An Introduction to Machine Design

Machine Design is the process of selection of the material, shape, size and arrangement of mechanical element so that the machine will perform its task.

A process which includes design of all parts of machine subjected to carry the forces without failure by transforming other form of energy into mechanical energy.

General Procedure in Machine Design:

  1. Need: Indicate the need for which purpose or aim the machine is to be used and design.
  2. Mechanism: Selection of suitable mechanism that will give the desired motion required.
  3. Analysis of forces: Finding the forces acting on each members of the machine.
  4. Selection of material: Selection of suitable material according to availability, cost and conditions for each member and suitable factor of safety.
  5. Design of element: Find the size of each member of the machine by considering the forces acting on the member and the stress.
  6. Modification: Modification is necessary of manufacturing to reduce overall cost modification of size of the member.
  7. Detail Drawing: Draw the detail of each component with tolerance and assembly of machine with complete specifications.
  8. Production: The components as per drawing are manufactured in the workshop.
Figure: Procedure in Machine Design

Mechanical Properties of Materials:

  • Strength: The ability of material to sustain the load without breaking or rupture is known as strength.
  • Fatigue: The property of material to resist the repeating load before it rupture is known as fatigue.
  • Elasticity: The property of material in which a deformation is done under the application of load and it vanished after removing the load is called as elasticity.
  • Plasticity: The property of material in which a deformation is done under the application of load but the deformation does not vanished is called as plasticity.
  • Ductility: The property of material due to which the wires can be drawn under the tensile force is called as ductility. Examples: Copper, Silver, Gold, Aluminum, etc.
  • Malleability: The property of material due to which the thin sheets can be drawn by rolling or hammering is called as malleability. Examples: Copper, Silver, Gold, Aluminum, etc.
  • Brittleness: The property of material due to which it breaks into small pieces under the application of ultimate load without prior indication is called as brittleness. Examples: Cast Iron, Ceramics, Glass, Concrete, etc.
  • Hardness: The property of material to resist abrasion, scratching, penetration or wear is called as hardness. Examples: Rockwell, Vickers, Brunel, etc.
  • Toughness: The property of material to absorb the shock energy, this capacity is called as toughness. It is also known as impact strength. Examples: Shock absorbers, Springs etc.
  • Stiffness: The ability of material to resist the deformation under the load is called as stiffness. It is also known as rigidity.
  • Creep: When a machine component is subjected to a constant load at high temperature for long time it shows permanent deformation is called as creep.
  • Resilience: The property of material to absorb the energy and to resist shock and impact loads is called as resilience. Examples: Springs, Toys, Wrist watches, etc.

Factor of Safety:

Factor of safety is defined as maximum stress to working stress or design stress.

For ductile material: Factor of safety is defined as the ratio of yield point stress to design stress.

Factor of safety = Yield Stress / Design Stress

For brittle material: Factor of safety is defined as the ratio of ultimate stress to design stress.

Factor of safety = Ultimate Stress / Design Stress

Selection of factor of safety: It depends upon number of consideration such as material, type of stress, mode of manufacturing. The factors to be considered while selecting the factor of safety are as follows:

  1. Effect of failure on human safety and financial loss.
  2. The reliability of applied load.
  3. The reliability of the properties of material.
  4. Quality of manufacturing and service conditions.
  5. The extent of localized stresses.
  6. The extent of simplifying application.

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