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Computer-aided design: use and misuse
G. Pitts
Computer-aided design: use and misuse The computer is a powerful aid to the designer and has made possible engineering design which would have been considered impossible only a decade or so ago. However it has not changed the design process but enhanced it in many ways. The author believes that the skill in using the computer as a design tool stems from knowing where it fits into the design procedure, and in this paper shows that the designer must design and not let his principal tool drive him to the wrong end.
In the last decade a considerable effort has been put into the development of software in order to refine and expedite design. In certain fields such as structural analysis and the design of electronic circuitry, complete packages have been developed to provide the designer with a ready means of rapidly evaluating possible alternatives. Unfortunately the development of software in other areas such as mechanical engineering has tended to be piecemeal in nature. This results from the general diversity of the topic and the difficulty of analytically predicting the overall performance of a mechanical system in terms of the performance of its individual components. The point is probably best illustrated by taking the particular subject of tribology (bearings and lubrication), at first sight this appears to be a relatively specific and well-defined area, but on closer inspection one finds that the task of producing a designer's software package becomes formidable. We have on one end of the scale the maker's catalogue of rolling element bearings, which at the present time is probably the best way of presenting this information to the designer, and at the other end, complex fluid film bearings requiring several hours of computation to specify their characteristics. The difficulty arises in the designer being frequently faced with the problem of evaluating between the choice of component available from a manufacturer's catalogue and a component which he himself must design, using either the available software or his slide rule; all too frequently the choice is made in favour of the catalogue, not because it offers the better solution, but because it is the one requiring the least personal risk and involvement. We must also keep the situation in perspective, until now we have only discussed a single element in the system, and although this element may be of prime importance, it is only one of many going to make up the complete unit, each one being assessed by a mixture of experience, analysis, intuition and component manufacturers' claims, etc. Many designers keep well clear of computation since they are not quite sure when, where, and how to use it. On the other hand some designers have jumped in at the SUMMER 1970
deep end, without first considering the full design implications, only to emerge with burnt fingers, and to be ridiculed by the more conservative of their colleagues. The answer comes in a careful appraisal of the design process, and an awareness of where the computer may be usefully fitted into the process structure.
The Design Process Figure ] illustrates diagramatically the steps which are carried out to produce a satisfactory design. On a large design project some of the steps become obvious as they may be handled by separate departments, but even at the detail design stage the good designer is subconsciously aware of this procedure. Before assessing the role of a computer in design, let us first consider the elements which make up the design process. Initially, we have the requirement which may take many forms: it may be generated within the sales department of the manufacturer being a result of a close knowledge of the market potential, as would be the case with domestic products such as vacuum cleaners and refrigerators, or it may come from an outside organisation, for instance an airline or a shipping company putting forward
Requirement ] Speclflcotlon ]
t
[
Alternative solutions
]
I
E,a,uat,oo
I
t t Detail design & manufacture
J
I s,nt"e '' .I
Fig. 1.
