Unit I

Fundamentals of Design
1.1 Design Philosophy: Design is the process of creation of new and
better machines or improvement in existing machines. The design process is
a long and time-consuming process. In many design processes, new idea is
generated from existing design/ideas. In design process various factors need
to be considered during design process like commercial success of the design,
availability of resources (Man, Material and Money). In designing a machine
component, it is necessary to have a good knowledge of many subjects such
as Mathematics, Engineering Mechanics, Strength of Materials, Theory of
Machines, Workshop Processes and Engineering Drawing.
1.1.1 Classification of Machine Design: -
The machine design may be classified as follows:
1) Adaptive design
2) Development design
3) New design.
1) Adaptive design: In many cases, the designer’s work is concerned with
adaptation of existing designs. This type of design needs no special knowledge
or skill to the designer. The designer only makes minor alternation or
modification in the existing designs of the product.
2) Development design: This type of design needs considerable scientific
training and design ability in order to modify the existing designs into a new
idea by adopting a new material or different method of manufacture. In this
case, though the designer starts from the existing design, but the final product
may differ quite markedly from the original product.
3) New design: - This type of design needs lot of research, technical ability
and creative thinking. Only those designers who have personal qualities of a
sufficiently high order can take up the work of a new design.
1.2 General procedures of machine design: -
In designing a machine component, there is no rigid rule. The problem may
be attempted in several ways. However, the general procedure to solve a design
problem is shown in flow chart as follows:

General Procedure in Machine Design

Need or Aim of Design

Selection of Mechanisms

Analysis of Forces

Selection of Material

Design of Machine Elements

Modification in Design

Detailed Drawing

Production

1. Need of Design: - First of all, make a complete statement of the problem,

indicating the need, aim or purpose for which the machine is to be designed.
2. Selection of Mechanisms: - Select the possible mechanism or group of
mechanisms which will give the desired motion.
3. Analysis of forces: - Find the forces acting on each member of the machine
and the energy transmitted by each member.
4. Material selection: - Select the material best suited for each member of
the machine.
5. Design of machine elements (Size and Stresses): - Find the size of each
member of the machine by considering the force acting on the member and
the permissible stresses for the material used.
6. Modification in design: - Modify the size of the member to agree with the
past experience and judgment to facilitate manufacture. The modification may
also be necessary by consideration of manufacturing to reduce overall cost.
7. Detailed drawing: - Draw the detailed drawing of each component and the
assembly of the machine with complete specification.
8. Production: - The component, as per the drawing, is manufactured in the
workshop.
1.3 General Considerations in Machine Design
Following are the general considerations in designing a machine component:
1. Type of load and stresses caused by the load- The load, on a machine
component, may act in several ways due to which the internal stresses are set
up. The various types of load and stresses are discussed later.
2. Motion of the parts or kinematics of the machine- The successful
operation of any machine depends largely upon the simplest arrangement of
the parts which will give the motion required.
3. Selection of materials- It is essential that a designer should have a
thorough knowledge of the properties of the materials and their behavior
under working conditions. Some of the important characteristics of materials
are: strength, durability, flexibility, weight, resistance to heat and corrosion,
ability to cast, welded or hardened, machinability, electrical conductivity, etc.
4. Form and size of the parts- The form and size are based on judgment. The
smallest practicable cross-section may be used, but it may be checked that
the stresses induced in the designed cross-section are reasonably safe. In
order to design any machine part for form and size, it is necessary to know
the forces which the part must sustain.
5. Frictional resistance and lubrication- There is always a loss of power due
to frictional resistance and it should be noted that the friction of starting is
higher than that of running friction. It is, therefore, essential that a careful
attention must be given to the matter of lubrication of all surfaces which move
in contact with others, whether in rotating, sliding, or rolling bearings.
6. Convenient and economical features- In designing, the operating features
of the machine should be carefully studied. The starting, controlling and
stopping levers should be located on the basis of convenient handling. The
economical operation of a machine which is to be used for production or for
the processing of material should be studied, in order to learn whether it has
the maximum capacity consistent with the production of good work.
7. Use of standard parts- The use of standard parts is closely related to cost,
because the cost of standard or stock parts is only a fraction of the cost of
similar parts made to order. The standard or stock parts should be used
whenever possible; parts for which patterns are already in existence such as
gears, pulleys and bearings and parts which may be selected from regular
shop stock such as screws, nuts and pins. Bolts and studs should be as few
as possible to avoid the delay caused by changing drills, reamers and taps and
also to decrease the number of wrenches required.
8. Safety of operation- Some machines are dangerous to operate, especially
those which are speeded up to insure production at a maximum rate.
Therefore, any moving part of a machine which is within the zone of a worker
is considered an accident hazard and may be the cause of an injury. It is,
therefore, necessary that a designer should always provide safety devices for
the safety of the operator. The safety appliances should in no way interfere
with operation of the machine.
9. Workshop facilities- A design engineer should be familiar with the
limitations of this employer’s workshop, in order to avoid the necessity of
having work done in some other workshop. It is sometimes necessary to plan
and supervise the workshop operations and to draft methods for casting,
handling and machining special parts.
10. Number of machines to be manufactured- The number of articles or
machines to be manufactured affects the design in a number of ways. The
engineering and shop costs which are called fixed charges or overhead
expenses are distributed over the number of articles to be manufactured. If
only a few articles are to be made, extra expenses are not justified unless the
machine is large or of some special design. An order calling for small number
of the product will not permit any undue expense in the workshop processes,
so that the designer should restrict his specification to standard parts as
much as possible.
11. Cost of construction- The cost of construction of an article is the most
important consideration involved in design. In some cases, it is quite possible
that the high cost of an article may immediately bar it from further
considerations. If an article has been invented and tests of handmade samples
have shown that it has commercial value, it is then possible to justify the
expenditure of a considerable sum of money in the design and development of
automatic machines to produce the article, especially if it can be sold in large
numbers. The aim of design engineer under all conditions should be to reduce
the manufacturing cost to the minimum.
12. Assembling- Every machine or structure must be assembled as a unit
before it can function. Large units must often be assembled in the shop, tested
and then taken to be transported to their place of service. The final location of
any machine is important and the design engineer must anticipate the exact
location and the local facilities for erection.
1.4 Modern Design considerations: Ergonomics and aesthetics.

