"Light Design" in Commercial-motor Construction*
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Theory and Practice for Minimum Deadweight with Maximum Permissible Loading. Relationship Between "Light Design" and "Light-material Design' is Examined
THE account presented here is,' perhaps, opportune, in that it will assist not only, in clearing up many misconceptions already existing. with respect to the theory Of light construetion, but, furthermore, in that it will prevent further confusion arising from uncritical study of the rival claims made on behalf of newer materials.
Much data has already been published on the value of aluminium and magnesium alloys in the building of the commercial motor. According to the viewpoint taken, this may be predominantly of a technical or economic nature, or both of these aspects may be of equal importance, but it will be observed that, whilst the use of materials of low specific weight, such as light or ultra-light alloys or plastics, may be considered as favouring from the outset the achievement of minimum deadweight, this end may in theory at least, also be reached with heavier materials via the route of scientifically conceived "light design."
How far these two aspects are complementary and how far each may be set in opposition to the other will now be considered.
Light design in road vehicles became of real significance only with the development of devices for supplying motive power by mechanical means, . and with the evolution of refinements. in construction and operation which
this necessitated. ' ". •
For stationary. 'machinesand those having rigid foundations, low Weight is, as a rule, not of prime importance. In transport vehicles, however, weight saving must never be considered as a side issue and left to chance. In nearlyevery case, the results of reduction in weight on running' costs are, found to balance the increasecrekpenies incurred in light design.
"Light Design" Defined
The expression "light' design " irritates that the materials of construction employed in the assembly under discussion are, at every point, stressed to the maximum possible limit; thus in order that, in the complete assembly,every part shall be stressed to the maximum, service conditions must be accurately determined, and, furtherrnore, the structuil itself must be based on accurate calculation; the more accurate the calculations, the more fully May the material employed be exploited, hence the greater the saving which may be effected. •
Where weight reduction is obtained by simplification, such as the omission of every comfort and convenience in a vehicle, there is no justification for labelling the resultant product as an example of light design. Even the use of light metals as constructional_ materials does not necessarily imply light design, but, rather, " lightmaterial" design. The fullest realization of the concept "light design " implies that every part which goes to make up the complete vehicle corresponds in itself to theterm. For .example, if a unit has been developed approximating to a supporting shell structure, then clearly internal attachments, weighing perhaps two or three times as much as framework and skin together,, must not be fitted without the strongest justification. _ The treatment of the entire assembly as a component or a unit leads to the ideas of structural strength and light shape.
Light design in vehicle construction can be considered ir terms of chassis and bodywork. Both of these units are fundamentally self-suppbrting. So far as machine-shop practice is concerned, the design of the framework presents the easier problem; minimum weight is readily obtained by the use of special forms 'of structural elements and their connection to 'girders, . cross-members and the like. In stressed-skin structures a skin of tllin sheet metal is stiffened so that it becomes suitable for withstanding the application of specific internal and external forces. The special forming of the structural elements and the adequate strengthening of the skin both demand a knowledge of the '-' tricks " of light design.. There is, for instance, the intentional and logical use of hollow sections in the forth of box girders; the superiority of the Closed section as compared with the open section is well known. In the case of longitudinal members designed .to resist torsion, these need not necessarily lie in one plane.
A further criterion of light design is that .the stress capacity of each point in the assembly shall not only be, high, but also shall be uniform, hence sudden changes in cross section of structural members are to be avoided. Again, the use of sheet metal for the transmission of shear stresses is a characteristic' of light design. Sheet-metal girders stressed in shear represent girders, the height of which, in relationship to the thickness of the metal used, is very great. A sheet-metal strip of the -width b and thickness s can be stressed in shear without collapsing, only up to a limited ratio of b to s. The shear force (T) is given by the expression T c
In the case of avional, for a shear stress of 14 kg./sq. mm„ the critical -value of bls is about SO according to BollenrathGroseck, thus an avional web 2 mm. in thickness cannot be higher than 100 mm. if. the ratio b 1 s = 100, le., for a height of 200 mm., the critical stress which causes collapse of the web falls to about 3 kg./sq. nam. If the web be made still higher no practical use can be made of the material at all.
