Design of parts for static loading

Modes of failure

Static load A static load is defined as a force, which is gradually applied to a mechanical component and which does not change its magnitude or direction w.r.t. time

Engineering materials Ductile material Has relatively large tensile strain before fracture Structural steel, aluminium Brittle material Has relatively small tensile strain before fracture Cast iron

Modes of failure Failure by elastic deflection Example: Transmission shafting: Max force acting on the shaft is limited by the permissible elastic deflection: lateral or torsional rigidity is the design criterion Failure by general yielding Example: component made of ductile material Yield strength is design criterion.

Modes of failure Failure by fracture Example: component made of brittle materials Ultimate tensile strength is design criterion

Factor of safety It is necessary to provide sufficient reserve strength in case of an accident Achieved by taking a suitable factor of safety

Factor of Safety (fs) or For ductile materials, the allowable stress σ is obtained by For brittle material Where Syt and Sut are the yield strength and the ultimate tensile strength of the material.

There are a number of factors which are difficult to evaluate accurately in design analysis: i) Uncertainty in the magnitude of external force acting on the component. ii) Variations in the properties of materials like yield strength of ultimate strength. iii) Variations in the dimensions of the component due to imperfect workmanship.

The magnitude of factor of safety depends on: i) Effect of Failure ii) Type of load iii) Degree of Accuracy in Force Analysis iv) Material of Component v) Reliability of Component vi) Cost of Component vii) Testing of machine Element viii) Service Conditions ix) Quality of Manufacture

The selection of magnitude of the factor of safety i) For cast iron components – Large factor of safety 3 to 5 ii) For ductile material components which are subjected to external static forces - factor of safety 1.5 to 2 iii) For ductile material components which are subjected to external fluctuating forces - factor of safety 1.3 to 1.5 iv) For components such as cams and followers, gears, rolling contact bearings etc. – factor of safety 1.8 to 2.5 v) For components of buckling considerations such as piston rods, power screws or studs – factor of safety 3 to 6

Conditions for selecting higher factor of safety: i) The magnitude and nature of external forces acting on the machine component cannot be precisely estimated. ii) It is likely that the material of the machine component has a non- homogeneous structure. iii) The machine component is subjected to impact force in service. iv) There is possibility of residual stresses in the machine component. v) The machine component is working in a corrosive atmosphere. vi) The machine part is subjected to high temperature during operation.

vii) The failure of the machine part may hazard the lives of the people and substantial loss of property. viii) It is not possible to test the machine component under actual conditions of service and there is variation in actual conditions and standard test conditions. ix) Higher reliability is demanded in applications like components of aircrafts. x) There is possibility of abnormal variation in external load on some occasions. xi) The quality of manufacture of the machine part is poor. xii) The exact mode of failure of the component is unpredictable. xiii) There is stress concentration in a machine component.