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LITTLE-KNOWN POMP. RACTOR-LORRY DESIGN.

11th July 1922, Page 14
11th July 1922
Page 14
Page 15
Page 16
Page 14, 11th July 1922 — LITTLE-KNOWN POMP. RACTOR-LORRY DESIGN.
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Which of the following most accurately describes the problem?

THE SUCCESS of the Scamixiell tractor-lorry and the fact that this type of machine can carry much heavier loads than the ordinary four-wheeled motor vehicle, together with the official recognition which has endorsed the legality of such combinations, have naturally drawn a great deal of attention to the problem of building other types of tractor-lorries, or_of converting ordinary vehicles into machines of this type. Already' successful efforts have been made in this direction, but certain models which we have seen both in practice and on paper, would appear to show that there are certain features of tractor-lorry design which have not received the mature consideration for which they call.

The elucidation of the problem of the tractor-lorry presents difficulties quite distinct from those found when designing the more conventional type of vehicle. Even the ScanuneII, well thought-out and well-designed as was the original model, had to be strengthened in certain parts, particularly the turntable, in order to meet the stringent requirements of everyday service.

• Difficulties in Design.

Ono of the great difficulties in most designs is that the tractive effort and braking stresses have to be taken in an indirect manner. For instance, the turntable spigot may be subjected to tensional and double bending stresses, which latter may be reversed quite frequently, whilst the length of the body and of the frame upon which the body is ,mounted, and the fact that they bear a load considerably greater than is usual, render them-particularly liable to racking and bending stresses, especi • ally as in eertain types they not only carry the load and take the torsional stresses due to any twisting movement between the wheels of the tractor portion . and those of the trailer portion, but they also have to take the major amount of the stresses due to braking and the tractive effort.. To analyse the various points, we must remember that although the original Knox tractor-lorry, and the Searnmell which was a development of the former, are of a particular type, this is not the only one that has been produced. There are now four main types as follow:— (1) The ordinary Scarnmell or Knox type with turntable supported on stout semi-elliptic springs, permitted to oscillate on the axle load spring brackets and with the front portion of the body frame resting on this turntable and secured by its spigot., through which the driving, certain of the braking, and other stresses are taken, whilst the trailer frame has to bear other stresses than those imposed by the load.

(2) A type resembling that detailed above, but in which the turntable is secured rigidly to the frame of the tractor portion.

(3) Also resembling Nos. I and 2, but with the lower portion of the turntable pivoted to the frame of the tractor portion. (4) The articulated type as recently developed by Ransomes, Sims and Jefferies, Ltd., in which the driving and braking stresses are taken by a separate frailer chassis, and the body, which is mounted on turntables on the tractor and trailer portions, does not hear any stresses except those imposed by the load and possibly twisting stresses caused by relative movement between the trailer Avhe.els • and those of the tractor portion.

We will now deal with the various stresses to which the tractor-lorry is, or may be, subjected, and see how they apply to the four types specifically mentioned. Possibly those due to uneven road surfaces are the most important, and they must be considered before any questions aa to the stresses imposed by braking, tractive effort, or load are taken into account..

One of the features of the ideal six-wheeled tractorlorry should be its capacity for _running on uneven surfaces which may lift or twist one portion of the vehicle in relation to the other without causing excessive torsional or other stresses on arty portion of the machine.

The next consideration are those stresses 'caused by spring deflection, which itself may be a result of load or of uneven road surfaces. Apart from these we have those directly attributable to the load, and others due to the tractive effort and to braking. The latter are particularly important, as they may considerably exceed the stresses due to the tractive effort, and at the same time cause reversals of stress, as in the case where the tractor wheels are braked so that the trailer portion tends to overrun the tractor portion. Dealing with the types of tractor-lorries in the order in which we have given them, we will commence with the Scammell type. Here the whole pull or tractive effort of the tractor portion is concentrated on the rear axle and is then transmitted through the turntable springs to the spigot of the latter ; thus the tractive effort and the inertia of the load, plus the rolling friction, cause a powerful rock lug action on the turntable and spigot due to the " couple " thus set up. The rocking is, of course, resisted by the frame of the tra.i.ler portion, which results in focusing at the front the pressure between the two plates of the turntable, and imposing both tensional and bending stresses on the apigot. .These stresses are, however, reduced in violence owing to the cushioning action.of the springs._

