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 is an iterative process - at various stages it will be necessary to go back and re - consider some decisions. However at some point an agreed 'freeze' date must be specified, after which no changes will be permitted. Otherwise project deadlines will not be met and costs will rise - often catastrophically.

The following are major steps in the design process:

  • recognition of a need, or idea for a new product
  • brainstorm possible solutions
  • select a solution principle
  • outline the solution
  • market survey
  • detailed design, selection of materials and manufacturing processes
  • analysis of performance
  • manufacturing plans
  • sales promotion
  • launch
  • monitor progress
  • develop mark 2 version.

Design problems are normally complex, even if they can be very simply stated, hence appropriate methodology needs to be understood and followed.

Almost invariably an important objective is to minimise time to market.

A serial approach has been traditionally used. Now simultaneous, or concurrent engineering is the standard, much more efficient approach.

Communicating any design accurately and economically is vital. This can be done by drawing, CAD, 2D, or Solid model data base - that can be shared (with appropriate access security) throughout the company and possibly with customers and suppliers.

The choice of manufacturing processes and materials must be considered to be an integral part of the design process.

Design problems are frequently multidisciplinary.

Some related important issues are:

Safety needs to be designed in at the outset.

Building in quality at the design stage is essential.

Quality must be owned by every level of the organisation.

A good understanding of the loads and operating conditions are essential.

Design review is an important part of the design process to monitor progress.

Analysis and simulation using sophisticated computer software are increasingly used to speed design while making the process more accurate.
Using modern software it is possible to carry out a range of 'studies' to assist in achieving an efficient, or optimum, design. For example:

Design Optimisation: Typically a component will be required to have a minimum mass while at the same time ensuring the maximum stress is below a specified value. The software can vary the sizes of features on a component while ensuring the specified conditions are met.
Sensitivity Study: The effect on varying a particular dimension on the maximum stress occurring in a component may be of interest. Software can investigate this helping to determine how important particular dimensions are.

Design for manufacture / assembly / maintainability / recycling, is becoming increasingly important as consumers become more sophisticated and environmentally aware. Modular design is an important way of meeting these requirements, however this usually results in increased weight compared to an integrated design.

David J Grieve, Revised: 25th January 2014, original: 9th March 2005.

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