WHAT THE OIL GAUGE TELLS.
A TECHNICAL TALKWHAT THE OIL GAUGE TELLS.
THAT little instrument which purports to inform us at what pressure the bearings of our motor are receiving their lubricants, often causes a great deal of misunderstanding. One often hears owners of different makes of vehicles comparing notes about their oil pressure, when actually, owing to the fact that the systems are totally different their comparisons are quite meaningless. There are many popular fallacies connected with lubrication, and the high pressure fetish is one of them. Many people seem to think that there is something specially desirable in a high oil pressure, without really considering what the pressure signifies, or why the maker has seen fit to put an instrument on the dashboard at all for registering this..
Ruling out systems incorporating drip feeds from a separate tank to the more sorely tried engine parts, as belonging more to the realm of history than practical mechanics, there remain two chief systems of lubricating the modern engine. That most used on modern high-duty engines is the arrangement in which oil is fed under pressure to each bearing, while the other is that in which a pump supplies sundry reservoirs or channels by means of which the bearings are kept oiled, but rather on the principle of “take it or leave it.”
The latter type ensures that each bearing is lubricated, in the sense that it contains oil, and therefore an oil film will remain between the surfaces as long as the loads do not become excessive, and cause a temperature which the bearing cannot stand.
This system, although now confined chiefly to rather touring engines, has many things to recommend it, as we shall see when we consider the matter of wear on bearings.
A Vicious Circle.
Unfortunately it cannot adequately cope with very high bearing loads and speeds, owing to the increased heating we have just mentioned. To attempt to overcome this without altering the system brings us in a vicious circle. To decrease the load factor—that is the product of load and rubbing speed of the bearing, we shall reed a larger and especially longer bearing. This means increased weight, not only because of the actual size increase, but also due to the fact that a bigger bearing is more liable to local distortion, and therefore must be more rigid in proportion. Increased weight, especially when it occurs in big ends, and connecting rods, means lower revs. As what we want are higher revs, this sort of thing is put out of count at the start, and we are back at the beginning once more, with a small bearing of which we want more than it can stand at present.
This brings us to pressure lubrication proper, which is employed on all high speed engines, and owes its inception to the increased load factors in common use.
Some Facts and Fallacies Explained.
The point which most people fail to realise, is that with this system the lubrication remains identical in principle, that is an oil film between two moving surfaces, and that the whole point of the pressure side of the question is to cool the bearings so that they will not break down under load.
Then we come back again to the question of the pressure required, and we are up against another vague idea in the minds of many that it is the oil pressure, as duly recorded on our nice little dial, that keeps the bearing surfaces apart as it were, and stops seizure. Of course what actually keeps them apart,—and in theoretically perfect lubrication there is no contact at all, is the oil, not the pressure. All you have got to do is to pour oil into a bearing, and no pressure you can apply, let alone a wretched 50 lbs. per square inch or so, can ever break down the film.
Pressure and Quantity.
Therefore the answer to the question, ” How much pressure ? “is simply this,— enough to force such a quantity of oil through the bearings in a given time to carry away the heat generated by them in that time, and so stop the temperature rising above that required for normal working. Therefore, the pressure gauge Is really not useful as a measure of pressure, but as a measure of the quantity of oil being fed through the bearings. Unfortunately in this capacity it has many limitations and these cause many owners to get worried over their lubrication.
