The design process

Introduction and synopsis

It is mechanical design with which we are primarily concerned here; it deals with the physical principles, the proper functioning and the production of mechanical systems. This does not mean that we ignore industrial design, which speaks of pattern, colour, texture, and (above all) consumer appeal - but that comes later. The starting point is good mechanical design, and the role of materials in it.

Our aim is to develop a methodology for selecting materials and processes which is design-led; that is, the selection uses, as inputs, the functional requirements of the design. To do so we must first look briefly at design itself. Like most technical fields it is encrusted with its own special jargon; it cannot all be avoided. This chapter introduces some of the words and phrases - the vocabulary - of design, the stages in its implementation, and the ways in which materials selection  links with these. 

The design process

Design is an iterative process. The starting point is a market need or a new idea; the end point is the full specifications of a product that fills the need or embodies the idea. It is essential to define the need precisely, that is, to formulate a need statement, often in the form: ‘a device is required to perform task X’. Writers on design emphasize that the statement should be solution-neutral (that is, it should not imply how the task will be done), to avoid narrow thinking limited by pre-conceptions. Between the need statement and the product specification lie the set of stages shown in Figure 2.1: the stages of conceptual design, embodiment design and detailed design.

The product itself is called a technical system. A technical system consists of assemblies, subassemblies and components, put together in a way that performs the required task, as in the breakdown of Figure 2.2. It is like describing a cat (the system) as made up of one head, one body, one tail, four legs, etc. (the assemblies), each composed of components - femurs, quadriceps, claws, fur. This decomposition is a useful way to nalyse an existing design, but it is not of much help in the design process itself, that is, in the synthesis of new designs. Better, for this purpose, is one based on the ideas of systems analysis; it thinks of the inputs, flows and outputs of information, energy and materials, as in Figure 2.3. The design converts the inputs into the outputs.

An electric motor converts electrical into mechanical energy; a forging press takes and reshapes material; a burglar alarm collects information and converts it to noise. In this approach, the system is broken down into connected subsystems which perform specific sub-functions, as in Figure 2.3; the resulting arrangement is called the function structure or function decomposition of the system. It is like describing a cat as an  appropriate linkage of a respiratory system, a cardio-vascular system,

a nervous system, a digestive system and so on. Alternative designs link the unit functions in alternative ways, combine functions, or split them. The function-structure gives a systematic way of assessing design options.

The design proceeds by developing concepts to fill each of the sub-functions in the function structure, each based on a working principle. At this, the conceptual design stage (Figure 2.1 again), all options are open: the designer considers alternative concepts for the sub-functions and the ways in which these might be separated or combined. The next stage, embodiment, takes each promising concept and seeks to analyse its operation at an approximate level, sizing the components, and selecting materials which will perform properly in the ranges of stress, temperature and environment suggested by the analysis or required by the specification, examining the implications for performance and cost. The embodiment stage ends with a feasible layout which is passed to the detailed design stage. Here specifications for each component are drawn up; critical  components may be subjected to precise mechanical or thermal analysis; optimization methods are applied to

components and groups of components to maximize performance; a final choice of geometry and material is made, the production is analysed and the design is costed. The stage ends with detailed production  specifications.Described in the abstract, these ideas are not easy to grasp. An example will help -it comes in

Section 2.6. First, a look at types of design.

MATERIALS SELECTION IN MECHANICAL DESIGN
SECOND EDITION
MICHAEL F. ASHBY
Department of Engineering, Cambridge University, England
OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI

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