Chapter 1 design stages

MACHINE DESIGN
CHAPTER 1:
THE DESIGN PROCESS
INTRODUCTION
1.0 DEFINITIONS:
1.1 Design:
In general, to design is to formulate a plan using a systematic method, for the satisfaction
of a human need.
In particular Machine Design is the creation of new and better machines and improving the
existing ones. A new or better machine is one which is more economical in the overall cost of
production and operation. The process of design is a long and time consuming one. From the
study of existing ideas, a new idea has to be conceived. The idea is then studied keeping in
mind its commercial success and given shape and form in the form of drawings. In the
preparation of these drawings, care must be taken of the availability of resources in money, in
men and in materials required for the successful completion of the new idea into an actual
reality.
In designing, the particular need to be satisfied may be quite well defined on the one
hand, for example, a gear box is giving trouble, redesign it so it can work better, while on
the other it may be vague and ill defined that a considerable amount of thought and
effort is necessary if the problems to be clear, for example many people are killed in
aeroplane accidents.
In contrast to Mathematical problems, design problems have no unique answers. A good
solution today may turn out to be a bad one tomorrow.
Human needs and desires are related to time and money which must be taken into account
when considering the optimal solution out of a wide range of possible solutions.
1.2 Classification of Machine Design
Machine design may be classified as follows :
a) Adaptive design. In most cases, the designer’s work is concerned with adaptation of
existing designs. This type of design needs no special knowledge or skill and can be
attempted by designers of ordinary technical training. The designer only makes minor
alternation or modification in the existing designs of the product.
b) 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.
c) 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.

The designs, depending upon the methods used, may be classified as follows :
(a) Rational design. This type of design depends upon mathematical formulae of principle of
mechanics.
(b) Empirical design. This type of design depends upon empirical formulae based on the
practice and past experience.
(c) Industrial design. This type of design depends upon the production aspects to
manufacture any machine component in the industry.
(d) Optimum design. It is the best design for the given objective function under the specified
constraints. It may be achieved by minimising the undesirable effects.
(e) System design. It is the design of any complex mechanical system like a motor car.
(f) Element design. It is the design of any element of the mechanical system like piston,
crankshaft, connecting rod, etc.
(g) Computer aided design. This type of design depends upon the use of computer systems to
assist in the creation, modification, analysis and optimisation of a design.
1.3 General Considerations in Machine Design
Following are the general considerations in designing a machine component :
a) 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 in chapters to follow.
b) 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.
The motion of the parts may be :
(i) Rectilinear motion which includes unidirectional and reciprocating motions.
(ii) Curvilinear motion which includes rotary, oscillatory and simple harmonic.
(iii) Constant velocity.
(iv) Constant or variable acceleration.
c) Selection of materials. It is essential that a designer should have a thorough knowledge of
the properties of the materials and their behaviour 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. The various types of engineering materials and their properties have been
discussed in at National certificate (NC).
d)Form and size of the parts. The form and size are based on judgement. 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. It is also important to
anticipate any suddenly applied or impact load which may cause failure.
e) 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.
f) 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 adjustment for wear must be provided employing the
various take up devices and arranging them so that the alignment of parts is preserved. If
parts are to be changed for different products or replaced on account of wear or breakage,

easy access should be provided and the necessity of removing other parts to accomplish this
should be avoided if possible.
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.
g) 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.
h) 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.
i) Workshop facilities. A design engineer should be familiar with the limitations of his
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.
j) 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.
k) 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 hand made 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.
l) 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 Mechanical Engineering design
Mechanical engineering design means the design of things and systems of the
mechanical nature such as machines , products ,structures , devices ,and instruments. As
applied to machines alone the usual name given is machine design.

For most of the part , machine design utilizes mathematics ; the materials sciences
(Materials, Workshop Technology and workshop practice) ; the engineering mechanics
sciences (Engineering Science, Applied Mechanics, Mechanics of Machines and Engineering
Mechanics); thermal (heat and thermodynamics) and fluid sciences. Hence in design
students are called upon to utilise all the theoretical knowledge learned in other subjects in a
logical and systematic way to solve a particular problem.
In a nut shell design involve
(i) Theoretical knowledge( Experimental training, strength of Materials,
Thermodynamics, Fluid Mechanics. Economics, Law etc.)
(ii) Human Skill (Creativity, imagination, inventiveness etc.)
The design process therefore entails the establishment of a concept, using creative skills. The
concept is then divided into smaller components that can be solved using the knowledge
gained in relevant subjects. Some of these smaller components my require additional
knowledge of other subjects such as electronics, of which one may not have sufficient know
how. It is very important for the design to recognise his own limitations and know when to
request assistance from persons with the necessary additional skills. In fact in practice more
than one person will be involved in a design project, therefore teamwork will be of utmost
importance.
1.5 Phases of design process:
What is meant by a good designer ?
A good designer’s most important attribute is that of logical decision making. Every
step in a design requires decision - decision not based on prejudices and feelings , but
on available facts.
In practice the engineering designer must give an economic solution to the presented
problem within a reasonable period of time. The way in which the designer sets
about finding this solution should involve a logical design process to satisfy a specific
need or requirement.
At the end of each step a decision must be made whether continuation to the next stage is
justified or whether a return to the previous stage in necessary. This is dependent on whether
the objectives of the previous stage have been met or not. A design process seldom advances
from one stage to the next without going back (iteration) to the previous stages to eliminate
shortcomings. This should not be considered as a waste of time or a failure but as a built-in
mechanism to ensure that the design is improved as the process continues.
Figures 1, 2 and 3 show models of systematic design process.
RECOGNITION OFNEED( REQUIREMENT)