The actual quantity of oil required is determined by practical experience of the engine in question, and by the methods used of cooling the oil after it has been through the bearings. Normally this happens in the sump, which is exposed to the stream of air past the car itself, and is often ribbed to increase this cooling effect, though the ribs one sometimes sees have more ornamental than practical value. Really thorough cooling of the oil is best obtained by use of a dry sump system, as exemplified on British sports cars by the Aston Martin, which carries its oil tank between the front dumbirons. On racing cars, such as the 1i-litre racing Talbots of a few years ago, oil radiators, carried on the side of the car, were used to increase the cooling effect. In a normal design something in the order of 25 gallons of oil per hour would be circulated in an engine developing 50 b.h.p. which is about the output of a sports l Flare. This quantity will of course vary considerably in various designs. In a new engine, or one which has recently had all its bearings taken up there is only a small clearance—usually too
small,—between the journal and its bearing, and therefore the pressure required to force enough oil through for cooling purposes is very high, in fact it may easily be higher than the pump in use can supply, unless the engine is run gently. Hence another reason for running in carefully after an overhaul in which the bearings have been fitted tight. Most engines, when the bearings have just been fitted, are definitely stiff, and many people think this is correct. On a high speed engine using pressure lubrication, it is definitely wrong. Bearings require definite clearance to accommodate their lubricant, and the ideal method of bearing fitting is not by scraping at all, but by machining each to the correct size. There should be a clearance of approximately 2 /1000ths of an inch in the bearing for oil when pressure lubrication—or rather oil-cooling —are employed for main and big-end bearings.
In the case of new engines this should be easy enough to arrange, but v,,hen overhauling an old engine it is no use attempting anything of this sort unless the crankshaft is also trued up on all journals. These are normally considerably oval, and unless they are restored to their original shape the whole job is rather a waste of time.
Unfortunately, in most cases spare bearings, when supplied, are usually on the small side, being intended to be scraped in. The scraping process is an excellent one if well carried out, and is in most cases the only possible way of doing it, but even then the mistake is so often made of fitting the job too tight. A fairly tight fit, in the case of ordinary splash, or trough, lubrication will settle down quite nicely, but in pressure systems merely restricts the flow of oil.
Change the Oil.
When an engine is so tight that the pressure when thoroughly warm—naturally the reading when cold is no guide— is excessively high, the way ‘not to reduce this is alter the oil supply. Systems have various methods of altering the oil pressure, and so the quantity flowing, either by bye-passing a varying quantity of the main supply back into the sump or by some adjustment to the pump itself. In this case the high oil pressure is not an indication of all being well, but a visible sign that insufficient oil is flowing through the bearings. Leave the supply at its normal setting, and change to a thinner oil until the bearings have settled down. Then the pressure will gradually fall, meaning that the flow of oil has increased. This will probably mean an excessive oil flow, and as a corollary a high oil consumption, and the original brand of oil is once more indicated. From this it can be seen that oil pressure must always be considered in relation to the condition of the bearings. When they are new and comparatively tight a higher pressure will be required to get the oil through them than will be the case when
they are getting worn. Therefore, if over some thousands of miles the oil pressure in normal running gradually falls from say 50 lbs. to 25, there is no cause for alarm. If the pump setting has not been altered the same quantity of oil will be flowing through them and they will be perfectly in order. If, at this stage, the supply is adjusted to bring the pressure up again to 50 lbs., it will merely mean that more work is being done, far too much oil is being flung all over the place, and the consumption considerably and needlessly increased.
The thing to be looked for is a sudden change of pressure, and it might here be noted that sudden change to a higher pressure is just as important as a sudden drop. A sudden drop means as a rule that the careless owner has let the level get down too low so that the pump cannot get hold of any. If the owner is not careless, however, it probably means that the metal of a bearing has failed, and that the oil is pouring through the one bearing without resistance, with the result that the others are being starved. In the case of a big-end this state of affairs will usually be subject to a very audible confirmation from the mechanism itself, but in the case of a main bearing, whose sudden failure is much less common, there may be little or no noise. In the case of the centre main bearing of a stiff 3-bearing 4-cylinder crankshaft, it is quite likely that audible comment will not occur, but this does not mean that it does not greatly matter.
Bearing Failure.
