Mechanical Design and Analysis class room notes

1. Dept. of Mechanical & Materials Engineering
The University of Western Australia
NOTES ON
DESIGN AND ANALYSIS
OF
MACHINE ELEMENTS
Douglas Wright
February 2001

2. INTRODUCTION AND EXPLANATION FOR STUDENTS
The current course in mechanical engineering design at UWA spans three years ( II, III, IV) in which it
- introduces the concept of design in the engineering environment (II) and provides hands-on experience of the design
process
- reviews failure mechanisms under steady loading (II) and examines failure under fluctuating loads (fatigue) and in
unstable situations (buckling, fracture mechanics) (III)
- considers (III) the analysis and safe design of various common elements of engineering systems such as pressure ves-
sels, shafts, gears and the like
- provides experience (IV) in the design of systems under the guidance of practising professionals.
These Notes form a resource for all years of the course, though they do not cover every topic (eg. fatigue is not yet
included). Most chapters appear also online at www.mech.uwa.edu.au/DANotes/ The coloured diagrams (and a few
animations) of the website are generally much easier ro understand than the black and white copies in the printed Notes.
It is presumed that students commencing the course are familiar with the concepts of equilbrium, 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.
For these reasons the course first explains what is meant by design and why we go to the trouble of designing. A proce-
dure for creative problem-solving, referred to as the rudimentary design process, is described in depth. This procedure is
simple enough to be understood and to be applied successfully by newcomers to design, yet forms a more-than-
adequate basis for solving any open-ended problem likely to be encountered by students. The process should be used
for design projects in the course.
Design is exemplified in particular through students taking part in a Design & Build competition where the task
demands a creative solution realisable by kitchen table-top materials and methods of construction rather than by sophis-
ticated metal working. This experience should convince students that solving real problems demands both creativity
and criticism.
In view of students' lack of exposure to machine elements (belt drives, springs etc.) the Notes adopt a simple mathemati-
cal 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 on 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 implementa-
tion 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 career - not necessarily
as 'designers' but as ingenious solvers of real-life 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 appreciat-
ing 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 online, 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 unencoun-
tered, problems - the easy, mundane ones can be solved by someone less qualified and less expensive to hire than them.
Douglas Wright

3. NOMENCLATURE
Symbols are defined when they first appear in
each chapter; however the forms shown here are
in general use.
PROGRAM DIRECTORY
The following Mac programs, referred to in the Notes, have been prepared to assist in the design task and may be down-
loaded from the website : www.mech.uwa.edu.au/DANotes/ Copies may be available also on other departmental serv-
ers from time to time.
Pascal source code is freely available on request, provided that authorship credit is retained always with the code.
COMPILED APPLICATIONS FOR COMPONENT DESIGN & ANALYSIS
brakes analyses twin shoe brakes for sensitivity, torque, bearing loads &c
FEM1 analyses two-dimensional linear elastic systems (with sample data file)
fillet welds analyses fillet welded planar joints consisting of a number of straight runs
motors assists selection of a squirrel cage motor for a given duty & acceleration time
springs facilitates the fatigue design of round wire steel compression springs
steel spur gears analyses steel spur gears for safety against strength and wear failure
tooth generator simulates manufacture of an involute gear tooth by rack generation
V-belts selects V-belt drives suitable for a defined duty
TEXT FILES
FEX00 specimen datafile for FEM1
heads.txt data base for pressure vessel design
FRACTURE MECHANICS
Fatigue of ductiles; stress concentration; linear elastic fracture mechanics; plasticity; yielding fracture mechanics -
the R6 technique; fatigue crack growth. Crack growth kinetics.
FINITE ELEMENTS
Linear 1-networks; extension to 2- and 3-networks. The Rayleigh-Ritz method. Finite element theory applied to
elasticity - equilibrium of the discretised body; element stiffness. Implementation; condensation and bandwidth;
discretisation. Appendices - the refining process; 'FEM1' User's Guide.
UNITS, DIMENSIONS AND CONVERSION FACTORS
x ≡ x ≡
x ≡ x ≡ x
xmin lo
x ≡ x ≡ xtilde a
~
x ≡ x ≡ xbar m
-
max hi
x
~
x-
x
x
CONTENTS
Most chapters include a bibliography and problems whose solutions are online at www.mech.uwa.edu.au/DANotes/
DESIGN
What is Design ?; why do we design ?; how do we design ?; problem statement; generation of ideas; criteria & con-
straints; practicalising the candidates; evaluating the candidates; the feasibility study; where do we go from here ?;
more advanced considerations.
Appendices A - improvement problem; B - tube end problem; C - lessons in frustration; D - analysis of a spring
driven vehicle; E - springs as energy stores; F - JCH Roberts on Creativity.
STRESS, STRENGTH AND SAFETY
Safety factor; stress concentration; static indeterminacy; elementary load building blocks; stress resolution; strain
resolution; failure theories; putting it all together; design equations for static shafts; power transmission shafts.
Appendix - indeterminate assemblies of multiple components.
MISCELLANEOUS STRENGTH TOPICS
Castigliano's theorem; thin curved beams; thick curved beams; asymmetric bending; contact stresses.
SPRINGS
Close coiled round wire helical compression springs; the spring characteristic; stresses & stiffness; buckling; wire
materials; presetting; fatigue loading; spring design.
Appendix - presetting a torsion bar.
THREADED FASTENERS
Thread geometry; screw thread mechanics; static failure; loads in an elastic bolted assembly; preload and its con-
trol; fluid pressurised joints; bolt fatigue; non-uniformly loaded bolt groups.
WELDED JOINTS
Fillet welded joints; geometric properties of lines; traditional analysis; throat stresses and joint safety; unified anal-
ysis; resolution.
Appendices - the compliant lap joint; extract from AWRA, Technical Note No. 8.
CYLINDERS
Axial stress; thin cylinders; thick cylinders; design equations; thin cylinder errors; strains; autofrettage; compound
cylinders; torsional loading.
PRESSURE VESSELS
Corrosion; welded joint efficiency; thin shells of revolution - heads; compensation; pipes and flanges; inspection
openings; supports; design.
SQUIRREL CAGE MOTORS
Characteristics of a steady load and of a motor; matching a motor to a given steady load; periodic loading; acceler-
ation; hydraulic couplings.
Appendix - integration in practice.
V-BELT DRIVES
Overall geometry; kinetics; fatigue; effectiveness; drive selection; approximate solutions; V-flat and pivoted motor
drives. Traction mechanics.
Appendix - commercial selection tables.
BRAKES
System dynamics; linings; brake shoe analysis - short translational shoe; long translational shoe; short rotational
shoe; shoe figures of merit; long rigid shoe; long hinged shoes; twin shoe brakes. The braked wheel; braking of
vehicles; wheel lock - vehicle characteristic; brake control characteristic.
SPUR GEARS
Overall kinetics of a gear pair; epicyclic trains; conjugate tooth action; the involute tooth; the generation process -
tooth systems and profile shift; gear meshing. Gear failure - reliability; tooth forces; bending strength; pitting resis-
tance; periodic duty.
Appendices - continued fractions; geometry of the involute gear tooth.
BUCKLING
Buckling of thin walled structures; stability of equilibrium; effects of imperfections; submerged pipelines; practical
columns - design equations.

