ken youssefi mechanical engineering dept. 1 mechanism machines are mechanical devices used to...

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.

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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 MechanismsStair climbing mechanism

A box that turns itself off

Airplane landing gear mechanism

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Example of Mechanisms

Conceptual design for an exercise machine

Rowing type exercise machine

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Example of Mechanisms

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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

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

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Four-Bar Linkage Categories

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

b) Shortest link is fixed. The result is a double-crank mechanism. Both, input and output links rotate 360.

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Four-Bar Linkage Categories

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

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

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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).

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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

Input link, crank Coupler link,

connecting rod

Output link, piston (slider)

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Slider-Crank Mechanism

Offset slider-crank mechanism

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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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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 CategoriesFunction Generation Mechanisms

A four-bar drive linkage for a lawn sprinkler

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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.

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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 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

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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

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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 positionMoving a storage bin from an accessible position to a stored position

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Motion Generation Mechanisms

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

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Straight line Mechanisms

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Straight line Mechanisms

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Straight Line Mechanism

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Straight Line Mechanism - Application

Straight line mechanism with dwell

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Scotch Yoke Mechanism

Example – car window

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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, or linear actuators.

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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, 2003,

• Machines and Mechanisms by Uicker, Pennock and Shigley, third edition, Oxford, 2002.

• Machines and Mechanisms by Myszka, Prentice-Hall, 2003