Analysis of Used Oil in Mechanical Maintenance (Part I)

In the modern industry, an effective lubrication program is essential in order to achieve a reliable production process; this requires continuous control of both conditions and performance for all lubricating and hydraulic oils. In recent years, Maintenance Engineering has intensified the use of diagnostic tools, including data from oil analysis, with particular regard to its chemical and physical conditions, the presence of contaminants and wear metals. The importance of those data was proven to be fundamental in order to make decisions regarding the maintenance of machinery: when, how and on what to intervene.


When it comes to classic maintenance strategies, we can identify two fundamental approaches to oil analysis, which have different but interconnected goals.

– The first approach is “proactive“: proactive or improved maintenance aims to keep the equipment in the best possible operating conditions. At this stage, oil analysis is one of the essential tools to establish sustainable margins for improving the equipment’s operating conditions, especially when it comes to contamination levels, without neglecting lubrication control, operating temperatures, thermodynamic parameters, etc. Proactive maintenance deals with setting targets for some parameters related to the root causes of failures, implementing preventative actions in order to reach the target audience. For example, improving the degree of online filtration, or programming cyclical offline filtration interventions, while monitoring the parameter in order to measure the effectiveness of the implemented actions. A proactive strategy is mainly concerned with monitoring the presence of contaminants and the conditions of the lubricant, ensuring that these parameters remain within strict limits of acceptability. The ultimate purpose of these actions is the prevention of wear, corrosion and general degradation of the machinery.

– The second approach is “predictive“: in this phase, oil analysis (together with ultrasonic, thermographic, vibrational analyses, etc.) aims to provide information on the machinery’s health status, in order to estimate its reliability and to allow maintenance to intervene when its conditions start to decline, rather than when it has already stopped working. This information is obtained from a parameter trend analysis (e.g. wear metals, particulate matter, byproducts of lubricant degradation), rather than using predetermined attention thresholds.


While searching for the causes of failure, blame often falls on the oil, which is sometimes defined as “degraded” or “not suitable” on the basis of analyses carried out retroactively, when it becomes difficult to distinguish the cause from the effect of the failure. In these situations, oil becomes a scapegoat, on which it is often convenient to pin the blame. In reality, experience shows that only a small part of the failures due to lubrication problems are attributable to a lubricant that has degraded or has lost its physical and performance characteristics. In terms of performance, modern lubricants are almost always far above the needs of the machinery, especially in the industrial sector; there is also a certain safety margin considered by machinery manufacturers, which indicates the use of specific lubricants (for example, in terms of viscosity). This observation should not neglect the verification of the lubricant’s chemical and physical conditions, among the various aspects to be taken into account when analyzing an oil sample. The condition of the oil is all the more important when the lubricant has a longer life expectancy (for example in steam turbines or large hydraulic systems), or with more difficult operating conditions in terms of thermal, oxidative or tribological stress (for example in diesel engines or air compressors).

The main analyses that are performed to determine the chemical and physical state of a lubricant are:

– Viscosity, which indicates the flow resistance of the lubricant. In general, it is essential that viscosity is always under control; many of the lubricant’s functions depend on this feature, such as keeping metal surfaces separated, reducing friction and eliminating heat from areas of maximum load.

– Fourier-Transform Infrared Spectroscopy (FT-IR), strategic in engine oil analysis, provides guidance on degradation, the presence of dissolved contaminants and the residual presence of some additives.

– Total Base Number (TBN), critical in oil analysis of gas engines or engines that use aggressive fuels; it indicates the residual ability of the lubricant to withstand degradation caused by the acids produced by combustion.

– Total Acid Number (TAN), which is particularly important in the analysis of oil from turbines and refrigeration units, is a fundamental indicator of the health of the base oil.

– Membrane Patch Colorimetry (MPC), critical for hydraulic and gas turbine oils, measures the amount of insoluble oil degradation that can form deposits (called varnishes).

The objectives of the analysis to evaluate oil conditions are essentially two:

– In a “proactive” sense, to monitor the chemical and physical characteristics of the lubricant and ensure that the oil meets the performance specifications provided by the manufacturer of the machinery: in short, the goal is to determine whether the oil is still suitable for operation or if it needs to be replaced.

– In specific applications, for example in gas engines, these analyses have become universally recognized, as they help to calibrate the lubricant replacement frequencies ; manufacturers of such machinery have to provide detailed procedures for parameter monitoring, including limits for used oil, and related formulas to extrapolate the replacement frequency.

– In a “predictive” sense, we look for signs of degradation or alteration of the oil that indicate a non-optimal operation of the machinery: the goal is therefore to know the health conditions of the machine, indirectly, through the modifications that the machinery itself (when not fully functional) induces on the chemical and physical conditions of the oil. These are evaluations that can hardly be performed only on the basis of fixed limits: rather, the analyst assesses the trend of the oil condition and, if it identifies a significant change in trends, it can suggest the need to perform efficiency checks of the machinery. For example, an increase in the amount of soot in an oil used by a diesel engine, with the same number of hours or km of operation of the oil, may lead to a deterioration in the quality of combustion, or a loss of compression due to wear of the segment/cylinder assembly.

Ultimately, the analysis of the condition of the used oil allows for the optimization of the costs associated with the operation of the plant, including the purchase, storage and disposal of a lubricant still in good condition, as well as the costs of labor and non-production related to the oil change itself. In addition, there is also the advantage of obtaining indications on the health of the machinery, with consequent savings in terms of prevention of unexpected downtime.

The next article will cover the analysis of contaminants and wear metals in used lubricants.

Edited by: Alessandro Paccagnini, Emanuele Croci

Extract from Maintenance issue of March 2011

Analyze, decide, improve