THE MOTOR LORRY ON ACTIVE SERVICE.
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Some Observations on Design. Dealing with the Weak Points Revealed by the Undue Strains to which the Vehicles Were Subjected.
WHEN THE NUMBER and variety of heavy Motor vehicles employed by the War Department 'and the extraordinary nature of the work performed are considered, it is evident that much valuable information relating to the success or failure of different designs may be obtained.
Of course, the abnormal conditions which must obtain on active service must be taken into Account.
The state of the roads, loads, speed, supervision and lubrication by drivers and repairs effected by maintenance 'staff are all points which have an important bearing on the behaviour of these vehicles. The author has had exceptional opportunities of noting.the manner in which the two last points were or were not attended to by different M.T. units. Nevertheless, after making the necessary allow-. ances, many interesting observations may be made and, no doubt on account of experience so gained, many modifications in future designs will result:
A large proportion of the defects which arose might be traced to abnormal wear and tear or to poor maintenance. On the other hand; a number of weak points might be ascribed to poor design, but were of such a nature as to be rectifie.d at small cost and with no radical alteration in the design of the vehicle as a .whole. •
It was a matter of considerable interest to note the extent to which Variceis makers endeavoured to eliminate the .various d'efects, which nattually arose under such conditions.
To deal adequately with such items as engine, trans-. mission and springs would require more than a whole number of this journal, but a_ few general remarks may he of interest. An outstanding feature of engine. design was the value of ample bearing surfades. In cases where these were not adequate, endless trouble was caused in scored and worn Journals, necessitating the despatch of the crankshaft to the base for truing , up. It would appear worth while to increase the distance between cylinder centres slightly in order to allow of wider bearings. This would sligitly increase the weight of the balanced revolving parts, but need make very little difference to the partially balanced reciprocating masses. • In this connection, the use of-ball bearings for the . main engine bearings.was a succssful feature.. The . length of the big-end journals so obtained justified their use, and the crankshaft of the particular make of vehicle which the author has in mind gave very little trouble.
Engine lubrication was always a source of anxiety, hut a large amount.of this was due to the varieties of oils used and the variation of climatic conditions.
In the case of an American engine. wHch was designed for the use of light oil, if the oil was u+.. all thick the worm drive Of the oil pump Was liable to strip.
The design, or rather the position, of the oil ,pump. rendered an otherwise excellent engine of British make unreliable unless in good hands. The pump being coupled direct to the inlet-camshaft was not , submerged, and, if the oil was heavy, the pump would often fail to circulate the oil unless primed. In the hands of a good driver this was a troublesome operation, and a .negligent driver could do much damage unless closely watched. joints oints in transmission gave a certain amount of trouble. The leather coupling gave satisfaction especially where it was not subjected to undue telescopic action. This effect is most pronounced at the rear end of the propeller shaft, and the use of a leather coupling combined with a fork sliding on. splines on the propeller shaft should give good xesults. The question of the best manner of transmitting the drive from the back axle to the frame, in the case of the live axle, was not advanced very much nearer a decision and there is still room for opinion on the matter. Several British makers were very successful with the use of the road springs for this purpose, the result being a neat design and the elimination of spring seat seizures, etc.
Torque members generally gave. litte trouble, but it is interesting to note that this proved to be the' most troublesome feature on two good American makes.
The torque tube arrangement has many advan:, tages, but, in practice, it does not always give the best results. In the case ,ofmake, considerable trouble was experienced owng to the flange either at the rear end of the tube or on the worm housing breaking away. (See Fig. 1.) The provision of a 'fuller radius in the corners or, as was later done, the addition to the patterns of ribs, would have avoided failure. It cannot be urged that this slight alteration would add materially to the unsprung weight of the vehicle. Another design of torque tube gave trouble owing to the rivets securing the rear flange to the torque tube working loose. Re-riveting and brazing got oyez thedifficulty. Considering the conditions, difficulties with steering gears were not so great as might have been expected. Being.the more exposed, the cross-rod joints often wore badly. In the use of the cup-and-ball type of joint a comparison between different makes on a small but important point is interesting: It is in. tended that the pin should be a tight fit in the steering arm, the shank of the pin being turned and a nut fitted underneath. The majority of designers allow a thickness of 11 in. for the arm, thus giving an ample bearing surface for the pin. In the case where only in. was allowed (see dimension A, Fig.
2), there wastrouble owing to the pin working loose and, if ,not checked soon enough, resulting in the pin being dragged out of the arm owing to the hammeriag action Stripping the thread
at the bottom of the pin. This has been remedied in later machines. Poor fitarms in a number of cases.
drop arms and steering ting accounted for loose
One outstanding feature ness of steering arm (at A) was the success of the
roller type of bearing for road wheels, especially the adjustable type-having tapered rollers.
For a time the simple floating gunmetal bush was satisfactory but., after several renewals of the bushes,' both the wheel centres and the axles became worn irregularly, ultimately necessitating machining and the fitting of odd-sized bushes.
The study of flame design provided a great deal of interesting data, and this alone would well form the subject for a separate article, which may be -written very shortly.
A great success was scored by -the flitch plate frame, i.e., wood reinforced on either side by steel plates. Of the other types, the rolled section was probably the most satisfactory, On account of the lower initial cost and the frame being leSs liable to crack longitudinally in the corners.
This raises another interesting point. In fixing brackets of various kinds there was a tendency to allow too small a bearing surfate, resulting in the frames being torn away. The practice of fixing dumb irons to the bottom flange only in the ease of the pressed steel frame did not give good results—the frames splitting in the corner in a longitudinal direction. On the other hand, on a lorry where the rolle.a section was used, this same fixing gave no trouble throughout the , whole of our experience.
Where vehicles were working on such roads as were common in France, it is obvious that the side members of the frames were severely stressed and a number of fractures occurred, this being common to both sections of frame. In practically every case this could be traced to the presence of holes drilled in the top or bottom flanges for the purpose of securing body bolsters, frame cross-members, etc., which had the effect of weakening the secticin where the metal was most required.