More advanced considerations

When designing for manufacture (or, better, for the manufacturER ) . . . .

• Specify components to be made as roughly as possible, consistent with their ability to perform their duty.
  Thus, if you estimate that a certain 25 mm dia shaft can work satisfactorily with a tolerance of +/- 0.1 mm (ie. any diameter between 24.9 and 25.1 mm is workable) then a tolerance of +/- 0.01 mm should not be specified because the increased accuracy requires
  • a more skilled operative,
  • a more sophisticated machining process,
  • a more accurate measuring device,
  • more time spent in checking size during manufacture, etc

          - all of which lead to unnecessary expense.

  The same philosophy applies to surface finish - surfaces should be as rough as possible consistent with duty, with fatigue life, with sales appeal and so on.
  Use the least skilled labour and the least sophisticated processes possible - though this outlook does not apply when mass production is contemplated.

• Maximise the use of stock sizes and existing components.
  If the size of a certain shaft to satisfy fatigue strength, lubrication and other necessities is calculated to be 23.4 mm, then the use of 25 mm dia bar stock should be considered - provided the surface finish of off-the-shelf bright stock is adequate, and the larger size does not compromise other components. The resulting 'waste' of material is usually more than offset by savings in manufacturing time.
  If a design calls for a component which is very similar to the existing part of another assemblage then you should examine the possibility of changing the design to adapt that part, perhaps modified slightly. This Lego / Meccano outlook is referred to as Value Analysis - a tool for reducing machining and the variety of parts which must be held in stock for later assembly and/or spares. Stock represents an investment which is just sitting there, requiring interest to be paid on the investment, and not earning profits.

• Pay attention to details.
  If you are not aware of what these could be, find out. For example, the steps in casting a flat belt pulley are essentially as follows. A wooden or plastic pattern is surrounded by a flask as shown in cross-section, Fig A below. This is filled with moulding sand which is consolidated by ramming before the lid is fitted, Fig B. The flask is then inverted and the pattern carefully removed, Fig C, to form the lower part of the mould cavity - the drag (in practice, a separate core would probably be used for the central hole). The cope or top portion of the mould, whose sand has a plane parting surface here, is made separately and mounted on the drag. Holes for the flow of molten metal are cut in the cope, Fig D. Metal is poured into the cavity through the feeder until seen in the riser after which it is allowed to solidify. The casting is removed when solid, Fig E, and the sprues cut off prior to final machining in way of adjacent components - the shaft and the belt, Fig F.
  A designer who was not conversant with this process might have specified the design of Fig G which will be more expensive as it has the following disadvantages :
  • there is no draft which allows pattern withdrawl from the sand without damage, Fig C
  • the thin web solidifies first due to heat transfer, and will be torn by massive sections which solidify and contract later, unless special conductors are inserted; this tendency is exacerbated by large blending radii here
  • the non-flat parting surface requires awkward consolidation of sand around the pattern in the cope, rather than easily on a flat surface, Fig D

  . . . . and so on - there are many texts devoted to the details of casting design.

  Students were asked to design a sheet-metal domestic letterbox to be made in reasonable quantities by a jobbing shop, using conventional manual techniques - mass production processes, such as heavy presswork, were not available. A typical design, involving slanting sides and a curved top for aesthetics and a hinged rear panel for access, is illustrated. You should criticise this design from the point of view of the manufacturer and users. How many different shortcomings can you identify ?

When designing for sale. . . .

• You should refer again to Seymour'sarticle on Competition Analysis and put yourself in the buyer's shoes. The economics in particular must be attractive. Remember always the ubiquity of the cost equation :
                  total costs = first costs + running costs
                          in which costs are typically discounted cash flows.
  First costs include interest accrued by borrowing for the initial purchase, or loss of potential earning power if existing capital is used, costs associated with the purchase (special foundations or buildings), and so on. Running costs include fuel, labour, outage due to breakdown, cost of maintenance, etc.
  Although the assignation of insurance, depreciation and the like - either to 'first' costs or to 'running' costs - is essentially an accounting exercise, the implications of these costs must not be forgotten at the design stage. Minimisation of total costs thus requires design optimisation in all areas - optimum 'design for manufacture' for example, primarily affects first costs, whereas optimum 'design for use' reflects principally in running costs.