4. Design 1
much thought. This is true enough - if the solution can be based on direct experience. However we
shall soon come to realise that without experience such a thoughtless solution usually comes to grief
sooner or later - the more involved the problem and the more folk affected by the solution, the more
likely is the solution going to fall in a heap.
Any old solution will not do - we must strive for the optimum solution.
We expect that the design process, if properly carried out, will show a high probability of disclosing
a solution which is optimum or close-to-optimum, if indeed a unique optimum exists.
The prime aim of this chapter is to develop a structured approach to design - an approach which
will promote confidence in effectively solving real life problems. We shall focus on problems involv-
ing engineering hardware - particularly for Design and Build (D&B) Competitions - however the
approach is perfectly general and applicable to problems arising from a marketing sortie or a labour
wrangle for example. The approach is thus very relevant to managers for example - not just to 'hard-
ware designers'.
Before presenting the method however, let us look briefly at why we go to the trouble of designing.
Why do we design ?
In a nutshell . . . . TO SURVIVE.
Most people these days exist by providing 'things' to others; in the case of engineers these 'things'
are technical muscle-power or know-how, or physical artefacts - that is solutions to buyers' or hirers'
particular problems. If these clients are not completely satisfied with the 'thing' provided then they
will dismiss the provider, go somewhere else for their next 'thing', and tell everyone about the pro-
vider's unsatisfactory 'things'. If this happens often enough to a particular provider then he will
cease to exist as a market force - nobody will want to know.
So clearly, if 'things' are not designed with care and attention to clients' needs then the provider will
have problems - just like Jane and John . . . .
• Jane worked as an engineer for a firm of consulting engineers, one of a number of such firms
specialising in minerals processing.
A certain mining company intended to develop a new deposit and therefore required plant to
process the mineral. It called for 'tenders' that is for plans and cost estimates for construction
and operation of the plant. The various consulting firms simultaneously each set about design-
ing the plant - ie. solving the client mining company's particular problem - and then reported
to the client outlining its proposed optimum solution. Consultants receive no remuneration for
this service.
The client reviewed the solutions submitted by the various consulting firms, and awarded the
contract for ongoing project management to the firm which had best satisfied its perceived
needs. The successful consulting firm therefore had ongoing work for a year or two.
Jane's firm was not successful in this instance. But this was unexceptional - consultants do not
normally expect to win every contract which is put out to tender.
But Jane's firm did not win the next job to come up either.
Or the next . . . . . . Or the next !
So what happened eventually ? Predictably, with no successful designs and with no money
coming in, Jane's firm folded and Jane is now out looking for work. There could of course be
reasons aplenty for Jane's firm sinking - but it could not hope to exist with designs which were
DESIGN
The word 'design' means different things to different people - a wallpaper pattern, a fashionable
dress, the appearance of a racing car and so on. We therefore start by defining what we mean by
'design' in the present context - ie. What design is all about.
This understanding will lead to an examination of
• Why we need to 'design', particularly in an engineering environment, and
• How we might best go about 'designing'.
What is design ?
The Concise Oxford Dictionary explains design as 'a mental plan, a scheme of attack, end in view,
adaptation of means to ends, . . . preliminary sketch for picture, . . . invention.' Evidently there is a
lot more to design than mere visual aspects, and design is not restricted to engineering. Key compo-
nents of this explanation are as follows :-
• Means to ends implies that we design not for the abstract mental exercise, but with a definite
goal in view - some action or some physical object (artefact) will result from the design.
• Mental suggests that design is a thinking process. When we design we deal primarily with
ideas, with abstractions rather than with numbers - and computers for example cannot do the
job for us, though they can help in certain tasks.
No matter what we design, it is vital that we develop and apply our imagination to visualise
realistically the future form of the artefact or action, how it will eventually come into being
and most importantly how it will thereafter interact with people and other artefacts or actions.
• Plan, scheme infers that design is distinct from implementation. Designers especially in engi-
neering seldom execute their plans, but rather communicate them to others - either by word of
mouth, or visually (sketches, engineering drawings, computer simulations &c), or through the
written word. Again, note the lack of emphasis on numbers.
• Invention means just that, we are coming up with something NEW - at least partly. Creativity
is crucial as we shall see later.
So, can we now define design? No ! and neither do we need to. A rigid definition implies a rigid pro-
cess, and design is anything but that. We shall adopt the following interpretation as it incorporates
the above concepts and conveys a reasonably clear idea of what design is all about -
Design is the application of creativity to planning the optimum solution
of a given problem and the communication of that plan to others.
Apart from the communication aspect therefore, we understand the essence of design to be prob-
lem-solving, though the type of problem encountered in design is not like a typical textbook mathe-
matics problem for example in which the unique 'correct' solution is guaranteed by following,
automaton-like, a series of learned solution steps. A design problem on the other hand is a real-life
problem with many solutions, some of which meet the problem requirements better, some worse,
and where the process of discovering the solutions is not mechanistic.
Some problems might appear not to need 'design' as a solution can be cobbled together without

5. Design 2
A problem is not a problem if it has been solved successfully in the past - it is trivial. Conversely if
the solution to a problem is not known prior to design, then the problem is new and the solution
also must be new.
The necessity for novelty in design is obvious where a number of competing providers of the same
'thing' coexist by continually providing new 'things'. Computer-'things' are a case in point - provider
A first launches a completely new type of memory, provider B counters by making it half the size,
provider C attacks via a drastic price cut enabled by a novel manufacturing technique, provider D
edges ahead with a much faster operating system, and so on. Nobody can afford to stand still;
nobody can exist by slavish copying; novelty is a necessity for good design, for survival.
Survival = Good design = Creativity
This does not imply that all aspects of a successful design have to be novel; you need not re-invent
the wheel.
It is useful to view design in the context of a typical artefact which evolves from initial conception,
through the distinct stages shown below, to eventual obsolescence. A planned action undergoes an
analogous sequence, however we shall concentrate on hardware.
• A need is recognised, ie. a problem is posed, so
• a certain artefact is designed to meet the problem - thereafter
• the artefact is manufactured, and
• sold/delivered to the user . . . .
• . . . . who operates it, causing wear and
• requiring maintenance to restore its effectiveness, until
• eventually it reaches the end of its economic life and is retired.
Various people are involved in the various stages - the designers, the manufacturers, the salespeo-
ple, the operators, the maintainers and the eventual dismantlers of an artefact are all completely dif-
ferent folk carrying out completely different tasks.
Feasibility Study
Operations Research
Detail Design
Research & Development
Industrial Design, Ergonomics
Industrial Relations
&c &c
Murphy's Law
Tech. Specs.
Safety
STAGES IN THE LIFE OF
A TYPICAL ARTEFACT
Creativity
Economics
Conception ( need )
DESIGN
Manufacture
Distribution, Sale
Operation
Maintenance
Retiral
feedbackofanticipatoryideas


demonstrably not competitive. Jane now realises that, while a score of 80% in a University
examination might be regarded as excellent, in real life there are no marks whatsoever for
coming second.
• John's firm, which makes and installs large industrial ovens, was approached by a client who
wanted to install such an oven in its existing factory. John was delegated to look after the con-
tract, so he examined thoroughly the myriad technical issues, including the most suitable
choice of . . . .
- energy source necessary to raise the oven's temperature,
- location for the oven in the factory with regard to minimising transport of products from/to
other manufacturing operations in the factory,
- control mechanism needed to ensure that the oven's temperature stays within bounds,
- fail-safe safety procedures which prevent any employee from being inadvertently locked in
the oven,
- insulation thickness to optimally balance the first and ongoing costs of insulation,
- chimney dimensions for projecting the exhaust gases high enough to ensure clean air for the
surrounding environment
- . . . . and so on, there were lots of other aspects to consider.
John was technically competent. He carried out all his sums correctly - though we need not
worry about the details at this stage. He was very satisfied when his recommended optimum
solution (a gas-fired oven) was accepted and the oven was designed in detail, built and put
into service.
All went well until reports fil-
tered in that the client's office
staff were reporting headaches
due to vibration of the office
structure. Expensive investiga-
tion proved that the culprit was
the oven's fan which drew air
for gas combustion through an
intake duct crossing the office
ceiling. After further work John
eventually had the duct re-
routed from the adjacent wall
as shown in the factory plan.
John's firm had to pay for the investigation, for the modifications, for a number of medical
bills, and for losses in production while the oven was out of service being modified.
Again all went well until the gatekeepers started to complain bitterly about noise from the re-
positioned air inlet next their hut. So it was back to the drawing board once again for John . . . .
he was not popular !
Lessons that John learned from this experience included :
- There is a lot more to design than mere technical calculations.
- An incomplete design which does not take everyone's viewpoint into consideration is a
recipe for trouble.
- It is the designer's reponsibility to seek out these viewpoints.
- A solution must be close-to-optimum to start with, as retrospective fixes are never wholly
satisfactory.
officefactory
floor
ORIGINAL PLAN
WITH AIR INTAKE DUCT
THROUGH OFFICE CEILING
factory
floor
oven
office
MODIFIED
ARRANGEMENT OF
AIR INTAKE DUCT
hut
oven intake
intake

6. Design 3
If something can go wrong then it will go wrong - and at the worst possible time
Murphy cannot be ignored - there is no excuse for designers throwing up their hands and
exclaiming 'How were we to foresee that happening ?' . . . . but they must foresee it (whatever
it might be) and make allowance at the design stage to minimise its deleterious effects.
Murphy is especially hard on beginning designers who have yet to learn that Nature does not
always follow simple theoretical predictions. But Murphy is no respecter of persons, and many
an experienced designer has suffered at his hands !
We wrap this section up by drawing attention to two articles reproduced below from the technical
press which throw further light on why we design :
• Seymour in 'Competition Analysis' lists many criteria which are commonly used by clients to
compare the products of competing providers. Designers must be aware of all these criteria
and design accordingly, and not focus solely on the 'technical specifications'.
• Somerville reports that 'Woodside Critical of Aust Suppliers' following construction of the NW
Shelf LNG facilities. Local industry is lambasted for poor performance : 'Some 70% of the tend-
ers (ie. feasibility studies) received from Australian companies were technically inadequate'
and '. . related to lack of effort in preparation . .' This is certainly an indictment that designers
ignore at their peril. However it’s the very last sentence in this article which is particularly
damning - Why ?
Having emphasized the importance of design, it is now time to look at how we go about it . . . .
How do we design ?
Newcomers to design often feel unsure of themselves
because the problems encountered are unlike those pre-
viously solved successfully in other units such as
Dynamics, Strength of Materials &c. Although there is
no mechanistic series of steps leading to the 'correct'
solution of a design problem, there are techniques which
may be learned for tackling design with confidence and
with reasonable expectation of achieving a 'close-to-
optimum' solution.
The illustrated Rudimentary Design Process is one such
technique, and will form the model for design through-
out this course - however the success of the process, like
that of any human endeavour, depends largely on the
attitude, skill and effort of the practitioner(s).
The rudimentary model is the engine lying at the very
heart of all professional engineering design processes
such as the Pahl & Beitz model (illustrated later) typify-
ing Continental practice, or the SEED model common in
the UK. Despite the complexity of these, the rudimen-
tary model is itself sufficiently simple to be used effec-
tively for problem solving by the rawest tyro.
It requires the designer(s) to carry out sequentially five
BANK OF SOLUTION CANDIDATES
uncritical - quantity not quality
OPTIMUM
SOLUTION
communicate
maybe
RUDIMENTARY
DESIGN
PROCESS
AVAILABLE
KNOWLEDGE
GENERATE
IDEAS
methods
STATE THE
PROBLEM
broad, complete
SPECIFY
CRITERIA &
CONSTRAINTS
RENDER
PRACTICABLE
EVALUATE
CANDIDATES
Design is the springboard for all subsequent stages, and so it is at the design stage that the later sat-
isfaction of each and every one of these folk is, or is not, effectively set in stone. That is why the
'feedback of anticipatory ideas' is highlighted in the sketch, as it is vital that designers foresee - in
every last detail - the interaction of the planned artefact with all these people, and endeavour to ful-
fill their wishlists. A designer must put herself in other folks' shoes, close her eyes and realistically
imagine their interactions with the artefact.
Do not get carried away by technicalities. Remember always that it is people who make decisions to
purchase; it is people who have to live with your design. A designer's primary goal is the satisfac-
tion of people, not of elegant mathematical expressions.
Design is keeping everybody happy.
. . . or at least as happy as possible. Sometimes it may be nigh on impossible to please everyone, but
you'll never-never know if you never-never have a go at trying to please them. That is why we
design ! We'll see later how to factor in conflicting criteria and different agenda. If you are not sure
what these are likely to be in a particular case, then don't be like John - find out.
The importance of good design is underlined by the fact that in Australian manufacturing industry
around 70% of product costs are defined at the design stage. As the average profit is only some 7% it
will be appreciated that indifferent design is commercially intolerable, as Jane's employers discov-
ered to their cost. The life stages sketch emphasises the importance of creativity and economics in
design, and of the technical specifications and safety in operation. We shall return to this critical
safety issue later.
Also shown in the sketch are some facets of the design process which it is useful to introduce here :
• A feasibility study is a report describing in broad but realisable terms the optimum solution.
An important component of a real life feasibility study is the solution's cost, but detailed cost-
ing is generally not expected in this course.
• Operations research is the name given to the branch of mathematics which models industrial
and commercial processes such as queuing, distribution, scheduling &c.
• Detail design completes all details necessary for the next stage, manufacture, details which are
omitted in the deliberately broad-brush treatment of the feasibility study. In practice a solution
must first be confirmed as feasible and the decision made to proceed with it, before detailing
commences.
• If a design lies at the cutting edge of known practice or science then it may not be possible to
accurately model certain aspects of its behaviour. Further research and development (R&D)
involving experimentation must then be conducted before these aspects of the design can be
finalised with confidence.
• Industrial design deals with artefacts' aesthetics, safety and ergonomics among other things.
The principles of ergonomics are used to optimise human-machine interaction when designing
eg. the controls of a bobcat (a mini bulldozer) so that the operator and the bobcat are essentially
seamless with the operator's eyes, two feet and hands integral non-fatigued components of the
control loops for turning, accelerating, reversing, braking, blade lifting, blade orienting and so
on.
• Industrial relations together with occupational health and safety are obvious and important
considerations in design - they are just facets of 'keeping everyone happy'.
• Murphy's Law states that :