41
a requirement to be met by the manufacturer, or, at the other end of the scale, it m a y , ~ e the culmination of earlier design considerations~ for example, an engine mounting would result from the decision to have an engine of a particular form mounted upon a specified chassis. The requirement in itself is not usually very specific, it essentially recognises a need, and at the same time it may contain redundancies unnecessary to the need. Requirements frequently carry values, which at first sight would seem more appropriate to the specification, such values should be treated with caution, they may be given by the customer without a careful consideration of the implications, and warrant further investigation, or by improving upon the quoted values the designer can increase the market potential. A specification results from quantifying the requirement and recognising any redundancies. A typical requirement might be for a centrifugal pump to clean out oil tanks with a sea water hose; a flow rate considered suitable by the customer being given. In drawing up the specification from this requirement, the mention of centrifugal would be an unnecessary restriction, this does not preclude the use of a centrifugal pump, but permits the consideration of other alternatives, this is therefore a redundancy in the requirement. The flow rate may be completely unrealistic; the rate in the specification should result from the availability of water, the time allowed for cleaning, the amount of oil left in the tank, and the acceptable proportion of oil to water required in the cleaning operation. The numerical values appearing in such a specification would include, head, flow rates, acceptable size, acceptable weight, etc., and the written specification would involve specifying the compatibility of the materials with sea water, the materials of the outer casing with oil, the permissible fire risk and the acceptable frequency of servicing, etc. A poor specification can result in wasted design effort and an expensive and badly designed product. It is usually the source of an unreliable product, an over-designed and as a consequence, expensive product. The third and fourth stages in the design process present the greatest challenge to the designer. The third stage involves the generating of alternative means of meeting the specification. The greatest difficulty is the overcoming of pre-conceived ideas. In this respect it is often easier to work on a revolutionary new requirement rather than one resembling something already in existence, a nagging temptation to copy the traditional design arises without first finding alternatives against which to make a valid judgment. In order to generate ideas it is advantageous to have some form of human interaction. One common form is known as brain-storming, a technique widely applied in value analysis. With this technique, a small group of engineers, in a relaxed atmosphere, are encouraged to put forward any ideas, no matter how ridiculous they may seem, and at this stage are not permitted to discuss the ideas in detail, but they merely list them for future evaluation. The result, is that an idea put forward by one person triggers ideas from the others. In the fourth stage the object is to reduce the number of alternatives, not only on technical grounds, but also on a commercial and economic basis. This step will be discussed in greater depth later, whilst bringing the role of the computer into perspective. 42
Synthesis involves the engineering of the product to a realistic technical and commercial venture. The designer has now reached the stage where calculations are of a routine nature, and he becomes intent on finalising values for such quantities as stress, stiffness, temperature and geometric characteristics, in so doing however, he must be aware of generating new requirements for further detailed design work. In Fig. I detailing and manufacturing have been included under one heading. The reason for this being that a good detail draughtsman must have a sound knowledge of manufacturing methods. Far too often the detailing of a well-conceived design is given to the new inexperienced recruit, when in fact the detail design can financially make or break a product, depending on whether or not the designer has exploited to the full his knowledge of economic manufacture.
The Role of the Computer in Design We will now reconsider the design process stage by stage, discussing the computer's contribution at each step starting with the requirement. Although the formulation of the requirement may, in many cases, be outside the control of the designer himself, the computer may become a useful asset to this operation. Usually its function is as a tool for analysing statistics. A good example is in aircraft design; the airline, before formulating a requirement, has to decide the routes upon which the new aircraft will be operating. Having selected suitable routes, an analysis has to be made of the current operating conditions, to extract such information as traffic flow and the peak travel periods. This information must then be related to social habits, such as meal times, working hours, and holidays, and to natural conditions such as time zones, and seasons. Steady traffic flow suggests a greater service frequency with a smaller aircraft, which in turn demands a rapid turn around. On the other hand a peak travel period suggests the converse, but will probably not permit any appreciable increase in the turn around. An obvious question is 'how many are required?'