These data are in conformity with the collapse of compressed angular bars with relatively thin flanges, and denionstrate the low degree of support given by smooth-sided wall sheets, hide. pendent of the method employed to fix them.
Stressed-skin Structures
In .'practice. stressed-skin design implies that both tensile and shear loads must be carried,and supporting structures are so shaped that full utilization is made of the coinplete-skin apart from narrow fields or webs; Nevertheless, vertical and horizontal strengthening is requirbd here as in the case of simple
thin-webbed beams. -• •
In the shear-stressed sheet-metal girder, or in stressed-skin structures; it calcitic:A be said that full ,utilization is yet made; in general, Of the sheet metal itself. According to Wagner, metal in the form of very thin sheet is so highly stressed in shear that it exhibits what are called tension-field:diagonals; which strengthen 'the structure and are themselves Capable of carrying forces. -In aircraft design the counterpart of 'Wagner's thin 'sheet theory is realized in the framework of Wallis. With geodetically arranged interloCking tension and cdinpression members. •
Light metals, in the forth' Of extruded sections.introduced new pOSsibility into the field Of light-design framework bY reason of the fact:that, at relatively low siniple Or complex sections of innumerable designs can be produced, enabling almost any required distribution. of mass' to beobtained. Thus the prefabricated units of light design are lattice girders, box • girders, shear. stressed (sheet) metal girders, stressedskin structures, plates and multi-purpOse sections Of various forms. • The definition "light design " implies, in itself, no limitations with respect to the structural materials used, and these do not affect the theory we have outlined. In practice, however, the use of this or that material does bring with it its own problems. Heavy metals, aluminium alloys, magnesium alloys, plastics and wood have all to
• be taken into' account. Fortunately, our knowledge of the mechanical, chemical and -physical characteristics of these materials has broadened considerably in the past few years, and we can calculate with some certainty the effect of static and dynamic loads, temperature, and of corrosive agents upon them. Thus, refinements may be effected in light design; for example, we may utilize differences in tensile and compression strength for fluctuating loads between zero and maximtim by employ-' log girders of asymmetric cross-section.
In commercial practice, factors such as price, capacity. for working, availability and ease of supply are of importance. Light design, for its complete realization, again, will often demand special service from the materials, according to the exigencies of the moment. For instance, consider the use of arc welding in the assembly of steel structures, or the need to avoid corrosion in the, case of very thin metal structures (this, at one time, led to the widespread use of thin-gauge stainless steels).
Another fundamental requirement of light design is maximum energy-storing capacity. According to the expression • S A = c E this increases inversely with the modulus of elasticity (E) and the stress (S). The low modulus of elasticity of light alloys is not always advantageous to their employment for the achieving of the maximum possibilities of light 'design.
Light design also carries with it car-tam n complications regarding jigs, fix-.
tures and tools. Requirements are called for which can only be fulfilled by appropriate specialized equipment and a personnel trained in its use. It is not easy, in one shop, to handle simultaneously box structures, shear-stressed metal-sheet girders and thin corrugated sheet-metal skins. MachineShop practice for light design in the -fullest meaning of the term presumes a capacity for handling work almost of every type, and the manipulation of sheet metal and extruded sections by bending, expanding, drawing, welding, riveting; and sO owls comprehensively envisaged.
Precision Called For -Fundamentally. as opposed, to work drawn up according to the more usual schools of thought, light design probably calls for more accurate calculation in the drawing office, higher precision in execution in the shops, and possibly the application of precision machining methods not, as a rule, called for in mass productidn as commonly understriod.
In contrast to aircraft construction, -the manufacture of transport vehicles, according to the tenets of light design, must, on the one hand, cater for highly accurate work, and, on the other, avoid the excessive costs frequently entailed in the realization of the aeroplane-designer's schemes; hence the more complete (and so more expensive) the light-design machine-shop equipment is, the higher should be the autput.in order to amortize capitaloutlay on plant.