7. range of piping materials required
and in many instances could not meet
the required schedule," he said.
Overall Mittertreiner estimates $226
miliion worth of sales were "lost" by
Australian industry through lack of
capability and experience. Essential
items purchased overseas include
cryogenic heat exchangers, gas tur-
bine generators, gas turbine and elec-
tric drive compressors and cryogenic
pumps.
Other factors pushing up the cost,
he claims, are decisions to reject the
lowest overseas price offered and the
acceptance of Australian tenders.
Rankling Mittertreiner is the choice
of Australian suppliers for transform-
ers, air fin coolers for the LNG trains,
and power cables.
These purchases came after discus-
sion with the State and Federal Gov-
ernments through the National Liai-
son Group (NLG), a body set up to
monitor and boost Australian partici-
pation in the project.
In the case of the power transform-
ers it appeared to the NLG, after rep-
resentations from the WA Govern-
ment, manufacturing groups and the
trade unions - who threatened indus-
trial action if nothing was done to
stop the tender going offshore - that
an anti-dumping inquiry would be
undertaken into the price submitted
by the originally successful foreign
tenderer once the transformers
landed in Australia.
A spokesman for Woodside said the
overseas tenderer withdrew from the
contract rather than face costly litiga-
tion and Westralian Transformers, a
WA-based Westinghouse subsidiary,
picked it up.
The general manager for Westralian
Transformers, Beavan Oakes, said the
original winner was substantially
below the 4 other tenderers.
"On the information we had it was
quite clearly a dumping case. After
duty the overseas price was 20%
below the next tendered price," he
said. Oakes is not convinced that Mit-
tertreiner is correct in claiming Aus-
tralian participation pushed up the
cost of the project.
"Sure Australian costs are high but
Woodside chose the site so far from
civilisation and imposed the stringent
requirements," he said.
- Paul Somerville
Many Australian suppliers to the
Northwest Shelf liquid natural gas
project have difficulty meeting the
high quality standards set down for
the LNG plant, are often behind
schedule and are sometimes not price
competitive with overseas suppliers,
said Woodside's LNG project man-
ager, Frans Mittertreiner.
He made the remarks when review-
ing the performance of Australian
companies on the $3 billion project at
a briefing in Perth, staged by Wood-
side Offshore Petroleum Pty Ltd at
the end of last year.
Mittertreiner said the review of
industry's performance had been at
the request of the WA State Govern-
ment, the Federal Government and
Australian industry.
By October 1987 $2240 million, or
75% of the total estimated cost of the
LNG plant, had been committed.
Of the $638 million worth of equip-
ment and materials ordered for the
LNG plant, 54% was acquired locally.
Overall, he said, the performance of
Australian industry had been good
but required improvement in specific
areas.
Despite some problems the plant is
expected to be completed on time
with the first shipment of LNG head-
ing for Japan in October 1989.
According to Mittertreiner Austra-
lian companies had problems main-
taining consistently high quality.
"In particular difficulties in produc-
ing acceptable castings for both valves
and pumps caused numerous repairs
and recastings, which resulted in seri-
ous delivery delays," he said.
He went on to say that the most
common problem was the relatively
high incidence of dimensional errors
and fabrication misalignments in Aus-
tralian shops as evidenced by the rela-
tively large number of concession
requests.
He pointed to a lack of understand-
ing by vendors of the specifications
and quality assurance and quality con-
trol requirements within orders. This,
he said, was related to a lack of effort
in the bid preparation stage and tend-
ers received from Australian compa-
nies were generally below the stan-
dard required and poor in comparison
to overseas tenders.
"Some 70% of the tenders received
from Australian companies were tech-
nically inadequate" he said.
Some local suppliers, he observed,
placed more emphasis on quantity
rather than quality and often lacked
attention to detail.
Getting Australian made or sup-
plied items to the plant site on time
presented Woodside with a few head-
aches.
"Australian vendors have not per-
formed as well on equipment deliver-
ies as overseas vendors. While 75% of
overseas equipment items were deliv-
ered within 3 months of the promised
date, only 52% of the Australian
sourced items were delivered in the
same time. For Australian vendors
more than 26% of deliveries were
more than 6 months late" said Mitter-
treiner.
All overseas vessels were delivered
on time while only 18% of vessels
from Australia were delivered on
time. For columns, 60% were deliv-
ered within 3 months of promised
deliveries for overseas vendors, while
no columns from Australia were
delivered in the same period," Mitter-
treiner said.
He also noted the need to stay cost
competitive with overseas vendors.
"Where orders [for columns and
vessels] were won by overseas manu-
facturers, the lowest Australian ten-
dered prices were between 25% and
110% higher," he said.
The prices for locally made piping,
flanges and process valves were some
100% to 200% higher than those of
similar overseas items, he said. So far
local industry has picked up 41% by
value of potential orders.
For the purchase of items such as
pipes and valves $67 million worth
could not be purchased here.
Australian industry did not have
the capability to produce the piping
and flanges in many of the sizes and
types required, he said.
"In general Australian industry
could only produce a small part of the
"Some 70% of the
tenders received from
Australian companies
were technically
inadequate"
NORTHWEST SHELF
Engineers Australia February 5th 1988 17
First, it is essential to define and identify competing
products. Usually, these will be products that either oper-
ate by the same mechanisms, or by utilising different
mechanisms achievc the same end result. The latter is par-
ticularly influential today with the rapid advances in tech-
nology. In consequence, has your company's market intel-
ligence isolated a competitor developing a new process or
technology which may greatly affect your product's mar-
ketability?
For each competing product, it is necessary to identify
(1) the technical advantages and disadvantages, bearing
in mind the function of the product
(2) the range of applications
(3) those products protected by patents, and the advan-
tages created by the patents
(4) the extent of adherence to British Standards
(5) the relative superiority on performance, reliability,
quality, finish and serviceability
(6) the reasons for any modifications which have been
carried out in the past four years
(7) those which must be used in conjunction with
another product, and may be dependent on factors
outside the control of the manufacturer.
For a product to sell successfully, it must appeal to the
needs of the potential customer. In order to orientate the
design and marketing effort, those factors considered
important by the potential customer in making a purchase
decision should be identifïed, eg :
price
delivery
after-sales service
maintenance cost including spares
product and company reputation
brand loyalty
personal contacts
sales history
intra-company trading restrictions
technical specifications
political considerations
number of purchase outlets
product range.
In anticipating the potential growth of each competitor,
it is necessary to identify which
- have excess production capacity
- have facilities for factory expansion
- are well placed to obtain capital.
Marketing not only involves selling products in the most
effective manner, it demands feedback to various func-
tions within a company, such as design. Hence it is advisa-
ble for engineers to be aware of the commercial position of
competitors.
Increasing sales in a rapidly growing market is obvi-
ously more straightforward than in a mature, slow growth
CME January 1981 39
market. It should be appreciated, however, that mature
markets can often provide a stable source of return. What-
ever the dynamic position, the market type and structure
must be determined to enable marketing plans and bud-
gets to be developed. Therefore the following information
should be collected
(1) the competing companies
(2) the market size in value and volume, by product
(3) the competitors' market shares by value and volume,
by product
(4) the market growth rate over the past four years,
together with the anticipated growth rate over the
next three years
(5) the changes in competitors' market shares in relation
to market size alterations
(6) the percentage of sales exported, by product, by
competitor
(7) the major competitors which dominate the market,
and any changes in leadership over the last four
years
(8) the number of companies entering and leaving the
market over the last four years, and the reasons for
those movements.
This detail should give a picture of the market structure.
It is now necessary to to analyse the marketing operations
of competitors. When conducting this, always make a
comparison to your own company. You should include
- distribution systems and effectiveness
- disribution agreements and margins
- discount and credit facilities
- stocking policy, at plant, distributors and users
- sales promotion, the mix of techniques and budget
- number of salesmen
- basis for salesmen's remuneration
- basis for territory allocation
- licensing and franchise agreements
- tendering policies.
The above will detail major similarities and differences
to your own company. It may also illustrate whether the
product type offered by your company is considered pri-
mary or secondary to your competitors. Identifying the
product which provides each competitor with its greatest
profit will give some insight into the possible reactions of
those competitors to technological improvements or
changes in marketing strategy by your company. Which of
these companies take competition and market research
seriously, and hence will be able to react quickly to any
market changes?
Answering the above questions will indicate what
strengths, currently enjoyed by the market leader, can be
developed by your company.
Competition Analysis
by S Seymour
This article is a checklist of the major factors
that should be considered in conducting an
analysis of competitors and their products.
Woodside critical of Aust suppliers