. The immediate answer would seem to relate directly to the passenger and freight traffic, but any engineer must consider down-time, or in the case of aircraft servicing time, this will rely to a great extent on the system reliability. With these statistics to hand, the operator is then in a position to analyse trends, and to predict future traffic demands. In a small company manufacturing domestic equipment, the designer can be involved in a management decision to undertake a market survey, although it is likely that marketing consultants would be employed, the designer may be required to assist in the preparation of the questionnaire, for only he knows the likely role of his new brain-child. The drawing up of the specification is an obvious application for the computer. A large proportion of the preparation is usually taken up with the evaluation of physical conditions, e.g., part of the specification for an underwater device would involve the hydrostatic pressure at the immersion depth. Such a trivial situation would not normally involve the use of a computer in its evaluation, but supposing the device is some form of underwater survey instrument, using the hydrostatic pressure as a reference, then it may be necessary to take account of the variation in water density resulting from compressibility, aeration, the immersion of plant and animal life, involving COMPUTER AIDED DESIGN
accurate calculation and a degree of statistical sampling. Rarely do we find that we can draw up an exact specification and then proceed to the second stage of generating alternative ideas without making further modifications to the specification. Taking once again the example of an aircraft; the initial specification might include the payload, passenger capacity, speed, operating cost, overall lengths, forms of power unit, etc. The designer then considers various configurations which would meet this specification, and selects a final form. This produces a further specification to take account of the accelerations experienced by various parts of the aircraft due to changes in flight attitude, such as pitching and landing. This in turn allows the designer to specify the forces resulting from these conditions. At this juncture the specification may become an iterative process. In order to support the loads the designer has to consider alternative structures, these structures produce additional weight, which again modifies the load and makes necessary further structural considerations. Here the computer can perform a very important design function in enabling the designer to speed up the process of iteration. Because of the stringent safety precautions taken in aircraft design, and the low factors of safety involved, the designer is limited to a series of well tested structural arrangements, and in most cases software exists for calculating their performance characteristics. The computer can therefore be used in a direct comparison between alternatives. The generating of alternative ideas is not a natural bent to the digital computer, and certainly at this stage in its development the third step in the design process is best left to the human mind and brain-storming sessions. Evaluation of ideas resulting from the previous stage occurs at different levels. The initial consideration is usually a thoughtful inspection, dismissing those ideas, such as 'sky hooks" which cannot function, but leaving alternatives for which there may be some shred of hope. We now consider the remaining ideas looking at orders of magnitude. For example consider the design of a powered cave rescue hoist, required for pot-holing. The specification requires that the overall dimensions of the winch motor must be less than 250 mm, the weight being less than I0 kg and the motor to be capable of 30 kW output. Supposing the two alternatives being considered are an electric motor, powered by a generator at the surface and a hydraulic motor powered by a remote pump. An approximate calculation of the power produced by an electric motor would give some idea of its size, which would probably preclude it from this particular application, again an approximate calculation of the flow rate and pressure in the hydraulic line would give an indication as to whether the hydraulic system was feasible. It can be seen that to involve digital computation at this point, would only be to add unnecessary complication. But if however reliable software does exist, for all the alternatives under consideration, then the computer might be used to eliminate impossible solutions. We have now arrived at the point where all the remaining possibilities are technically feasible, the next step to consider is the economic feasibility of the alternatives. Theae considerations should not only include manufacturing costs, but should also take account of operation and maimenance. Economic considerations frequently eliminate more alternatives than do the technical ones. The previous step has either resulted in one final soluSUMMER 1970
tion, in which case the problem lies in engineering this to a satisfactory conclusion, or we are still left with a number of possible alternatives, between which there are only marginal differences. It is at this point where the computer becomes a very useful design tool. It enables an accurate technical assessment to be made of each alternative in a relatively short period of time, and allows performances to be balanced against economic considerations. These technical assessments can be made basically in one of two ways. If the alternatives are not directly related, then the performance of one solution may be compared manually with the performance of the others. Where the alternatives are related, then they may be optimised numerically, but even then it is necessary for the designer to inspect the optimised solution and to have the overriding authority to replace it, with what may appear mathematically to be an inferior one.
