From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical article on lubrication, the use of colloidal graphite, and oil properties in car maintenance.
| Identifier | ExFiles\Box 145\3\ scan0016 | |
| Date | 13th February 1934 | |
| Reprinted from The Motor February 13, 1934. LUBRICATION—VITAL TO CAR MAINTENANCE—Contd. Acheson Colloidal Graphite THERE is an increasing number of oils and greases on the market containing colloidal graphite. This material is so finely divided that it will pass readily through oil-ways and filters. When carried by the oil through the various bearings the graphite particles form what is called a "graphoid" surface on the metals, thus tending to smooth out the irregularities and to provide a greasy film over which oil spreads very readily. It has been proved by careful tests that such a film is an excellent safeguard against bearing seizure should the supply of lubricant fail temporarily, as when starting a stiff engine on a cold morning. Graphited oils are of particular value when "running-in" a new or reconditioned chassis. Although smooth to the eye, the surfaces of freshly machined shafts, bearings, pistons, etc., present microscopic irregularities and, when pressed into contact, produce very high pressures over the high spots, which are prejudicial to the maintenance of a lubricating film. The exceedingly tiny graphite particles in the oil improve lubricating conditions during the process of breaking down these irregularities, and probably also tend to fill in the microscopic valleys which separate the high points. Used by Car Manufacturers That this action is no mere theory has been proved by various scientific tests, and oils containing colloidal graphite are now being employed during the assembly of engines by many car manufacturers. They are equally valuable in the case of a reconditioned engine, which, of course, must be run-in in much the same way as a new engine. While the surfaces are still rough—if one may use the term to describe such minute irregularities—the amount of heat which is generated is abnormal, so that if the car be over-driven there is risk of the oil film breaking down, followed by metal-to-metal contact and material damage. Here again the use of colloidal graphite forms a safeguard, because it has been proved that under conditions of scanty lubrication a graphoid surface will provide sufficient lubrication to prevent seizure for quite a long period. Running-in compounds, consisting of oil carrying colloidal graphite, can be obtained for addition to the lubricant in the sump. Preferably, the oil carrying the graphite should be of a kind similar to that used normally in the engine. Thus, with few exceptions, the running-in compound is a straight mineral oil and will give the best results when mixed with a similar "straight" lubricant. A new engine can be overloaded both by excessive speed and by too free a use of the throttle. Both these faults should be avoided by the driver, but, on the other hand, if he is too kind to the mechanism he will prolong the running-in period to an undue extent. The great point to remember is that it is excessive temperatures which result in harm; thus, while a short burst at full speed will cause no damage, and will, indeed, help to free the engine, a prolonged run at full throttle may easily result in seizure or bearing failure. [Graph Caption] Comparative tests on a bearing after cutting off the oil supply. Curve A shows seizure after 36 minutes when oil had been used, but colloidal graphite gave free running for 27 hours (curve B). Upper cylinder lubricants mixed with the fuel are being used by many thousands of motorists, and most of these contain a proportion of colloidal graphite. In a desire to limit oil consumption many designers have resorted to the use of very stiff piston rings, with the result that piston lubrication is often inadequate. Furthermore, as already mentioned, very little oil reaches the bores during the first few minutes after a cold start. Consequently, there is much to be said for the idea of introducing a lubricant with the fuel, although the quantities which can be used are necessarily small. An excess of lubricant would, of course, tend to give trouble with sparkling plugs and carbon deposits. Upper Cylinder Lubrication Colloidal graphite is particularly suitable for upper cylinder lubrication, owing to its resistance to heat, its affinity for oil and its ability to form a film over metal surfaces. In some cases it is also found to have a beneficial effect in keeping the valves working freely. As already mentioned, oils containing colloidal graphite can also be employed in the gearbox and back axle. Another application is to the penetrating oils now so largely used for lubricating leaf springs. After cleaning accumulations of mud from the spring leaves, it is only necessary to spray them with lubricants of this type, the penetrating qualities of which enable them to find their way between the metal surfaces. Similar oils are of value for lubricating coachwork parts, such as door hinges and locks, while they can also be spread around body mounting bolts if these are producing squeaks. Filtering Devices AT present the metallic or gritty impurities carried in the oil are present in the oil by the forced lubrication system generally used in modern engines. They are therefore liable to become embedded in the relatively soft metals employed for the main bearings, big-ends and pistons. Particles so caught are apt to score the crankshaft and cylinder bores with which they are in rubbing contact. Consequently, efficient filtration is of very great assistance in minimizing wear. The first line of defence is a good air cleaner on the carburetter intake, which will prevent a considerable quantity of road grit from entering the power unit. Various types of cleaner are in use, the three principles employed being filtration through cloth, the removal of particles by giving the air a whirling motion and, thirdly, the use of metallic gauze treated with oil to which dust and grit adhere. The car maker's instructions should be followed with regard to the periodic removal of dust from the cleaner; for example, in the third type the gauze should be removed at intervals, washed