THE SCIENCE OF :TENT LUBRICATION
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IT is, of course, well known that the application of a lubricant to a journal bearing reduces frictional resistance and allows the shaft to run more freely. When oil is the lubricant used, the reason for the elimination of most of the friction is apparently obvious, in that it can be assumed that the oil makes the mating surfaces more slippery, but this explanation provides only a small part of the answer.
Lubrication may be divided into two classes, namely, viscous and boundary. The former relies entirely on the presence of a liquid or fluid layer of lubricant 'between the journal and bearing, whilst the latter is dependent, to a large degree, on what is known as the oiliness of the lubricant.
In Fig, I is shown, in dia,rEammatic form, a simple journal bearing in which the n.earance has been greatly exaggerated and in which lubricant is present. It will be noted that the journal does not lie in the centre of the bearing and that, contrary to expectation, it does not tend to climb the bearing wall, but takes up a position to the side of the vertical centre line.
This phenomenon is simply explained by Reynolds's hydrodynamic theory, which is based on experiments conducted by Tower, who found that the pressure to the lett of line, AB (Fig. 1), was positive, and could reach a figure of twice that calculated from the load and projected area of the bearing, whilst -the pressure on the right of the line -could be lower than atmospheric.
Basing his arguments on these results, Reynolds put forward the theory that the oil film converged, thus creating a pressure at the apex, B, which was released after this point was passed. Modern lubrication practice has established Reynolds's theory and carries it a stage farther. It has been proved that the film of lubricant is divided into three separate layers, two' of which are solid and adsorbed on the surfaces of the journal and bearing respectively, the third being liquid and sandwiched between the solid layers (Fig. 1).
When a lightly loaded Shaft revolves, it creates pressure in the film, as has already been explained. This separates the metallic surfaces and permits free rotation, the only retarding force being that required to shear the liquid layer of lubricant. That this force can reach quite high values can be appreciated when the action of a poppet valve in its guide is studied, If no oil be present, the valve stem slides frbely with a clearance of as little as .0005 in„ but, immediately oil is introduced, an appreciable force must be applied to make the valve move.
Greater Fluid Resistance with Increase In Viscosity Were a grease or an oil of high viscosity employed, the resistance would be greater than if a light oil were used, but an increase in the clearance between the valve stem and the guide bore would result in a decrease of the force resistirtg sliding Thus the load which can be supported by a viscous film of oil is increased as the viscosity goes up and frictional resistance decreased by increased bearing-tojournal clearance.
Unfortunately, this latter law cannot be put to practical use, for, when the clearance becomes too great, the oil leaks out at the ends of the bearing, so that, unless a forced system of lubrication be employed, it is possible for metal-to-metal contact to occur. Even with forced feed, the carrying capacity of the oil film in a sloppy bearing is considerably reduced. .
When the bearing is heavily loaded the liquid layer is squeezed out and the solid, adsorbed layers come into contact with each other. In consequence, the friction is dependent on the relative slipperiness between these layers. The ease with which the two adsorbed layers of lubricant slide over each other is governed by their oiliness, a property of a lubricant which still remains something of a mystery. It can be defined, in general terms, as that property which controls sliding friction but which has no influence on viscous friction. Usually, organic lubricants, such as lard oil, have more oiliness than mineral oils, and,
for this reason, the blended oils are extremely popular for heavy-duty work. No bearing, in practice, ever conforms solely with the conditions governing either viscous or boundary lubrication, each type taking its turn as the loading or speed varies.
In relatively high-speed work, the pressure in the converging oil film is correspondingly large, a fact which should guarantee a heavier load being carried than can be accommodated at lower speeds. But, with high-speed operation, the temperature of the lubricant is raised, which lowers its viscosity and, as high viscosity is of paramount importance to the strength of the oil film, an increase in temperature means a decrease in film strength.
This disadvantage has engaged the attention of lubrication engineers for a considerable time, and many steps have been taken to obviate it. The most common method of preventing seizure of the bearing is to incorporate suitable additives in the lubricant. .