M isuse The principal short-coming in computer use from the point of view of the designer, is an action common to all aspects of design, that is to proceed directly from the requirement to the design synthesis without first formulating a sound specification and also neglecting alternative solutions to the problem. We must be aware of the pressures brought upon the designer to cause him to take this action. The most obvious pressure, is that of a designer when confronted with a requirement, having a flash of inspiration and seeing The Solution. If a similar problem has been tackled before, it is likely that the computer software used in solving it, has been suitably filed and catalogued, and stands there like a beckoning temptress offering a solution inferior to a neglected alternative. We must, however, keep the problem in perspective. If large quantities of the resulting product are required, then the preparation of the alternative software for its solution, becomes a realistic proposition, if on the other hand it is a one-off design of a minor component, the fact that software exists for one solution may rightly influence the designer to make this his choice from the alternatives. Another form of influence to which the designer is exposed, is the enthusiasm of specialist organisations. Much of the software development is carried out by very competent specialists. Although these organisations have a comprehensive knowledge of one element in the overall design, they are not in the position of the designer to see the system as a whole, i.e. they can usually be relied upon to predict quite accurately the technical feasibility of the element, but may be unaware of the economic or commercial implications. For this reason the designer must first satisfy himself, by a preliminary evaluation of the situation, that a solution appropriate to the problem lies in this field, since involvement usually leads to financial commitment, and very soon to a point of no return. It is usually sufficient in assessing such a situation to get some idea of the orders of magnitude involved, A good example is that of bearing selection. The Engineering Sciences Data Unit (Mechanical) produce a very useful data sheet which enables the designer to select the type of bearing most appropriate to his specification. This is shown in Fig. 2. If he decides that his requirements fall well within the range e r a particular fluid film bearing, then he can proceed to seek expert advice from one of the organisations specialising in this field, and as a result will be able to utilis¢ to 43
advantage any software they may have available. There are other areas of user ris~ which must be considereal. If we take the statistical [inalysis of the market, we must be careful to ensure that any questionnaire which should be compiled produces a sensible result after analysis.
One of the difficulties here, is establishing the nccd for a new product, it is clearly ridiculous to go to the prospective customer and ask him if he will buy the new product, there is no analogy here with party politics and elections. It is up to the engineer who originates the idea to project it via
Note: Diameters ore given in inches on eoch curve. \ Except for Rolling Beorin~s, curves ore drown for beorings with width equol to diometer. A medium viscosity minerol ell is ossumed for the hydrodynomic beorings.
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Frequency of Rototion (rev/min) ~m
. . . . .
Rubbing Beorir~s Oil Impreqnoted Porous Metal Beotings Rolling Bearings Hvdrodynomic Oil Film Beorir~
Fig. 2. Engineering Sciences data Sheet No. 65007 (Courtesy of the Engineering Sciences data unit) 44
COMPUTER AIDED DESIGN
the market research team to the prospective customer, and on the basis of the market analysis decide whether or not he has projected the correct product image. When variables are simply related, it is sensible to optimise the problem so far as is possible, using a standard optimisation routine, leaving the computer with the formidable task of finding the maximum or minimum value. The danger lurking here is the final correct solution. When it has been proved beyond any mathematical doubt that the solution obtained is the optimum, there seems to be very little point in checking it. But are we confident that the constraints, we so quickly expressed in mathematical terms, truly represent the relevant physical conditions ? We do not doubt the computers ability to handle the distribution problem, but can we estimate the risk offered to a fuel pipe by an operator's foot in mathematical terms, or the draught coming from a door irritating an assembler on an optimised production line? The result is that the optimum solution must be inspected carefully,
not to ensure that the computer has done its job, but to make sure that it was given the right problem to optimis¢ in the first place. If the solution is sensible, then fine, but if it is not, look at the data you thought described the problem.
In C o n c l u s i o n The computer is a powerful aid to design, it has made possible engineering feats considered ridiculous only a decade or so ago. It has not changed the design process, as some would have us believe, but in many ways it has enhanced it. The skill in using the computer as a de,sign tool, stems from knowing where it fits into design. Like most applications where the computer has found uses, it can easily swamp its user with output. So let us be sure that when we are designing, we are designing, and not being driven to the wrong end by our principal tool.
Received February 1970
Dr. G. Pitts, B.Sc.(Eng.), Ph.D., C.Eng., M.I.Mech.E., is a Lecturer in Engineering Design in the Department of Mechanical Engineering, University of Southampton. He served his apprenticeship in Production Engineering with De Havilland Aircraft becoming a jig and tool designer with that company. He has worked in the Stress Office with B.A.C., Flight Service Engineering with Seaboard World Airlines and the Advanced Products Division of the Worthington Corporation. His Ph.D. topic was steady-state characteristics of tilting-pad gas-bearings and he also acted as a consultant on this subject to the International Dragon Project, U.K.A.E.A.