  Subtle interplay between cost components often arises - for example better 'quality of manufacture' may increase first costs but decrease running costs through improved and/or more reliable performance. Suppose for example a buyer requires a water heater for a certain application, and the choice lies between gas-fired and solar heaters. Knowing the initial prices of various sizes of each, the price of gas, the expected solar insolation, and making some allowance for foreseeable maintenance etc, the buyer can build up a picture as shown here. The solar heater will come into its own in the larger sizes as the 'fuel' costs are negligible. From this picture buyers can determine that, if their installation is below the cross-over size then they should select a gas-fired heater, otherwise they should select a solar. Buyers will try to assess what they are buying before parting with hard earned cash, so designers must weigh up carefully what they are trying to sell and how buyers will view it.

  A further example of economic analysis is shown; it concerns a range of automatic machine tools for mass production. As the models become more sophisticated the initial price rises but the time spent in machining a given batch of components decreases, leading to a reduction in running costs. The total costs for machining a particular part exhibit a clear minimum, so the model corresponding to this degree of sophistication should be bought to machine that part - though other non-economic arguments would have to be considered of course.
  Further examples of cost curves similar to these abound. Consider the choice of a heat exchanger. If the exchanger is small, so is its first cost. So also is its thermodynamic effectiveness - which leads to high running costs. Conversely a large exchanger implies a high first cost with lesser running costs. So the above graph applies again with exchanger size as the abscissa.

  The larger the machine or item of plant, the more efficient is it likely to be
  - this is a general truism - that's why gas turbines, though acceptable in large sizes for airplanes, have not caught on for automotive applications - the efficiency in the smaller sizes is unacceptably low.

  Another general trend is illustrated here. The size or capacity of a machine is usually indicated by a figure which represents the rated continuous load which the machine can deliver or handle - thus a '150 kW' engine means that the engine can deliver a steady 150 kW continuously. This is also known as 'full load'.
  The efficiency (or effectiveness) of a machine increases fairly steadily as the load on the machine is increased, until a maximum efficiency is reached at or close to the rated load. Further load increase causes a sudden drop in efficiency as shown.
  It is rare for the load on a machine to remain constant, so although a designer might choose a machine on the basis of its rating (a single figure) the load cycle should be understood clearly so that the effects of part- and over-load operation can be quantified.

  A final instance of economic reasoning concerns the supply of power to the electricity grid by a number of steam-powered turbo-alternators, some of which are large, some old, some out of service for (hopefully routine) maintenance, and so on. A major problem is in deciding which turbo-alternators should be generating at any one time for minimum overall running costs, given the expected daily variation of demand in the grid as shown. The newest, largest and most efficient turbo-generators are driven constantly at their rated load, supplying the grid's base load. The load on the smaller less efficient machines varies, as dictated by demand peaks. Other machines are on hot standby, ready for unexpected demand or breakdown of other generators.

• Do not ignore the contribution which industrial design can make towards aesthetics and sales appeal. For example, the functional design of a drill press, Fig A below, is not in tune with modern fashion which accents simplicity in form (the expensive computer lookalike). A sheet metal cowl, Fig B, is therefore added to give an uncluttered illusion whilst still allowing adequate motor ventilation and permitting the belt to be easily swapped between pulleys to alter the drilling speed. This modification also enhances the design's safety by reducing the chance of fingers being accidentally trapped in the moving belt. (Consideration of industrial safety is sometimes regarded as the prerogative of industrial designers, but safety is so vital that everyone must be involved, not just a select few.) Further styling might conclude that the cowl makes the drill seem top-heavy, so the base is enlarged and rendered in a contrasting colour to appeal through an enhanced impression of stability, Fig C.

When designing for use. . . .

Remember that your design will not act in isolation, but in concert with other   MEN, MATERIALS & MACHINES- plan accordingly.
Other considerations include :

• The required technical specifications must be met with confidence - the solution must be practical in other words. This calls for detailed application of theory from other subject areas (Dynamics, Thermodynamics etc).