It often happens that a bearing is overloaded, or that it is starved of oil. fn the case of overloading, a condition is reached beyond boundary lubrication. The adsorbed layers of oil are broken down and metal-to-metal contact occurs, Were the surfaces perfectly smooth, this would have little immediate effect, but, in reality, metallic faces which have been ground by ordinary methods and which appear perfect to the naked eye, are actually rough and have a contour as shown, exaggerated, in Fig. 2. When two surfaces of such a nature come into close contact with each other the peaks of one become locked in the valleys of the other, the metal is torn away, a great deal of heat is generated, expansion takes place, arid, eventually, the hearing seizes. The employment of additives in a lubricant prevent, to some extent, metallic contact, the degree of efficiency depending upon the ingredients used Additives take two forms—one in which the substances used combine chemically with the bearing and journal metal to form a skin of metallic salt which keeps the two apart, and the other, an inert substance, such as soapstone or graphite, which fills sip the irregularities in the surfaces and which forms on them a further adsorbed skin possessing certain lubricating properties which are inherent in the additive. With the best of the chemical ingredients, it is claimed that the film strength is increased to the extent of two and a half times at normal temperatures and by more than five times at temperatures above 200 degrees C.
Of the chemically inert additives, graphite is by far the best. Whilst in its natural state it possesses certain lubri cating properties, it is unsuitable for general use as it readily separates out from any carrier with which it is
mixed and sludgy deposits -are formed. In its colloidal
form, however, it remains in suspension in the carrier indefinitely, unless it be brought into contact with . an electrolyte, such as sulphuric acid.
In addition to its capacity for filling in the irregularities of bearing surfaces and preventing seizure, graphite possesses lubricating qualities of its own, which can be accounted for by a study of its atomic and molecular structure. The graphite molecule consists of six atoms spaced at the corners of a hexagon. These hexagons may be likened to a series of flat plates, one on top of the other, the distance between the plates being about three times that between adjacent atoms in each hexagop (Fig. 3).
Because of the greater distance between the plates, they are more loosely joined than are the individual atoms and readily slide over each other when force, such as the shearing force in a rotating bearing, is applied to them. Hence, even after all traces of oil or grease have been removed from a bearing in which graphite is present, seizure will not take place for some time. This lubricating property of graphite is made use of in
oil-less bushes, the graphite, in combination with other materials, all in a finelydivided state, being compressed to
shape and size. The bushes are of particular use in hightemperature applications as the graphite retains its lubricating ability at temperatures up to, and sometimes exceeding, 600 degrees C.
It may be gathered from the foregoing that the frictional resistance in a plain bearing is proportional to the degree of fineness of the 'surface finish. To a certain extent this is true, but only when the mating surfaces have become lapped together, or when the lubrication is truly viscous., Even with lapped surfaces, however, the action of the most minute flaw in the material causes swirls' and eddies in the oil which react on the metal and form undulations.
This formation of an irregular surface has been pat to practical use by engine builders in America' who super-finish bearings to 60 micro. ins. That is, the average depth ofthe infinitely small depressions in the mating surfaces is 60 millionths of an inch. The finish, it is claimed, reduces initial frictional resistance, decreases' running-in time and gives longer life to the bearings.
So successful have the results proved that cylinder bores are treated chemically to produce an artificially roughened surface equivalent to that provided by super-finishing methods, the results claimed being equivalent to those for bearings.
The reason for the increase in efficiency can be explained by reference to Fig. 4, which represents the surface of a cylinder after chemical treatment. The material, not being uhiform in density and hardness, is affected to a different degree at each point on its surface, so that there are comparatively icing stretches of fiat surface interspersed with depressions. The side pressure of the piston may break down the viscous and adsorbed layers of lubricant but the depressions are still filled with oil, so that lubricant is continuously supplied, if to only a small degree, to the sliding. faces.