8. Design 5
ties repeated for the whole initial bank of candidates.
It is important to complete each activity over all the candidates before starting the next activity.
The outcome of the design process is the feasibility study, which in the present context describes
succintly the major decisions made whilst arriving at the optimum solution.
You, the designer, have to sell your ideas. You will have to convince others that your optimum solu-
tion is in fact THE optimum, and this requires justification of all significant design decisions. Justifi-
cation does not mean unsubstantiated opinions - you must be prepared to defend your arguments
with demonstrable facts.
When you have acquired the necessary engineering judgement you will be able to shortcut steps in
the rudimentary design process - for example you might not need to consciously practicalise all can-
didates or to fully justify each and every decision, relying instead upon experience. However if your
engineering judgement is still at the toddling stage then you should carry out all the activities
exactly as described.
Design & build competitions provide practice at applying the design process by those whose knowl-
edge of practical solutions, manufacturing techniques and the like is generally limited. The competi-
tions thus demand creative solutions, and enable student designers to appreciate the implications of
their designs when it comes to the later stages of manufacture, of operation and of maintenance.
Details of the design process will be explained with particular emphasis on D&B competitions.
Let us now look at the individual activities in more detail, starting with the problem statement.
Problem statement
Factors relevant to the problem statement include the following :
• Understand the problem - communicate.
Problems are often specified rather imprecisely because the client does not understand exactly
what's required, or because complete clarification requires initial exploratory work. An impre-
cise specification should not prevent a start being made on the problem, but it is important that
designer and client communicate at the earliest opportunity - and continue to communicate
throughout the design process - to make sure that they are both on the same wavelength
regarding what has to be done.
It is all too easy for a poorly briefed designer to go off at a tangent. It is the designer's responsi-
bility - not the client's - to initiate communication and to clear up any misunderstanding.
Specifications, for D&B competitions in particular, are often misinterpreted simply because
they are not read with enough attention to detail. There is no place for loose interpretation.
Each and every clause must be isolated, put under the microscope and examined critically to
deduce its ongoing implications. Especial care must be taken to . . . .
• Avoid artificial constraints.
The solution space for a problem must always be finite and
further constrained by the designer's limited knowledge.
Extreme care must be taken to avoid tighter artificial con-
straints. These are fictitious restrictions which are not intrin-
sic to the problem but which are introduced incorrectly and
usually unwittingly by the designer.
- In the SLAMDUNK competition, students had to design
and build a device to pick up a ball from a rest position on the ground, transport it to a verti-
cal pipe 3 m away, and drop it into the end of the pipe 1 m above ground - the accent was on
real boundary
fictitious
restrictions
your limited
knowledge
solution
space
solution
space
discrete activities. Each of these activities will be explained in depth later but it is useful first to
describe broadly what each consists of and how they fit together :-
• It is not enough to unthinkingly accept the problem as given, instead the designer must
amplify and state the problem in terms which are both :
- broad, in not being constrained unnecessarily, and
- complete, in identifying everyone and everything likely to be affected by any solution.
The problem statement contains information relevant to the next two activities :
• The designer, assisted by methods which enhance creativity, generates ideas for possible solu-
tions to the problem. This activity is inventive and often comprises a free association of ideas
based upon the designer's knowledge of solutions to analogous problems. Ideas are recorded
but not criticised in any way - the activity must be totally non-judgemental since criticism crip-
ples creativity.
When idea generation is exhausted, the result of this activity is a bank of many potential solu-
tion candidates - some are later found to be good, some bad, some ugly.
• The designer must then specify the various :
- constraints (such as 'must be shorter than 1m') to which every candidate must conform if it is
to be a viable solution, and
- criteria ( such as 'cost' and 'degree of safety' ) by means of which candidates may later be
compared with one another.
The constraints and criteria reflect the wishlists of all those identified as having a stake in the
problem, so this very detailed activity is really one of completing the problem in unequivocal
terms.
This activity is left until after ideation has finished, thereby avoiding the stifling of ideation by
prior nit-picking detail.
• A practical solution satisfies all the problem's constraints - it can be manufactured, it will oper-
ate, and so on. 'Practicalisation' is the process of rendering a candidate practical - of fleshing
out or transforming a raw idea, possibly recorded during the ideation activity only by a key-
word or rough sketch, into a practical solution to the problem.
During practicalisation the designer continually criticises each candidate, moulding it to con-
form to the constraints and to satisfy the criteria as much as possible. Practicalisation invari-
ably uncovers fresh secondary problems, some of which may be trivial while others must be
solved by further application of the rudimentary process.
Since ideas were generated non-judgementally, it will not be possible to practicalise every idea
to comply with all constraints - these ideas must therefore be trashed. But impractical ideas
often trigger other worthwhile ideas - that is why trashing is delayed until this late stage of the
design process. Trashing is due to the inability to meet constraints, not criteria.
On completion of this activity, every candidate must have been either :
- rendered entirely practical, or
- scrapped . . .
. . . . . . . there are no half measures.
• The designer finally evaluates the remaining, now fully practical candidates to see how they
compare with one another in meeting the complete set of criteria defined earlier. The optimum
solution to the problem is the candidate which best satisfies ALL the criteria, not just one or
two.
The evaluation activity may indicate that some of the criteria or constraints are inappropriate.
If this should occur then the criteria and constraints must be updated and the last three activi-

9. Design 6
etc
allow
insurance
to lapse
petrol
consumption
ignore
maintenance
exchange
Rolls for pre-
owned Lada
how to reduce car's running costs ?
or, more broadly :
MY POSER
etc
walk bike public
transport
car
how to travel between home and work at minimum cost ?
or, more broadly :
etc
I go to work
work
comes
to me
web
interface
tele-
conferencing
how to interface me and work at minimum cost ?
or, more broadly :
etc
how to make money ?
play the
gee-gee's
work at
University
retire on a
Government
pension
send my
family out
to work
!!
or, more broadly :
I am feeling the pinch financially, so I pose the problem . . .
how to reduce my car's petrol consumption ?
etc
improve
driving
habits
& narrower
check
tyre
pressures
remove
dinghy from
roof
tune engine
minimising the run-time from pickup to drop. Most students
divided this task into two components :
i carry the ball 3 m horizontally, and
ii lift the ball 1 m vertically.
This subdivision was perfectly legitimate because a separate source of energy was appropri-
ate for each of the two motion components. But many students artificially bounded the prob-
lem by separating also the kinematics - eg. the horizontal component was completed first, a
trigger thereafter activating the vertical. If the motion components had been carried out
simultaneously then a quicker run could have resulted.
- A pressure vessel is used to store a pressurised fluid. It usually consists of a cylinder with end
closures and with attached pipes through which fluid enters and leaves the vessel. Students
were asked to design such a vessel with a 'pipe diameter of 500 mm'. They took this to mean
that the pipe's bore (inside diameter) was 500 mm, but in fact this dimension referred to the
pipe's 'nominal diameter' - a different thing entirely. By introducing this artificial constraint
the students' optimum vessel was in fact some 25% unnecessarily expensive and would not
have gotten past first base in a real-life tendering situation.
The detailed logic of why this fictitious bound led inexorably to a cost blow-out need not
concern us here, but the point which must be made is that slack interpretation of the specifi-
cation was the root cause of the blow-out - exactly the sort of thing that Somerville (above)
warned against.
One insidious artificial constraint involves accepting the problem as presented without consid-
ering whether the solution to a broader problem might in fact give greater satisfaction. So you
should try to . . . .
• Broaden the problem.
Try to define a less specific problem which encompasses the given problem, thereby increasing
the potential advantages of a successful solution. My Poser shown opposite is a case in point -
rather than pussy-foot around adjusting my car's timing in an effort to reduce fuel costs and
improve my finances, I am likely to be substantially more satisfied by posing and solving a
broader problem.
Broadening is not always so attractive as it was in this example: it depends on the context. The
client may not wish it broadened; you may be given a very specific design task to carry out - no
frills, no fancy ideas, just solve the problem! While you do not have much option under these
circumstances, attempts to broaden the issue - if only for your own enlightenment - will assist
your understanding and solution of the specific problem.
• Complete the problem.
Completion is a very detailed exercise, and in order to prevent its inhibiting creativity it should
generally be left until after the bank of solution candidates has been generated.
During the problem statement therefore, completion should consist only of identifying folk
who are likely to be affected by, and who must be satisfied by the design. Their agenda will be
examined in detail under 'constraints and criteria' below.
• Consider subdividing the problem.
Sometimes a problem lends itself to splitting into sub-problems which are each easier to solve
than the original problem. For example if the problem were to collect a pile of grain at ground
level and deliver it to a bin 3 m high and some metres distant horizontally, then separate sub-
problems might be posed :
i how to collect the grain and lift it off the ground ?

