Lubrication and Lubricants
The underlying principles of friction between everyday objets of conventional smoothness seem to have been understood clearly by Leonardo da Vinci (ca. 1500). These principles were formulated by Amontons (1700) as follows:
Friction is proportional to the load normal to the rubbing surfaces.
It is independent of the area of contact.
The third and less significant rule was formulated by Coulomb (1800):
Friction is independent of the velocity of movement. Even the earliest investigators recognized that friction varies with the material and condition of the surfaces in contact; indeed, it is customary to regard the expression
Resistance to tangential motion/Force normal to the surfaces
as an approximate constant for each surface system; it is called the coefficient of friction. A useful distinction is that when motion between the surfaces is started from rest, the constant is known as the static coefficient; when motion is already established, it becomes the kinetic coefficient of friction.
Friction is an important phenomenon in everyday life, but most of the manifestations with which we are familiar are between soft, rough surfaces rather than the hard, polished ones occurring in the bearings of power-transmitting devices. Thus the high friction between a leather shoe sole and a stone pavement, which enables us to stand or walk without slipping, is due to the fact that the irregularities in the floor enter the comparatively soft leather surface pressed down on them. The friction here is due to the irregularities or asperities in the surfaces, which interlock. In a system of this nature, it will generally be found that the coefficient of static friction will increase with the time during which the surfaces have been pressed together and that the kinetic coefficient of friction changes with the velocity of motion. In addition, the static and kinetic coefficients are not the same in value. Where smooth hard surfaces are employed, the static coefficient for the surfaces at once reaches a steady value, which is not very different from that of the kinetic coefficient. It is obvious that what is involved is the slow change in shape of the nonrigid surface, supplemented by change in the degree of interlocking of asperities.
The general "laws" stated above were derived from observation on relatively smooth, relatively rigid surfaces of ordinary cleanness, thus presumably unlubricated. Actually, all surfaces prepared and handled without elaborate precautions bear, by touch or by condensation from the atmosphere, greasy films of marked lubricating value. For smooth metal surfaces so contaminated, coefficients of friction of the order of 0.1 to 0.3 have been observed. As cleanliness is improved, the coefficients rise to the point where relative sliding without damage becomes impossible and seizure occurs; this is discussed below.
The first and second laws need little change from the form in which they were derived by the early natural philosophers; the third needs restatement as follows.
Friction is practically independent of speed when this latter is above a certain minimum value, and decreases slightly with increase of speed a much higher values.
Any explanation of the nature of friction should offer reasonable opportunity for deduction of these rules. The two explanations which have been most attractive since the earliest days are based, respectively, on the resistance to sliding motion offered by interlocking roughnesses of the two surfaces and or the cohesive attraction, among molecules of the surfaces, across the interface. It is obvious that for rough surfaces such as wood, stone, or unfinished metal castings, gross asperities will be the determining factors.
FRICTION AND LUBRICATION
It has been pointed out that friction between carefully cleaned surfaces is quite high, tending to seizure, while the greasy surfaces of daily life will show coefficients near 0.1 to 0.3. Two further stages, in the progression from full lubrication to no lubrication, are recognizable; these are fluid film, thick film, or hydrodynamic lubrication, and thin film or boundary lubrication.
The mode of occurrence of thin-film and thick-film lubrication in ordinary practice may be indicated by the statement that the latter is regarded as the ideal which should prevail in all well-designed journal bearing systems when in normal motion; the former is a somewhat undesired condition existing when bearing systems are starting, stopping, undergoing oil starvation, or are under extremely severe conditions of duty. The various regions of friction and lubrication may then be listed as follows:
Dry friction of clean surfaces practically never prevails except under experimental conditions; the frictional resistance is high, and seizure occurs with extreme readiness. Dry friction of ordinary surfaces in daily life is lower than that of clean surfaces. Here also seizure occurs readily; the so-called laws of solid friction have been deduced from phenomena observed with surfaces of ordinary cleanliness.
Thin-film lubrication represents a transition stage between greasy dry friction and thick-film lubrication. It is an unstable condition and depends for its existence on what is apparently chemical reaction or secondary valence combination between the metals and the lubricant. It is most likely to prevail at times of low oil supply. In many bearing systems, lubrication is inadequate when the parts are moving at lower speeds than those for which they have been designed, as in starting or stopping. Under those conditions thin-film lubrication may prevail.
Thick-film lubrication represents a stable region in which the moving surfaces are separated by a complete film of lubricant, so maintained inspite of the pressure which constitutes tho load on the bearing system. The persistence of the oil film depends on the pumping action of the moving parts (supplemented by the supply pressure usually provided in actual machines), and the case with which this desirable condition is attained depends on the correctness of the bearing design and the proper choice of oil, particularly as to viscosity at the effective temperature.
Recognition of the dependence of friction in bearings upon the variables of the complete bearing system probably began with the observation by Petroff in 1883 that an oil of optimum viscosity could be selceted for each particular service. The voluminous studies of journal-bearing lubrication since that date have served to extend the list of controlling conditions until it includes.