When the tractor heels are braked this action is exactly reversed, the pressure :then focusing at the rear of the plates and the spigot being forced in the opposite direction ; on the other hand, owing to the lower plate of the turntable being given almost free movement, practically no stresses are imposed by spring deflection or uneven roads, except asregards twisting or torsional stresses, which must be taken up, partly by the springs of the turntable and those of the trailer portion, and partly by the stiffness of the trailer frame and body. It will be noted that the portion of the load carried by the tractor is not imposed. upon the tractor frame; but is concentrated on the rear axle, so that the tractor-itself may be built Compara.tively lightly, providing its engine and transmission are powerful enough satisfactorily to perform their work.

The Rigid Turntable.

Turning to the second type of tractor-lorry, in which the turntable is fixed rigidly to the frame of the tractor portion, we find a far different state of affairs. In this the tractor portion not only does the work of hauling and a certain amount of braking, but it also bears something like half the total weight of the load. This also applies to Types 3 and 4, but there are certain points about Type 2 with which we must first deal. To begin with, the fixed turntable is fundamentally wrong. The tractor and trailer portions cannot form a rigid unit, and, there' fare there is constant relative movement between the upper and lower portions of the turntable, and although the " couples" caused by the tractive and braking efforts are considerably reduced owing to the effective distance between them being lessened, yet, owing to the absence of springs there is no cushioning action, and all the torsional stresses come direct on the turntable, thus greatly increasing the stresses on the spigot. Some of these stresses can he reduced by fitting buffers to the chassis of the trailer portion, so that these bear on the frame of the tractor portion, but this is merely benefiting one section at the expense of the other, the stresses being then transferred to the frame and body of the trailer portion. In the event of uneven distribution of the load, rocking of the turntable must also inevitably ensue owing to the spring deflection, and every deflection due to inequalities of the road will cause the focus of pressure between the plates and the turntable to be first at one end and then at the other, so that 'there is continual reversal of stresses on the spigot.

In Type 3, where the turntable, although secured to the tractor frame, is permitted to swivel, which swivelling is usually restricted by balancing springs, some of the faults in Type 2 are eliminated. For instance, stresses due to lifting or lowering of the trailer portion are practically non-existent, as the lower portion of the turntable can accommodate itself to meet the requirements, but there is still the disadvantage that the load is carried on the tractor frame instead of direct on the axle.

Finally, we will deal with Type 4, which is the latest development in tractor-lorry Construction, and one which is certainly of considerable interest, as it embodies features which, although not new in themselves, have never been applied to this type of machine. We refer aliefly to the fact that tracking

of the trailer wheels with those of the tractor is obtained, without making the_ machine in any way complicated. To all intents and purposes, the outfit consists of an ordinary vehicle and a trailer. The novelty occurs in the fact that vehicle and trailer each carry a turntable or pivoted bolster, upon which the body is mounted. The turntables are arranged so that they can tilt as well as swivel, so that neither the body nor the load is distorted by the turning of the attachment, although, so far as we ean judge, it would appear that the sbody and its frame must be constructed strongly enough to resist the torsional stresses which occur owing to relative movement between the wheels of the tractor and trailer, and which must be transferred to the springs, as the swivelling bolsters cannot rock sideways, for they are additionally supported on the tractor and trailer frame respectively by buffers fitted to their ends. The swivelling action of the bolsters also takes care of any stresses due to road inequalities, lifting or falling of the trailer portion and any uneven distribution of load which might cause increased flexion on the springs of the driving wheels. In this design the tractive effort and braking stresses are taken through the chassis frame direct to the trailer frame, and vice versa, thus the body is relieved of all stresses other than those due to the torsional effort ; also the turntable spigots are not called upon to transmit these stresses, and a weak point is thus removed. • It will be seen from the foregoing that the design of a satisfactory tractor-lorry demands skill and great attention to detail. It must not be made a rush job, as, otherwise, a very promising type of machine may be rendered unpopular.

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