theory of machines
TRANSCRIPT
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Ken Youssefi UC Berkeley 1
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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Ken Youssefi UC Berkeley 2
Example of Mechanism
Can crusher
Simple press
Rear-window wiper
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Ken Youssefi UC Berkeley 3
Example of Mechanisms
Moves packages from an assembly bench to a conveyor
Lift platformMicrowave carrier to assist people on wheelchair
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Ken Youssefi UC Berkeley 4
Example of Mechanisms
Lift platform
Front loader
Device to close the top flap of boxes
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Ken Youssefi UC Berkeley 5
Example of Mechanisms
Conceptual design for an exercise machine
Rowing type exercise machine
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Ken Youssefi UC Berkeley 6
Example of Mechanisms
Six-bar linkage prosthetic knee mechanism
Extension position
Flexed position
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Ken Youssefi UC Berkeley 7
Four-Bar Linkage
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Ken Youssefi UC Berkeley 8
Four-Bar Linkage Categories
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Ken Youssefi UC Berkeley 9
Four-Bar Linkage Categories
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Ken Youssefi UC Berkeley 10
4-Bar mechanisms
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Ken Youssefi UC Berkeley 11
4-Bar mechanismsS + l > p + q
4 double rocker mechanisms
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Ken Youssefi UC Berkeley 12
The Slider-Crank Mechanism
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Ken Youssefi UC Berkeley 13
The Slider-Crank Mechanism
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Ken Youssefi UC Berkeley 14
Slider-Crank Mechanism - Inversion
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Ken Youssefi UC Berkeley 15
Mechanism CategoriesFunction 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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Ken Youssefi UC Berkeley 16
Mechanism Categories
Function Generation Mechanisms
A four-bar drive linkage for a lawn sprinkler
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Ken Youssefi UC Berkeley 17
Mechanism CategoriesMotion 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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Ken Youssefi UC Berkeley 18
Mechanism CategoriesMotion Generation Mechanisms
Four-bar automobile hood linkage design
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Ken Youssefi UC Berkeley 19
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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Ken Youssefi UC Berkeley 20
Primary Joints
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Ken Youssefi UC Berkeley 21
Higher Order Joints
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Ken Youssefi UC Berkeley 22
Motion Generation Mechanisms
Rotating a monitor into a storage position
Moving a storage bin from an accessible position to a stored position
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Ken Youssefi UC Berkeley 23
Motion Generation Mechanisms
Lifting a boat out of water
Moving a trash pan from the floor up over a trash bin and into a dump position
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Ken Youssefi UC Berkeley 24
Function Generation MechanismsGraphical 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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Ken Youssefi UC Berkeley 25
Function Generation MechanismsGraphical Solution
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Ken Youssefi UC Berkeley 26
Crank-Rocker Mechanism
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Ken Youssefi UC Berkeley 27
Motion Generation MechanismsGraphical Solution
Two position synthesis – C1D1 and C2D2
• Draw the link CD in its two desired positions, C1D1 and C2D2
• Connect C1 to C2 and D1 to D2.
• Draw two lines perpendicular to C1 C2 and D1D2 at the midpoint (midnormals)
• Select two fixed pivot points, O2 and O4, anywhere on the two midnormals.
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Ken Youssefi UC Berkeley 28
Motion Generation MechanismsGraphical 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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Ken Youssefi UC Berkeley 29
Motion Generation MechanismsGraphical 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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Ken Youssefi UC Berkeley 30
Motion Generation MechanismsGraphical Solution
Two position synthesis (coupler output) with Dyad added.
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Ken Youssefi UC Berkeley 31
Motion Generation MechanismsGraphical Solution
Two position synthesis (rocker output) – C1D1 and C2D2
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Ken Youssefi UC Berkeley 32
2 Position Motion – rocker output
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Ken Youssefi UC Berkeley 33
Motion Generation MechanismsGraphical Solution
Three position synthesis – C1D1, C2D2 and C3D3
• Draw the link CD in its three desired positions.
• Follow the same procedure,draw construction lines, C1C2, C1C3 and C2C3. Same for point D.
• Draw three midnormals for each point.
• Locate the intersection of the three midnormal, O2 and O4 are the two fixed pivot points.
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Ken Youssefi UC Berkeley 34
Motion Generation MechanismsGraphical Solution
Three position synthesis – C1D1, C2D2 and C3D3
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Ken Youssefi UC Berkeley 35
3 Position Motion
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Ken Youssefi UC Berkeley 36
3 Position Motion
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Ken Youssefi UC Berkeley 37
3 Position Motion
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Ken Youssefi UC Berkeley 38
Slider-Crank Mechanism
In-line 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
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Ken Youssefi UC Berkeley 39
Slider-Crank Mechanism
Offset slider-crank mechanism
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Ken Youssefi UC Berkeley 40
Straight line Mechanisms
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Ken Youssefi UC Berkeley 41
Straight Line Mechanism
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Ken Youssefi UC Berkeley 42
Scotch Yoke Mechanism
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Ken Youssefi UC Berkeley 43
Geneva Mechanism
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Ken Youssefi UC Berkeley 44
Linear Geneva Mechanism
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Ken Youssefi UC Berkeley 45
Ratchet Mechanism
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Ken Youssefi UC Berkeley 46
Straight Beam Walking Mechanism
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Ken Youssefi UC Berkeley 48
Roller and Flat Follower Cams
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Ken Youssefi UC Berkeley 49
Cylindrical Cam Mechanism
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Ken Youssefi UC Berkeley 50
Gears – Rack and Pinion
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Ken Youssefi UC Berkeley 51
Gears
Worm Gear Sets Bevel gears Planetary Gear set
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V-8 Engine
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Ken Youssefi UC Berkeley 53
Type of Motion and Mechanisms
Translation to Translation
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.
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Ken Youssefi UC Berkeley 54
Type of Motion and MechanismsRotational to Rotational
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Ken Youssefi UC Berkeley 55
Type of Motion and MechanismsRotation to Translation
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Ken Youssefi UC Berkeley 56
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, Prentice-Hall, 1999