Mechanical Engineering Design Notes

Design Contents

Preliminary Matters
Design Methodology
..brain storming
..evaluation matrix
Statistical Considerations
..variability in materials
..variability in dimensions
..variability in loading
..preferred sizes
Design Factor
Introduction to Failure
Failure Theories
Application of von Mises
..criterion in 2 D

Stress Concentration
..and notch sensitivity
Failure Under Combined Loading
..combined bending and torsion
Failure Under Cyclic Loading
..fracture mechanics
Instability - Buckling
Concentrically Loaded Strut
..slender columns
..Euler formula
..effective length
..short and intermediate columns
Eccentrically Loaded Strut
.. theory
Shock Loading

Design Methodology

1. Brainstorming

This is the first stage in the design process once a 'need' has been established. A variety of people meet to generate ideas. All ideas, however unlikely should be written down. At this stage, which is idea generation not appraisal, care must be taken not to cast doubts on any idea. Such a meeting should be brief, half an hour may be sufficient. Once the meeting extends beyond about an hour, there is a danger that people present may switch off.

The brainstorming session is followed up - a couple of days later - to allow some time for reflection on the generated ideas, by a meeting which assesses the ideas that were listed. At this stage the obviously non - feasible ideas are rejected. This then leaves a number of ideas or concepts that are potentially viable.

2. Evaluation Matrix

An evaluation matrix is used to select the best of the potentially viable ideas. At this stage an element of quantification is brought in. The concepts are listed accross the top of the matrix or table and down the left hand side are listed criteria by which the concepts are to be assessed. In a simple application of the evaluation matrix, each concept is awarded marks agaist the criteria and these are summed at the bottom of the matrix, the best concept having the highest total. There are several variations on this theme, weightings may be applied to criteria that are believed to be more important.

It should be noted that not only is an evaluation matrix used in assessing the concepts, such matrices may be used many times in the design of a system, for example when considering what transmission system or type of motor is to be used, or essentially anywhere where a choice has to be made between possible alternatives.

3. Quality Function Deployment (QFD).

In recent years manufacturers have become increasingly aware that if their designs are to be successful, their customers need to be carefully considered during the design process. Quality Function Deployment (QFD) is a way of achieving this.

There are four stages in QFD:

i) Customer requirements, gleaned from market research and technological forecasts, are translated into product characteristics.

ii) The product specification needed to meet the customer requirements for the product characteristics is derived. As well as assessing how the 'in house' design meets the customer requirements, it is common to assess rivals competitive products to gauge how the proposed product will fare in the market place.

iii) Manufacturing (including quality) plans identify the design and process parameters that are essential if the requirements are to achieved.

iv) Process sheets, derived from the manufacturing plans, provide instructions for the operators.

This QFD procedure is increasingly being used in automobile design and manufacture and is doubtlessly one of the reasons why cars are widely perceived as becoming 'better'.

For further information about QFD see: Quality Function Deployment Institute

Further Reading - 'Total Design', by S Pugh

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