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MILITARY DESIGN IN RE NSTRUCTION TRANSPORT

28th May 1943, Page 26
28th May 1943
Page 26
Page 27
Page 28
Page 26, 28th May 1943 — MILITARY DESIGN IN RE NSTRUCTION TRANSPORT
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Which of the following most accurately describes the problem?

By A. W. Haigh,

A.M.I.A.E. DURING the recent. past a great deal of publicity. has been given to post-war planning committees and their work We are constantly reminded of the perfect cities in which we are to enjoy our adequately (?) pensioned old age, but no mention is ever made of the means by which these ities are.to be built. Particularly corkspicuous by its absence is the root of all

: building operations,.namely the means for transporting the building materials to the site .

The provision of an efficiently designed vehicle for this work should 136 of equal importance to the planning of the cities themselves. Be-cause nothing has been officially published on the subject of post-war chassis design, it can be assumed that the efforts of war-time automobile designers are sufficiently appreciated for them to be entrusted with the work of producing a vehicle, after the war, which will be subject to road conditions practically identical with those to which military chassis already -produced are now subjected. Local authorities have done a wonderful job in clearing up their blitzed areas, but . once the building gangs have taken possession, to begin their work of reconstruction, the ground will once more resemble a battlefield. Ferther, large quantities of stone will b e required, f r o m quarries, which again will demand' a vehicle designed more for cross-country work than for normal conditions.

It would appear, then, that designs which have already proved more efficient under war conditions than -those of the enemy (if the German statement that only captured British vehicles were to be used fcir. desert reconnaissance—their own models stuck in the sand too often—is to be believed) should be analysed and the results incorporated in a chassis for the especial use of reconstruction contractors after the war.

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The 3-ton general-purpose lorry would be the most suitable choice. This vehicle is. of four-wheel-drive design, which at once lends itself to criticism. When four-wheel drive for cross-country vehicles was adopted at the beginning of the war, the difference in the distance travelled by the front and rear wheels (Fig. 1) and the consequent wear of the front tyres was pointed out.

It was assumed that when the vehicle was negotiating • rough country the front tyres would be off the ground for a sufficient length of time to even out any discrepancy in distance travelled, by rotating backwards, thns preventing excessive tyre wear. In practice, how: ever, it has been found that this does not occur to any • great extent and that the front tyres wear far more rapidly than those on the rear wheels.

• WeTe four-wheel drive to be adopted for our post-war vehicle it would be imperative that the front drive be used only as a last resort, for the sake of tyre economy, if for no other reason. The difficulty of deciding what constitutes a last resort would result in drivers using • four-wheel drive when there was no real need for it, consequently it would be far better to have a vehicle • with orthodox rear-wheel propulsion.

• Standard front axles on those military vehicles employing only rear-wheel drive have ptoved unsatisfactory, in many cases, for rough-country work. The A24 beams became bent and the stub axles were broken at the point illustrated in Fig. 3. Beams, therefore, should be made more robust and stub axles thickened at the weak point. The latter item may cause a slight increase in centre distance, with resultant heavier steering, -but the very small increase in steering effort is justifiable if breakage be obviated.

Few complaints have been made in respect of rear axles. The casings are specially ribbed up for heavy 'duty, and the tubes designed with a factor of safety of 4 to 1. One fault which was found with the initial designs was the tendency of axle-casing ends to open up under constant hammering over rough ground (Fig. 3A), but this was cured by incorporating a flange in the design, as shown in Fig. 313.

A factor of safety of 2.5 to 1 on the axle shafts was

high enough to ensure no abnormal failure figures, but trial shafts with a factor of safety of 2 to 1 proved inadequate for the job, breakage occurring at the splined end after only a very short trial. Normal crown wheels and pinions are used, designed for maximum torque, and are found to be satisfactory.

Before the war colonial purchasers of British cornmercial vehicles which were to be operated over rough tracks stipulated that they should be fitted with only single tyres. This practice has been adhered to with success on our heavier military vehicles, the deep ruts on soft tracks encountered during operations, precluding the use of twins, as the possibility of all the rear-axle load being taken on two tyres instead of all four (it being possible for the others to ov4rhang ruts in the roads) made tyre failure almost certain.

