faculty development programme on design of machine elements

Faculty Development Programme on Design of Machine Elements

P. Emmanuel Nicholas,Department of Mechanical Engineering,

PSNA CET, Dindigul

2

Session 1- Contents

• Design Process• Tolerances• Stress• Failure Theories

PSNA College of Engineering and Technology


3

Design Process

Idea Real product

Design Process


4

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.


5

Design Process -Steps1. Recognition of need. 2. Synthesis (Mechanisms). 3. Analysis 4. Evaluation. 5. Detailed drawing. 8. Production.


6

Factors influencing design process

• Mechanical Properties• Working conditions• Cost


7

Material selection - factors

• Strength - to resist the load• Hardness - to resist indentation /abration • Toughness - to resist shock loads• Stiffness - resistance against deflection


8

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


9

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


10

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)


11

Find the stresses acting on diff. elements


12

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.


13

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


14

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


15

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


16

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.


17

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.


18

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


19

Tolerances


20

Tolerances (Contd..)


21

Tolerances (Contd..)


22

Fits

• The degree of tightness & looseness between the mating parts


23

Hole/Shaft basis system

• Hole/Shaft is kept constant and the shaft/hole is varied to obtain different types of fits.


24

Hole/Shaft basis system (Contd..)

faculty development programme on design of machine elements

Documents