10. Design 7
Generation of ideas
This inventive phase of design is unlike other activities in the design process - and indeed differs
from other subjects in the engineering course - in being absolutely non-critical. You may find it diffi-
cult to switch off your analytical faculties, but it is vital that you completely separate creation from
criticism, since nothing inhibits creativity more than does nit-picking criticism. There is plenty of
scope for criticism during later activities in the design process.
Avoid ALL criticism while creating
Don't be disheartened because you think you lack any powers of invention. Krick op cit concludes
that inventiveness depends upon :
• Inherited qualities : we cannot all inherit Leonardo's genes, but read on . . . .
• Method : you can employ proven techniques to increase your inventive prowess
• Attitude : you must be positive, you can invent !
• Knowledge : your understanding of how related problems are solved can be increased
• Effort : Edison (inventor of the light bulb) was spot-on when he observed that
‘Invention is 1% inspiration and 99% perspiration’
Apart from the first, these are skills, and like all skills they develop only with practice. This course
cannot make you a good designer, it can only point you in the right direction.
Ideas cannot emerge from a complete vacuum. Our minds generate ideas, however indirectly, from
our store of knowledge - moulding, modifying, and trying on for size the myriad items buried away
in our subconscious which might conceivably be relevant to the problem in hand. This necessary
knowledge base is indicated in the rudimentary design process - the deeper and wider the base, the
more useful ideas are likely to be generated. Student designers are therefore advised to continually
increase their general, scientific and technical knowledge by taking advantage of the resources avail-
able in university libraries eg. Some interesting and not-too-heavy periodicals are cited in the Bibli-
ography at the end of this chapter.
There are numerous techniques to enhance creativity - Synectics, Morphological Analysis, and the
Theory of Inventive Problem Solving (TIPS), to name a few. Some techniques are more structured than
others; some are more hit-and-miss - but they all suggest ways of modifying and/or combining
existing partial or related solutions in order to devise new solutions. Which is all very well, pro-
vided the inventor knows about existing solutions. The knowledge component is therefore crucial
when ideating.
Structured techniques are suited to more bounded problems for which detailed knowledge of
related solutions in a specialised field is particularly important. For example the designers of a new
industrial vacuum cleaner would need to know about the fluid dynamics of multi-stage fans, the
intricacies of manufacture with various plastic materials, the latest trends in visual appeal, what sort
of jobs the cleaner might tackle (slurp up liquids? back-pack transportability?), cost/performance of
potential competitors, ease of emptying, etc etc.
The present course is aimed at jolting the tyro designer away from ingrained analytical habits, and
so the problems introduced here - typified by D&B tasks - are amenable to less structured methods
such as brainstorming, based on general knowledge of how similar problems are solved by Nature
for example. Brainstorming is essentially a free association of ideas where each idea is recorded
when first thought of, and actively encouraged to beget further ideas. A designer can brainstorm
alone, or preferably with others who have different interests, different experiences . . . . ie. essentially
ii how to elevate the grain to a height of 3 m ?
iii how to transport the grain some metres horizontally ?
iv how to energise these various transportation components ?
v how to control these various transportation components ?
vi from what materials should a device be constructed ? etc. etc.
Although this divide-and-rule philosophy can lead to a more tractable solution process, it is
not a universal panacea because :
- If you think about the problem too critically with a view to subdivision then your mind may
be pre-configured into the critical mode, to the detriment of subsequent ideation.
- If subdivision leads to a plethora of problems then you might unconsciously throw in the
towel - the prospect of having to solve so many problems is just too daunting.
- Subdivision of the problem may obscure unified solutions. For example if each grain
transporter sub-problem were individually addressed, then it would be more difficult to
come up with solutions in the form of a large industrial vacuum cleaner (answering sub-
problems i-v in one fell swoop), or a couple of guys equipped with wheelbarrows and shov-
els (answering all the above).
- Sub-problems are seldom of equal difficulty. You must learn to distinguish between those
which are relatively difficult and those which are not, putting most of your energy into solv-
ing the former. For example in D&B competitions the question of materials is often quite
minor compared to the difficulty of figuring out a mechanism to carry out the required task -
so the choice of materials might advisedly be left until later detailing.
• Beware the 'improvement' brief.
Look out when you are asked to improve an existing solution! An improvement is often sought
because someone else's design is less than perfect, and by sticking to the letter of your instruc-
tions you may automatically retain drawbacks which are inherent in the existing solution -
recall John's oven and the inevitable limitations of retro-fixes.
Another class of 'improvement' brief arises when an existing solution must be adapted to con-
ditions which differ from those for which it was designed in the first place - conditions for
which it may be totally unsuited. Appendix A describes an unfortunate request for improve-
ments to a computer workstation.
As a problem-solver, you cannot afford to accept uncritically whatever information is handed to
you. In many cases you have to figure out the best problem before figuring out the best solution !
And for this . . . .
You must ask the right questions.
It wasn't until after John had stated and solved his oven problem that he appreciated the need to ask
around. And by then of course it was too late.
Before progressing to the next stage of the design process therefore, the problem statement must be
as broad and as complete as you can make it. Let's now consider this next stage - ideation.

11. Design 8
( 1 )
( 2 )
( 3 )
( 4 )
( 5 )
( 6 )
( 7 )
( 8 )
( 9 )
( 10 )
( 11 )
( 12 )
( 13 ) ( 14 )
fixed
diaphragm
different spans of knowledge.
Any hint of criticism during ideation is strictly taboo - there is no such thing as a 'crazy' idea whilst
generating ideas. This is the key. If you think that an idea is unworkable or laughable, then the
thought MUST be suppressed. Any idea which gives birth to other ideas cannot be wholly daft, and
as this spawning is not weighed up until post-ideation criticism, there cannot be any justification
during ideation for concluding that the idea is crazy or not. There is plenty of scope for criticism
after the ideation activity has ceased.
Roberts, in Appendix F, presents a number of idea-generation techniques. You should read this
extract to obtain an overview of the various methods available for enhancing creativity - if nothing
else it indicates that the topic is sufficiently regarded to have attracted a lot of interest over the years.
We shall concentrate on brainstorming as described by Roberts - you should practice this wherever
possible to gain proficiency.
Some hints which are generally applicable to the ideating phase and which have not yet been men-
tioned include the following :
• Two activities follow the problem statement in the rudimentary design process :
- ideating to create the bank of solution candidates, and
- completing the problem by defining the constraints and criteria.
It is suggested strongly that ideation be carried out first, before one's thoughts
turn critically to constraints and criteria. The reason for this recommendation
may be seen from the fluid filled pipe -
"How can fluid be prevented from escaping from the end of the tube ?"
What is the effect of defining the constraints and criteria before ideation ? App-
endix B illustrates the benefits from delaying critical thinking till after ideation.
• Ideas in engineering are often but not always recorded more succintly by sketches than by
keywords.
• Although the more ideas you generate the better, you must recognise
when to stop inventing. Design costs money; so if you find that your
creativity is drying up, then it may be time to quit.
• Further candidate solutions often pop spontaneously into one's head
some days after a deliberate brainstorming session has been termi-
nated. If possible, time should be allowed for this subconscious gesta-
tion as it can't be hurried.
• Finally, remember that it takes effort to search for new ideas, and, as later criticism invariably
throws up lesser problems, you must be prepared to confront these lesser problems. We'll look
at this more closely when we come to the practicalisation activity.
EXAMPLE Generate ideas for the operating principle of a mechanical device to raise and retain a
1 kg unopened can of baked beans as high as possible from rest. Prior to raising, the device must fit
inside a cubic envelope of 0.4 m side.
Possible mechanisms are sketched opposite- the descriptions are amplified by comments (in italics)
which were added after ideation had been completed :
( 1) large spring like a Jack-in-the-box - conical for stability, possibly using inter-coil anti-toppling
restraints
( 2) spirally wound compression spring - from flat stock for enhanced inherent stability
fluid
time
cumulative number
of ideas generated
rate of idea
generation
QUITTIME?

