faculty development programme on design of machine elements
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Faculty Development Programme on Design of Machine Elements
P. Emmanuel Nicholas,Department of Mechanical Engineering,
PSNA CET, Dindigul
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Session 1- Contents
• Design Process• Tolerances• Stress• Failure Theories
PSNA College of Engineering and Technology
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Design Process
Idea Real product
Design Process
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Design Process Types1. Adaptive design -concerned with adaptation
of existing designs.2. Development design - needs considerable
scientific training and design ability in order to modify the existing designs into a new idea.
3. New design - needs lot of research, technical ability and creative thinking.
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Design Process -Steps1. Recognition of need. 2. Synthesis (Mechanisms). 3. Analysis 4. Evaluation. 5. Detailed drawing. 8. Production.
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Factors influencing design process
• Mechanical Properties• Working conditions• Cost
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Material selection - factors
• Strength - to resist the load• Hardness - to resist indentation /abration • Toughness - to resist shock loads• Stiffness - resistance against deflection
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Stress
• The internal force per unit area at any section of the body is known as unit stress or simply a stress.
• Tensile/ Compressive (Direct & Bending) – Young’s Modulus
• Shear – Shear Moduls• bearing stress or crushing stress - compressive
stress at the surface of contact
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Normal Stress (Tensile & Compressive)
• Direct Stress– Due to axial load (P)
σ = P/A
• Bending Stress– Due to Transverse load (F)
σ = (M/I ) * y
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Shear Stress
• Due to torqueτ= (T/J )*r
• A compressive stress at the surface of contact between two members of a machine part, that are relatively at rest is known as bearing stress or crushing stress.
σ_c= P/(d*t*n)
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Find the stresses acting on diff. elements
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Find the stresses at points A and B.A 100 mm diameter off-set link is transmitting an axial pull of 30 KN as shown in the figure.
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Principal Stresses
• The plane at which no shear stress but only direct stresses – Principal planes
• The direct stresses acting on principal planes – Principal stresses
• The plane at which maximum shear stress is acting – Plane of shear
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Normal & Shear on the inclined plane
• Normal stress acting on a plane inclined ‘θ’ with vertical
• And the shear stress
1 2 1 2 cos 2 sin 22 2n
1 2 sin 2 cos 22
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Principal stresses
• Maximum principal stress
• Maximum principal stress
• Maximum shear
2 21 21 1 2
1 42 2p
2 21 21 1 2
1 42 2p
21 2 2max 1 2
1 42 2
p p
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Failure Theories
• Maximum principal stress (or) maximum normal stress (or) Ranking theory – Failure occurs when the maximum normal stress is
equal to the tensile yield strength.– Max.principal stress=yield strength/n (for ductile
material)– Max.principal stress=ultimate strength/n (for
brittle material)– mostly used for brittle materials.
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Failure Theories (Contd..)
• Maximum shear theory (or) Guest’s theory (or) Coloumb theory– Failure occurs when the maximum shear stress
developed in the machine member becomes equal to the maximum shear stress at yielding
– Max.shear stress=yield strength/n– mostly used for ductile materials.
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Failure Theories (Contd..)
• Maximum strain theory (or) Venant’s theory– Failure occurs when the maximum strain in the
member equal in the tensile yield strain
• Maximum strain energy theory – Failure occurs when the strain energy stored per
unit volume of the member becomes equal to the strain energy per unit volume at the yield point
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Tolerances
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Tolerances (Contd..)
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Tolerances (Contd..)
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Fits
• The degree of tightness & looseness between the mating parts
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Hole/Shaft basis system
• Hole/Shaft is kept constant and the shaft/hole is varied to obtain different types of fits.
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Hole/Shaft basis system (Contd..)