
the following ramp can
be obtained.
L
dm – mean diameter
dm
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L
tan 
d m
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Forces on Screw Threads
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Identical to pushing an object up along
a thread
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Forces on Screw Threads
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Considering a small element of the nut
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
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δP - small force on the element due to the
torque,
δW - a small part of the load which the
element is supported,
δN - the normal force,
δW
δP
δF - the friction force
Motion
δF
λ
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δN
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Forces on Screw Threads
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Considering the force
equilibrium, when the
screw is about to
rotate
δW
δP
δF
Motion
λ
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δN
N = P sin + W cos 
F = P cos  - W sin
F = N
P cos   W sin
=
P sin  W cos 
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Forces on Screw Threads
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However, the friction
angle φ;
  tan 
 tan  tan 
P = W 

1
tan

tan




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Therefore, the total
torque;
T=
R
Wtan(  )
m
The torque required
 tan  tan 
T = R mW

1
tan

tan



T = R m Wtan(   )
Rm – mean radius
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Swivel Head
F
Rc
Torque required to turn the collar
T c  c Rc F
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Mechanical Engineering
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Design of the Nut
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Shear and bearing
failure of threads
Bearing area
nut
Shear area
screw
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Drawing of Screw Threads
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Design of the frame
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The frame is subjected to combined
bending and direct stresses
W
W
W
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Wx
x
W
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Design of the frame
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An I-section can be used for the frame
P
6t
t
σ
9t
P
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Mechanical Engineering
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Design of a C - Clamp
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Major Design Steps
1. Calculate the torque required at the handle
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To overcome the friction at the screw
To overcome the friction at the collar
2. Calculation of stress on the screw
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Maximum torsional stress
Maximum direct stress
3. Bearing pressure on threads
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Mechanical Engineering
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Design of a C - Clamp
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Major Design Step
4. Selection of number of starts
5. Check the shear failure of threads
6. Calculation of the stresses on the frame
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Moment due to couple
Direct stresses
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Mechanical Engineering
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