Subject: Machine Design -I

Topic: Machine Design consideration

Difficulty: Low

The selection of a proper material, for engineering purposes, is one of the most difficult problem for the designer. The best material is one which serve the desired objective at the minimum cost.

Factors should be considered while selecting the material :

1. Availability of Material,

2. Cost of Material,

3. Mechanical Properties,

4. Manufacturing Considerations.

Availability of Material:

• The material should be readily available in the market, in large enough quantities to meet the requirement.
• Cast iron and aluminium alloys are always available in abundance while shortage of lead and copper alloys is a common experience.

Cost of Material:

• For every application, there is a limiting cost beyond which the designer cannot go. When this limit is exceeded, the designer has to consider other alternative materials.
• In cost analysis, there are two factors, namely, cost of material and the cost of processing the material into finished goods.
• It is likely that the cost of material might be low, but the processing may involve costly manufacturing operations.

Mechanical Properties:

• Mechanical properties are the most important technical factor governing the selection of material. They include strength under static and fluctuating loads, elasticity, plasticity, stiffness, resilience, toughness, ductility, malleability and hardness.
• Depending upon the service conditions and the functional requirement, different mechanical properties are considered and a suitable material is selected.
• For example, the material for the connecting rod of an internal combustion engine should be capable to withstand fluctuating stresses induced due to combustion of fuel.

Manufacturing Considerations:

• Machinability of material is an important consideration in selection. Sometimes, an expensive material is more economical than a low priced one, which is difficult to machine.
• Free cutting steels have excellent machinability, which is an important factor in their selection for high strength bolts, axles and shafts.
• In fabricated assemblies of plates and rods, weldability becomes the governing factor. The manufacturing processes, such as casting, forging, extrusion, welding and machining govern the selection of material.