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Vojnotehnički glasnik
2010, vol. 58, iss. 1, pp. 83-112
article language: Serbian
document type: Professional Paper
doi:10.5937/vojtehg1001083P


Modern methods of oil analysis in technical systems
Vojna akademija, Katedra vojnih mašinskih sistema, Beograd

e-mail: sretenperic@yahoo.com

Abstract

Different technical systems require an appropriate lubricant to be used at an appropriate place, at appropriate time and in appropriate quantity. Determination of technical systems condition has a very important role in the development of theory and practice of friction, wear and lubrication. Lubricant is, as a contact element of tribomechanical systems, a carrier of information about the state of the whole system, from the aspect of tribological and other ageing processes. The analysis of oils, based on a properly defined program, thus represents a very effective method for monitoring the condition of technical systems, which ensures early warning signals of potential problems that could lead to failure and break down of technical systems. Introduction It is not always simple to determine a type of lubricant, frequency of lubrication and the quantity of lubricant to be used. The optimal recommendation would be to follow specifications of technical system manufacturers, experience, lab research or professional recommendation of lubricant suppliers. Rational lubricant consumption can be obtained by timely oil replacement, which then enables a maximum possible period of use as well as high-quality lubrication. Since the primary role of lubricants is to reduce negative effects of tribological processes related to friction, wear and temperature increase in tribomechanical systems, all types of maintenance include lubrication as a very important part of the whole procedure. On the other hand, lubricant is, as a contact element of the system, a carrier of information about the condition of the whole system, from the aspect of tribological and other ageing processes. Therefore, an analysis of oils, based on a properly defined program, represents a very effective method for monitoring the condition of technical systems, which ensures early warning signals of potential problems that could lead to failure and break down of technical systems. Besides mechanical components in a system structure, the condition of lubricant itself is also affected, which leads to a loss of lubricating properties. Contamination and degradation of lubricating oils There are numerous opportunities for contamination and degradation of lubricating oils. Contamination and degradation of oil exploitation cannot be completely prevented, but can be significantly reduced, which is very important both for oil and for a technical system itself. The rate and degree of degradation of oil are proportional to the rate and extent of contamination. It is therefore important to prevent rapid contamination of oil, before and during use. The spectrum of oil contaminants is considerably wide. Any contaminant destructive impact on oil, reducing its physical-chemical and working properties, results in shortening its service life as well as the service life of the technical system in question. During oil exploitation, changes occur in: chemical compositions and properties of base oils, chemical compositions and properties of additives, and consequently chemical compositions of oils in general, as a result of contamination and degradation. The most significant oil contaminants are base oils degradation products, additives degradation products, metal particles as a result of wear processes, solid particles from the environment, water and products of fuel combustion. During the operation the following changes occur: contamination of oil by the products of its own degradation, by products of incomplete combustion of fuel and by contaminants of various origin. The main objectives of the analysis and monitoring of oil exploitation in vehicles The main objectives of the analysis and monitoring of oil exploitation in vehicles are: - analysis of system element wear processes, - analysis of lubricant contamination processes, - monitoring changes in the properties of lubricants in order to optimize the life of system functionality control (penetration of contaminants, temperature and pressure, filter efficiency, etc.) and - determining the extent of damage and causes of failure. The analysis of the contents of different metals in lubricants is very important. Metal particles are abrasive, and act as catalysts in oil oxidation. In motor oils, they can originate from additives, wear processes, fuel, air and cooling liquid. Metals from additives may be Zn, Ca, Ba, or Mg and they indicate additive deterioration. Metals originating from wear are: Fe, Pb, Cu, Cr, Al, Mn, Ag, Sn, and they point to increased wear in these systems. The elements originating from cooling liquids are Na and B, and their increased content indicates the penetration of cooling liquid in the lubricant. The increased content of Si or Ca, which originate from the air, points to a malfunction of the air filter. Condition monitoring through oil analysis tests There are many different types of oil analysis tests that are used to evaluate lubricants. The tests must cover three areas: technical system condition, contamination condition, and lubricant condition. From the technical system condition aspect, attention should be paid to the presence of any metal particles in oil and the tendencies in their change. The second focus would be the lubricant condition, especially viscosity change, increase in oxidation, and signs of additive depletion. The third focus would be impurities, where the emphasis should be placed on particle number, water content and metal impurities. Theoretically, oil analyses are divided into three classes. In reality, all three condition-monitoring classes are interrelated and must be considered as a whole. For example, an increase in viscosity could be an indication that a lubricant is oxidizing. But oxidation could be an incorrect conclusion, if there is no indication of an increasing oxidation tendency obtained either by the acid number (AN) values analysis or the Fourier Transform-Infrared (FT-IR) analysis. Lubricant monitoring enables its refreshing or replacement before serious technical system damage occurs. If damage is noticed in the course of operation, and is caused by impurities or lubricant problems, the technical system condition can be monitored and the system may be shut down immediately to minimize damage. There are the two types of alarms, i.e. warning signs used in oil analysis: absolute and statistical alarms. An effective oil analysis relies on the combination of both types. The warning limit is the absolute alarm. The statistical tendency takes into account variability based on oil sampling and its contamination and represents the standard deviation. The deviation from the normal variability indicates serious problems, which is the first signal for taking action and dealing with the problem. As the deviation tendency approaches the warning limit, oil replacement, oil purification or a system inspection is required. Metal particles content and viscosity or some other parameters can be tested. The normal variability range takes into account minor variations caused by analytical accuracy, sample homogeneity, etc. Statistical alarms, which provide the earliest possible warning without false alarms, are difficult to achieve. The factors such as oil adding or changing, filter changes and a sampling technique can distort the results. The following tests are most frequently used in technical system condition monitoring: - Spectrometric Analysis, - Analytical Ferrography, - Rotrode Filter Spectroscopy (RFS), - Infrared Analysis (FT-IR), - Viscosity, - Total acid number (TAN), - Total Base Number (TBN) - Water and Particle Count.

Keywords

monitoring; maintenance of technical systems; oil analysis

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