The conditions which will obtain on building sites and stone quarries will demand adequate tyre equipment, and singles will prove to be the more efficient. Naturally', with only single tyres, their diameter will be tncreased, but loading height is not of .paramount importance, and the extra height -demanded by, using larger-diameter tyres will not be any disadvantage. Satisfactory adhesion between tyre and -road is ensured by fitting the 'Nay famous Trak-Grip tyre on military lorries, but they should hardly be necessary for peacetime use.

On a loaded lorry, negotiating rough country, wheel deflection is abnormally high. To cope with this, independent suspension on lightervehicles Was adopted (even some trailers incorporate a system of torsion-bar springing), but on the heavier chassis orthodox semi

elliptic springs were adhered to. Many designs of spring and spring mounting were put forward, and many of them failed to deal with the task imposed on them. Perhaps the most efficient system combined a frame using channel cross-members and trunnion-mounted springs, which when deflected were not twisted.

There are two types of orthodox "grid-iron" frame, one using tubular cross-members, which type is strong in torsion and is extensively used for double-deck passenger-vehicle chassis, and the other having only channel or U cross members. The latter is comparatively weak in torsion, in that any abnormal loads on the wheels, besides causing the springs to deflect, twist the frame and relieve the suspension of some of its stresses.• • Frame design, . for cross-country vehicles, is an extremely important item which ensures either success or failure of the whole vehicle. Therefore, it would be ' wise to accept the non-rigid type. which has proved itself in practice, for use On our post-war chassis. Springs will, likewise, be subject to heavier stresses, and the adoption of trunnion mounting should, as in the case of military vehicles, go a long way to prevent failure. SrR5NGTRENIA16 FLANGE The abnormal deflection of the wheels demands careful thoughtin the location of the steering drop arm. On normal civilian vehicles steering interference is practically unknown and the tolerance allowed in a drop-arm position is fairly elastic. Fig. 2 shows a layout of a, front spring and steering drag link, with the latter pivoted at a distance from the articulation point of the former.

For normal deflections, interference is negligible, but on the higher deflections it becomes quite serious. It is imperative, therefore, that the steering-drop-arm ball should be as close to the articulation point of the spring as is compatible with the satisfactory. working of the system, that is, the angles between the drag link and axle steering arm should be kept within reasonable limits with the wheels on full lock.

A further point which must be watched when considering wheel deflection is the height of the engine in the chassis. Whilst standard production engines and their nor':dal mountings have proved excellent in service, the distance between the sump and the axle had to be increased to ensure clearance on full bump.

The practice of drivers, when their lorries have stuck in mud or sand, of revving up the engine and fiercely engaging the clutch', causes high stresses both on that unit and on the pinion teeth in the gearboxes. Fortunately, the clutches were capable of withstanding the extra load imposed on them and gearboxes, both main and auxiliary, being specially designed with larger and thicker-rooted teeth, gave no cause for anxiety. It is as well to note, however, that teeth in main gearboxes were designed 'stronger than normal in order to accommodate shock loads, and in auxiliary boxes, to take care of increased torque plus shock loads.

The question as to whether auxiliary gearboxes should be utilized on our post-war chassis is perhaps the most difficult item to be settled in the whole design: Increased tractive effort will most certainly be needed and, unless special' fiveor even six-speed boxes are to be designed, it would almost certainly be necessary to use an auxiliary box to provide it.

Finally, two minor items are worthy of mention. Persistent shocks to the chassis not only affect the main items of the units mentioned, but also the parts which are normally dismiSged as routine-design practice. For instance, particular difficulty is still being experienced in preventing oil leakage from nearly every joint and oil seal in the chassis. Bolts: are stretching just sufficiently to allow a gradual seepage of lubricant between the faces of the components, whilst dirt, grit and general infinitesimal distortion are having the same effect on many oil seals.

These disadvantages may not occur under peace-time conditions, as drivers will be able to take a little more care• when negotiating . particularly bad patches of ground, but it would be as well to obviate them by slightly more robust initial design.

l3,odies and cabs-, in general, have been designed on austerity lines, so far as look S go, but have been greatly increased in strength. The matter of body types is one for the individual operator and cannot be discussed to any advantage here. A chassis designed with the above points in mind, however, should form an efficient and reliable base upon whickto mount any body the operator may require, and it will be to the operator's advantage to see that he obtains such a chassis to provide him with trouble-free transport.

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