The U.S. Military specifies the use of Mil-PRF-2104 engine oil in the hydraulic system of certain nontactical military vehicles. Skid-steer loaders and other heavy equipment also use engine oils in their hydraulic systems. These vehicles are required to meet roll-off cleanliness specifications in order to improve hydraulic equipment reliability. Automatic particle counters are used to verify the cleanliness of these systems. Occasionally, particle counters detect phantom particles that cannot be removed by filtration. This paper examines the possible role of base oil and additive selection in the appearance of phantom counts. Filtered Group I and Group III base oils were doped with the components of an engine oil formulation. Particle levels were monitored before and after filtration using an on-line automatic particle counter. The results show that base oil selection has minimal bearing upon appearance of phantom counts while additive selection is a significant factor. Results from three different particle counters are compared. Two laser particle counters that operate by the light-blockage principle were found to produce phantom counts from polydimethylsiloxane antifoam additives. A direct-imaging laser particle counter classified antifoam particles as water droplets and was less susceptible to phantom particle interferences from silicone antifoam additives.
Hydraulic motors convert the flow produced by a hydraulic pump into rotary motion. These motors are often used to propel skid steers, excavators, loaders, feller bunchers, and other heavy equipment. While hydraulic pumps operate under relatively constant high-speed conditions, motors frequently come to a complete stop and reverse direction, particularly when the payload is engaged. As a result, hydraulic motors operate under boundary lubrication conditions which can promote wear-particle generation. In this study we examined wear particles generated by a geroler-type hydraulic motor under high-load conditions using online particle counters, direct-imaging laser particle analysis, ferrography, and atomic force microscopy (AFM). Relatively high quantities of particles were discharged from the hydraulic motor case drain and return-line during the first 30 min of break-in. After 30 min, the particle count decreased to an ISO 17/15/12 level. At the end of the 6-h break-in process case drain samples were approximately four-times cleaner than return-line samples. Under low-speed and high-speed, high-torque conditions, case drain samples were also very clean. Return-line particle counts were five-times higher at 1 rpm than at 450 rpm. Ferrographic analysis revealed that most of the wear particles were less than 15 μm in longest dimension and therefore may be classified as “normal” wear. AFM imaging of ferrograms was performed in contact and tapping mode using a phosphorus doped silicon probe that had a 10 nm tip radius. The largest particles produced during low-speed, high-torque operation tended to be a few hundred nanometers longer and thicker than those produced under milder break-in conditions. Ferrography proved to be a particularly useful method for preparing wear particles for AFM analysis because the ferrogram substrate is flat in the submicron range.
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