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

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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..)