Fig. 1
Alternatively
IDIDE
Fig. 2
1.5.1 Recognition (Identification) of need (requirement)
All designs must start with a need or requirement, from this the designer decides what the
objectives are. The requirement outlines the need to be satisfied.
Recognition of need and phrasing it often constitute a highly creative act, because the need
may be only a vague discontent, a feeling of uneasiness or a sensing that something is not
right.
IDEFICATION OFTHE PROBLEM
GATHERING OF INFORMATION
ANALYSIS
IMPLEMENTATION OF THE
DESIGN
EVALUATION OFIDEAS
PRELIMINARYIDEAS
EVALAUTION
PRESENTATION ANDCOMMISIONING
STNTHESIS,ANALYSISAND
OPTIMASATION
SETTING OF OBJECTIVES(SPECIFICATION)

For recognition of need the designer should keep an open mind and attempt to see not only
the problem but also to understand its relationship with its environment. Sometimes a small
adjustment or alteration outside the problem area may eliminate or alleviate a symptom that
was thought to be a problem, thereby preventing major undertaking. Once the problem is
understood, it must be formulated in writing. Objectives to be met, specific requirements,
unspecified conditions and factors to be considered when the eventual performance of the
designed object is to evaluated, must be noted and written down. The following must
considered:
i) Statement of the problem:
The problem should be stated clearly in one sentence. This consists of three basic
components:-
(i) ‘What needs to done’ should be addressed, e.g. The purpose of this project is
to design a conveyor belt with supporting structure.
(ii) The standard, code or principles on which the design will be based must be
stated, e.g. According to BS structural steel code.
(iii) ‘Why the design needs to be done’ should be answered, e.g. so that 50kg wire
reels can be loaded on to trucks of varying heights.
a) The sub-problems
When projects are large they are divided into smaller sub-projects that will be easier
to comprehend and solve. Each sub-project should be a complete project.
Sub project 1. Design a conveyor with a total travel of 30m that can carry 10, 50kg
wire reels at velocity 0.2m/s.
Sub-project 2. Design a structure for the conveyor belt with an inlet 0.6m above
ground level and adjustable outlet.
b) Delimitations
It is also important that after stating the problem and sub-problems clearly, the
designer specifies what he will not intend to do.
The designer will not incorporate
(i) A variable speed for the conveyor
(ii) A means whereby the structure can be moved from one place to another.
c) Assumptions
Factors that will be taken for granted and that will not be incorporated into the design
must also be clearly stated, e.g.
(i) The existing floor will be able to carry the structure.
(ii) The number of reels per given time should be 10
The following should also be taken into account;

i) Check the reliability of the need source
ii) Make a preliminary assessment of the need to see if it is economically realistic
iii) Can the requirement be simply modified to expand the market potential?
iv) Do not fail to recognize a subdivision of a larger design as a requirement in its
own right.
1.5.2 Setting the objectives (specification) or gathering information
In design and definitely the real word situations enough information is not normally
given and therefore the gathering of relevant information is not always that obvious.
The first important source of information is people. Information can be gathered from
the people who will be involved with the final product, people who were previously
involved in a similar project and representatives from various supply companies.
Another source is written material (catalogues, reports, experimental data, design
magazines and books and the internet). Work by other designers can also be a source
of relevant information.
When collecting data you might find that your initial problem statement was not
complete. Do not hesitate to alter the problem statement to eliminate uncertainties.
This process of amending previous stages is very important and eliminates the
possibility of major changes in your design at the final stage because on an initial
misconception.
A specification is a listing of all parameters essential to the design.
For correct specification the following must be considered:
a) Listing of design parameters such as:
- Physical characteristics e.g. output power required, ambient temperature and
pressure variations, the load to be supported, reliability life span etc.
- Subjective aspects such as appearance, styling, surface finish etc
- Objective aspects such as user skill, safety, standards and service requirements,
b) The specification must be so framed as to avoid unnecessarily influencing the
possible range of solutions,
c) An order of priorities within a specification must be recognized, among cost,
weight, reliability, appearance just but to mention a few,
d) Any item or value transferred from the requirement recognition to the specification
must be critically examined before the transfer is permitted.
1.5.3 Synthesis , analysis and optimization
Synthesis involves the engineering of design to the state of preparing working
drawings, component schedules and other media of communication between the

