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A.N.KHUDAIWALA (L.M.E) G.P.PORBANDAR

Machine Design

What is the importance of Machine Design for engineers?

What is Machine Design?

Creation of new and better machines AND

Improving existing ones

So that it is economical in the cost of production and operation.








3. Selection of MATERIALs

Physical properties: Density, Melting point, Elec/thermal properties

Mechanical properties:• STRENGTH – resist externally applied loads without breaking

or yielding• STIFFNESS – resist deformation under stress• ELASTICITY – regain original shape once the force is removed• PLASTICITY – property which retains deformation (required for

forging etc)• DUCTILITY – ability to be drawn into a wire by a tensile force• BRITTLENESS – sudden breaking with minimum distortion• TOUGHNESS – resist fracture due to high impact load• CREEP – deformation under stress and high temperature• FATIGUE – ability to withstand cyclic stresses• HARDNESS – resistance to wear, scratching, deformation,

machinability etc

Metal Non-metal

Ferrous Non-ferrous











Theoretical stress concentration factor, Kt

Maximum stress at the discontinuity

Nominal stress, max stress with no discontinuity

Kt is used for normal stresses and Kts for shear stresses.


Consider an elliptical hole in an infinitely large plate loaded in uniform tension. Using the theory of elasticity the theoretical stress concentration factor is given by the equation below.

Elliptical hole

2b

2a

• For a circular hole Kt = 3

• The equation can be applied to a longitudinal crack where b << a

Guidelines for design.Abrupt changes in cross-section should be avoided.

Fillet radii or stress-relieving groove should be provided. Fig. 11.3(d)

Slot and grooves should be provided with generous run-out radii and with fillet radii in all corners. Fig. 11.3(b)Stress relieving grooves or undercut should be provided at the end of threads and splines. Fig. 11.3(c)Sharp internal corners and external edges should be avoidedWeakening features like bolt and oil holes, identification marks, and part number should not be located in highly stressed areas.Weakening features should be staggered to avoid the addition of their stress concentration effects, Fig. 11.3(d)

Stress Concentration








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