Mechanism
Machines are mechanical devices used to accomplish
work. A mechanism is a heart of a machine. It is the
mechanical portion of the machine that has the
function of transferring motion and forces from a
power source to an output.
Mechanism is a system of rigid elements (linkages)
arranged and connected to transmit motion and/or
force in a predetermined fashion.
Mechanism consists of linkages (rigid bars,
gears, belts & pulleys, chains & sprockets and
cams) and joints.
Ken Youssefi
Mechanical Engineering Dept.
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Mechanism Synthesis and Analysis
Synthesis – design a mechanism to perform a
desired function
Analysis – given a mechanism, perform kinematics
and kinetics analysis
Graphical techniques
Analytical methods
Four-Bar Linkage
θ3
θ2
θ4
Ken Youssefi
Mechanical Engineering Dept.
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The Slider-Crank Mechanism
Member 4 is converted to a slider
Input link,
crank
Coupler link,
connecting rod
Output link,
piston (slider)
Ken Youssefi
Mechanical Engineering Dept.
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Example of Mechanism
4
2
4
3
2
1
3
5
6
Can crusher
1
Simple press
4
1
2
3
Rear-window wiper
Ken Youssefi
Mechanical Engineering Dept.
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Example of Mechanisms
6
5
4 Moves packages from an assembly
2
bench to a conveyor
3
Six bar
1
Six bar
Six bar
5
1
4
6
3
6
5
4
1
2
Microwave carrier to assist
people on wheelchair
Lift platform
Ken Youssefi
2
3
Mechanical Engineering Dept.
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Example of Mechanisms
Lift platform
Front loader
Device to close the
top flap of boxes
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Mechanical Engineering Dept.
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Stair climbing mechanism
Example of Mechanisms
A box that
turns itself off
Airplane landing
gear mechanism
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Mechanical Engineering Dept.
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Example of Mechanisms
Rowing type exercise machine
Conceptual design for an
exercise machine
Ken Youssefi
Mechanical Engineering Dept.
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Example of Mechanisms
6-Bar mechanism, automatic spoiler
Ken Youssefi
Mechanical Engineering Dept.
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Example of Mechanisms
Extension position
Flexed position
Six-bar linkage prosthetic
knee mechanism
Ken Youssefi
Mechanical Engineering Dept.
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Four-Bar Linkage Categories
l = longest link
s = longest link
p and q = intermediate links
Case I
a)
shortest link + longest link < summation of the other two links
Shortest link is the driver with
either one of the adjacent links
as the ground link (fixed). The
result is two different types of
crank-rocker mechanisms, input
link rotates 360 (crank) and
output link oscillates (rocker)
Ken Youssefi
Mechanical Engineering Dept.
fixed
fixed
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Four-Bar Linkage Categories
Case I
shortest link + longest link < summation of the other two links
b) Shortest link is fixed. The
result is a double-crank
mechanism. Both, input and
output links rotate 360.
Ken Youssefi
Mechanical Engineering Dept.
13
Four-Bar Linkage Categories
Case I
c)
shortest link + longest link < summation of the other two links
The longest link is fixed and
either one of the intermediate
links is the driver. The result is a
double-rocker mechanism.
Both, input and output links
oscillate.
Grashof condition – one link rotates 360
Ken Youssefi
Mechanical Engineering Dept.
14
Four-Bar Linkage Categories
Case II
shortest link + longest link > summation of the other two links
There are four possible
mechanisms depending on
which link is fixed. All
mechanisms are double-rockers.
Case III
shortest link + longest link = summation of the other two links
Same mechanisms as in case I
with the addition of a
branching point (all links
become collinear).
Ken Youssefi
Mechanical Engineering Dept.
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Case I: s + l < p + q
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4-Bar mechanisms
Mechanical Engineering Dept.
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4-Bar mechanisms
S+l>p+q
4 double rocker
mechanisms
Ken Youssefi
Mechanical Engineering Dept.
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The Slider-Crank Mechanism
Member 4 is converted to a slider
Input link,
crank
Coupler link,
connecting rod
Output link,
piston (slider)
Ken Youssefi
Mechanical Engineering Dept.
18
Slider-Crank Mechanism
Offset slider-crank mechanism
Ken Youssefi
Mechanical Engineering Dept.
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Slider-Crank Mechanism
The mechanism has a
stroke B1B2 equal twice
the crank length r2.
Locations B1 and B2 are
called the extreme
positions (limiting) of
the slider
In-line slider crank mechanism
Ken Youssefi
Mechanical Engineering Dept.
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The Slider-Crank Mechanism
Ken Youssefi
Mechanical Engineering Dept.
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Slider-Crank Mechanism - Inversion
Ken Youssefi
Mechanical Engineering Dept.