SUMMER 1970
45
Computer-aided design: use and misuse The computer is a powerful aid to the designer and has made possible engineering design which would have been considered impossible only a decade or so ago. However it has not changed the design process but enhanced it in many ways. The author believes that the skill in using the computer as a design tool stems from knowing where it fits into the design procedure, and in this paper shows that the designer must design and not let his principal tool drive him to the wrong end.
In the last decade a considerable effort has been put into the development of software in order to refine and expedite design. In certain fields such as structural analysis and the design of electronic circuitry, complete packages have been developed to provide the designer with a ready means of rapidly evaluating possible alternatives. Unfortunately the development of software in other areas such as mechanical engineering has tended to be piecemeal in nature. This results from the general diversity of the topic and the difficulty of analytically predicting the overall performance of a mechanical system in terms of the performance of its individual components. The point is probably best illustrated by taking the particular subject of tribology (bearings and lubrication), at first sight this appears to be a relatively specific and well-defined area, but on closer inspection one finds that the task of producing a designer's software package becomes formidable. We have on one end of the scale the maker's catalogue of rolling element bearings, which at the present time is probably the best way of presenting this information to the designer, and at the other end, complex fluid film bearings requiring several hours of computation to specify their characteristics. The difficulty arises in the designer being frequently faced with the problem of evaluating between the choice of component available from a manufacturer's catalogue and a component which he himself must design, using either the available software or his slide rule; all too frequently the choice is made in favour of the catalogue, not because it offers the better solution, but because it is the one requiring the least personal risk and involvement. We must also keep the situation in perspective, until now we have only discussed a single element in the system, and although this element may be of prime importance, it is only one of many going to make up the complete unit, each one being assessed by a mixture of experience, analysis, intuition and component manufacturers' claims, etc. Many designers keep well clear of computation since they are not quite sure when, where, and how to use it. On the other hand some designers have jumped in at the SUMMER 1970
deep end, without first considering the full design implications, only to emerge with burnt fingers, and to be ridiculed by the more conservative of their colleagues. The answer comes in a careful appraisal of the design process, and an awareness of where the computer may be usefully fitted into the process structure.
The Design Process Figure ] illustrates diagramatically the steps which are carried out to produce a satisfactory design. On a large design project some of the steps become obvious as they may be handled by separate departments, but even at the detail design stage the good designer is subconsciously aware of this procedure. Before assessing the role of a computer in design, let us first consider the elements which make up the design process. Initially, we have the requirement which may take many forms: it may be generated within the sales department of the manufacturer being a result of a close knowledge of the market potential, as would be the case with domestic products such as vacuum cleaners and refrigerators, or it may come from an outside organisation, for instance an airline or a shipping company putting forward
Requirement ] Speclflcotlon ]
t
[
Alternative solutions
]
I
E,a,uat,oo
I
t t Detail design & manufacture
J
I s,nt"e '' .I
Fig. 1.
