On the Characteristics of Cylindrical Journal Lubrication at High Values of the Eccentricity

The object of the investigation described in the present paper was to obtain more definite information than is at present available on the nature of the action between two lubricated surfaces when the value of the intensity of the pressure as defined by load/(area of lubricated surface) is considerably higher than that normally attained in current practice with cylindrical journals. In the bath lubrication experiments of Mr. Beauchamp Tower in 1883, the load at seizing was about 600 lbs. per square inch of the projected area of the bearing. In recent practice with forced lubrication and special lubricants, it has been found possible to increase the load up to 2000 lbs. per square inch without seizing. In the case of worm gears, however, high efficiencies of power transmission are obtained at values of the pressure between the surfaces which are of the order of those of the limiting compressive stresses of the materials, and it has been suggestedf that perfect lubrication, by means of a film of oil completely separating the surfaces, does not obtain in such cases, but that the thickness of the oil between the surfaces is of molecular dimensions, so that the interaction of these molecules with the molecules of the surfaces will determine the nature of the frictional resistance to motion which is experienced. This condition has been termed Boundary Lubrication by Mr. W. B. Hardy The evidence in favour of the existence of boundary lubrication in the case of worm gears is that the efficiency of power transmission by such gears is very little affected by the viscosity of the lubricant, but depends appreciably on the constitution of the oil and nature of the solid surfaces. That this evidence is conclusive cannot, in the opinion of the writer, be regarded as certain, since it is conceivable that in these tests the thickness of the film and the area of it which is under pressure should vary in such a way as to produce changes in the resistance of the order of those due to variations of viscosity of the lubricant, and further, that the flow of the material of the worm wheel, which undoubtedly takes place under the extremely high pressures, may account for the observed effect of the nature of the material.