12. Design 9
Problem completion - constraints & criteria
The problem has been treated broadly up till this stage in order to derive as many candidate solu-
tions as possible. It is now time to complete the problem, to focus on exactly what is required of
solutions by way of constraints and criteria.
• Constraints
A constraint is a bound, a limit with which every candidate must comply if it is to be a valid
solution. Problems are usually characterised by a number of constraints.
Satisfaction of a constraint by a candidate is binary - the candidate either does or does not sat-
isfy the constraint, there are no ifs, no buts, no maybes and no 85% about it.
Typical constraints are :
i “To solve my transport problem I need a vehicle which must cost less than $10000"
ii “Our device for the D&B competition has to fit inside a 400 mm cube”
iii “Any solution to the greenhouse gas problem must not put any of our people out of work”
There are constraints - often implied and not spelled out - which are obvious and particularly
important to D&B competition groups. Each group designs, constructs, operates and repairs its
own solution device for the competition. The group therefore must be able to build the device,
the device must operate as intended, and so on.
• Criteria
A criterion is a yardstick by which the suitability of candidates may be judged - some candi-
dates may satisfy the criterion well, some poorly. Problems are usually characterised by a
number of criteria.
The degree to which the candidate i satisfies the criterion j may be expressed by the utility,
ui.j - often a real number between 0 and 100%, though other scales are used :
- A high utility indicates that the candidate satisfies the criterion to a high degree.
- A low utility signifies that the candidate satisfies the criterion poorly.
- A utility of 100% means that the candidate satisfies the criterion perfectly.
- Zero utility means that the candidate is completely useless - but only as far as the relevant
criterion is concerned.
NB : A zero utility does NOT imply that the candidate is useless as a whole, so a zero utility
for one candidate with respect to one criterion must NOT be cause for trashing the candidate.
Typical criteria are :
iv “To solve my transport problem a vehicle will have to be fast, cheap to run, manoeuvera-
ble, safe and commodious”
v “Simple manufacture without powertools” will be a requirement for any device we adopt
to solve the D&B problem“
vi “Any solution to the greenhouse gas problem must minimise the number of our people put
out of work”
A constraint is more limiting than an equivalent criterion - the $10000 limit of constraint ‘i’ above
leaves less scope for a solution than a criterion along the lines of 'cheap as possible' or 'minimum
cost'. You should therefore try to enlarge the solution space wherever possible, by converting con-
straints into equivalent criteria. For example if Australia had adopted the criterion ‘vi’ above rather
than the constraint ‘iii’ during the 1997 Kyoto talks then its stance would likely have attracted less
criticism - ie. more folk might have been kept happy. Unfortunately many political decisions appear
to be argued on the basis of constraints rather than criteria, thus antagonising whole sections of the
community.
In this completion activity we are not interested in whether a candidate complies with a constraint
( 3) constant force spring - very thin steel tape pre-formed to circular cross-section becomes a tube when
unwound flattened from a roll (similar to a steel tape-measure but substantially more pre-forming)
( 4) party blow-up - or more practically, fluid pressurised coiled hose
( 5) fluid pressurised telescopic tubes - need careful practicalising to render leakproof with negligible
friction
( 6) sectionalised extending mechanism - eg. wire braced for lightness, extended like fireman's ladder
#8 below
( 7) rolling diaphragm instead of sliding seal to stop leakage between telescopic tubes (practicalising
detail)
( 8) fireman's ladder - operated by pulling on a single cord (not sketched)
( 9) roll of 'tank tracks' (one-way bending chain) or flat belts - probably arranged as in #14 for
stability
(10) unwinding arm like elephant's trunk, possibly straightened by pulling a cord (how are mus-
cles/tendons arranged to enable elbow bending for example?)
(11) cord-operated straightening arm - consisting of identical pin-jointed bars with pulleys at the joints
(12) lazy tongs - operated by tension springs or elastic bands
(13) foaming agent expanded through a nozzle becomes rigid when exposed to air
(14) triangular arrangement of three effectors such as #3 (or 4 or 9) for mutual support against buckling
(15) thermal expansion of vapourising fluid used instead of pressurised air
(16) very flexible fishing rod(s) bent and coiled initially within a box
(17) air-operated bellows similar to a bamboo/tissue paper Chinese lantern that folds flat
(18) screw jack eg. pump-action screwdriver
(19) project the can - with packed parachute if necessary (depending upon the elevated retention time
desired)
(20) project can upwards, trailing cords which solidify on exposure to air - note similarity with pre-
vious idea
(21) ditto, but trailing chains whose specially shaped links lock when aligned thus supporting
compression - note how this followed directly from the previous impractical idea
(22) suspend from a balloon
(23) utilise magnetic repulsion/levitation
(24) plant a seed of Jack's QuikGro Beanstalk under the can, water it and stand clear - not all crazy
ideas bear further fruit, however . . .
(25) train a snake to balance a can on its nose like a seal - is crazy too, but triggers the following
which isn't . . .
(26) feed coiled wet thick rope vertically out of a container above which dry ice freezes rope, ren-
dering it rigid
- and so on.
A common failing on the part of budding designers is to abandon ideation before sufficient candi-
dates have been identified - during ideation you should always aim to
Generate as many ideas as possible - quality doesn't matter at this stage
If a candidate bank is not large enough to start with, then a few unsuccessful attempts at practicalis-
ing could see you with nothing left to work with.
Having generated a bank of solution candidates, let's see how we go about setting up the problem's
constraints & criteria.

13. Design 10
The practicalisation activity takes the form of an assessment and development of the candidate by
the designer, who must modify the candidate to surmount if possible all barriers to a practical solu-
tion. The activity is seldom instantaneous but rather requires the designer to repeatedly traverse the
practicalisation loop :
• proactively searching for and identifying ALL subsidiary problems, and
• solving these subsidiary problems (if possible).
Practicalisation is not mere criticism, it is positive remedial action
A problem is trivial - it is no problem - if it has previ-
ously been encountered and solved satisfactorily.
For real problems . . .
. . . if we adopt an existing solution then there is no
problem, but if we try a novel solution then further
problems are likely to arise . . . for each of which . . .
. . . if we adopt an existing solution then there is no
problem, but if we try a novel solution then further
problems are likely to arise . . . for each of which . . .
. . . and so on - the problems becoming smaller and
smaller until they reach triviality.
Practicalisation is not so exhausting as might be infer-
red from this description, but it must be exhaustive. One major lesson learned by students in D &B
competitions is that real life does not forgive “practicalisation” which is not thorough and complete.
One example of problems leading to other problems involves the primary problem "to design a new fac-
tory for producing a given chemical". There may be a number of different solutions to this, each with its
own set of raw materials, chemical reactions, economics and so on.
If one of these solutions is adopted then the various necessary heat exchangers, pressure vessels
(containers for pressurised fluids), pipes, pumps, cooling towers etc. each forms a secondary prob-
lem - some of which may be trivial as the items may be procured off-the-shelf.
The solution to one of these non-trivial secondary problems - let's say a pressure vessel - would
throw up tertiary problems involving the choice of material, of dimensions, lagging to reduce heat
loss, etc. etc. leading eventually into detail design.
Large problems like this require considerable resources involving large design teams for their solu-
tion. But whether the problem is large or little - if it is not known that the solution is practical before
comparing it with others, then obviously the comparison is an utter waste of time as it could result
in an 'optimum' which can't be made or which won't work !
Practicalisation does not equate to detail design. For example, two parts may have to be joined
together demountably. Looking into the means of joining, it might be concluded that a set screw is
perfectly feasible as the loads are unremarkable, access for tightening and loosening is not restricted,
and so on. This is practicalisation.
Further analysis involving fatigue loading, materials and safety might lead to a solution in the form
of an M10x1.5 class 10.9 socket-headed set screw, length overall 80 mm, length of thread 25 mm,
unlubricated and torqued to 90% proof. This is detail design; it has no place in this introductory
chapter on design.
During practicalisation the designer must foresee and must overcome (if possible) all drawbacks to
secondary
primary
PROBLEM
SOLUTION SOLUTION
PROBLEM PROBLEM
SOLUTION SOLUTION
PROBLEM
tertiary
PROBLEM
SOLUTION
PROBLEM
SOLUTION
PROBLEM
or not, neither are we interested in how well a candidate satisfies a criterion - all we are doing is
identifying and recording the constraints and criteria, so that they might be applied to candidates
during later activities in the design process.
One of the most crucial tasks in the whole design process is to :
Ensure that ALL constraints and criteria are identified.
The constraints and criteria are determined by the wishlists of everyone who, and everything which
will interact with the solution. A designer must therefore realistically visualise the solution as it
progresses through its life stages, and anticipate these interactions together with the correspond-
ing constraints and criteria. If other sources can assist with this task then clearly they should be con-
sulted. A designer's incomplete knowledge of manufacturing processes for example is good reason
for talking with a fitter and turner, for further reading, or for experimenting personally.
One of the reasons for mounting D&B competitions is that members of student design groups are
involved with all life stages of their solution, and so must themselves establish the completion wish-
list. Although other folks' agenda are not addressed, students still have to foresee the solution's
future and to set constraints and criteria accordingly. These should not have to be inferred during
later activities, but must be written down in black and white in the present completion activity.
One common criterion which is often overlooked is the need for simplicity. Appendix C provides a
couple of graphic illustrations where complexity equates to frustration.
Keep it as simple as possible
If you don't identify and write down all constraints ("we must be able to make it") and criteria ("it's got
to be as simple as possible") then you'll most probably overlook them when it comes to the next step,
practicalisation, resulting in a less than optimum solution.
Practicalisation - rendering candidates practical
A solution candidate is practical if it complies with all the problem constraints and well satisfies the
problem criteria which have been identified previously. Practicalisation is the activity in which an
embryo candidate solution (in all probability described only by a very hazy sketch or a few key-
words) is transformed into a practical solution - if this is possible- before candidates are compared
with one another and certainly before any manufacture is contemplated.
The outcome of practicali-
sation for a particular can-
didate is that :
• either the candidate
is practical, that is
complies with all
constraints,
• or the candidate is
not practical as it
cannot meet every
constraint
. . . . there are no half
measures.
YES YES
NO
Are you
certain that candidate
cannot comply with all
constraints
?
practical solution to
evaluation activity
impractical
candidate to trash
candidate solution from bank
Solve secondary
problems, modify
Are you
certain that candidate
can comply with all
constraints
?
NO
PRACTICALISATION

