Introduction and synopsis
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
which speaks of pattern, colour, texture, and (above all) consumer
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
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
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
often in the form: ‘a device is required to
perform task X’. Writers on design emphasize that the statement should be
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
The product itself is called a
technical system consists of
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
ceps, claws, fur. This decomposition is a useful way to analyse 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.
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
of the system. It
is like describing a cat as an appropriate linkage of a respiratory system, a cardio-vascular system,