Investigation on on-line monitoring method for lubricating oil deterioration

Lubricating oil deterioration in operating machinery is a complicated and dynamic process with combined effects of working conditions, contaminations and wear conditions, which makes the process unpredictable in the whole life-span cycle, and the deteriorated lubricating oil may increase wear rate and cause severe wear afterwards. On-line monitoring provides the only solution to evaluate the overall variation of lubricating oil conditions by supplying real-time data. Focusing on this issue, this paper presents an investigation on the process of lubricating oil deterioration with an on-line multi-object monitoring system. Three monitoring indexes, including viscosity, permittivity and wear debris, were adopted. A four-ball tester was applied to produce accelerated oil degradation, and was monitored with the on-line monitoring system. In addition, off-line oil analysis was also adopted for verification. The main results demonstrate that the selected three indexes can reflect the aging process of lubricating oil from different aspects quantitatively. The variation rate of viscosity can represent different working conditions of the tribo-pairs, the permittivity indirectly represents the oxidation degree of lubricating oil, and the wear characteristics indicates three wear stages of tribo-pairs including run-in, normal wear and severe wear, which meets the typical "Bathtub Curve" very well.