41
a requirement to be met by the manufacturer, or, at the other end of the scale, it m a y , ~ e the culmination of earlier design considerations~ for example, an engine mounting would result from the decision to have an engine of a particular form mounted upon a specified chassis. The requirement in itself is not usually very specific, it essentially recognises a need, and at the same time it may contain redundancies unnecessary to the need. Requirements frequently carry values, which at first sight would seem more appropriate to the specification, such values should be treated with caution, they may be given by the customer without a careful consideration of the implications, and warrant further investigation, or by improving upon the quoted values the designer can increase the market potential. A specification results from quantifying the requirement and recognising any redundancies. A typical requirement might be for a centrifugal pump to clean out oil tanks with a sea water hose; a flow rate considered suitable by the customer being given. In drawing up the specification from this requirement, the mention of centrifugal would be an unnecessary restriction, this does not preclude the use of a centrifugal pump, but permits the consideration of other alternatives, this is therefore a redundancy in the requirement. The flow rate may be completely unrealistic; the rate in the specification should result from the availability of water, the time allowed for cleaning, the amount of oil left in the tank, and the acceptable proportion of oil to water required in the cleaning operation. The numerical values appearing in such a specification would include, head, flow rates, acceptable size, acceptable weight, etc., and the written specification would involve specifying the compatibility of the materials with sea water, the materials of the outer casing with oil, the permissible fire risk and the acceptable frequency of servicing, etc. A poor specification can result in wasted design effort and an expensive and badly designed product. It is usually the source of an unreliable product, an over-designed and as a consequence, expensive product. The third and fourth stages in the design process present the greatest challenge to the designer. The third stage involves the generating of alternative means of meeting the specification. The greatest difficulty is the overcoming of pre-conceived ideas. In this respect it is often easier to work on a revolutionary new requirement rather than one resembling something already in existence, a nagging temptation to copy the traditional design arises without first finding alternatives against which to make a valid judgment. In order to generate ideas it is advantageous to have some form of human interaction. One common form is known as brain-storming, a technique widely applied in value analysis. With this technique, a small group of engineers, in a relaxed atmosphere, are encouraged to put forward any ideas, no matter how ridiculous they may seem, and at this stage are not permitted to discuss the ideas in detail, but they merely list them for future evaluation. The result, is that an idea put forward by one person triggers ideas from the others. In the fourth stage the object is to reduce the number of alternatives, not only on technical grounds, but also on a commercial and economic basis. This step will be discussed in greater depth later, whilst bringing the role of the computer into perspective. 42
Synthesis involves the engineering of the product to a realistic technical and commercial venture. The designer has now reached the stage where calculations are of a routine nature, and he becomes intent on finalising values for such quantities as stress, stiffness, temperature and geometric characteristics, in so doing however, he must be aware of generating new requirements for further detailed design work. In Fig. I detailing and manufacturing have been included under one heading. The reason for this being that a good detail draughtsman must have a sound knowledge of manufacturing methods. Far too often the detailing of a well-conceived design is given to the new inexperienced recruit, when in fact the detail design can financially make or break a product, depending on whether or not the designer has exploited to the full his knowledge of economic manufacture.
The Role of the Computer in Design We will now reconsider the design process stage by stage, discussing the computer's contribution at each step starting with the requirement. Although the formulation of the requirement may, in many cases, be outside the control of the designer himself, the computer may become a useful asset to this operation. Usually its function is as a tool for analysing statistics. A good example is in aircraft design; the airline, before formulating a requirement, has to decide the routes upon which the new aircraft will be operating. Having selected suitable routes, an analysis has to be made of the current operating conditions, to extract such information as traffic flow and the peak travel periods. This information must then be related to social habits, such as meal times, working hours, and holidays, and to natural conditions such as time zones, and seasons. Steady traffic flow suggests a greater service frequency with a smaller aircraft, which in turn demands a rapid turn around. On the other hand a peak travel period suggests the converse, but will probably not permit any appreciable increase in the turn around. An obvious question is 'how many are required?'