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Machine Design   Mechanical

Introduction:
Design is essentially a decision-making process. If we have a problem, we need 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 an example, design of a chair. A number of factors need 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) Material for the chair, its strength and cost need to be determined.
 (d) Finally, the aesthetics of the designed chair. Almost everyone is involved in design, in one way or the other, in our daily lives because problems are posed and they need to be solved.

Basic concept of machine design:
Decision making comes in every stage of design. Consider two cars of different makes. They may both be reasonable cars and serve the same purpose but the designs are different. The designers consider different factors and come to certain conclusions leading to an optimum design. Market survey gives an indication of what people want. Existing norms play an important role. Once a critical decision is made, the rest of the design features follow.

Types of design:
Adaptive design: This is based on existing design, for example, standard products or systems adopted for a new application. Conveyor belts, control system 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 an entirely new one but based on existing scientific principles. No scientific invention is involved but requires creative thinking to solve a problem. Examples of this type of design may include designing a small vehicle for transportation of men and material on board a ship or in a desert. Some research activity may be necessary.

Types of design based on methods:
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.

References:
1.     Wikipedia
2.     Elements of Machine Design, William Cawthrone Unwin
3.     Design of Machine elements, NPTEL
A M
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IIT Kharagpur

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