Introduction to Machine Design

Introduction to Machine Design || What is Machine Design?

Design is essentially a decision-making process. To solve a problem, it is necessary to design a solution. In other words, to design is to formulate a plan to satisfy a particular need and to create something with a physical reality. Consider, for example, the design of a chair. A number of factors need to be considered first:
(a) The purpose for which the  chair is to be designed, such as whether it is to be used as an easy chair, an office chair, or to accompany a dining table.
(b) Whether the chair is to be designed for a grown-up person or a child.
(c) The material for the chair, its strength, and its cost need to be determined.
(d) Finally, the aesthetics of the designed chair.
Almost everyone is involved in design, in one way or another, in our daily lives because problems are posed, and they need to be solved. [Click Here to Watch video] [Click Here for PDF]

Basic Concepts of Design

Decision making occurs at every level of design. Consider two vehicles of different makes. They may both be reasonable cars with the same objective, yet their designs differ. The designers evaluate several elements and reach specific conclusions, resulting in an optimum design. A market survey gives an insight of what people want. Existing norms play a vital role. Once a major decision is made, the remaining design features will follow.

For example, once we’ve decided on the engine capacity, shape, and size, the rest of the design process will follow. A faulty judgment results in a bad design and product.
Design can be for a variety of products, and with our current expertise and knowledge bank, we have a lengthy list of design disciplines, such as ship design, building design, process design, bridge design, clothing/fashion design, and so on.

The emphasis here is on machine design. We now define a machine as a group of resisting bodies with successfully constrained relative motions that are utilized to convert other forms of energy into Mechanical energy or to transport and change available energy to perform productive work. A heat engine is defined as something that turns heat into mechanical energy. 

In many cases, however, the machines receive mechanical energy and use it to perform a specified duty, such as a hoist, a bicycle, or a hand-winch.
This modification or transformation of energy necessitates the use of a variety of machine elements, both tiny and large. Machine design is largely concerned with developing these pieces so that they may safely transmit forces and execute their functions. Consider the following simple mechanisms.
(a) A hand winch; (b) A small press with a power screw.
Each of these mechanisms produces some meaningful work through certain combinations of machine parts. Designing these mechanisms would include first designing the elements and then assembling them in order.

Types of Machine Design

There may be several types of design such as:

Adaptive design

This is based on existing design, for example, standard products or systems adopted for a new application. Conveyor belts, control systems of machines and mechanisms, or haulage systems are some of the examples where existing design systems are adapted for a particular use.

Developmental Design

Here we start with an existing design, but finally a modified design is obtained. A new model of a car is a typical example of a developmental design.

New Design

This type of design is entirely new but based on existing scientific principles. No scientific invention is involved, but it requires creative thinking to solve a problem. Examples of this type of design may include designing a small vehicle for the transportation of men and materials on board a ship or in a desert. Some research activity may be necessary.

Types of Design based on method

Rational design
This is based on determining the stresses and strains of components and thereby deciding their dimensions.

Empirical design
This is based on empirical formulae which in turn is based on experience and experiments. For example, when we tighten a nut on a bolt the force exerted or the stresses induced cannot be determined exactly but experience shows that the tightening force may be given by P=284d where, d is the bolt diameter in mm and P is the applied force in kg. There is no mathematical backing of this equation but it is based on observations and experience.

Industrial design
These are based on industrial considerations and norms viz. market survey, external look, production facilities, low cost, use of existing standard products.

Factors to be considered in machine design

There are many factors to be considered while attacking a design problem. In many cases these are a common sense approach to solving a problem. Some of these factors are as follows:
(a) What device or mechanism to be used? This would decide the relative arrangement of the constituent elements.(b) Material
(c) Forces on the elements
(d) Size, shape and space requirements. The final weight of the product is also a major concern.
(e) The method of manufacturing the components and their assembly.
(f) How will it operate?
(g) Reliability and safety aspects
(h) Inspectibility
(i) Maintenance, cost, and aesthetics of the designed product.

This is best judged by understanding the problem thoroughly. Sometimes a particular function can be achieved by a number of means or by using different mechanisms and the designer has to decide which one is most effective under the circumstances. A rough design or layout diagram may be made to crystallize the thoughts regarding the relative arrangement of the elements.

(b) Material

This is a very important aspect of any design. A wrong choice of material may lead to failure, over or undersized product or expensive items. The choice of materials is thus dependent on suitable properties of the material for each component, their suitability of fabrication or manufacture and the cost.