the film being between one and three-thousandths of an inch. Luckily, the rotation of the shaft tends to draw the oil into the clearance space, so maintaining the film even when a very heavy load is tending to squeeze it out of the bearing. The tenacity with which the film "hangs on" increases with its viscosity (other properties being equal), but the friction produced by shearing the film also goes up. Consequently, the oil should be as thin as possible consistent with safety. Engine Lubrication Systems In the case of the engine a good viscosity curve is of particular importance, owing to the forced feed system of lubrication now so generally employed. In this system a pump draws oil from the sump, or lower part of the crankcase, where there is a supply of one to two gallons of lubricant, according to the size of the car. The pump delivers this oil under pressure through pipes leading to the main crankshaft bearings, and it then passes through holes in the crankshaft to the big-end bearings. Naturally, this system provides a very considerable resistance to the flow of oil, so that when the lubricant is cold and stiff its movement tends to be very sluggish. The pistons and, in many cases, the gudgeon pins depend for their lubrication upon surplus oil, which, emerging from the big-ends and main bearings, is thrown around the crank-case by the rotation of the crankshaft. Consequently, when the flow is meagre piston lubrication is inadequate, and these conditions obtain just at the very time when the cylinders are cold and the products of combustion are apt to condense on the polished surfaces of the bores. Defeating Cylinder Wear Recent researches have shown that a large proportion of cylinder-bore wear occurs under these cold-starting conditions, and that such wear can best be prevented by copious lubrication. Consequently, there are many arguments in favour of employing a reasonably thin oil, so as to promote as ready a flow as possible under cold-starting conditions. In the choice of an oil from the viewpoint of viscosity, the motorist should also be guided by the way in which he is using his car. For example, in an engine employed mainly for short business runs it is beneficial to use a lubricant of a grade much thinner than that recommended for fast main-road work. The condition of the engine is also important, as the parts become worn the clearances increase, and a thick grade of oil can profitably be used. Advice in any specific case can always be obtained from the car manufacturers or oil marketing concerns. [Diagram Caption: Viscometer] Diagrammatic view of the Redwood Viscometer; 50 c.c. of oil is allowed to flow through the jet at various temperatures, the time required being recorded in seconds. Here we may remark, in passing, that the oil pressure gauge as ordinarily fitted merely records back pressure, so that often enough a high reading indicates a low rate of flow. Thus it is a common experience for the gauge to show a very high pressure just after a cold start, when the amount of oil in circulation is actually at a minimum. On the other hand, with a fully warmed engine on a fast run at a steady speed, the pressure gauge readings should not fluctuate nor tend to decrease; any unusual falling off may be taken as an indication of a low oil level or the use of a lubricant unsuited to the conditions. Before leaving this part of the subject we may mention that any means that can be taken to protect the metal surfaces of the bores with a film of lubricant should tend to reduce wear when starting from cold. This is an important argument in favour of upper cylinder lubricants which are introduced into the cylinders by admixture with the fuel. Many engineers believe that colloidal graphited oils are also particularly valuable for this purpose, of the kinds which are discussed later in this article. Oil Consumption Figures Many car owners use a thicker oil than is really desirable, because they fear that under arduous working conditions the oil may become so thin (by heating) as to involve a risk of bearing failure. Provided that the lubricant is of really good quality, this risk is almost negligible, the only drawback of using too thin an oil being that the consumption will increase considerably. An engine which is in good working condition should run a distance of not more than 1,500 to 2,000 miles on a gallon of oil if it is to be effectively lubricated. While a smaller mileage usually indicates a much bigger mileage should be regarded as a fault, because it may represent inadequate lubrication and the possibility of undue wear. These figures relate to normal cruising speeds of 40-45 m.p.h.; at higher speeds the consumption of oil increases very rapidly owing to many factors, such as higher temperatures, the greater quantity of oil which is thrown around the crankcase by the crankshaft, oil vapour lost through the breather, direct losses by leakage, etc. [Diagram Caption: Shaft] How a shaft draws in a film of oil which prevents metal-to-metal contact. The film is continually sheared, in layers, by the rotational movement. Properties of Lubricating Oils In giving particular attention to viscosity, as being a property which can be estimated by eye when the oil is poured, we do not wish to give the impression that this is the only property that matters to the effectiveness of a lubricant. Mineral oils are all hydro-carbons, but their molecular composition varies very greatly according to their origin and the processes to which they are subjected. The more stable the molecular structure the less likely is the oil to deteriorate in the sump. Here, mixture with air at high temperatures is liable to produce oxidization, which, if it is to be effectively lubricated. While a smaller mileage usually indicates a much bigger mileage should be regarded as a fault, because it may represent inadequate lubrication and the possibility of undue wear. These figures relate to normal cruising speeds of 40-45 m.p.h.; at higher speeds the consumption of oil increases very rapidly owing to many factors, such as higher temperatures, the greater quantity of oil which is thrown around the crankcase by the crankshaft, oil vapour lost through the breather, direct losses by leakage, etc. | ||