Condition monitoring of lubricating oil is a preventive tool that can be used to schedule machine maintenance downtime and predict impending machine failure. Techniques and apparatus for continuous on-line debris detection and oil analysis are become more and more sought after in these modern times.
During normal machine operation small wear debris particles of on the order of 1 to 10 microns are generated. When abnormal wear begins, large debris particles in the range of 10 to 150 microns are produced. The particle size and concentration will increase gradually until machine failure. Also, during prolonged or extreme usage conditions various forms of contamination and additive depletion can begin to degrade the physical properties of the lubricant to an unsatisfactory level. Therefore, continuous monitoring of wear debris and critical oil properties is essential to prevent catastrophic system failure of machines.
This thesis demonstrates the development of a comprehensive procedure for detecting debris and analyzing physical parameters associated with lubricating oil degradation using ultrasonic and capacitance based measurements. It was found that both ultrasonic and capacitance measurements can detect particles as small as 1.75 mils (44.5 μm) in diameter. The ultrasonic system detects debris by measuring the decrease in ultrasonic intensity caused by scattering of the wave by the presence of debris in the oil. As lubricating oil is non-conductive, the capacitance based system monitors increases in the effective capacitance of the system brought on by the presence of conductive wear debris.
In an effort to expand this technique to include analysis of critical oil properties, ultrasonic and capacitance measurements are done to examine changes in the viscosity and pH of the oil. Comparative viscosity measurements were taken using the ultrasound equipment. A relationship between differences in viscosity and the amplitude and transit time of an ultrasonic wave were developed. The data shows that increases in oil viscosity lead to increased acoustic velocity and attenuation. To analyze acid content levels in contaminated oil, the capacitance measurement system has been utilized to show the correlation between increases in lubricating oil’s pH level and the increase in the capacitance of the system.