Mechanisms

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 Ken Youssefi

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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 Ken Youssefi

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Example of Mechanisms

Extension position

Flexed position

Six-bar linkage prosthetic knee mechanism Ken Youssefi

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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 Ken Youssefi

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Mechanism Categories Function Generation Mechanisms

A four-bar drive linkage for a lawn sprinkler  Ken Youssefi

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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 Ken Youssefi

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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 Ken Youssefi

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  Ken Youssefi

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Function Generation Mechanisms Graphical Solution Two position synthesis – Design a four-bar crank and rocker mechanism to give 45 o of rocker rotation with equal time forward and back, from a constant speed motor input. 1 – Draw the rocker O 4B 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) Ken Youssefi 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 •

Draw the link CD in its two desired  positions, C1D1 and C2D2

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Connect C1 to C2 and D1 to D2.

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Draw two lines perpendicular to C1 C2 and D1D2 at the midpoint (midnormals)

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Select two fixed pivot points, O 2 and O4, anywhere on the two midnormals.

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Motion Generation Mechanisms Graphical Solution Two position synthesis (coupler output) – C 1D1 and C2D2

The rigid body to be moved from position 1 to 2 is secured to link 3. Ken Youssefi

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Motion Generation Mechanisms Graphical Solution Two position synthesis (coupler output) with Dyad added. 1 – Select any point B 1 on link  O2C1. Locate B2. 2 – Choose O 6 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) – C 1D1 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 •

Draw the link CD in its three desired positions.

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Follow the same procedure,draw construction lines, C1C2, C1C3 and C2C3. Same for point D.

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Draw three midnormals for each  point.

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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 r 2. 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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