Machine Design

What is the importance of Machine Design for engineers?

What is Machine Design?

Creation of new and better machines AND

Improving existing ones

So that it is economical in the cost of production and operation.

Machine Design

Requirement

Model(Rough idea)

Creation

How a design is born

marketability

Availability ofFUNDS

Availablematerial

Manufacturingresources

Analysis

Market survey

Aesthetic

Ease ofhandling

Safety

Economical

Recyclability

Force/stress Material/sused Sizes

What is the basic knowledge required for Machine Design?

•Mathematics

•Engineering Mechanics

•Strength of Materials

•Mathematics

•Engineering Mechanics

•Strength of Materials

•Workshop Processes

•Engineering Drawing

• Mathematics

• Engineering Mechanics

• Strength of Materials

• Workshop Processes

• Engineering Drawing

• Computing

• Finite Element Analysis, Computational Fluid Dynamics etc

• Mechanics of Machines• Mechanics of Materials• Fluid Mechanics & Thermodynamics

Important considerations in Machine Design

1. Type of LOAD and STRESSes caused by the load

Steady loads

• Dead loads

• Live loads

Variable loads

• Shock loads (suddenly)

• Impact loads (applied with some velocity)

• Stress and strain (Tensile, compressive, shear)

• Thermal stresses

• Torsional stresses

• Bending stress

Important considerations in Machine Design…..

2. KINEMATICS of the machine (Motion of the parts)

Find the simplest arrangement that would give the most efficient motion that is required.

3. Selection of MATERIALs

Knowledge of the properties of the materials and their behaviour under working conditions is required.

Strength, hardness, durability, flexibility, weight, resistance to heat and corrosion, electrical conductivity, machinability, etc.

Important considerations in Machine Design…..3. Selection of MATERIALs

Physical properties: Density, Melting point, Elec/thermal properties

Mechanical properties:• STRENGTH – resist externally applied loads without breaking or yielding• STIFFNESS – resist deformation under stress• ELASTICITY – regain original shape once the force is removed• PLASTICITY – property which retains deformation (required for forging etc)• DUCTILITY – ability to be drawn into a wire by a tensile force• BRITTLENESS – sudden breaking with minimum distortion• TOUGHNESS – resist fracture due to high impact load• CREEP – deformation under stress and high temperature• FATIGUE – ability to withstand cyclic stresses• HARDNESS – resistance to wear, scratching, deformation, machinability etc

Metal Non-metal

Ferrous Non-ferrous

Important considerations in Machine Design…..

4. Form and size of the parts

Use I-beam or Angle-iron?

The size will be determined by the forces/torques applied (stresses on the object) and the material used such that failure (fracture or deformation) would not occur

Design Process

General procedure in Machine Design…..

Detailed drawing

Need or aim

Synthesis

Analysis of the FORCES

Material selection

Design of elements

Modification

Recognize and specify the problem

Select the mechanism that would give the desired motion and form the basic model with a sketch etc

Determine the stresses and thereby the sizes of components s.t. failure or deformation does not occur

Modify sizes to ease construction & reduce overall cost

Production

STRESS AND STRAIN

DIRECT OR NORMAL STRESS When a force is transmitted through a body,

the body tends to change its shape or deform. The body is said to be strained.

Direct Stress = Applied Force (F) Cross Sectional Area (A)

Units: Usually N/m2 (Pa), N/mm2, MN/m2, GN/m2 or N/cm2

Note: 1 N/mm2 = 1 MN/m2 = 1 MPa

Direct Stress Contd. Direct stress may be tensile or compressive,

c and result from forces acting perpendicular to the plane of the cross-section

c

tTension

Compression

Direct or Normal Strain When loads are applied to a body, some

deformation will occur resulting to a change in dimension.

Consider a bar, subjected to axial tensile loading force, F. If the bar extension is dl and its original length (before loading) is L, then tensile strain is:

Direct or Normal Strain Contd.

dlFF

L

Direct Strain ( ) = Change in Length

Original Length

i.e. = dl/L

Direct or Normal Strain Contd. As strain is a ratio of lengths, it is

dimensionless. Similarly, for compression by amount, dl:

Compressive strain = - dl/L Note: Strain is positive for an increase in

dimension and negative for a reduction in dimension.

Shear Stress and Shear StrainShear stresses are produced by

equal and opposite parallel forces not in line.

The forces tend to make one part of the material slide over the other part.

Shear stress is tangential to the area over which it acts.

Shear Stress and Shear Strain Contd.

P Q

S R

FD D’

A B

C C’

L

x

Shear strain is the distortion produced by shear stress on an element or rectangular block as above. The shear strain, (gamma) is given as:

= x/L = tan

Shear Stress and Shear Strain Concluded

For small ,

Shear strain then becomes the change in the right angle.

It is dimensionless and is measured in radians.

Elasticity and Hooke’s Law

All solid materials deform when they are stressed, and as stress is increased, deformation also increases.