Although progress has been made in bearing design by the introduction of super-finishing methods, the rough surface employed is not an excuse for neglecting bearings which have become scored, on the assumption that the scored depressions will act in the same way as the machined grooves. It must be remembered that the size of these grooves is infinitely sm-all but, in addition, their form is controlled on the grinding machine, so that each one may be assumed to be identical with its neighbour, enabling arguments, similar to those put forward for the chemically treated cylinder bores, to be used in proving the reduction of friction. Worn bearings, however, have an extremely irregular surface which is prone to cause seizure rather than to improve lubrication.
Ball and Roller Races Must Have a High Degree of Finish
Where ball or roller bearings are used, it is imperative that the surface finish of both races be as smooth as possible. Were roughened surfaces employed they would act as a serious deterrent to free running, producing an effect similar to the action of cobble stones on a road wheel.
T,he provision of large quantities of lubricant to ball or roller bearings increases their frictional resistance, as will be appreciated by reference to Figs. 5 and 6. Under light loads the lubricant forms a skin under the balls or rollers, in much the same way as on a journal bearing, whilst under heavy loads the film is ruptured and either boundary lubrication is affected ormetal-to-metal contact takes place. With either light or heavy loading there is insufficient space under the rolling inertia for all lubricant to pass through, so that it assumes the shape illustrated, forming a wave in front of the balls or rollers, whilst excess oil must pass around the sides.
With halls there is a larger area free from the, passage of lubricant than with rollers, hence the force resisting rolling of the former is less than that of the latter but, in either case, the greater the amount of lubricant the more must find its way around the sides, hence the greater the rolling resistance.
Unfortunately, it is impossible to arrange for the exact amount of lubricant which would provide the lowest resistance combined with a minimum warking temperature.
• Therefore, the bearing must either run free and hot (with very little lubricant), in which case it will soon fail, or it must run more sluggishly, but at a reasonable temperature.
There are no hard and fast rules governing the selection of a lubricant. One brand of oil, with exactly the same viscosity and other characteristics as another lubricant,
• may prove inferior to its rival in a practical test. General indications as to the type of lubricant to use can, however, be given. For low-speed high-pressure application, where the oil film is frequently ruptured, a solid lubricant with added graphite would be suitable.
For lighter slow-speed work a plain grease should be • used. When selecting a grease, care should be taken to ensure that it melts as a whole. If the oil in the grease melts first the soap will be left to cause overheating in the bearing. The cheaper ranges of grease can contain an excess of water and such adulterants as china clay to make up bulk, both of which should be avoided.
On most components, other than wheel bearings and the steering box, oil is employed, the types to be used being specified by the vehicle manufacturer. Whilst these recommendations should always be adhered to where possible, it sometimes happens that the specified lubricant cannot be obtained. In this event the following particulars
may be of assistance in choosing a substitute. The best results are usually obtained with oils containing a small percentage of free fatty acid (not more than 2 per cent.). The cheaper lubricants often contain more than this and, in consequence, the bearing metal becomes corroded. The viscosity of the substitute oil, and the range of temperature through which such viscosity is maintained, should be equal to that of the specified oil.
• Viscosity is the property of a lubricant which governs • fluid friction and the load-carrying capacity of the fluid layer; therefore, if the oil chosen has a higher viscosity than the original it will have a greater frictional resistance and, if the viscosity, although the same at normal temperature, decreases more rapidly in service, there is more likelihood of metal-to-metal contact.
Oiliness is a valuable feature and, for high-pressure work, an oil possessing a high oiliness value should be used. As regards gumming and sludge formation the former is dependent on the type of oil. The paint oils dry rapidly and leave a gummy skin, so that they are useless as lubricants. The semi-drying oils, of which rape is the least oxis,lizable and, therefore, used extensively for lubrication, should be avoided where possible. Although they do not gum as quickly as the paint oils they do eventually dry Out and form the customary elastic skin. All animal and vegetable oils dry out in time and should not be used exclusively, but should be blended with a nondrying mineral oil. Sludging is caused by deposition of the resinous and asphaltic constituents to be found in mineral oils.