• One factor which is often overlooked is the   inherent variabilityof supposedly constant data given to the designer as part of the problem statement.
  The variation may be significant, or it may not, but the designer has got to know one way or the other. Students who designed a stair climber often ignored variations in tread height from one step to the next. In many cases this did not matter because the climbers' modes of propulsion were insensitive to this parameter - the students were lucky. But if they had used a Sherpa-type of device then variations in nominally constant dimensions would have had a profound effect on operation.

  Typical occurrences of non-constant loading include :

  • Fatigue loading of components : for example, the bending moment on a rotating shaft may be constant, but the stress on an element of the shaft must vary from tensile to compressive.
  • Busy road bridges are provided with peak lanes (analogous to the electricity grid above).
  • Manufacturing and mineral processing plants have stockpilesbetween the various operations to cater for differing operating rates and outage of individual machines.
  These examples are widely recognised and, where relevant, may be analysed by the techniques of operations research - the name given to the branch of mathematics which deals with industrial problems - queuing, routing and sequencing, minimising and optimising, learning and the like (Bibliography). But mathematical models are no substitute for common sense.

  Other examples may not be so obvious; therefore it is important to plan for untoward variations. Is there any chance of localised buckling, of vibration resonance, of imperfection interaction, or of foundation settlement ? What are the consequences of applying elastic theory when the material behaves plastically, and so on ? Altering the position of a heat exchanger baffle led to the establishment of an extremely intense local vortex, which went undetected until high velocity liquid (like a cyclone) had eroded its way right through the 50 mm thick steel cover. Designers must visualise fluid and stress flows; designers have to be subtle. Again - design is not merely a matter of applying a few formulae.

  A certain mineral processing plant is operating at just below its rated throughput when a strike occurs, halting production in all operations. On resolution of the labour difficulties, management - which has little empathy with things engineering - decrees that all processing machines will operate flat out until production has caught up with the backlog. The result ? Breakdown of machines. No.2 crusher is the first to wilt under the strain of being asked to do more than it was designed to do. Stockpiles may allow the rest of the plant to continue to operate for a limited period while this machine is repaired, as suggested by the sketch, but further breakdowns can obviously be expected.
  The moral here ? This scenario should have been foreseen by the designer(s). Stockpiles offer little help here; the only hope of protecting the machines from abuse lies in education. In other words, users must be provided with sufficient information to allow them to predict the results of their actions. If users want the ability to increase production, then they must be prepared to buy larger items of plant. And this brings us on to the next point . . . .

• The life of a component, machine or item of plant depends upon its load -
  the greater the load, the shorter the life.
  A commonly accepted approximation is
              life * (load)**index = constant
                    in which the positive index also is constant, both constants being characteristics of the machine under consideration. Typical indicative values of the index are 3 for ball bearings, 7 for ore-hauling railways, and 12 for elastomeric V-belts. The magnitude of life reductions caused by increased loads, in view of these large indices, can readily be appreciated. The characteristic constants can often be predicted from fatigue or crack propagation analyses (based on material properties from tests) or from maintenance records.

• Safetyis at all times of paramount importance.
  Failure of hardware, through fracture or buckling for example, must be prevented by correct detail design along the lines of the following chapters. Designers must be satisfied that any theory which they apply is in fact applicable to the case in hand, and that no variations in loading for example render the theory irrelevant. Liberty ships operated satisfactorily in WW2, as did Comet aircraft in the postwar years - both designed by the elastic theory which we still use today. But they still fell to bits (literally) for no reason which was understood at the time. Elastic theory has since been shown to be inadequate by itself for certain combinations of manufacture, materials, environment and load (see the later chapter on fracture mechanics).

  Avoidance of potential danger to life and limb is more significant than hardware breakdown. Damages for injury commonly reach six figures, and the courts seem to accept little frailty on the part of designers, who can be found liable for compensation. Guards around dangerous machines are usually specified by Codes and are therefore mandatory. But they may not be enough. People must be guarded against themselves, since they can be bloody stupid- and this adjective is used advisedly. Loss of fingers resulted from circular saw guards being deliberately immobilised in a railway woodworking shop.
  You have to foresee all possible interactions between your design and people, rational and idiotic, and plan accordingly. Peoples' safety awareness must be fostered.