If a material returns to its original size and shape on removal of load causing deformation, it is said to be elastic.

If the stress is steadily increased, a point is reached when, after the removal of load, not all the induced strain is removed.

This is called the elastic limit.

Hooke’s Law States that providing the limit of

proportionality of a material is not exceeded, the stress is directly proportional to the strain produced.

If a graph of stress and strain is plotted as load is gradually applied, the first portion of the graph will be a straight line.

The slope of this line is the constant of proportionality called modulus of Elasticity, E or Young’s Modulus.

It is a measure of the stiffness of a material.

Hooke’s Law

Modulus of Elasticity, E = DirectstressDirectstrain

Also: For Shear stress: Modulus of rigidity or shear modulus, G = ShearstressShearstrain

Equation For Extension

This equation for extension is very important

From the above equations:

E F Adl L

F LA dl

dlF LA E

//

Factor of Safety The load which any member of a machine

carries is called working load, and stress produced by this load is the working stress.

Obviously, the working stress must be less than the yield stress, tensile strength or the ultimate stress.

This working stress is also called the permissible stress or the allowable stress or the design stress.

Factor of Safety Contd. Some reasons for factor of safety include the

inexactness or inaccuracies in the estimation of stresses and the non-uniformity of some materials.

Factor of safety = UltimateoryieldstressDesignorworkingstress

Note: Ultimate stress is used for materials e.g. concrete which do not have a well-defined yield point, or brittle materials which behave in a linear manner up to failure. Yield stress is used for other materials e.g. steel with well defined yield stress.

Lateral Strain and Poisson’s Ratio 

Under the action of a longitudinal stress, a body will extend in the direction of the stress and contract in the transverse or lateral direction

(see Fig. below). The reverse occurs under a compressive

load.

Stress Effects

PP

Longitudinal Tensile Stress Effect

Longitudinal Compressive Stress Effect

PP

Poisson’s RatioLateral strain is proportional to the longitudinal strain,

with the constant of proportionality called ‘Poisson’s ratio’ with symbol, .

Mathematically, LateralstrainDirectorlongitudinalstrain

For most metals, the range of is 0.28 to 0.33.

Classification of Engineering Materials

Metals and their alloys 1. Ferrous metals 2. Non-ferrous

metals cast iron, steel, copper, brass,

Zinc Non metals such as glass, rubber,

Plastic

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Choice of Materials for Engineering purposes

1. Availability of materials2. Suitability of materials for working conditions in

services 3. Cost of the materials

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Mechanical Properties of Metals

Strength – ability of a material to resist the externally applied forces without breaking

Stiffness- ability of a material to resist deformation under stress

Elasticity – regain its original shape after deformation when external forces are removed

Ductility – property of a material enabling it to draw into wire when tensile force is applied

Brittleness – property of the material opposite to ductility

Mechanical Properties of Metals

Toughness- property of the material to resist fracture due to high impact loads

Resilience – property of the material to absorb energy and to resist shock and impact loads

Creep – when subjected to constant stress at high temperature for a long period it will undergo a slow and permanent deformation

Malleability – special case of ductility which permits materials to be rolled or hammered into thin sheets

Hardness – important property of the metals – it embraces many different properties such as resistances to wear, scratching, deformation, and machinability etc.,

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Enhance the properties

Heat Treatment

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Interchangeability The term interchangeability is

normally employed for mass production of identical items within the prescribed limits of sizes.

Certain variations are recognized and allowed in the sizes of the mating parts to give the required fitting.

This facilitates to select a random from a large number of parts for an assembly and results in a considerable saving in the cost of production

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Standardization Minimize variety Standard components and parts can

be obtained readily and economically from the market

Makes the repair and maintenance of machine simple

Reduce the time and effort to design and manufacture new machines

Mass production also becomes possible due to standardization

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Preferred Numbers Preferred numbers are nothing bet a

series of numbers in a geometric progression

Used for standardization Reduce unnecessary variations in sizes

of an articles Used for fixing the speed, power

capacities, structures By experience that discrete increment of

particular measure, if maintained in GP gives a logical, uniform and proportionate characteristic variation pattern.

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_____

5√10 = 1.6 → R5 ___

10√10 = 1.25 → R10 ___

20√10 = 1.12 → R20 ___

40√10 = 1.06 → R40

Important properties of preferred numbers

Numbers Close to each other at the lower end

The magnitudes of number increase they are more widely dpaced

Simple and easily remembered They are unlimited towards the lower

and well as higher numbers They include all the decimal multiples

and sub multiples of any terms

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Standardizing organisations India – ISI Indian standards institution Great British – BSI British Standards

institution Europe – ISA international Federation

of National standardizing Associations

ABC conference- America, Britain and Canada for cooperative discussions in the field of standardizing

National Physical Laboratories – NPL certification marks for quality instruments

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Failure

Failure

Failure- Excessive deformation

Failure- Excessive deformation

Stress-strain dia.

Failure - Fracture

Failure - Fracture

Failure - Wear

Failure- Deterioration