Structural Problems of the Commercial Vehicle
Page 39
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Points from an Interesting I.A.E. Paper by Mr. V. W. Pilkington, M.B.E., M.Eng., Director of Engineering, Leyland Motors, Ltd., rrillS paper, read on October 1 1 before the Institution of Automo bile Engineers, deals mainly with vehicles as a whole—not with sglecialized units and components—and concerns goods and passenger types, excluding military and unusual forms.
The historical section of the paper brings to the fore an outstanding point. In the early days of development of vehicles there was a clear line of demarcation where the engineer finished and the coachbuilder carried on to achieve the complete vehicle.
On the railways it is common prattice to build a chassis, the principal purpose of which is to take the " buffing " loads. Above this the coachbuilder erects a timber structure. In the heyday of horsed vehicles this was reversed; the coachbuilder completed the structure, and the power unit—the horse—was flexibly attached to it, as were the axles, through the peculiar form of suspension then adopted.
• Designing a Vehicle as a Unit Fot 50 years the basic conception appears to have held of a structure on which the component units are assembled and which is then handed to the coachbuilder to complete. In the development of other means for transport. by air or water, the opposite is the case, the whole vehicle being designed as a unit, whether it be an aircraft or a ship.
• To-day, standardization, to achieve rational production, must be accompanied by technical development, and if a vehicle is to be functionally designed, it appears to be an axiom that the whole structure should be taken by itself. Single-deckers in America have definitely followed this trend. In Britain, commercial vehicles fall within two categories—those within the motorcar classification and those within the defi nition of a heavy motorcar. This difference has an important bearing on the general design of vehicles and their structure.
Legal Anomaly In the first class, if a vehicle does not exceed 3 tons unladen, then it can operate at 30 M.p.h., and be driven by • a relatively unskilled man under 21, this without any relation to its maximum ' gross weight, until it reaches the next permissible step of 12 tons gross weight with a vehicle having two axles.
The problems Of the structure and for the designer have been amplified by this artificial classification, in that the attempt is made to carry a maximum pay-load on the least tare weight. Certain of these vehicles approximate to motorcars, and overloading, with consequent failue both of structure and other units, has been marked in the past.
The structural problems, ignoring overload, are, however, similar to those of the next grade of vehicle, the heavy motorcar. There are prescribed conditions for these, which are limiting factors in design; these cover overall width and length, overhang, tyre size, turning circle, etc. • Front overhang has not yet been defined, but it is the intention of the Ministry to limit this, particularly on public-service vehicles, to avoid the sweep diagram beaming unduly large.
Thus, the basic dimensions of vehicles are fairly well defined. Even suspension cannot be catered for purely as such, on account of regulations.
It is prescribed that on the publicservice vehicle the step height must fall between the upper and lower limits, whether the vehicle be laden or unladen. Further, it has to pass an arduous test, in that the tilting angle when laden, under the worst conditions, shall not exceed, in the case of the double-decker, 28 degrees, and the single-decker 35 degrees, with some concession for the trolleybus. Other restrictions on ground clearance, maximum height, and internal clearance for passengers exercise a vital bearing on the structure.
Area and Load In the goods vehicle, about 2 ft. 6 ins. of platform length is allocated per ton of load, whilst the driver must have about 4 ft. It has become conventional to arrange the engine in the cab to economize in length, as it concerns payload.
The chassis frame is usually simple, to allow cheap manufacture and the most varied applications. This gives the operator freedom of action with regard to the type of body, and for export permits easy dismantling, whilst affecting the duty to be paid.
The first problem is to decide how far stiffness shall weigh against strength considerations, and the simple side member must be so arranged with its attachments and cross-members that the whole structure will be of the lightest form and able to operate for many thousands of miles without fatigue, taking into account the duty for which it is designed.
If all vehicles operated on what was. in effect, a billiards table. there would be no problem. Load distribution is important, yet for some strange mason the combination of chassis frame and body structure nearly always results in a stress concentration point. This is particularly true of a vehicle carrying a tank for liquid, which itself is torsionally and horizontally rigid, also of an all-metal body on a bus or coach. If the frame be not torsionally rigid, then. obviously, it must be looked upon as part of the suspension system. If we depart from the conventional frame to some unified structure, then the preconceived notion of front-end suspension must be modified.
Side members can be evolved which are reasonably stable, and Attachments should be so designed that in themselves there is no metal flexure; otherwise, fatigue cracks will start from holes in them. The metal must be of such thickness, and bolts or rivets of such proportions as to ensure that the bearing area for either in shear does not cause compression fatigue of the surrounding material. Bolts in any frame attachment should be nsed for drawing the parts together, and the attachment ' should be spigoted to relieve the bolts of shear stresses.
Torsional Weakness
The normal channel cross-member is torsionally weak, whilst within the space in which a tubular member can be fitted. its modulus is so low as not to make a real contribution to high torsional rigidity. It is, therefore, preferable to assume some twist to ensure that stress concentration is avoided.
Platform and tipping bodies do not present real structural problems, but the load on the first cross-bearer behind the cab must be distributed to prevent a concentration point. Coachbuilt passenger bodies are not torsionally rigid, each joint giving a little flexibility.
The problem of the all-metal body is more important, and any undue flexibility in mounting makes the tilt test more difficult. Carrying the main passenger load on the upper deck provides a difficult condition; that is, the shear load in the horizontal plane, due to breaking and acceleration. This has to be met by the comparatively light-section pillars needed to obtain the maximum glass area in the lower saloon. Staircase and platform structure is, therefore, important.
Stressed-skin Construction In riveting highly stressed gusset plates in fairly thin material, it is an advantage to countersink both portions to relieve the rivets of shear loading. In American buses and coaches stressedskin construction is widely adopted.
The auth.t pleads strongly for research into methods of attachment. Some new plastic adhesives appear to offer scope for investigation. Under favourable conditions they have a higher capacity than the ordinary riveted or bolted section. We are too apt to look upon carbon steel as the only suitable material for fabrication. Yet in America they are often strongly wedded to aluminium.
A point in the author's reply to the discussion was that each additional ton of tare weight represents a reduction of I m.p.g.