• Ergonomicsis the study of the human/machine interface (Bibliography) with a view to optimising the effectiveness of both human and machine. A typical application is the provision of a working environment for a bulldozer driver who, for maximum effectiveness, should :
  • be located in a cabin to minimise fatigue from airborne noise, dust and heat
  • sit on a sprung cushioned seat to minimise vibration and shock transmission
  • have excellent all-round vision for tractor manuevering and bladework
  • have all controls - for blade height, angle etc - adjacent to fingertips

  . . and so on.

  These requirements would be obvious to any designer who cared to observe what the average bulldozer driver experiences and does from one second to the next. Unfortunately, ergonomics often plays second fiddle to economics in such instances. The fact that it is probably false economy is often not recognised - not only are the delayed effect of prolonged exposure to a harmful environment legally actionable (eg. mesothelioma), but enhanced comfort often leads to improved efficiency - ie. decreased running costs despite increased first cost.

When designing for maintenance. . . .

• Again, it is a case of imagining yourself at the coalface. If a repairer's fingers are continually being bruised through lack of room to wield a spanner, then the resulting frequent trips to first aid will earn the overseer's wrath and lead to an determination not to order another design from the same stable.

• Maintenance is rather like safety - the designer's task is to foster an awareness of the need and to make things as easy as possible, not just for carrying out maintenance, but also for monitoring to reveal the need for maintenance - testabilityand maintainabilityare mandatory requirements of most defence equipment for example.
  If continuous monitoring of performance is impractical, then intermittent monitoring at least should be carried out to expose incipient malfunction. Preventative maintenance can then be planned and executed as with the turbo-generators mentioned above. This approach is much superior to waiting until something actually breaks down before fixing it. Obviously, the more critical a machine in the overall scheme of things, the closer must be the watch over it. Such thinking leads to the provision, as a matter of course, of thermometer pockets in heat exchanger pipework to check for the build-up of deposits over a period of time. Even little exchangers are often so equipped, since their failure could trigger a disastrous chain reaction - though if they are a vital link then they are usually replicated.

• Providing spare sub-assemblies as opposed to individual spare components may be an attractive proposition.
  For example the water pump on a car engine usually fails because the seal wears, allowing water to leak past it to corrode a shaft bearing, which then seizes. Depending on the manufacturer, it is often cheaper to replace the complete pump rather than just the bearing and seal, because
  • the level of skill required to bolt on the complete replacement pump is much less than that needed to dismantle the pump and rebuild it with new components
  • if one component has reached the end of its useful life then other components are probably not far behind
  • the turnaround time is much less.

  Extending this concept, the designer should investigate the advisability of the manufacturer or his specialist agent assuming the responsibility for all maintenance. This becomes a matter of necessity rather than choice when the equipment becomes too sophisticated for the user to repair, as with computers and domestic refrigerators.
  The provision of complete reconditioned machines is another option along the same lines. Throw-away razors and throw-away cigarette lighters are evidently commercially feasible. . . . what next?

When designing for retiral. . . .

• Open cut mining operations in environmentally sensitive areas usually begin with removing and storing the topsoil for eventual replacement and revegetation when mining has ceased. The carcases of other industries may not be so easily retired - nuclear power stations and offshore oil rigs are cases in point.

• There is probably not much general advice to give here, except once again, to think of the retiral phase, to think of those who must effect it. There are many instances of successful ventures based on recycling retired products - car tyres transformed into non-slip sports surfaces, marine life nuclei and so on.

  The disposal of waste products in an acceptable fashion must be solved during the design of processing plant. A fossil fuelled power station for example excretes large quantities of hot water, gases and dust, whose effective dispersal does not come cheaply.

It is realised that you may not understand all the ramifications of this section, but the arguments should have made you aware that the art of engineering design is not merely a series of calculations. You must plan ahead.

Conclusions

Well, that's it ! You should now have the fundamentals of designing under your belt. But recall that knowing the fundamentals of anything is pretty useless unless you can apply them advantageously.
Design is no different - you've got to practice application of the fundamentals, particularly when the problem doesn't seem to require any design process, rudimentary or otherwise.

Remember always that when you've got to solve a real life problem, it's the optimumsolution that you're looking for. Good luck in your search !