It is important to know what the internal arrangements of your engine are, on occasions of this sort. If, as is likely, Nos. 2 and 3 big ends are fed from the centre bearing’s supply of oil, the failure of this main bearing means the failure of their oil supply, as well as the partial failure of the supply to the other engine bearings. When the main bearing is O.K. the pressure to it will force a certain quantity of oil through it and out into the sump, while about the same quantity will, due to the same pressure, be forced via the drilled crankshaft to the two big ends, from which it also escapes and is flung off to the cylinder walls. When the bearing fails, say by part of the metal breaking away, there will no longer be any resistance to the oil at this point and
therefore no pressure to supply the big ends. Thus by driving home very gently it should be possible to avoid farther damage, as the oil in the big ends will be sufficient for very light running.
A sudden rise in pressure is just as important, as it is almost certainly due to a choke in the system. If the pressure goes suddenly clean off the scale this will mean a choke near the pump, and is likely to be noticed easily. What can happen, however, is that the pressure might suddenly jump from say, 40 lbs. to about 60 and remain fairly normal at 60. Such a movement might easily escape notice, but will cause trouble if speed is maintained. This means a partial choke or, more likely, a complete stoppage to one bearing only, due to some foreign matter getting in that portion of the system. Assuming a similar engine to the last case, such a big jump would probably mean a main bearing blocked up, with the result that the big ends fed from it are also in danger. This shows the importance of keeping a watchful eye on the pressure gauge, and also of being absolutely sure what its normal pressure is with the oil being used and under given conditions.
Sudden changes of this sort must not, of course, be confused with the changes caused by difference of temperature or quantity of oil. After a long hard run, the oil level will have dropped and as the reduced quantity of oil will be getting a pretty ” thin ” time, in more senses than one, the pressure will gradually fall. If fresh oil is then added, the pressure will of course jump up again and stay up. This will be obvious to most readers, but I have only mentioned it to show that there are some things to be guarded against in reading oil gauges, as I have known people get suddenly excited over some change through attributing what is normal and logical, to some unseen and complicated imaginary trouble. In all these remarks it has been assumed that the bearings of the engine in question are in similar condition, having all been fitted at the same time. This state of affairs is, of course, what should be aimed at, as it is only by this means that the oil flow through all the bearings will be similar. In the life of a sports car, however, there is often considerable incident, and not a little speed. Therefore, one or other of the unfortunate occurrences mentioned earlier in this article may disturb the even
tenor of its existence, and a new bearing may be required, and here care is needed. ‘the wise remarks concerning new wine in old bottles, or whatever it is, apply with even greater force to the case of a new bearing among older bearings in a pressure fed system. When the whole job is new, even if it has been fitted rather too tight, an equally large (or small) quantity of oil will go through all bearings, and if more flow is required a thinner oil can be used.
When one new bearing is fitted the case is altered. If a big end fails through temporary lack of oil or some similar cause, it will probably not be much worn. Very likely the other bearings have just settled. down nicely and are in their prime, and do not want to be disturbed at all. If the new bearing is fitted, not actually tight, but as tight as the others were when new, it will be out of step, as it were, and owing to its smaller clearances, will not get as much oil as the others. If an attempt is being made to get the utmost power from the engine with safety—and some racing engines still have plain big end bearings—this is likely to mean failure of the new bearing again. Therefore, the condition of the old bearings must be measured up and the new one fitted so that it is as nearly as possible in the same condition as the others.
If the new bearing is a big end, as suggested above, incorrect fitting will only cause its own failure, but if it is a main bearing, it will have a greater effect, as other bearings are being fed through it.
New ” Mains ” and Oiled Plugs.
Oiled plugs and new bearings do not in general seem to have much to do with each other, but a new main bearing, fitted with smaller than correct clearances, will cause extra oil to flow to the big end bearing or bearings connected with it. If they are getting slightly slack this will mean an abnormal quantity of oil being flung out to the walls of those cylinders. Hence, possibly, oiling up as the result of a new main bearing being fitted too tight. Not so far-fetched when reasoned out ! After all the manufacturer of the car did not put the oil gauge on the dash purely as an ornament, and a watchful eye on that little needle may save quite a lot of expense, and also make motoring more interesting, when its functions are more
fully understood. W.S. B.