designer and manufacturer. Synthesis can be divided into preliminary and final
synthesis:-
Preliminary Synthesis:
Preliminary ideas (synthesis), is where imagination and creativity play a major role as
one should think of a number of different ways in which to solve the problem. Make
sketches of the different ideas and write down their advantages and disadvantages.
Brainstorming can play a pivotal role if people are working in groups.
Preliminary calculations and discussions with the end user of the design will now help
to eliminate some of the ideas. Factors such as cost and availability of materials use of
standard components and manufacturing techniques required should also be considered
to lift the best idea on which the design will be based.
Analysis:
Physical sizes and dimensions of various components must be calculated. Calculations
must be based on the theory of various engineering subjects. Before you calculate an
unknown value, try to estimate the value and if the calculated answer is not of the same
order or magnitude as your estimation, you should check your reasoning and
calculations. Be critical of you answers.
Unknown values can also be determined through experiments, testing of models and
testing of prototypes. Sophisticated tools for analysis are also available on the
computer.
Optimisation:
Optimisation means standardisation of calculated dimensions as standardised it
catalogues, books manufacturer’ s manuals or available components in the stores. In
optimising calculated values it is always recommended to take values above the
calculated values as taken smaller values result in weakening the section.
The synthesis of the optimum solution cannot take place without both analysis and
optimization.
The analysis may reveal that the solution is not an optimum one. If the design fails
either or both of these tests the synthesis procedure must begin again. Proper synthesis
must include the following:
i) Detailing is as important as the other design stages and requires a specialized
knowledge of manufacturing methods,
ii) Synthesis is the correct place for detailed analysis, not the evaluation stage,
iii) Be aware of new requirements arising during synthesis and treat them, as
designs in there own right.
Final Synthesis:

This is where detailed working drawings are produced. These drawings will
communicate your final design to your customer and to the people involved in the
manufacturing process. Documentation on starting up procedures, mechanics,
adjustments, maintenance requirements and other relevant information should also be
compiled. Time and money will normally put major restrictions on your design,
tempting you to take shortcuts. Try to avoid this and adhere to the steps outlined above
as they have proved themselves to produce the beet results.
1.5.4 Evaluation and Iteration
Evaluation is a significant phase of the total design process. It is the final proof of a
successful design and usually involves the testing of a proto-type in the laboratory.
Evaluation looks at the satisfaction of the need or needs, reliability, competence of the
product, economics, maintenance and adjustment, profit, etc
In short evaluation must include the following;
a) Careful assessment avoids throwing away the effort made in earlier stages of the
design process,
b) Evaluation should start at a superficial level, increasing in detail as the elimination
proceeds,
c) Economic as well as technical considerations should be assessed at each stage of the
elimination.
Iteration:
Although the stages of the design process have been laid out as discrete steps, iteration
must exist between them. Design is an iterative process, particularly on large projects.
1.5.5 Presentation and Commissioning.
Presentation is a selling job. In presentation the design tries to sell or to prove to the
interested parties that the solution is a better one.
Poor presentation may prove time and effort spend on obtaining the solution have been
largely wasted.
Basically, there are only three means of communication; These are written, oral and
graphical.
The designer must be technically competent versatile in all three forms of
communication.

More specifically machine design follow steps shown in figure 3 below:
Figure 3: Specific procedure in Machine Design.
Specific Procedure in 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 as follows :
a) Recognition of need. First of all, make a complete statement of the problem, indicating the
need, aim or purpose for which the machine is to be designed.
b) Synthesis (Mechanisms). Select the possible mechanism or group of mechanisms which
will give the desired motion.
Recognition of
need
Synthesis
(Mechanisms).
Analysisof forces
Material selection
Design of elements
(Sizeand Stresses).
Modification
Detailed drawing
Production.

c) Analysis of forces. Find the forces acting on each member of the machine and the energy
transmitted by each member.
d) Material selection. Select the material best suited for each member of the machine.
e) Design of 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.
It should be kept in mind that each member should not deflect or deform than the permissible
limit.
f) Modification. 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.
g) Detailed drawing. Draw the detailed drawing of each component and the assembly of the
machine with complete specification for the manufacturing processes suggested.
h) Production. The component, as per the drawing, is manufactured in the workshop.
1.6 DESIGN CONSIDERATIONS OR CHARACTERISTICS WHICH INFLUENCE
THE DESIGN OF AN ELEMENT OR ENTIRE SYSTEM
Usually a number of these have to be considered in a given design situation. Sometimes one
of these will turn out to be critical, and when it is satisfied, the others no longer need to be
considered.
These can be grouped as follows;
1.6.1. Mechanical factors
These include strength and stress; factor of safety; friction, wear and lubrication;
serviceability; reliability; longevity; maintainability; etc
1.6.2 Thermal Properties
Such as corrosion; heat conductivity and refractory properties.
1.6.3 Physical considerations
e.g. weight; volume; noise - vibration ; size ; stiffness and flexibility etc
1.6.4 Electrical and magnetic properties
Such as conductors and non-conductors of electricity
1.6.5 Consumer considerations
Such as Ergonomics (control, shape); styling; appearance etc.
1.6.6 Economic considerations
Such as: cost of materials, labour, transportation, and productivity
1.6.7. Legal considerations
- Codes of practice standards

Chapter 1 design stages

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Chapter 1 design stages