CHAPTER 1GENERAL

CONSIDERATION IN DESIGN OF MACHINE ELEMENTS

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Introduction To Machine DesignDefinition of Machine Design

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• Machine design is defined as the use of scientific principles, technical information & imagination in the description of a machine or a mechanical system to perform specific functions with maximum economy & efficiency.

• Machine Design is defined as the creation of new design or improving the exist one.

Design Process:

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Introduction To Machine DesignBasic Requirement of Machine Elements

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• Strength and Rigidity

• Wear Resistance

• Minimum Dimensions & Weight

• Manufacturability

• Safety

• Conformance to standards

• Reliability

• Maintainability

• Minimum Life-cycle Cost

Introduction To Machine DesignBasic Procedure of Design of Machine Elements

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Engineering Materials and their Properties

Material Selection

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• Selection of proper material for the machine components is one of the most important steps in process of machine design

• The best material is one which will serve the desired purpose at minimum costs

• Factors Considered while selecting the material:

i) Availability: Material should be readily available in market in large enough quantities to meet the requirement

Material Selection

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ii) Cost:

iii) Mechanical properties:

Manufacturing Considerations:

• In some applications machinability of material is an important consideration in selection

• Where the product is of complex shape, castability or ability of the molten metal to flow into intricate passages is the criterion of material selection

• In fabricated assemblies of plates & rods, weldability becomes the governing factor

Properties of Material

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Strength: Ability of the material to resist, without rupture, external forces causing various types of stresses

Elasticity: Ability to regain its original shape & size after deformation, when the external forces are removed

Plasticity: Ability to retain the deformation produced under the load on a permanent basis

Stiffness or Rigidity: Ability to resist deformation under the action of an external load

Properties of Material

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Toughness: Ability to absorb energy before fracture takes place

Malleability: Ability to deform to a greater extent before the sign of crack, when it is subjected to compressive force

Ductility: Ability to deform to a greater extent before the sign of crack, when subjected to tensile force

Brittleness: Property of the material which shows negligible plastic deformation fracture takes place

Hardness: Resistance to penetration or permanent deformation

Engineering MaterialCast Iron

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• Cast iron is an alloy of iron & carbon, containing more than 2% of carbon

- Typical composition of ordinary cast iron is:

• Carbon = 3-4%

• Silicon = 1-3%

• Manganese = 0.5-1%

• Sulphur = up to 0.1%

• Phosphorous = up to 0.1%

• Iron = Remainder

Engineering MaterialCast Iron

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• Advantages:

• Available in large quantities,

• higher compressive strength,

• components can be given any complex shape without involving costly machining operations,

• excellent ability to damp vibrations,

• more resistance to wear even under the conditions of boundary lubrication,

• mechanical properties of parts do not change between room temperature and 350 degree centigrade

Engineering MaterialPlain Carbon Steel

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• Depending upon the percentage of carbon, plain carbon steels are classified as:

i) Low carbon steel – Less than 0.3% carbon, popularly known as mild steel, its soft & ductile, easily machined & welded, however due to low carbon content unresponsive to heat treatment

ii) Medium carbon steel – carbon content in the range of 0.3% to 0.5%, popularly known as machinery steel, easily hardened by heat treatment , stronger & tougher than low carbon steel, well machined, respond readily to heat treatment

Engineering MaterialPlain Carbon Steel

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iii) High carbon steel – more than 0.5% carbon, popularly known as hard steels or tool steels, respond readily to heat treatment, when heat treated attain high strength combined with hardness, less ductile than low carbon steels & medium carbon steels, difficult to weld, excessive hardness accompanied by excessive brittleness

• In applications like automobile bodies & hoods, the ability of the material to deform to a greater extent or ‘ductility’ is the most important consideration so a plain carbon is preferred

Engineering MaterialPlain Carbon Steel

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• In applications like gears, machine tool spindles & transmission shaft, strength toughness & response to heat treatment are important considerations, medium & high carbon steels are preferred

• Spring wires are subjected to severe stress & strength is the most important consideration so high carbon steel is selected for helical & leaf springs

Engineering MaterialAlloy Steel

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• Carbon steel to which one or more alloying elements are added to obtain certain beneficial effects

• The commonly added elements include silicon, manganese, nickel, chromium, molybdenum and tungsten

• The term ‘alloy steels’ usually refers to ‘low’ alloy steels containing from 1-4% of alloying elements

• Alloy steels have higher strength, hardness & toughness, higher hardenability, retain their strength & hardness at elevated temperatures, higher resistance to corrosion and oxidation

Engineering MaterialAlloy Steel

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- Silicon increases strength & hardness without lowering the ductility. Silicon is added in spring steel to increase its toughness

- Manganese increases hardness and toughness and also increases the depth of hardening

- Nickel increases strength, hardness and toughness without sacrificing ductility

- Chromium increases hardness & wear resistance, steel containing more than 4% chromium have excellent corrosion resistance

- Molybdenum increases hardness & wear resistance, resists softening of steel during tempering and heating

Introduction To Machine DesignStandardization

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• Standardization is the obligatory norms, to which various characteristics of a product should conform. The characteristics include materials, dimensions and shape of the component, method of testing and method of marketing, packing and storing of the product.

• Following standards are used in Mechanical Engineering Design

Standards for materials, their chemical compositions, Mechanical properties & heat treatment

Introduction To Machine DesignStandardization

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Standards for shapes and dimensions of commonly used machine elements

Standards for fits, tolerances and surface finish of component

Standards for testing of products

Standards for engineering drawing of components

Introduction To Machine DesignPreferred Numbers

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• The system is based on the use geometric progression to develop a set of numbers

• There are five basic series denoted as R5, R10, R20, R40, and R80 series which increases in steps of 56%, 26%, 12%, 6% and 3% respectively

• Each series has its own series factor as shown below