DESIGN AND ANALYSIS
OF MACHINE ELEMENTS
INTRODUCTION AND EXPLANATION FOR STUDENTS
These Notes have been prepared specifically for the current course in mechanical engineering design at UWA, which
- introduces the concept of design in the engineering environment and provides hands -on experience of the design process
- reviews failure mechanisms under steady loading and examines failure under fluctuating loads (fatigue) and in unstable situations (buckling, fracture mechanics)
- considers the analysis and safe design of various common elements of engineering systems such as pressure vessels, shafts, gears and the like
- provides experience in the design of systems under the guidance of practising professionals.
It is presumed that students commencing the course are familiar with equilibrium, stresses etc. but have
- very little or no practical background involving engineering hardware and construction methods,
- very little or no experience in solving open-ended design problems in which an expressed need must be transformed into a physical artefact or an action.
Design is exemplified in particular through students taking part in a Design & Build competition where kitchen table- top materials and methods of construction are used in preference to sophisticated metal working. This experience should ensure students realising that solving real problems demands both creative and critical thinking.
In view of students' lack of exposure to machine elements (belt drives, springs etc.) the Notes adopt a simple mathematical approach to explain elements' behaviour and safety - however it should be realised that although computers and mathematical models may help in this regard, their ability to reflect all nuances of real behaviour cannot be guaranteed. Engineers cannot do without sound engineering judgement based upon practical knowledge acquired through experience.
Students may wish to supplement the often necessarily brief descriptions of the Notes by consulting the many library texts and references. The web is an increasingly useful descriptive resource.
In practice, some components are designed to guidelines laid down in standard Codes of Practice whose implementation could be disastrous if they are treated like recipe books. While there is nothing intrinsically wrong with recipes - provided that they do not replace or inhibit creativity and provided that their limitations are clearly understood - it is a fact that Codes are often applied indiscriminately by students. To help avoid this, the Notes provide background to assist intelligent application of some important Codes, as undergraduate texts usually offer little help in this regard.
An extremely important objective of the design course is to prepare students for their subsequent careers, not necessarily as 'designers' but as 'ingenious solvers of real problems' ( pronounced 'engineers' ! ) - so this course differs somewhat from other University subjects in that it does not serve up a host of facts and figures for memorising, with subsequent regurgitation in examinations. Rather students are expected to demonstrate
- an understanding of why a certain approach is used to throw light on a particular component's behaviour,
- an appreciation of the general trends of that behaviour, and
- an ability to modify the component economically to suit the design problem in hand.
For these reasons
- Lectures will trace out only the broad arguments.
- Students are expected to read the Notes in detail, to follow through the development of the theory whilst appreciating its assumptions - that is, generally, to flesh out the lecture material.
- Examinations are open-book and test ability to adapt course material to new situations. Students should therefore attempt many tutorial examples, to become adept at adaptation. Having answers for some problems to hand in an examination (and in real life) is useless unless it is known how to modify the solution processes intelligently.
- Detailed answers to all tutorial problems are provided, but they should not be consulted until the problems have been tried and the solution steps appreciated.
Students should realise that when they graduate they must be prepared to tackle the difficult, ie. previously unencountered, problems - the easy, mundane ones can be solved by someone less qualified and less expensive to hire than them.
NomenclatureSymbols used to characterise a typical variable 'x' are shown below. The text equivalents also are employed since the symbols currently cannot be rendered conveniently in HTML.
|x-bar||steady or mean component||xm|
|x-tilde||varying component; amplitude||xa|
|x-hat||maximum value||xmax, xhi|
|x-vee||minimum value||xmin, xlo|
- What is design ?
- Why do we design ?
- How do we design ?
- The Rudimentary Design Process:
- Where do we go from here ?
- More advanced topics
STRESS, STRENGTH AND SAFETY
- The concept of safety factor
- Stress concentration
- Resolving static indeterminacy
- Elementary load building blocks - tension, shear, bending and torsion
- Stress resolution and principal stresses; strain resolution
- Failure theories - distortion energy, maximum shear stress and modified Mohr theories
- Putting it all together - the design equation for static shafts of circular cross-section
- Power transmission shafting - an introduction
- Analysis of statically indeterminate assemblies with multiple components