A Study of the Effect of the Macrogeometry of Rubbing Steel Bodies upon Their Point Contact on the Tribological Characteristics of Lubricants under Boundary Lubrication Conditions

The effect of the shape of contacting nonconformal elements of friction units that form an initial point contact on the tribological characteristics of lubricants during friction in the boundary lubrication mode is studied. A point contact of rubbing bodies is realized on four-ball friction machines when testing the tribological characteristics of lubricants in accordance with GOST (State Standard) 9490-75 and GOST 23.221-84. Such tests are carried out upon friction of standard bearing balls, which does not allow taking into account the impact of the materials of the rubbing bodies on the results of the experiment. Therefore, two alternative designs of mandrels are developed which make it possible to implement two friction schemes: rotating ball clamped in the spindle of the machine–three face planes of rollers and rotating ball–side surfaces of three cylindrical rollers. The loads on both the friction unit and simultaneously tested mandrel in which three balls are installed are the same (108 N), the spindle rotation speed is 1 rpm, and the duration of testing is 60 min for both friction schemes. The diameter of all the used balls is 12.7 mm; the diameters of rollers are 5 and 8 mm, respectively; and the material of the samples is steel ShKh-15 or its analog 100Cr6. The designs of all three mandrels and dimensions of the corresponding samples are chosen in such a way that the contact areas of the samples under study with the wearing ball would lie on a circle with a diameter of 7.32 mm in all cases. The tests are carried out in the absence of a lubricant and upon friction in a medium of a PAO-4 polyalphaolefin oil, as well as in the medium of the same oil with oleic acid and DF-11 (a solution of zinc dialkyldithiophosphate in a low-viscosity oil) additives. It is found that the composition of the lubricant has a decisive effect on the test results (values of the friction coefficients and areas of wear scars), while the geometry of the rubbing bodies and the maximum Hertz stresses upon a tribological contact affect the antifriction and antiwear properties of oils to a substantially lesser extent.