1.5 Load: - It is defined as any external force acting upon a machine part.
Types of loads-
1. Dead or steady load- A load is said to be a dead or steady load, when it
does not change in magnitude or direction.
2. Live or variable load- A load is said to be a live or variable load, when it
changes continually.
3. Suddenly applied or shock loads- A load is said to be a suddenly applied
or shock load, when it is suddenly applied or removed.
4. Impact load- A load is said to be an impact load, when it is applied with
some initial velocity.
5.2 Concepts of stress-
Stress: - When some external system of forces or loads act on a body, the
internal forces (equal and opposite) are set up in the body, which resist the
external forces. This internal force per unit area at any section of the
body is known as a stress.
It is denoted by a Greek letter sigma (σ).
 Mathematically,
Stress, σ = P/A
where P = Force or load acting on a body, and
A = Cross-sectional area of the body.
In S.I. units, the stress is usually expressed in Pascal (Pa) such that
1 Pa = 1 N/m2
Types Stresses:
Direct Stress
Shear Stress
Bending Stress
Torsional Shear Stress
Crushing Stress
Bearing Pressure
1) Direct Stress: - When body is subjected under a direct load, body offers
the resistance to the load, that stress (Resistance) is called as direct Stress.
Types of Direst Stress-
✓ Tensile Stress-

When a body is subjected to two equal and opposite axial pulls P (also called
tensile load), then the stress induced at any section of the body is known as
tensile stress.
Mathematically, Tensile stress is calculated as,
Tensile stress, σt = P/A
where P = Force or load acting on a body, and
A = Cross-sectional area of the body.
✓ Compressive Stress-
When a body is subjected to two equal and opposite axial pushes P (also called
compressive load), then the stress induced at any section of the body is known
as compressive stress.

Mathematically, Compressive stress is calculated as,

Compressive stress, σc = P/A
where P = Force or load acting on a body, and
A = Cross-sectional area of the body.
2) Shear Stress: - When a body is subjected to two equal and opposite forces
acting tangentially across the resisting section, as a result of which the body
tends to shear off the section, then the stress induced is called shear stress.

Mathematically, Shear stress is calculated as,

Shear stress, τ = P/A
where P = Force or load acting on a body, and
A = Sheared area (Cutting Area) of the body.
3) Bending Stress: - When a body is subjected to eccentric load, as a result
of which the body tends to bend, then the stress induced is called Bending
Stress.
Mathematically, bending stress is calculated as,
Bending stress,

where M = Maximum Bending Moment

I = Moment of Inertia about a Neutral Axis.
Y= Distance of outermost Layer from neutral axis.