. The immediate answer would seem to relate directly to the passenger and freight traffic, but any engineer must consider down-time, or in the case of aircraft servicing time, this will rely to a great extent on the system reliability. With these statistics to hand, the operator is then in a position to analyse trends, and to predict future traffic demands. In a small company manufacturing domestic equipment, the designer can be involved in a management decision to undertake a market survey, although it is likely that marketing consultants would be employed, the designer may be required to assist in the preparation of the questionnaire, for only he knows the likely role of his new brain-child. The drawing up of the specification is an obvious application for the computer. A large proportion of the preparation is usually taken up with the evaluation of physical conditions, e.g., part of the specification for an underwater device would involve the hydrostatic pressure at the immersion depth. Such a trivial situation would not normally involve the use of a computer in its evaluation, but supposing the device is some form of underwater survey instrument, using the hydrostatic pressure as a reference, then it may be necessary to take account of the variation in water density resulting from compressibility, aeration, the immersion of plant and animal life, involving COMPUTER AIDED DESIGN
accurate calculation and a degree of statistical sampling. Rarely do we find that we can draw up an exact specification and then proceed to the second stage of generating alternative ideas without making further modifications to the specification. Taking once again the example of an aircraft; the initial specification might include the payload, passenger capacity, speed, operating cost, overall lengths, forms of power unit, etc. The designer then considers various configurations which would meet this specification, and selects a final form. This produces a further specification to take account of the accelerations experienced by various parts of the aircraft due to changes in flight attitude, such as pitching and landing. This in turn allows the designer to specify the forces resulting from these conditions. At this juncture the specification may become an iterative process. In order to support the loads the designer has to consider alternative structures, these structures produce additional weight, which again modifies the load and makes necessary further structural considerations. Here the computer can perform a very important design function in enabling the designer to speed up the process of iteration. Because of the stringent safety precautions taken in aircraft design, and the low factors of safety involved, the designer is limited to a series of well tested structural arrangements, and in most cases software exists for calculating their performance characteristics. The computer can therefore be used in a direct comparison between alternatives. The generating of alternative ideas is not a natural bent to the digital computer, and certainly at this stage in its development the third step in the design process is best left to the human mind and brain-storming sessions. Evaluation of ideas resulting from the previous stage occurs at different levels. The initial consideration is usually a thoughtful inspection, dismissing those ideas, such as 'sky hooks" which cannot function, but leaving alternatives for which there may be some shred of hope. We now consider the remaining ideas looking at orders of magnitude. For example consider the design of a powered cave rescue hoist, required for pot-holing. The specification requires that the overall dimensions of the winch motor must be less than 250 mm, the weight being less than I0 kg and the motor to be capable of 30 kW output. Supposing the two alternatives being considered are an electric motor, powered by a generator at the surface and a hydraulic motor powered by a remote pump. An approximate calculation of the power produced by an electric motor would give some idea of its size, which would probably preclude it from this particular application, again an approximate calculation of the flow rate and pressure in the hydraulic line would give an indication as to whether the hydraulic system was feasible. It can be seen that to involve digital computation at this point, would only be to add unnecessary complication. But if however reliable software does exist, for all the alternatives under consideration, then the computer might be used to eliminate impossible solutions. We have now arrived at the point where all the remaining possibilities are technically feasible, the next step to consider is the economic feasibility of the alternatives. Theae considerations should not only include manufacturing costs, but should also take account of operation and maimenance. Economic considerations frequently eliminate more alternatives than do the technical ones. The previous step has either resulted in one final soluSUMMER 1970
tion, in which case the problem lies in engineering this to a satisfactory conclusion, or we are still left with a number of possible alternatives, between which there are only marginal differences. It is at this point where the computer becomes a very useful design tool. It enables an accurate technical assessment to be made of each alternative in a relatively short period of time, and allows performances to be balanced against economic considerations. These technical assessments can be made basically in one of two ways. If the alternatives are not directly related, then the performance of one solution may be compared manually with the performance of the others. Where the alternatives are related, then they may be optimised numerically, but even then it is necessary for the designer to inspect the optimised solution and to have the overriding authority to replace it, with what may appear mathematically to be an inferior one.
