Abstract
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.
Highlights
Lubricating oil, which is composed of base oil and varieties of additives and contains significant tribological information on an operational process, plays a vital role in the full life-span performances of the machine [1]
Due to oxidation and thermal stresses in working conditions, physical and chemical properties of lubricating oil will degrade and eventually cause the increase of wear rate, which begins with additives depletion followed by oxidation of the base oil [3,4,5]
2 Experiment setup generalized as: (1) numerous as the methods have been proposed to describe the condition of lubricating oil, sensors can only provide some physical value, while the mapping relationship between the value and real-time lubricant conditions is undetermined, which makes on-line application difficult to realized; (2) the deterioration of lubricating oil should be analyzed as a dynamical process, instead of sampling and probed in some time points; and (3) studies on lubricating oil monitoring have been confined in wear debris analysis, the degradation mechanism of lubricating oil remains indistinct in principle
Summary
Lubricating oil, which is composed of base oil and varieties of additives and contains significant tribological information on an operational process, plays a vital role in the full life-span performances of the machine [1]. According to the previous attempt of lubricant analysis in machine condition monitoring, off-line approach is proved to be useful but limited in industry [7]. On-line oil monitoring is proposed and regarded as a more suitable method for condition monitoring of machine health [9]. For this purpose, various on-line methods based on physical and chemical principles are proposed to evaluate oil condition with numerous indicators, such as viscosity, permittivity, temperature, water content and index of specific contamination [3]. A coupling relationship between lubricant properties and wear performance of machine is determined in laboratory analysis [10]. The results from on-line monitoring are examined with off-line analysis to verify the accuracy of on-line data
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