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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
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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.
Dr. P. Jeyaraj NITK Surathkal
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
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
Materials
Materials
Designmade out of some material
able to manufacture that material
Through understanding ofmaterial propertiestreatments
manufacturing process
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
Tensile Test
Tensile Test
The result is a stress-strain curve
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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.
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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.
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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.
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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.
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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.
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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.
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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.
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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.
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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.
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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.
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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.
Dr. P. Jeyaraj NITK Surathkal
What is Mechanical Design? Machine, Mechanism and Structure Materials Cotter Joint
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
Dr. P. Jeyaraj NITK Surathkal
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.