ON-LINE CONDITION MONITORING OF HYDRAULIC OILS – UNDERSTANDING THE RESULTS

On-line condition monitoring of hydraulic oils and other lubricants can not only protect the machine from sudden breakdowns, but can also be used as a tool for predictive maintenance of the machine. In order to monitor and track changes in hydraulic oil on-line the user should have good knowledge in complete hydraulic and monitoring system, such as sensors being used, operation of the hydraulic system, location and mounting of condition monitoring system, lubricant being used... In the first part the paper presents several sensors and quantities that are most commonly used to on-line measure the properties of hydraulic oils, such as temperature, viscosity, relative humidity, electrical conductivity, relative permeability and cleanliness class. Besides presenting the quantities paper also discusses potential miss-readings and inaccuracies of the sensors. In the second part the paper presents and discusses several real case studies from on-line condition monitoring systems of hydraulic oils from industry. The study shows that some changes, such as increase of water content in hydraulic oil, can be detected on-line very reliable, whereas some other changes, such as increase or decrease of viscosity cannot be detected so easily and trustworthy.