14. Design 11
former might provide adequate reinforcement.
(d) Tubes could be centrifugally cast by quickly rotating a tubular mould into which a setting
plastic eg. is poured. Centrifugal force causes the liquid to form a uniform film which sets
inside the mould. A tube so formed could become the mould for the next smaller tube.
(e) Tubes might be made from rubber hose, allowing radial expansion to prevent air leakage.
(f) The large radial gap between adjacent sizes of tube available in the shops - which led to the
proposed scrapping of the telescoping tube idea - might be put to
good use as a mould cavity in which to cast an intermediate
tube. There are many casting plastics and latex rubbers availa-
ble on the market. The fishing line reinforcement mentioned
above might be incorporated again here.
And so on. The foregoing doesn't pretend to be an exhaustive list of ideas, and all the secondary
problems have not yet been resolved - ie. telescoping tubes have not yet been fully practicalised -
but certainly we now have sufficient confidence to retain the underlying idea and to invest in the
building and development of prototype tubes.
EXAMPLE Here is another example of what can be done with even lower-tech materials
and manufacture. Students were asked to design and build a stair climber. One common result
of initial ideation was a vehicle equipped with caterpillar tracks like those on military tanks.
The tracks were perceived as rubber belts with treads, but because nobody could conveniently
lay their hands on such peculiar components the whole idea was trashed - students didn't try
to practicalise . . . .
How may caterpillar tracks be made from scratch using commonly available materials ?
Some ideas are sketched :-
Both these examples are typical of D&B problems in which practicalisation is incomplete because
our knowledge/experience cannot predict every detail - we don't know if we can manufacture a
tube from wound fishing line or a tank track from bent cardboard. However the above ideation has
indicated some possible solutions to the secondary problem of manufacture, and importantly has
demonstrated that manufacture is not necessarily impossible. Note the glimmer of hope here, com-
pared to students' knee-jerk reaction to trash both telescoping tubes and tank tracks because they
couldn't immediately lay their hands on them.
Practicalisation can be completed only by acquiring the necessary knowledge - with D&B devices
this is usually best done by direct experimentation. The investment of time and effort in experimen-
in operation
as cast
bent cardboard cut-out
paper clip hinge pins
match glued on for traction
stapled upholstery webbing
lengths of plastic tube, riveted on
narrow strip of flexible carpet forms track,
or attached around wheel periphery
form chain
track from
bent wire
the artefact's practical realisation. Secondary problems which the idea's novelty has introduced must
be solved satisfactorily. This again is time-consuming. You have to work at it. You will have to start
the problem-solving process all over again - "How can this drawback be overcome so that this candidate
can be rendered practicable ?" You will have to consider the manufacture and operation of the candi-
date IN DETAIL. You will need to know something about materials and how you yourself can fash-
ion them. You may have to set up mathematical models of the device's operation, and so on.
With D&B competitions for example, the major constraints relate to the two significant life stages
after design - the constraints concerned with manufacture and the constraints relative to operation.
We shall demonstrate practicalisation first with respect to manufacture.
EXAMPLE Candidate #5 of the above can raising device involves telescoping tubes. From
where might we obtain these ? Are we able to manufacture them ? Practicalise this, given that
the pressure of any fluid used must not exceed 100 kPa for safety reasons.
An all too common approach here is to visit the nearest hardware shop on the lookout for plas-
tic or light alloy tubes of different diameters and thicknesses which may nest inside one
another, and on being unable to unearth suitable tubes to scrap the candidate as impracticable.
This lack of effort is deplorable. What one might do is . . . .
A simple p = F/A demonstrates that a pressure of 100 kPa acting over a circular area of only 11
mm diameter will support a mass of 1 kg. Alternatively, if the whole 0.16 m2 plan area of the
device is available to lift the can then the necessary air pressure is a paltry 60 Pa (6 mm H2O).
These limits indicate that intermediate pressures and areas could be used successfully, and that
metal tubes may not be necessary to withstand the operating pressure. So, recalling that the
environment is dry ambient, we further practicalise by asking what other materials/
manufacture might be used?
(a) We might glue cardboard or paper or plastic, winding up tubes helically
like the support tubes for toilet rolls eg., using the next smaller tube as a
mandrel. Each tube might consist of multiple layers of differing hand,
built up on a temporary innermost layer which is later removed to ensure clearance
between adjacent tubes. The thickness of each tube would then be easily adjusted to with-
stand the internal pressure.
(b) A very light tissue paper tube could be close wound with nylon fishing line eg. - the former
component ensures a leakproof tube, the latter provides reinforcement to withstand the burst-
ing effect of the internal pressure. This idea might be criticised because
of the difficulty of a leakproof sliding seal outside the tube due to the
corrugations formed by the fishing line. But further practicalisation
might reveal that the interstices could be filled with smoothed setting
plastic - or they might be put to good use as a reservoir for a honey-
like substance doing double duty as a speed retarder and an air seal (due to its surface tension
and viscosity).
(c) Composite tubes could be formed in the same manner as Saturn fuel tanks, by winding rein-
forcing fibres helically around a mandrel (possibly using a lathe for uniform pitch) then
impregnating/spraying them with a solidifying plastic. Subsequent machining for a smooth
external surface might be necessary. A silk stocking or a long knitted sock stretched over a
composite tube
half-section of

15. Design 12
a device before building, it may be possible to assess operation by means of a mathematical model -
though it must be emphasised that mathematical modelling is no substitute for direct experimenta-
tion if that is possible. Students generally seem reluctant to study hardware by mathematical models
of their own devising - they're excellent at analysing existing models, but this ability is completely
useless if no reasonably accurate model of the device exists.
It is not suggested that mathematical modelling always be attempted - the simple party blow-up
(candidate #4 of the can-raising device above) is far too complex theoretically - but rigid body
dynamics eg. is often very useful in enabling prediction of operation. Students should practise the
construction of such models - ensuring that free bodies are correct, a common source of error in stu-
dents' work. A few illustrative mathematical models are now given.
EXAMPLE An idea proposed for the stair climber involves motorised wheels equipped
with lobes which engage with the steps to allow the device to progress smoothly from one step
to the next. What shape of lobes should be used?
The 'Sherpa' model shown overleaf considers lobe geometry based on the involute for smooth
ascent. Although this model may be practical from the point of view of kinematics, it says nothing
about forces or ascent speed, so the idea has not been practicalised fully. Rather than extend the
model to include kinetics and strength, the kinematic model may provide sufficient confidence to
build a device, test it and develop it physically to full performance capability.
EXAMPLE What size of spring should be used in the lazy tongs, candidate #12 for the can
lifter above.
The model of a rudimentary light lazy tongs is illustrated and analysed here. The expression
relating the load's acceleration 'a' to the arm's inclination 'θ' is found to comprise three terms,
only the first of which is positive. So unless the inclination is large enough to yield a positive
acceleration, it doesn't matter what size of spring is used - the device won't work!
This conclusion was found out the hard way by students who built the device then found that
they had to incorporate a compression spring perpendicular to the spring sketched in order to
start the device. Given the springs and dimensions, the model enables the compressive load P
to be found as a function of inclination, if required for later detail design to avoid buckling and
failure of members.
EXAMPLE This concerns practicalisation of the can lifter candidate
#11, the cord-operated straightening arm in which the load 'W' is ele-
vated through the distance 'h' by means of a cord pulled by tension 'F'
through the distance 's'.
W
S
S
a
eight W raised by light mechanism of strut length c,
with tension spring of stiffness k and free length L o :-
pring force : F = k ( L – Lo ) = k ( 2 c cos θ – Lo )
For light wheel : F – P cos θ = 0
For rising mass : 2 P sin θ – W = m a
o acceleration :
a
g =
4kc
W
sinθ –
2kLo
W
tanθ – 1
nd velocity : v2
2gc
= 2kLo
W
cosθ – kc
W
cos2θ – sin θ + const.
c x
P
F
W
a
P
P
c
θ
k
N (≥W/2)
W
device
W
F
s h
tation has been shown to be justifiable in the above examples, and would concentrate only on the
unknown aspects of manufacture, not necessarily on the complete build (recall that our main aim
during practicalisation is to become certain that we can or we cannot manufacture the candidate).
Let's now consider practicalising from the point of view of operation.
The principle of operation has been established during the previous ideation activity; the principle
now has to be fleshed out during practicalisation - again sufficiently to satisfy ourselves that the can-
didate either can or cannot operate. By far the most satisfactory basis for assessing operation is
direct experience of this operation. While we can't experience all aspects of operation before the arte-
fact is completed, in most cases it is possible to quickly and cheaply experiment with certain compo-
nents or sub-systems whose behaviour is particularly problematical, and to modify these to obtain
the operation required. You must be prepared to try it !
EXAMPLE A D&B project required collection of as many dried split peas from a pile as pos-
sible, using a device which could be battery powered but which had to be as light as possible.
The pile was situated 1.5m away from the device's initial position, and the peas had to be deliv-
ered to a collection container.
The initial problem could be broken down into sub-problems such as "how to transport device to
vicinity of pile?", "how to pick up peas?", "how to deliver peas to collector?", "how to power the device -
electric motor(s), clockwork motor(s), springs, compressed gas cylinders etc.?".
Concentrating here on the pick-up phase, there are many possible candidate solutions - rotat-
ing/sweeping brush, conveyor belt, bulldozer/scraper, vacuum, air jets and so on. The
vacuum was popular with contestants as a cheap vacuum unit for cleaning the interior of a car
using the car's electric system was available.
Some contestants dismissed this solution on the grounds that "it needed batteries which were too
heavy" (Note the unforgiveable error of trashing a candidate on the basis of a criterion), "its
suction was insufficient to pick up peas" etc. - without any justification whatsoever. They did not
actually test the unit, but based their conclusions on their preconceptions, which - not being
based on past experience - couldn't have been more wrong.
It would have been so easy to have actually tried the unit out, to have experimented with it by
itself. How close to the peas would the vacuum's nozzle have to be in order to pick peas up?
Could a snow-plough blade be attached to the moving nozzle to pick up more peas? What are
the lightest batteries required to give the desired period of operation? How can the unit be inte-
grated with other sub-systems such as transportation?
All these sorts of questions could have been answered definitely, and all the necessary modifi-
cations completed, ie. the candidate substantially practicalised by direct experimentation
before comparing it with other candidates or integrating it into the complete device. Even if a
group couldn’t afford to purchase one of these units without some confidence in its potential, it
might have been possible to borrow one, or to try one out in the shop with a plate of split peas.
This project was edifying as some competitors rejected the vacuum candidate without proper practi-
calisation, whereas other competitors produced successful devices based on it. Conversely, some
competitors rejected conveyors on the basis of mere arguments - mere hot air with absolutely no jus-
tification, no basis in the real world. The winning device was a conveyor.
Lacking the opportunity for direct experimental appreciation of the performance of a sub-system or