From the mechanical and physical standpoints, material is saved; nevertheless, the final assembly costs more to the manufacturer. From the buyer's point of view, however, taking into account lower operating costs and higher scrap values, then the lighter, scientifically designed vehicle is cheaper, for load-carrying capacity for any particular design or type is at a maximum, whilst deadweight is cut to virtually an irreducible minimum.
The paradox thus appears to be that the greater the saving in., material achieved, the greater the expense involved; the removal of the last pound of surplus weight may well prove more costly than the pruning of the obvious hundredweight!
Realization of Theory Stringent analyses have been made as to the economies which may be effected by the adoption of light design in
various types of vehicle. Investigations included fuel costs, tyre costs, effects on roads, ease of driving, etc. Economy in light design reaches a maximum when saving in deadweight can be replaced directly and'entirely by pay-load. Economies in this direction are more decisive than -those coupled with savingof fuel, reduced maintenance, reduced wear andtear, improved acceleration, and so on,
How far are the aims of light design, as we have summarized them, already realized? It seems at first sight simple enough to shape a vehicle, for instance, as a tube resistant to bending and torsion. Looking upon the unit in crosssection as a single member, the side walls form the web, whilst the floor and roof constitute flanges. In practice, the unit must be stiffened longitudinally and transversely in order to make it rigid against compressive loads tending to cause buckling and collapse. Until recently, a vehicle was compoged of two independent assemblies, namely, chassis and body. The organization of the motorcar industry tends to perpetuate this feature of design, more particularly as, in the evolution of the industry, the manufacture of chassis and bodywork has tended to become centralized in separate workS, or at least in separate, highly organized and specialized departments.
Conventional design has profited from improvements in materials and production technique, and has, on the whole, tended to become lighter, but the first step in the attainment of, real Tight design was the transformation of the chassis into a framework with supporting skeleton and supporting skin. In the " Leichtwagen ." of the Swiss State Railways (S.B.B.), a further improvement has been introduced; the internal skin is smooth and utilizes to the maximum the properties of the metal of which it is constructed: of the total weight of the vehicle, the proportion assigned to the skin has thus become correspondingly greater.
Even with comparatively recent vehicles constructed on the line of light design, limiting factors introduced by -machine-shop practice may still he observed. For example, in the design of Groseck for the combined rai motorcar for the Weimar-Berka line, Ahrens reports that, tq facilitate welding, a longibidinal -girder had to be introduced, whicis, according to drawing-office calculations, was certainly not necessary: Furthermore, as a result of insufficient stiffening of the skin, certain windows in the vehicle could not be lowered. It might be noted, incidentally, that this assembly was in steel, as, at the time, economies had to be effected in the use of aluminium.
In recent years, various technical and economic difficulties associated with the practical realization of light design in its entirety have furthered lightma terial design which enables the theory Of true light design to be in part, at least, achieved bY more primitive methods. This development has ,been fostered by rapid progress in the technique of the manufacture and .working of .light alleys.
Vehicle construction in light metals represents to-day a first step to the ultimate attainment of real light design,, not only because of the saving in deadweight effected, but also as regards the simplification in production and handling which aluminium and magnesium alloys have made possible. Where heavy metals have reached their limit in light desjgn, then light metals come into the picture, to extend the evolutionary process.
Other Aids to Success
Progress in the aChievement of maximum lightness, via light design and the use of intrinsically light materials, is likely to be affected considerably by developments in joining technique. The use of very thin' materials or of special alloys, frequently' carries with it, at present, the need for assembling by the relatively Clumsy Methods of riveting or. bolting, "the more 'logical process of welding being often inadmisiible owing to the adverse effects of heating on the structure and the impossibility of subsequently -re-heat-treating the whole.
Development along these lines ntay be expected as a result of the evolution of suitable non-heat-treatable, streng, wrought light alloys, by the production of really satisfactory low-temperattire soldering or brazing compounds and by the evolution of specialized welding techniques in which heating effects are confined to the smallest possible localized areas.
Finally, it should be realized that the light vehicle, whether the outcome of light design pushed to the unit demanded by theory, or attained in compromise by the use of light alloys, will be the resultant of a scientific blending of heavy metals, light Metals, plastics and wood, specifically used in those particular spheres where fullest advantage can be taken of each. •