Theory of machines
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Transcript of Theory of machines
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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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
Lift platformMicrowave carrier to assist people on wheelchair
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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
Conceptual design for an exercise machine
Rowing type exercise machine
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Example of Mechanisms
Six-bar linkage prosthetic knee mechanism
Extension position
Flexed position
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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 mechanismsS + 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 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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Mechanism Categories
Function Generation Mechanisms
A four-bar drive linkage for a lawn sprinkler
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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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Mechanism CategoriesMotion 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
Moving a storage bin from an accessible position to a stored position
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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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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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Function Generation MechanismsGraphical Solution
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Crank-Rocker Mechanism
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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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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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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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Motion Generation MechanismsGraphical Solution
Two position synthesis (coupler output) with Dyad added.
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Motion Generation MechanismsGraphical Solution
Two position synthesis (rocker output) – C1D1 and C2D2
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2 Position Motion – rocker output
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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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Motion Generation MechanismsGraphical 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
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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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 Bevel gears Planetary Gear set
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V-8 Engine
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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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Type of Motion and MechanismsRotational to Rotational
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Type of Motion and MechanismsRotation 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, Prentice-Hall, 1999