(c) Forces on the elements

The external loads cause internal stresses in the elements and these stresses must be determined accurately since these will be used in determining the component size. Loading may be due to:
i) Energy transmission by a machine member.
ii) Dead weight.
iii) Inertial forces.
iv) Thermal effects.
v) Frictional forces.
In other ways loads may be classified as:
i) Static load- Does not change in magnitude and direction and normally
increases gradually to a steady value.
ii) Dynamic load- a) changes in magnitude- for e.g. traffic of varying weight passing a bridge.
b) changes in direction- for e.g. load on piston rod of a double acting cylinder.
The nature of these loads are shown in figure below. Vibration and shock loading are types of dynamic loading.


Types of Loading


(d) Size, shape, space requirements and weight

Preliminary analysis would give an approximate size but if a standard element is to be chosen, the next larger size must be taken. Shapes of standard elements are known but for non-standard element, shapes and space requirements must depend on available space in a particular machine assembly. A scale layout drawing is often useful to arrive at an initial shape and size. Weight is important depending on application. For example, an aircraft must always be made light. This means that the material chosen must have the required strength yet it must be light. Similar arguments apply to choice of material for ships and there too light materials are to be chosen. Portable equipment must be made light.

(e) The method of manufacturing the components and their assembly

Care must always be taken to ensure that the designed elements may be manufactured with ease, within the available facilities and at low cost.

(f) How will it operate?

In the final stage of the design a designer must ensure that the machine may be operated with ease. In many power operated machines it is simply a matter of pressing a knob or switch to start the machine. However in many other cases, a sequence of operations is to be specified. This sequence must not be complicated and the operations should not require excessive force. Consider the starting, accelerating and stopping a scooter or a car. With time tested design considerations, the sequences have been made user-friendly and as in any other product, these products too go through continuous innovation and development.

(g) Reliability and safety aspects

Reliability is an important factor in any design. A designed machine should work effectively and reliably. The probability that an element or a machine will not fail in use is called reliability. Reliability lies between 0 ≤ R< 1. To ensure this, every detail should be examined. Possible overloading, wear of elements, excessive heat generation and other such detrimental factors must be avoided. There is no single answer for this but an overall safe design approach and care at every stage of design would result in a reliable machine.
Safety has become a matter of paramount importance these days in design. Machines must be designed to serve mankind, not to harm it. Industrial regulations ensure that the manufacturer is liable for any damage or harm arising out of a defective product. Use of a factor of safety only in design does not ensure its overall reliability.

(h) Inspectibility

This is also an important factor that is to be considered while designing a machine component or machine. There must be some provision of inspecting the various components of machine for proper working condition.

(i) Maintenance, cost and aesthetics of the designed product

Maintenance and safety are often interlinked. Good maintenance ensures good running condition of machinery. Often a regular maintenance schedule is maintained and a thorough check up of moving and loaded parts is carried out to avoid catastrophic failures. Low friction and wear is maintained by proper lubrication. This is a major aspect of design since wherever there are moving parts, friction and wear are inevitable. High friction leads to increased loss of energy. Wear of machine parts leads to loss of material and premature failure.
Cost and aesthetics are essential considerations for product design. Cost is essentially related to the choice of materials which in turn depends on the stresses developed in a given condition. Although in many cases aesthetic considerations are not essential aspects of machine design, ergonomic aspects must be taken into considerations.


Q.1: Define machine design.

Q.2: What is an adaptive design?

Q.3: Briefly suggest the steps to be followed by a designer.

Q.4: Discuss ‘factor of safety ‘ in view of the reliability in machine design.


A.1: A machine is a combination of several machine elements arranged to work together as a whole to accomplish specific purposes. Machine design involves designing the elements and arranging them optimally to obtain some useful work.

A.2: Adaptive design is based on an existing design adapted for a new system or application, for example, design of a new model of passenger car.

A.3: Machine design requires a thorough knowledge of engineering science in its totality along with a clear decision-making capability. Every designer follows his own methodology based on experience and analysis. However, the main steps to be followed in general are:

  • Define the problem.
  • Make preliminary design decisions.
  • Make design sketches.
  • Carry out design analysis and optimization.
  • Design the elements for strength and durability.
  • Prepare documentations to be followed for manufacture.

A.4: Reliability of a designed machine is concerned with the proper functioning of the elements and the machine as a whole so that the machine does not fail in use within its designed life. There is no single answer to this and an overall safe design approach at every stage of the design is needed. Use of factor of safety in designing the elements is to optimize the design to avoid over-design for reliability.

References or Source of Content

ABDULLA SHARIF, Design of Machine Elements, Dhanpat Rai Publications (P) Ltd, New Delhi, 1995.

V. B. Bhandari, Design of Machine Elements, Third Ed., The McGraw-Hills Companies, New Delhi

S. KHURMI and J.K.GUPTA, A Text Book of Machine Design, S. Chand and company ltd., New Delhi, 2000.

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