16. Design 13
This demonstrates that the cord displacement 's' increases in proportion to the bar inclination
'θ', the constant of proportionality being the pulley diameter 'd'. Thus for small forces in the
mechanism we need a large pulley diameter.
It is not difficult to extend this analysis to cover changing forces and buckling proclivities in
the device as bar inclination changes during the arm-straightening process. Again, we gain an
understanding of performance before building starts.
Students were asked to design and build a vehicle powered by a supplied rubber band to travel as
far as possible along a straight horizontal track. In a common device the rubber band, modelled as a
spring, was connected to a cord wrapped around a drum attached to
a large driving wheel. The spring was first wound up by rotating the
wheel by hand, the vehicle was then released on the track and trav-
elled due to the cord unwinding off the drum.
Students made the drum diameter small in an effort to achieve a
large travel from a given spring displacement - ie. they examined only the overall geometry of
motion. But the ground traction force accelerating the vehicle was proportional to the drum diame-
ter, so a small drum resulted also in a small accelerating force. This force was so swamped by fric-
tion (which students hadn't allowed for) that there was no acceleration whatsoever. The vehicle
refused to budge and caused much embarassment - or hilarity, depending on the point of view !
This inter-dependence of kinematics and forces occurs in all devices.
Beware the dangers of a kinematic analysis without looking also at the forces.
A more complete analysis involving the kinetics of this vehicle is presented in Appendix D, and pro-
vides insight into the interaction between the major design parameters such as weight, overall
geometry, spring characteristics, etc. and the resulting performance - all vital stuff when assessing
whether or not the device will operate.
A brief overview of springs as energy storage devices is given in Appendix E to illustrate the kind of
information available in the literature - a few simple measurements enable the stiffness of a given
spring to be estimated, taking for steel E = 207 GPa, G = 80 GPa. The later chapter on Springs should
be consulted for further information.
An even more realistic model of such a device must recognise further consequences of the choice of
spring with a given energy storage - either :
• a stiff (large k) spring requires a heavy vehicle with massive members
to withstand the buckling effect of large spring forces, and a large inef-
ficient speed increaser to amplify the small spring deflections, or
• a compliant (low k) spring which needs a vehicle of large dimensions
to accomodate the large spring deflections, though compactness may
be achieved by a clock spring rather than the tension spring foreseen.
Extension of the model to include these effects is not particularly difficult, but there often comes a
time when the preparation of a realistic mathematical model is too demanding given the significance
of the problem. Then is the time to experiment with hardware.
Mathematical models are not restricted to mechanics, as the following demonstrates.
EXAMPLE Some effort has been devoted already towards practicalising manufacture of the
k
khi
klo
deflection
force Springs storing
equal amounts
of energy
In first assessing feasibility we consider an ideal mechanism for which 'F' is constant. Work-
energy thus requires Fs = Wh so that if the geometry/kinematics require that the cord's dis-
placement 's' is small, then a correspondingly large tension 'F' is required in the cord, with con-
sequent implications on bar buckling etc.
So, what are the kinematics? They are examined in the box below for a single bar of the device.
Bar of length c carries two pulleys of diameter d around which a
cord is wrapped. To find the relation between cord length x-x and
bar inclination to the horizontal θ.
Inclination of cord to bar : γ = arcsin(d/c) which is constant
From geometry : π = φ + ( π
/2 – γ ) + θ and so
cord length between points 'x' = 2 ( 1/2 d.φ + √( (c/2)
2
– (d/2)
2
) )
= d ( π/2 + γ – θ ) + √( c2 – d2 )
= constant – d. θ
γ
φ
x
x
θ
c

17. Design 14
lel tongs in the device . . . . and so on.
If you don't ask these sorts of questions and answer them satisfactorily then don't be surprised when
- not if - Murphy appears on the scene! So, once again . . . . YOU MUST ASK THE RIGHT QUES-
TIONS.
We have illustrated practicalisation by means of devices built and operated by the designers - clearly
manufacture and operation are the most important life stages. But the same thoroughness and atten-
tion to detail are necessary in other aspects of more usual problems involving other folk. This is why
it's so important for the designer to approach any problem from the points of view of all those likely
to be affected by the solution - the lathe operator, the user, the sales person, the maintainer etc. Only
by visualising the step-by-step actions of these people can the designer appreciate the subtleties of
their interaction with the solution candidate.
This section concludes by emphasising the need for definite knowledge of a candidate's practicabil-
ity after the practicalisation activity. Theoretically, an accurate comparison (evaluation) cannot be
undertaken until all candidates have been designed completely. In choosing a new car for example,
all candidates are physically available - however this is hardly a design problem, it's purely a matter
of selection.
In the design context we cannot afford the luxury of designing in all detail every likely looking can-
didate in order to select a single 'best' solution. At the other extreme, what confidence can we have
in a choice between one candidate which we don't know will work and a second which we don't
know how to make? So the designer must continue around the practicalisation loop until the ulti-
mate practicality or uselessness of each candidate is known with some certainty. Failure to do so is
one of the most common shortcomings of students' designs. All decisions must be justifiable, and
they can't be if they are based on incomplete knowledge.
Candidates must be practicalised before evaluation is attempted.
Let's now see what this evaluation activity entails.
Evaluation - choosing the optimum candidate
At the conclusion of the practicalisation activity, we have a number of candidate solutions which
meet all the constraints. These now have to be evaluated - that is compared with one another on the
basis of the problem criteria - in order to select the optimum solution.
Evaluation consists of three distinct steps, carried out by the designer :
1. the relative importance of the various criteria is defined
2. the degree to which each candidate satisfies each criterion is established
3. the degrees to which the candidates satisfy the overall problem are finally worked out.
Let's examine these steps individually.
1. Relative importance of the criteria
Different sets of criteria are associated with different problems. One particular criterion which may
be common to different problems usually assumes different significance in these problems.
The following sketch illustrates the importance of five criteria common to two different artefacts.
In the design of a domestic vacuum cleaner, cost is probably the most important criterion since
potential purchasers' first thought is the effect on their pockets. Undiscerning consumers are not so
telescoping tubes, candidate #5 of the can raising device. The device is conceived as an air res-
ervoir at an initial high pressure, to which the unexpanded tube of cross-sectional area 'A' is
connected. On being released, the tube expands, raising the 1 kg can to a height 'H'. What
height may be expected?
The air undergoes an expansion process ( p1 + p0 ).V1
n = ( p2 + p0 ).V2
n where p0 is atmos-
pheric pressure and other pressures are gauge.
The constraints are :
- V1 (the reservoir initial volume) ≤ (0.4 m)3
- p1 ≤ 00 kPa
- V2 = V1 + HA
- p2 ≥ Wcan/A.
Assuming an adiabatic process ( n = γ ), this may be solved to obtain an idea of the height H
achievable - the feasibility of the device may thus be assessed.
Once a device has been built, a mathematical model can be a useful aid to understanding any unex-
pected behaviour which testing uncovers - eg. a vehicle will not start, or it flips over and kicks its
wheels in the air, or, if propellor driven, it rotates wildly while the propellor remains stationary, and
so on! Real life Research and Development entails the testing of physical models when available
mathematical models lack realism. Clearly the sophistication of any modelling, whether it be mathe-
matical or physical, must be in keeping with the importance and sophistication of the project.
Don't get carried away by mathematical or computer modelling, remember that a mathematical
model is a means to an end, not an end in itself.
An extremely important task in the practicalisation exercise is to thwart Murphy by foreseeing all
his worst tricks and sabotaging them. Again this requires effort on the part of the designer to visual-
ise each life stage step by step, and to ask questions about what could go wrongwith each. Thus in
the case of the lobed stair climber "What would be the effect on climber operation if the stairs were imper-
fect?", eg. stairs provided with an anti-slip bead, or built with a tolerance of ± 5 mm on tread dimen-
sions, or covered with a fluffy carpet, or copiously treated with slippery polish by the cleaner, and
so on. Or again, "If overall stability requires two identical lobed wheels on a single driving axle, what would
happen if the axle became misaligned to the stair treads?"
What could go wrong with the lazy tongs, candidate #12 of the can lifter? Elas-
tic bands would probably be used instead of the linear springs envisaged in the
mathematical model; the bands would introduce severe non-linear and hyster-
itic behaviour which would render useless any quantitative deductions from
the model - though qualitative findings would still be very useful.
If the tongs' struts were not 'identical' then they could bind, causing unexpected
friction in the mechanism. Manufacture of the struts by drilling the three holes in each through an
accurately pre-drilled jig would ensure adequate dimensional similarity.
Or, "Murphy would try to tip over the extended can lifter - what steps would minimise overturning tenden-
cies?" This might lead to practicalising ideas such as :
- a heavy wide base together with light struts to impart overall stability
- a speed retarder to slow the ascent and minimise dynamic effects
- close fits on all rotating joints to avoid excessive play in the mechanism
- use of lubricant to minimise friction together with the correct disposition of springs to improve
uniformity of forces internal to the device
- the possible use of cords within the mechanism to ensure ascent synchronism of the two paral-
2
A
H
1
V1
deflection
force