mechanisms newsj

8/12/2019 Mechanisms NewSJ

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Mechanism Synthesis and Analysis

Synthesis design a mechanism to perform adesired function

Analysis given a mechanism, perform kinematics

and kinetics analysis

Graphi cal techniques

Analytical methods


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Ken Youssefi Mechanical Engineering Dept. 2

M echanism

Machines are mechanical devices used to accomplishwork. A mechanism is a heart of a machine. It is themechanical portion of the machine that has thefunction 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 and/orforce in a predetermined fashion.

M echani sm consists of l inkages and joints.


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F our -Bar L inkage


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The Slider -Crank M echanism

Input link,crank Coupler link,

connecting rod

Output link,piston (slider)


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Example of M echani sm

Can crusher

Simple press

Rear-window wiper

132

4

5

3

4

2

1

6

4

321


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Example of M echanisms

Moves packages from an assembly bench to a conveyor

Six bar

5

3

42

1

6

Lift platform

Six bar5

34 2

1

6

Microwave carrier to assist people on wheelchair

Six bar

5

34

2

1

6


7/54Ken Youssefi Mechanical Engineering Dept. 7


Lift platform

Front loader

Device to close thetop flap of boxes



Example of M echani smsStair climbing mechanism

A box thatturns itself off

Airplane landinggear mechanism




Conceptual design for anexercise machine

Rowing type exercise machine


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Six-bar linkage prostheticknee mechanism

Extension position

Flexed position


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F our-Bar L inkage Categories

Case I shortest link + longest link < summation of the other two links

a) Shortest link is the driver witheither one of the adjacent linksas the ground link (fixed). The

result is two different types ofcrank-rocker mechanisms, inputlink rotates 360 (crank) andoutput link oscillates (rocker)

fixed

fixed


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F our-Bar L inkage Categories


b) Shortest link is fixed. Theresult is a double-crankmechanism. Both, input andoutput links rotate 360.


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F our -Bar L inkage Categor ies

c) The longest link is fixed andeither one of the intermediatelinks is the driver. The result is adouble-rocker mechanism.Both, input and output linksoscillate.

Grashof condition one link rotates 360



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F our-Bar L inkage Categor iesCase I I shortest link + longest link > summation of the other two links

There are four possiblemechanisms depending onwhich link is fixed. Allmechanisms are double-rockers.

Case I I I shortest link + longest link = summation of the other two links

Same mechanisms as in case Iwith the addition of a

branching point (all links become collinear).


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4-Bar mechanisms


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4-Bar mechanismsS + l > p + q

4 double rockermechanisms


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The Slider -Crank M echanism

Input link,crank Coupler link,

connecting rod

Output link,piston (slider)


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Slider -Crank M echani sm

Offset slider-crank mechanism


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Slider -Crank M echanism

In-line slider crank mechanism

The mechanism has astroke B 1B2 equal twicethe crank length r 2.

Locations B 1 and B2 are

called the extreme positions (limiting) ofthe slider


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The Slider -Crank M echani sm


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Slider -Crank M echani sm - I nversion


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Categories of M echanisms

Function generation mechanismsOutput is a specified function of input

Motion generation mechanisms

Path generation mechanisms

The orientation of the object is important,r igid body guidance

A point on a link moves on a specified path


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M echanism Categories

Function Generation Mechanisms

A four-bar drive linkage for a lawn sprinkler


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M echanism CategoriesFunction Generation Mechanisms

A four-bar function generation mechanism tooperate an artificial hand used for gripping.

A four-bar function generation

mechanism to lower an attic stairway.


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M echanism CategoriesMotion Generation Mechanisms

In motion generation, the entire motion of the coupler linkis of interest ( rigid body guidance ).

New Rollerblade brake system

Fixed pivot points

2

4

3Moving joints


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Motion Generation Mechanisms

Four-bar automobile hood linkage design


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M echanism Categor ies


Lift platform

Six bar5

3

42

1

6

Microwave carrier to assist people on wheelchair

Six bar

53 4

2

16


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Path Generation Mechanisms

In path generation, we are concerned only with the path of atracer point and not with the motion (rotation) of thecoupler link.

Crane straight line motion


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M echanism CategoriesPath Generation Mechanismswith timing

A four-bar path generation mechanism as part of an arm-actuated propulsion system

for a wheelchair

A four-bar path generation mechanism to guidethread in an automatic sewing machine


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Primary Joints

H i h O d J i


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H igher Order Joints


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M otion Generation M echanisms

Lifting a boat out of waterMoving a trash pan from the floor up overa trash bin and into a dump position


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Straight l ine M echanisms


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Straight l ineM echanisms


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Straight L ine M echanism


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Straight L ine M echani sm - Application

Straight line mechanismwith dwell

S hY k M h i


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Scotch Yoke M echanism

Example

car window


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Geneva M echanism


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L inear Geneva M echani sm


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Ratchet M echani sm


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Straight Beam Walking M echanism


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Roller and F lat F ollower Cams


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Gears Rack and Pinion

Gears


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Gears

Worm Gear Sets Bevel gears Planetary Gear set


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V-8 Engine

TypeofM otionandM echanisms


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Type of M otion and M echanisms

Translation to Translation

Most power sources that are readily available today are either of the purerotational motion type, such as electric motor or hand crank, or of the

pure translational type, such as pneumatic or hydraulic cylinder, or linearactuators.

TypeofM otionandM echanisms


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Type of M otion and M echanismsRotational to Rotational


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Type of M otion and M echanismsRotation to Translation


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References

Mechanism Design, Analysis and Synthesis byErdman and sander, fourth edition, Prentice-Hall,

Machines and Mechanisms by Uicker, Pennockand Shigley, third edition, Oxford

Machines and Mechanisms by Myszka, Prentice-Hall

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