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 through which an embryo candidate solution (in all probability described only by a very hazy sketch or a few keywords) 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 practicalisation for a particular candidate is that :

• either the candidate is practical (ie. complies with all constraints)
• or the candidate is not practical (ie. cannot meet every constraint)
  . . . . . . there are no half measures.

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 solution. 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).

One example of problems leading to other problems involves the primary problem "to design a new factory 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 problem - 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 solution. 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 80mm, length of thread 25mm, 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 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 candidate in detail. You will need to know something about materials and how you yourself can fashion 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 concerned with 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.

 

Would you have trashed candidate #5 because suitable nesting tubes were unavailable in your local hardware store ? Or did you come up with other secondary solutions?

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 . . . .

 

EXAMPLE         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, compared 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, and with D&B devices this is usually best done by direct experimentation. The investment of time and effort in experimentation 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 candidate 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 artefact is completed, in most cases it is possible to quickly and cheaply experiment with certain components 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 possible, 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 delivered 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 and so on?'.
Concentrating here on the pick-up phase, there are many possible candidate solutions - rotating/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 integrated with other sub-systems such as transportation?
All these sorts of questions could have been answered definitely, and all the necessary modifications 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 could not 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 practicalisation, 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 justification, 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 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 experimentation 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 students' 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 below 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 below. The expression relating the load's acceleration a to the arm's inclination q 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 elevated through the distance h by means of a cord pulled by tension F through the distance s.

 

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 displacement s is small, then a correspondingly large tension F is required in the cord, with consequent implications on bar buckling etc.

 

So, what are the kinematics? They are examined in the box below for a single bar of the device. This demonstrates that the cord displacement s increases in proportion to the bar inclination q, 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. 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 travelled 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 diameter, so a small drum resulted also in a small accelerating force. This force was so swamped by friction (which students hadn't allowed for) that there was no acceleration whatsoever. The vehicle refused to budge, causing much embarassment - or hilarity, depending on the point of view !

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 inefficient 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.

Mathematical models are not restricted to mechanics, as the following demonstrates.

 

EXAMPLE         Some effort has been devoted already towards practicalising manufacture of the telescoping tubes, candidate #5 of the can raising device. The device is conceived as an air reservoir 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+p_o).V₁ⁿ = (p₂+p_o).V₂ⁿ where p_o is atmospheric pressure and other pressures are gauge.
The constraints are :
                  V₁ (the reservoir initial volume) ≤ (0.4 m)³
                  p₁ ≤ 100 kPa
                  V₂ = V₁ + HA
                  p₂ ≥ W_can/A

Assuming an adiabatic process ( n = g ), 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 unexpected 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 mathematical 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 visualise each life stage step by step, and to ask questions about what could go wrong with each. Thus in the case of the lobed stair climber 'What would be the effect on climber operation if the stairs were imperfect?', eg. stairs provided with an anti-slip bead, or built with a tolerance of +/- 5 mm on tread dimensions, 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? Elastic bands would probably be used instead of the linear springs envisaged in the mathematical model; the bands would introduce severe non-linear and hysteritic 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 tendencies?' 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 parallel 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 QUESTIONS.

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 attention 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 practicability 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 candidate 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 ultimate practicality or uselessness of each candidate is known with some certainty. Failure to do so is a 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.