Introduction to design of machine elements for mechanical engineering students.

1. 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.

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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

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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.

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

20. 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 avoided
Weakening 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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