THE MEASUREMENT OF THE PROPERTY OF OILINESS 220009
The term “oiliness” is defined as that property of lubricants by virtue of which one fluid gives lower coefficients of friction (generally at slow speeds or high loads) than another fluid of the same viscosity. Its importance under practical operating conditions is shown to be greater than is generally recognized. Unfortunately, however, no satisfactory method has ever been developed for the quantitative measurement of this property in comparing different lubricants.
The paper describes the variety of possible methods of measuring the property of oiliness and of throwing light on the mechanism of partial lubrication, including (a) the use of a Deeley-type machine to measure coefficients of friction between plane surfaces at slow speeds; (b) a refined and reproducible method of determining static coefficients of friction between partially lubricated metal surfaces; (c) the measurement of the interfacial nergy between oil and mercury; (d) measurements of the electrical resistance and the rate of formation of adsorbed films on metal surfaces; and (e) the clogging of fine metal capillaries through which lubricants are forced. Some other interesting preliminary experiments also are described.
In the light of the results obtained by the above-mentioned methods, it is believed that the static-friction test, with proper refinements, is the best single measure of the properties of oiliness, but that it should be supplemented by measurements of the thickness of the adsorbed films at high pressures, in order to throw more light on the mechanism of the action of different constituents in lubricating oils.
Animal and vegetable oils are almost invariably superior in oiliness to straight mineral-oils. The blending of considerable proportions of these neutral glycerides with mineral oils greatly improves their oiliness, but the same results may be accomplished by adding much smaller proportions of other materials, such as fatty acids or oil-soluble soaps.
These experiments confirm entirely the customary hypothesis that oiliness is due to selective adsorption of constituents in the oil by the metal surface, but the common conception of a mono-molecular adsorbed film that acts merely by masking the attractive forces of the metal surfaces for one another appears to be incorrect. The adsorbed film is shown to be of colloidal rather than molecular dimensions, is a plastic solid rather than a fluid-film, and apparently acts by smoothing over surface irregularities, carrying much of the load, and minimizing metal-to-metal contact and abrasion. The structure and physical characteristics of this film seem to be more important than its thickness in determining its efficiency in lowering friction.
The constituents of lubricants that form these adsorbed layers can be selectively adsorbed and largely removed from the oil by repeated treatments with very finely divided metals, such as iron-by-hydrogen.