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Categories of Mechanisms
 Function generation mechanisms
Output is a specified function of input
 Motion generation mechanisms
The orientation of the object is important,
rigid body guidance
 Path generation mechanisms
A point on a link moves on a specified path
Ken Youssefi
Mechanical Engineering Dept.
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Mechanism Categories
Function Generation Mechanisms
A function generator is a linkage in which the relative motion
between links connected to the ground is of interest.
2
1
3
4
A four-bar hand actuated wheelchair
brake mechanism
Ken Youssefi
Mechanical Engineering Dept.
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Mechanism Categories
Function Generation Mechanisms
A four-bar linkage for a lawn sprinkler
Ken Youssefi
Mechanical Engineering Dept.
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Mechanism Categories
Function Generation Mechanisms
A four-bar function generation
mechanism to lower an attic stairway.
A four-bar function generation mechanism to
operate an artificial hand used for gripping.
Ken Youssefi
Mechanical Engineering Dept.
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Mechanism Categories
Motion Generation Mechanisms
In motion generation, the entire motion of the coupler link (3)
is of interest (rigid body guidance).
2
Moving joints
3
Fixed pivot points
4
New Rollerblade brake system
Ken Youssefi
Mechanical Engineering Dept.
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Mechanism Categories
Motion Generation Mechanisms
Four-bar automobile hood linkage design
Ken Youssefi
Mechanical Engineering Dept.
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Mechanism Categories
Six bar
Six bar
5
1
4
6
3
6
5
4
1
2
Microwave carrier to assist
people on wheelchair
Lift platform
Ken Youssefi
2
3
Mechanical Engineering Dept.
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Mechanism Categories
Path Generation Mechanisms
In path generation, we are concerned only with the path of a
tracer point and not with the motion (rotation) of the
coupler link.
Crane – straight line motion
Ken Youssefi
Mechanical Engineering Dept.
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Mechanism Categories
Path Generation Mechanisms
with timing
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A four-bar path generation mechanism to guide a
thread in an automatic sewing machine
A four-bar path generation mechanism as
part of an arm-actuated propulsion system
for a wheelchair
Mechanical Engineering Dept.
31
Primary Joints
Lower Order Joints
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Mechanical Engineering Dept.
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Higher Order Joints
Ken Youssefi
Mechanical Engineering Dept.
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Motion Generation Mechanisms
Rotating an object into a storage position
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Moving a storage bin from an
accessible position to a stored position
Mechanical Engineering Dept.
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Motion Generation Mechanisms
Moving a trash pan from the floor up over
a trash bin and into a dump position
Ken Youssefi
Mechanical Engineering Dept.
Lifting a boat out of water
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Straight line Mechanisms
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Mechanical Engineering Dept.
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Straight line
Mechanisms
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Mechanical Engineering Dept.
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Straight Line Mechanism
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Mechanical Engineering Dept.
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Straight Line Mechanism - Application
Straight line mechanism
with dwell
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Mechanical Engineering Dept.
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Scotch Yoke Mechanism
Example – car window
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Mechanical Engineering Dept.
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Geneva Mechanism
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Mechanical Engineering Dept.
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Linear Geneva Mechanism
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Mechanical Engineering Dept.
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Ratchet Mechanism
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Mechanical Engineering Dept.
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Straight Beam Walking Mechanism
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Mechanical Engineering Dept.
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Ken Youssefi
Mechanical Engineering Dept.
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Roller and Flat Follower Cams
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Mechanical Engineering Dept.
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Cylindrical Cam Mechanism
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Mechanical Engineering Dept.
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Gears – Rack and Pinion
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Mechanical Engineering Dept.
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Gears
Worm Gear Sets
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Bevel gears
Mechanical Engineering Dept.
Planetary Gear set
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V-8 Engine
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Mechanical Engineering Dept.
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Type of Motion and Mechanisms
Most power sources that are readily available today are either of the pure
rotational motion type, such as electric motor or hand crank, or of the
pure translational type, such as pneumatic or hydraulic cylinder, or linear
actuators.
Translation to Translation
Ken Youssefi
Mechanical Engineering Dept.
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Type of Motion and Mechanisms
Rotational to Rotational
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Mechanical Engineering Dept.
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Type of Motion and Mechanisms
Rotation to Translation
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Mechanical Engineering Dept.
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References
• Mechanism Design, Analysis and Synthesis by
Erdman and sander, fourth edition, Prentice-Hall,
• Machines and Mechanisms by Uicker, Pennock
and Shigley, third edition, Oxford
• Machines and Mechanisms by Myszka, PrenticeHall
Ken Youssefi
Mechanical Engineering Dept.
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