Lubrication Systems for Commercial Vehicles.
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Representative Systems in Use on Modern Petrol-driven Machines.
We published an article on " Lubricants and Lubrication for Motor Vehicles " last week, and in that article certain interesting conclusions were given as the result of careful tests upon a number of samples of engine and gear oils, which samples were procured at haphazard from the principal manufacturers. A good case was, on that occasion, made out for the employment of high-grade oils of pure hydro-carbon characteristics. It must not be forgotten, however, that a highgrade or any other oil will not show economical and satisfactory results if the system on which the lubrication is effected is ill-considered, and if it is carried out on wrong principles. To =enable our readers still further to consider the question of efficient lubrication with special reference to the petrolpropelled commercial motor-vehicle, we are now taking the opportunity of supplementing our previous article with a descriptive account of the most noteworthy systems of application which are in use at the present time.
The lubrication system of any piece of mechanism, whether it be a gas engine or a steam dredger, bears something of a similarity to the circulation of blood in the arteries and veins of an animal. The engine, as the prime mover OD an automobile chassis, is naturally the detail which is the subject of the most careful deliberation in the matter of lubrication, but we shall have occasion also to consider the care which has, in many modern designs, been bestowed upon the arrangements for the efficient lubrication of change gears, clutches, live axles, and other moving parts on many of the better-class machines.
We may, with no great difficulty, divide the accepted methods of the lubrication of modern, high-speed petrol engines into a small number of classes, and this separation will serve to demonstrate clearly the lines upon which the modern manufacturer is working to secure efficient oiling. We may roughly state that the efficiency of the lubrication system of a high-speed internal
combustion engine depends upon the thoroughness with which the connecting-rod big-ends are provided with oil. The main and camshaft bearings, the -pistons, and even the gudgeon-pins, as a rule, present no great difficulties, and the feed to these parts has usually been satisfactorily effected by either gravity or pressurefeed to the main hearings
and subsequent distribution ; but the big-ends have always presented a serious problem, and it is largely in relation to these important details that we shall consider the main features of the various lubrication systems in use. The markedly ununiform method, which depends on the centrifugal whirling of an indefinite quantity of oil within the confines of the crank chamber, and which has been aptly known as the " splash " system, is being steadily pushed out of the field by the many methods of forced lubrication which have been expressly embodied in order to ensure definite reliability in this matter of lubrication. It is curious, however, to notice that the latest development is again in the direction of an improved splash form of lubrication, but the two systems are unlike in that the older method seldom provided for a constant level in the crank case, whilst the newer arrangement ensures regularity in this respect.
The original splash system, while ensuring the existence of a state of mist in the crank chamber when the engine was running, which mist consisted of finely atomised lubricating oil and dirt, made little pretence to ensure that the big-ends should always have the same depth of oil in the engine base, with the result that the scoops were at one time submerged, and at another time they were revolving quite clear of the oil surface. It is also presumable that,
• once the oil contained in the bottom was whirled up, it was centrifugally maintained all round the case, providing the engine-speed did not drop. In this way, although it must be assumed that the oily vapour had every inducement to percolate into the bearings of all the exposed moving parts, the scoops on the big-ends lifted very little oil; there was no fresh supply to maintain a constant level under the bigends. The actual height was maintained, in most cases, entirely at the discretion of the driver, and he had very little to guide him, as to the state of affairs in the crank chamber, other than the apparent overheating of something vital, which clearly indicated a Shortage in the supply, or the emission of dense clouds of smoke from the exhaust box,
or the mis-firing of oily plugs, in both of which circumstances he, of course, felt justified in assuming the engine had " got too much oil." Gauge glasses on the outside of the base castings were useless when the old form of splash system was employed, as, even if they were not obscured by dirt, they could never indicate a true level when the engine was running.
Several attempts have been made by manufacturers to devise a means .which should prevent the accidental overcharging of the base chamber, and which, at the same time, should provide a: ready means of decision as to whether there were sufficient oil in the case. The WOlseley Company, amongst others, provided small stand pipes, which were adjustable as to height, and which were screwed into the bottom of the crank-chamber base. Oil was then pumped into the case, before the engine was started, until such time as an overflow took place from both of the separate pits ; careful adjustment of the height of the stand pipes secured the correct level of the oil when at rest. This method enabled the driver to ascertain when he had not pumped in sufficient oil before starting, and to know when he had pumped in too much, but that was all. When he was on the road, he had to use his judg ment, and that is a condition of working which it is inadvisable to introduce, if means can be taken to avoid it.
Manufacturers and users, at the expense of numberless " big-ends " and scored shafts, realised that some more positive method must be adopted, and, while admitting that, apart from the difficulty of maintaining a constant level, the splash system was thorough, they remembered the experience of the builders of high-speed, enclosed, steam engines, and saw no alternative but a pump-forced system of oiling, which would ensure that the oil should reach the point for which it was intended.