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.
Ken Youssefi
UC Berkeley
1
Example of Mechanism
Can crusher
Simple press
Rear-window wiper
Ken Youssefi
UC Berkeley
2
Example of Mechanisms
Moves packages from an assembly
bench to a conveyor
Microwave carrier to assist
people on wheelchair
Lift platform
Ken Youssefi
UC Berkeley
3
Example of Mechanisms
Lift platform
Front loader
Device to close the
top flap of boxes
Ken Youssefi
UC Berkeley
4
Example of Mechanisms
Rowing type exercise machine
Conceptual design for an
exercise machine
Ken Youssefi
UC Berkeley
5
Example of Mechanisms
Extension position
Flexed position
Six-bar linkage prosthetic
knee mechanism
Ken Youssefi
UC Berkeley
6
Four-Bar Linkage
Ken Youssefi
UC Berkeley
7
Four-Bar Linkage Categories
Ken Youssefi
UC Berkeley
8
Four-Bar Linkage Categories
Ken Youssefi
UC Berkeley
9
4-Bar mechanisms
Ken Youssefi
UC Berkeley
10
4-Bar mechanisms
S+l>p+q
4 double rocker
mechanisms
Ken Youssefi
UC Berkeley
11
The Slider-Crank Mechanism
Ken Youssefi
UC Berkeley
12
The Slider-Crank Mechanism
Ken Youssefi
UC Berkeley
13
Slider-Crank Mechanism - Inversion
Ken Youssefi
UC Berkeley
14
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
Ken Youssefi
UC Berkeley
15
Mechanism Categories
Function Generation Mechanisms
A four-bar drive linkage for a lawn sprinkler
Ken Youssefi
UC Berkeley
16
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
Ken Youssefi
UC Berkeley
17
Mechanism Categories
Motion Generation Mechanisms
Four-bar automobile hood linkage design
Ken Youssefi
UC Berkeley
18
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
UC Berkeley
19
Primary Joints
Ken Youssefi
UC Berkeley
20
Higher Order Joints
Ken Youssefi
UC Berkeley
21
Motion Generation Mechanisms
Rotating a monitor into a storage position
Ken Youssefi
Moving a storage bin from an
accessible position to a stored position
UC Berkeley
22
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
Ken Youssefi
UC Berkeley
23
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).
Ken Youssefi
UC Berkeley
24
Function Generation Mechanisms
Graphical Solution
Ken Youssefi
UC Berkeley
25
Crank-Rocker Mechanism
Ken Youssefi
UC Berkeley
26
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.
Ken Youssefi
UC Berkeley
27
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.
Ken Youssefi
UC Berkeley
28
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.
Ken Youssefi
UC Berkeley
29
Motion Generation Mechanisms
Graphical Solution
Two position synthesis (coupler output) with Dyad added.
Ken Youssefi
UC Berkeley
30
Motion Generation Mechanisms
Graphical Solution
Two position synthesis (rocker output) – C1D1 and C2D2
Ken Youssefi
UC Berkeley
31
2 Position Motion – rocker output
Ken Youssefi
UC Berkeley
32
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.
Ken Youssefi
UC Berkeley
33
Motion Generation Mechanisms
Graphical Solution
Three position synthesis – C1D1, C2D2 and C3D3
Ken Youssefi
UC Berkeley
34
3 Position Motion
Ken Youssefi
UC Berkeley
35
3 Position Motion
Ken Youssefi
UC Berkeley
36
3 Position Motion
Ken Youssefi
UC Berkeley
37
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
UC Berkeley
38
Slider-Crank Mechanism
Offset slider-crank mechanism
Ken Youssefi
UC Berkeley
39
Straight line
Mechanisms
Ken Youssefi
UC Berkeley
40
Straight Line Mechanism
Ken Youssefi
UC Berkeley
41
Scotch Yoke Mechanism
Ken Youssefi
UC Berkeley
42
Geneva Mechanism
Ken Youssefi
UC Berkeley
43
Linear Geneva Mechanism
Ken Youssefi
UC Berkeley
44
Ratchet Mechanism
Ken Youssefi
UC Berkeley
45
Straight Beam Walking Mechanism
Ken Youssefi
UC Berkeley
46
Ken Youssefi
UC Berkeley
47
Roller and Flat Follower Cams
Ken Youssefi
UC Berkeley
48
Cylindrical Cam Mechanism
Ken Youssefi
UC Berkeley
49
Gears – Rack and Pinion
Ken Youssefi
UC Berkeley
50
Gears
Worm Gear Sets
Ken Youssefi
Bevel gears
UC Berkeley
Planetary Gear set
51
V-8 Engine
Ken Youssefi
UC Berkeley
52
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
Ken Youssefi
UC Berkeley
53
Type of Motion and Mechanisms
Rotational to Rotational
Ken Youssefi
UC Berkeley
54
Type of Motion and Mechanisms
Rotation to Translation
Ken Youssefi
UC Berkeley
55
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
Ken Youssefi
UC Berkeley
56