M isuse The principal short-coming in computer use from the point of view of the designer, is an action common to all aspects of design, that is to proceed directly from the requirement to the design synthesis without first formulating a sound specification and also neglecting alternative solutions to the problem. We must be aware of the pressures brought upon the designer to cause him to take this action. The most obvious pressure, is that of a designer when confronted with a requirement, having a flash of inspiration and seeing The Solution. If a similar problem has been tackled before, it is likely that the computer software used in solving it, has been suitably filed and catalogued, and stands there like a beckoning temptress offering a solution inferior to a neglected alternative. We must, however, keep the problem in perspective. If large quantities of the resulting product are required, then the preparation of the alternative software for its solution, becomes a realistic proposition, if on the other hand it is a one-off design of a minor component, the fact that software exists for one solution may rightly influence the designer to make this his choice from the alternatives. Another form of influence to which the designer is exposed, is the enthusiasm of specialist organisations. Much of the software development is carried out by very competent specialists. Although these organisations have a comprehensive knowledge of one element in the overall design, they are not in the position of the designer to see the system as a whole, i.e. they can usually be relied upon to predict quite accurately the technical feasibility of the element, but may be unaware of the economic or commercial implications. For this reason the designer must first satisfy himself, by a preliminary evaluation of the situation, that a solution appropriate to the problem lies in this field, since involvement usually leads to financial commitment, and very soon to a point of no return. It is usually sufficient in assessing such a situation to get some idea of the orders of magnitude involved, A good example is that of bearing selection. The Engineering Sciences Data Unit (Mechanical) produce a very useful data sheet which enables the designer to select the type of bearing most appropriate to his specification. This is shown in Fig. 2. If he decides that his requirements fall well within the range e r a particular fluid film bearing, then he can proceed to seek expert advice from one of the organisations specialising in this field, and as a result will be able to utilis¢ to 43
advantage any software they may have available. There are other areas of user ris~ which must be considereal. If we take the statistical [inalysis of the market, we must be careful to ensure that any questionnaire which should be compiled produces a sensible result after analysis.
One of the difficulties here, is establishing the nccd for a new product, it is clearly ridiculous to go to the prospective customer and ask him if he will buy the new product, there is no analogy here with party politics and elections. It is up to the engineer who originates the idea to project it via
Note: Diameters ore given in inches on eoch curve. \ Except for Rolling Beorin~s, curves ore drown for beorings with width equol to diometer. A medium viscosity minerol ell is ossumed for the hydrodynomic beorings.
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Frequency of Rototion (rev/min) ~m
. . . . .
Rubbing Beorir~s Oil Impreqnoted Porous Metal Beotings Rolling Bearings Hvdrodynomic Oil Film Beorir~
Fig. 2. Engineering Sciences data Sheet No. 65007 (Courtesy of the Engineering Sciences data unit) 44
COMPUTER AIDED DESIGN
the market research team to the prospective customer, and on the basis of the market analysis decide whether or not he has projected the correct product image. When variables are simply related, it is sensible to optimise the problem so far as is possible, using a standard optimisation routine, leaving the computer with the formidable task of finding the maximum or minimum value. The danger lurking here is the final correct solution. When it has been proved beyond any mathematical doubt that the solution obtained is the optimum, there seems to be very little point in checking it. But are we confident that the constraints, we so quickly expressed in mathematical terms, truly represent the relevant physical conditions ? We do not doubt the computers ability to handle the distribution problem, but can we estimate the risk offered to a fuel pipe by an operator's foot in mathematical terms, or the draught coming from a door irritating an assembler on an optimised production line? The result is that the optimum solution must be inspected carefully,
not to ensure that the computer has done its job, but to make sure that it was given the right problem to optimis¢ in the first place. If the solution is sensible, then fine, but if it is not, look at the data you thought described the problem.
In C o n c l u s i o n The computer is a powerful aid to design, it has made possible engineering feats considered ridiculous only a decade or so ago. It has not changed the design process, as some would have us believe, but in many ways it has enhanced it. The skill in using the computer as a de,sign tool, stems from knowing where it fits into design. Like most applications where the computer has found uses, it can easily swamp its user with output. So let us be sure that when we are designing, we are designing, and not being driven to the wrong end by our principal tool.
Received February 1970
Dr. G. Pitts, B.Sc.(Eng.), Ph.D., C.Eng., M.I.Mech.E., is a Lecturer in Engineering Design in the Department of Mechanical Engineering, University of Southampton. He served his apprenticeship in Production Engineering with De Havilland Aircraft becoming a jig and tool designer with that company. He has worked in the Stress Office with B.A.C., Flight Service Engineering with Seaboard World Airlines and the Advanced Products Division of the Worthington Corporation. His Ph.D. topic was steady-state characteristics of tilting-pad gas-bearings and he also acted as a consultant on this subject to the International Dragon Project, U.K.A.E.A.
SUMMER 1970
45