Volume 6, Issue 1 (January 2009)
Atomic Force Microscopy of Geroler Motor Wear Debris Ferrograms
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.