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Ken Youssefi Mechanical Engineering Dept. 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 and/or

force in a predetermined fashion.

Mechanism consists of linkages and joints.


Example of Mechanism

Can crusher

Simple press

Rear-window wiper


Example of Mechanisms

Moves packages from an assembly

bench to a conveyor

Lift platformMicrowave carrier to assist

people on wheelchair



Lift platform

Front loader

Device to close the

top flap of boxes


Example of MechanismsStair climbing mechanism

A box that

turns itself off

Airplane landing

gear mechanism



Conceptual design for an

exercise machine

Rowing type exercise machine



Six-bar linkage prosthetic

knee mechanism

Extension position

Flexed position


Four-Bar Linkage


Four-Bar Linkage Categories

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

a) Shortest link is the driver with

either one of the intermediate

links as the ground link

(fixed). The result is two

different types of crank-rocker

mechanisms, input link rotates

360 (crank) and output link

oscillates (rocker)

s + l < p + q




b) Shortest link is fixed. The

result is a double-crank

mechanism. Both, input and

output links rotate 360.



c) The longest link is fixed and

either one of the intermediate

links is the driver. The result is a

double-rocker mechanism.

Both, input and output links

oscillate.

Grashof condition – one link rotates 360



Four-Bar Linkage CategoriesCase II shortest link + longest link > summation of the other two links

There are four possible

mechanisms depending on

which link is fixed. All

mechanisms are double-rockers.

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

Same mechanisms as in case I

with the addition of a

branching point (all links

become collinear).


4-Bar mechanisms


4-Bar mechanismsS + l > p + q

4 double rocker

mechanisms


The Slider-Crank Mechanism

Input link,

crank Coupler link,

connecting rod

Output link,

piston (slider)


Slider-Crank Mechanism

Offset slider-crank mechanism


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


The Slider-Crank Mechanism


Slider-Crank Mechanism - Inversion


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



Function Generation Mechanisms

A four-bar drive linkage for a lawn sprinkler


Mechanism CategoriesFunction Generation Mechanisms

A four-bar function generation mechanism to

operate an artificial hand used for gripping.

A four-bar function generation

mechanism to lower an attic stairway.



Motion Generation Mechanisms

In motion generation, the entire motion of the coupler link

is of interest (rigid body guidance).

New Rollerblade brake system




Four-bar automobile hood linkage design



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


Mechanism CategoriesPath Generation Mechanisms

A four-bar path generation mechanism as

part of an arm-actuated propulsion system

for a wheelchair

A four-bar path generation mechanism to guide a

thread in an automatic sewing machine


Primary Joints


Higher Order Joints



Rotating a monitor into a storage position

Moving a storage bin from an

accessible position to a stored position



Lifting a boat out of water

Moving a trash pan from the floor up over

a trash bin and into a dump position


Straight line Mechanisms


Straight line

Mechanisms


Straight Line Mechanism


Straight Line Mechanism - Application

Straight line mechanism

with dwell


Scotch Yoke Mechanism

Example – car window


Geneva Mechanism


Linear Geneva Mechanism


Ratchet Mechanism


Straight Beam Walking Mechanism


Roller and Flat Follower Cams


Cylindrical Cam Mechanism


Gears – Rack and Pinion


Gears

Worm Gear Sets Bevel gears Planetary Gear set


V-8 Engine


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, or linear

actuators.



Rotational to Rotational



Rotation to Translation


References

• Mechanism Design, Analysis and Synthesis by

Erdman and sander, fourth edition, Prentice-Hall,

2003,

• Machines and Mechanisms by Uicker, Pennock

and Shigley, third edition, Oxford, 2002.

• Machines and Mechanisms by Myszka, Prentice-

Hall, 2003

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