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).
Paper by G. H. Baillie discussing the best systems of lubrication for internal combustion engines.
| Identifier | ExFiles\Box 16\1\ Scan017 | |
| Date | 17th March 1910 | |
| CONFIDENTIAL UNTIL READ. The Royal Automobile Club. The Best System of Lubrication for Internal Combustion Engines.* By G.{Mr Griffiths - Chief Accountant / Mr Gnapp} H.{Arthur M. Hanbury - Head Complaints} BAILLIE. SYSTEMS of lubrication may be divided into two broad classes :— (1) Without splash. (2) With splash. (1) Can be divided into two :— (a) With oil pressure on the bearings ; (b) Without oil pressure on the bearings. Might, as far as results are concerned, be regarded as a form of splash, as in both cases oil enters the bearings only by capillary action. A better division would be systems with and without pressure on the bearings, if it were not that so many systems have pressure on some and not on others. The object to be attained in lubricating a bearing is to prevent the two surfaces ever coming into contact by ensuring that there is always a film of oil between them. Take the case of a shaft with a heavy fly-wheel resting in bearings. If the shaft is at rest, and the bearings have a film of oil to start with this film will remain if the pressure per square inch is below a certain limit. Above this limit it will be squeezed out, and the shaft and bearings will come in contact. If the shaft is rotating, the pressure per square inch of bearing may be largely increased without squeezing out the oil. This has been shown by the fact that it is sufficient to force oil under pressure into the bearings at the moment of starting only. This forces the shaft out of contact with the bearing, and introduces a film of oil. So long as the shaft rotates this film persists, and it makes no difference to the lubrication whether the further supply of oil is under pressure or not. Without this initial film of oil, the oil cannot get between the surfaces in metallic contact, even though the shaft is rotating, before the bearing has been damaged. The case of big ends of connecting-rods of petrol engines is, however, somewhat different. Here the surfaces are not permanently in contact, since the alternating pressure puts the opposite sides of the bearing under pressure at the end of the exhaust stroke and at the end of the suction stroke. The play in the bearing then leaves a space into which the oil can creep under capillary pressure. In big ends, therefore, no attention need be paid to getting an initial film of oil into the bearings. In petrol engines it depends on the number of bearings, the arrangement of the cranks, and the order of firing, whether the same applies to the main bearings. When the pressures on two crank pins, one on each side of a bearing, change in direction simultaneously, there is no doubt that the inertia forces are enough to change the direction of the pressure in the intermediate main bearing also, and allow of the introduction of oil. In the most common arrangement of a four-cylinder engine, however, where there are three bearings and one order of firing, 1, 2, 4, 3, there is a change of direction in the pressure on one side only of the centre bearing, and a gap in the main bearing could be made only by the shaft bending and being inclined in the bearing. Also it is doubtful if the shaft is ever lifted in the bearing of the fly-wheel end. On the whole, then, there seems more need for a pressure supply to the main bearings than to the big-end bearings. Whether there is actual need for pressure in any bearing depends entirely on the pressure per square inch between the surfaces. Experience shows that in certain engines with no pressure lubrication the bearings wear just as well as in others with pressure. On the other hand, Mr. Hutton tells me he has found that in racing engines where the pressure is very high forced lubrication is quite essential. Apart, then, from any question of convenience and reliability, the only advantage of forced lubrication seems to be in enabling the bearing surface to be cut down. The disadvantages are :— (1) The amount of oil getting to the big ends depends on the condition of the main bearings, and to ensure there being enough when the main bearings are slack, the pressure must be sufficient to give more than enough when the main bearings are tight, and this leads to too much being thrown off into the cylinders and their sooting up. (2) The viscosity of the oil varies so much when cold and when hot that the gauge indications mean nothing at all, and more often than not the gauge shows no pressure at all when the oil is hot and bearings are slack. This defect could be remedied by putting the relief valve, not where it generally is, on the pump, but as near as possible to the bearings. Then, however, the pump has to deliver under very high pressure when starting cold, and squirts oil out of its glands, or may strain its drive. (3) The pressure system may break down. An indicator is always fitted to show when this happens, but no one looks at his indicator often enough to be certain of finding out a breakdown soon enough to prevent damage. The Orleans electric bell indicator shows that an ear as well as an eye should be kept on the lubrication system. When a pressure system breaks down the supply of oil ceases at once. When the supply to an ordinary splash system breaks down, it is a long time before the oil level falls enough to entirely stop lubrication. A not uncommon form of breakdown is when an oil-pipe, imperfectly fixed, rubs against some part of the engine or frame and wears itself through. The advantages are :— (1) Smaller bearing surfaces may be used. (2) Oilways do not choke up. This latter advantage, however, is not possessed by all systems. There is safety against chokage only when the system is so arranged that there is always an effective pressure in every oilway. Turning, now, to the different arrangements of forced lubrication, I know of two cars that have gone the length of forcing the oil through crank shaft, up the connecting-rod, into the gudgeon-pin and through it to the cylinder walls. I cannot imagine why it was done. The difficulty has generally been to keep the oil away from the cylinder, not to get it to. A good many cars force the oil up to the gudgeon-pin. This has been found in practice to be quite unnecessary. The throw-off from the connecting-rods is always enough to lubricate the gudgeon-pin, especially if a drip point is arranged on the top of the piston. The third class, forcing to the big ends only, is the usual one. Sometimes baffle plates are fitted below the cylinders, sometimes not. I am inclined to think that they are always necessary when an effective oil pressure is maintained throughout. The only other system without splash is the squirt feed. It is not common, and has no advantages that I can see. It does not give any pressure in the bearings, and sends a lot of oil on the outside surface of the big end which gets thrown off into the cylinder. The lubrication ceases at once on a breakdown. Splash Lubrication May be divided into splash in a large quantity of oil in the crank chamber, and splash in small troughs. Without some means of keeping or easily ascertaining the right level, it is the most horrible system devised. A pipe reaching up to the right level and ending in a tap below the crank chamber is sometimes put forward as such a means. Any maker who adopts it deserves imprisonment. The pipe is always full of oil, so oil always runs out when the tap is opened. One has to judge by the size of the pool on the floor whether the oil is running out of the crank chamber or only out of the pipe. Another alleged means which is nearly as bad is a gauge glass. The surface of the glass is always too dirty to see the oil level. A float in a side chamber is better. An extremely neat system was adopted in the Sunbeam cars, and is shown in Fig. 1. A pipe reaching up to the right level is connected to a hand-pump with a two-way tap. With the tap one way, working the pump takes oil from the tank and forces it into the crank chamber. On reversing the tap, the pump draws oil from the crank chamber and forces it back into the tank. More than enough oil is forced into the crank chamber, and the surplus is then drawn out; when the right level is reached it is indicated by the pipe gurgling. This is a system which might well be adopted in low-priced cars where it is desired * Paper read before the members of the Royal Automobile Club, March 17th, 1910. | ||