Transcript 9514925-theory-of-ma.. - NCERT BOOKS and CBSE BOOKS

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 in a
predetermined fashion.
Mechanism consists of linkages and joints.
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Example of Mechanism
Can crusher
Simple press
Rear-window wiper
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Example of Mechanisms
Moves packages from an assembly
bench to a conveyor
Microwave carrier to assist
people on wheelchair
Lift platform
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Example of Mechanisms
Lift platform
Front loader
Device to close the
top flap of boxes
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Example of Mechanisms
Rowing type exercise machine
Conceptual design for an
exercise machine
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Example of Mechanisms
Extension position
Flexed position
Six-bar linkage prosthetic
knee mechanism
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Four-Bar Linkage
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Four-Bar Linkage Categories
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Four-Bar Linkage Categories
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4-Bar mechanisms
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4-Bar mechanisms
S+l>p+q
4 double rocker
mechanisms
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The Slider-Crank Mechanism
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The Slider-Crank Mechanism
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Slider-Crank Mechanism - Inversion
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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.
A four-bar hand actuated wheelchair brake mechanism
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Mechanism Categories
Function Generation Mechanisms
A four-bar drive linkage for a lawn sprinkler
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Mechanism Categories
Motion Generation Mechanisms
In motion generation, the entire motion of the coupler link
is of interest (rigid body guidance).
New Rollerblade brake system
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Mechanism Categories
Motion Generation Mechanisms
Four-bar automobile hood linkage design
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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
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Primary Joints
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Higher Order Joints
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Motion Generation Mechanisms
Rotating a monitor into a storage position
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Moving a storage bin from an
accessible position to a stored position
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Motion Generation Mechanisms
Moving a trash pan from the floor up over
a trash bin and into a dump position
Lifting a boat out of water
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Function Generation Mechanisms
Graphical Solution
Two position synthesis – Design a four-bar crank and rocker
mechanism to give 45o of rocker rotation with equal time forward and
back, from a constant speed motor input.
1 – Draw the rocker O4B in both
extreme positions, B1and B2 in
any convenient location with
angle θ4 = 45o.
2 – select a convenient point O2
on line B1B2 extended.
3 – Bisect line B1B2 , and draw a
circle with that radius about O2.
4 – Label the two intersection of
the circle with B1B2 extended, A1
and A2.
5 – Measure O2A (crank, link2)
and AB (coupler, link3).
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Function Generation Mechanisms
Graphical Solution
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Crank-Rocker Mechanism
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Motion Generation Mechanisms
Graphical Solution
Two position synthesis – C1D1 and C2D2
1. Draw the link CD in its two desired
positions, C1D1 and C2D2
2. Connect C1 to C2 and D1 to D2.
3. Draw two lines perpendicular to
C1 C2 and D1D2 at the midpoint
(midnormals)
4. Select two fixed pivot points, O2
and O4, anywhere on the two
midnormals.
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Motion Generation Mechanisms
Graphical Solution
Two position synthesis (coupler output) – C1D1 and C2D2
The rigid body to be moved from position 1 to 2 is secured to link 3.
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Motion Generation Mechanisms
Graphical Solution
Two position synthesis (coupler output) with Dyad added.
1 – Select any point B1 on link
O2C1. Locate B2.
2 – Choose O6 anywhere on
B1B2 extension.
3 – Draw a circle from O6 with
radius B1B2/ 2. Mark the
intersection with B1B2 extension
as A1 and A2.
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Motion Generation Mechanisms
Graphical Solution
Two position synthesis (coupler output) with Dyad added.
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Motion Generation Mechanisms
Graphical Solution
Two position synthesis (rocker output) – C1D1 and C2D2
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2 Position Motion – rocker output
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Motion Generation Mechanisms
Graphical Solution
Three position synthesis – C1D1, C2D2 and C3D3
1.
Draw the link CD in its three
desired positions.
2.
Follow the same procedure,draw
construction lines, C1C2, C1C3 and
C2C3. Same for point D.
3.
Draw three midnormals for each
point.
4.
Locate the intersection of the three
midnormal, O2 and O4 are the two
fixed pivot points.
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Motion Generation Mechanisms
Graphical Solution
Three position synthesis – C1D1, C2D2 and C3D3
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3 Position Motion
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3 Position Motion
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3 Position Motion
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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
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Slider-Crank Mechanism
Offset slider-crank mechanism
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Straight line
Mechanisms
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Straight Line Mechanism
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Scotch Yoke Mechanism
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Geneva Mechanism
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Linear Geneva Mechanism
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Ratchet Mechanism
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Straight Beam Walking Mechanism
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Roller and Flat Follower Cams
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Cylindrical Cam Mechanism
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Gears – Rack and Pinion
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Gears
Worm Gear Sets
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Bevel gears
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Planetary Gear set
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V-8 Engine
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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.
Translation to Translation
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Type of Motion and Mechanisms
Rotational to Rotational
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Type of Motion and Mechanisms
Rotation to Translation
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References
• Mechanism Design, Analysis and Synthesis by
Erdman and sander, fourth edition, Prentice-Hall,
2001,
• Machines and Mechanisms by Uicker, Pennock
and Shigley, third edition, Oxford, 2002.
• Machines and Mechanisms by Myszka, PrenticeHall, 1999
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