ADMC Intro

What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint

Design and Analysis of Machine ComponentsIntroduction

Dr. P. JEYARAJ

Assistant ProfessorDepartment of Mechanical EngineeringNITK Surathkal, Mangalore 575 025

December 30, 2014

Dr. P. Jeyaraj NITK Surathkal


Design

The term design clearly encompasses a wide range of meaning.Wallpaper is designed. We are wearing designer clothes. Automobiles are designed in terms oftheir external appearance.

In these examples, design refers primarily to the object’s aesthetic appearance.

In case of the automobile, all of its other aspects also involve design.Itsmechanical components (piston, connecting rod, crank, brakes, suspension,etc.,) must be designed by engineers.

Engineering design- The process of applying the various techniques andscientific principles for the purpose of defining a device, a process, or a systemin sufficient detail to permit its realization.

Machine design - deals with the creation of machinery that works safely and

reliably.


Terminology

MechanismA device which transforms motion to some desirable pattern

may contain linkages, cams, gears, belts and chain

typically develops very low forces and transmits little power

Examples

Umbrella , Folding chair and Adjustable desk lamp

DefinitionA system of elements arranged to transmit motion in a pre determined fashion

Terminology

Machinetypically contains mechanisms

designed to provide significant forces and transmit significant power

Examples

Bulldozer, Robot,Engines

Definition

A system of elements arranged to transmit motion and energyin a pre determined fashion

Basic Procedure

Basic ProcedureConsists of a step by step approach from given specifications about the

functional requirements of a product to the complete description in the form of

drawings of the final product.

Design of Machine Elements

Design of Machine Elements

The design of machine elements is the most important step in the completeprocedure of machine design. In order to ensure the basic requirements ofmachine elements, calculations are carried out to find out the dimensions of themachine elements.

The basic procedure of the design of machine elements is illustrated as


Materials

Materials

Designmade out of some material

able to manufacture that material

Through understanding ofmaterial propertiestreatments

manufacturing process



Material Property

Material Property

determined through destructive testing of samples undercontrolled loadings

test loadings do not accurately duplicate actual serviceloadings

there will be some statistical variation in the strength of anyparticular sample compared to the average tested propertiesfor that material

many of the published data are given as minimum values



Material Property

Material Property

The best material-data will be obtained from destructive ornondestructive testing under actual service loadings ofprototypes of actual design, made from actual materials bythe actual manufacturing process.

Done only when economic and safety risks are highaircraft, automobiles

motor cycles, farm equipment



Tensile Test

Tensile Test

The bar is stretched slowly in tension until it breaks, while theload and the distance across the gage length (or alternativelythe strain) are continuously monitored. The result is astress-strain curve



Tensile Test

Tensile Test

The result is a stress-strain curve



Tensile Test

Tensile Test

Parameters measured are load and deflection but thoseplotted are stress and strain.

Stress (σ) is defined as the load per unit area, (σ) = PAO

,where P - Applied load at any instant, and Ao - original crosssection area of the specimen.

Stain is the change in length per unit length ε = l−lolo

, wherelo - original gage length and l - gage length at any load P.

Modulus of Elasticity E = σε . E is a measure of the stiffness of

the material in its elastic range.



Tensile Test

Tensile Test

Elastic Limit - The point beyond which the material will takea permanent plastic deformation. Boundary betweenelastic-behavior and plastic-behavior region of the material.

Yield strength - At a point y slightly above the elastic limit,the material begins to yield more readily to the applied stress,and its rate of deformation increases (note the lower slope).This is called the yield point, and the value of stress at thatpoint defines the yield strength, Sy of the material.

Materials that are very ductile like low-carbon steels, willsometimes show an apparent drop in stress just beyond theyield point.



Tensile Test

Tensile Test

Many ductile materials will not exhibit this apparent drop instress. The yield strength is then taken at the intersection ofthe stress-strain curve and the offset line of 0.2percent strain.



Tensile Test

Tensile Test

Ultimate Tensile Strength - The stress in the specimencontinues to increase non-linearly to a peak value at point u.This is considered to be largest tensile stress the material cansustain before breaking.

However for ductile steel curve shown, the stress appears tofall off to a smaller value at the fracture point f.



Specific Strength

Specific Strength

In comparing the properties of different materials it is quituseful to express those properties normalized to the material’sdensity.

Since light weight is nearly always a goal in design, we seekthe lightest material that has sufficient strength and stiffnessto withstand the applied loads.

The specific strength of the material is defined as thestrength divided by the density.

The strength-to-weight ratio is another way to express thespecific strength.



Ductility

Ductility

Ductility - tendency for a material to deform significantlybefore fracturing.

Figure shows a test specimen of ductile steel after fracture.The distortion called necking-down can clearly seen at thebreak. The fracture surface appears torn and is laced withhills and valleys



Brittleness

Brittleness

Brittleness - absence of significant deformation beforefracture.

Figure shows a test specimen of cast iron after fracture. Thebreak shows no evidence of necking and has the finer surfacecontours.



Ductile Vs Brittle Failure

Ductile Vs Brittle Failure

Brittleness - absence of significant deformation beforefracture.

Figure shows a test specimen of cast iron after fracture. Thebreak shows no evidence of necking and has the finer surfacecontours.



Compression Test

Compression Test

Tensile test machine can be run in reverse to apply acompressive load to a specimen that is a constant-diametercylinder as shown in Figure.

The ductile sample will not fracture in compression. If enoughforce is available from the machine, it could be crushed into apancake shape

Brittle materials will fracture when compressed. Note therough, angled fracture surface. Brittle materials have muchgreater strength in compression than in tension.



Bending Test

Bending Test

A thin rod, is simply supported at each end as beam andloaded transversely in the center of its length until it fails.

If the material is ductile, failure is by yielding. If the materialis brittle, the beam fractures.



Torsion Test

Torsion Test

The shear properties of a material are more difficult todetermine than its tensile properties. A specimen similar tothe tensile test specimen is made with non circular details onits ends so that it can be twisted axially to failure.

The helical twist in the ductile specimen’s line after failureshows that it wound up for several revolutions before breadingin ductile case.

The brittle, torsion-test specimen’s line is still straight afterfailure as there was no significant plastic distortion beforefracture.



Torsion Test

Torsion Test

The stress-strain relation for pure torsion is defined byτ = Grθ

lo

The shear modulus G = E2(1+ν)

The breaking strength in torsion is called the ultimate shearstrength or modulus of rupture Sus and it is calculated fromSus = Tr

J , where T is the applied torque necessary to breakthe specimen.



Torsion Test

Torsion Test

In the absence of available data for the ultimate shearstrength of a material, a reasonable approximation can beobtained from tension test dataSteel: Sus � 0.80 SutOther ductile materials: Sus � 0.75 SutSys � 0.80 Sy


Cotter Joint

Cotter Joint

Used to connect two co-axial rods, which are subjected toeither axial tensile force or axial compressive force.

Typical applications of cotter joint are

Between the piston rod and the crosshead of a steam engineBetween the slide spindle and the fork of the valve mechanismBetween the piston rod and the tail or pump rod

Cotter Joint

Cotter Joint

The principle of wedge action is used in a cotter joint. Acotter is a wedge shaped piece made of a steel plate. Thejoint is tightened and adjusted by means of a wedge action ofthe cotter.

The construction of a cotter joint, used to connect two rods Aand B is shown. Rod A is provided with a socket end, whileRod B is provided with a spigot end.

Cotter Joint

Tensile Failure of rods

The tensile stress in the rod is given by

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