Limit values for lubricants

Limit values provide an initial indication as to whether analysis results point to a problem. However, this requires detailed knowledge of the application, the lubricant used and sometimes even the lubricant analysis itself. Important aspects of oil analysis, such as wear or oil condition, also depend signifi cantly on the duration of use. Due to different operating conditions and times, limit values are therefore often diffi cult to defi ne. The trend line is helpful here, in that it makes a more detailed assessment possible. But one point is crucial; the analytical results must always be evaluated in relation to each other. The mere consideration of individual values can lead to considerable errors in judgement. Before a tribologist can evaluate the results of an analysis, he needs a basis for his assessment. In addition to his own experience, he makes use of absolute limit values, fresh oil reference values and the trend line for this. The adjoining sample spreadsheet shows the extent to which the limit values, e.g. of elements in engine oils, can vary.

Table of contents

  1. Where do limit values come from?
  2. Absolute limit values vs trends
  3. Trends – the tool of choice?
  4. The solution from OELCHECK

Where do limit values come from?

Limit values are determined by the following groups, often with very different intentions:

  • Component manufacturers
  • OEMs
  • Oil companies
  • Standards, technical groups/organisations
  • Laboratories

Manufacturers of components often defi ne limit values for individual parameters which affect the performance or the service life of the component. For example, manufacturers of hydraulic components, such as pumps and valves, give recommendations for oil cleanliness. In particular, specifications from OEMs relating to warranty cases should be strictly adhered to. This is because OEMs also define limit values to identify necessary repairs and maintenance, establish general conditions for safe operation, and assist in evaluating oil analysis values. The limit values of oil companies usually relate to the condition of the oil. These are used to judge the point at which additive degradation or ageing has progressed too far, and the oil can no longer reliably fulfi l its tasks. This will guarantee that the oil still meets the necessary conditions of use at the time of the oil change. Limit values and evaluation bases can also be standardised, as is the case, for example, with ASTM. These values issued by specific organisations relate primarily to areas of application and must be examined on a case-by-case basis for their applicability. The limit values used by the OELCHECK laboratory are statistically determined and validated by experienced tribologists using historical analytical data which has been collected in the company‘s own database for more than 25 years. Special features such as oil and machine type, manufacturer and sampling point can also be included in the assessment in order to obtain particularly reliable and specific limit values. This is because OELCHECK asks for detailed information about every lubricant sample in its Sample Information Forms.

Absolute limit values vs trends

Absolute limit values provide quick and easy orientation (see Fig. 1). They are, for the most part, based on statistical analyses of machines operated under comparable conditions. As long as these operating conditions are in agreement, the limit values used are also applicable. Under differing conditions, such as start-and-stop versus continuous operation, the absolute limit values lose their significance. For all statistics, limit values must always be validated by experienced experts. If they are set too high, it may be that the lubricant has changed significantly or that the machine has been damaged without this being noticed. If they are too low (see Fig. 2), then the accumulating alarms are eventually ignored, because they occur regularly and often without good reason. Then, when an actual problem does arise, no one will respond quickly enough. Analytical results often show no ideal trend such as that shown in Fig. 1. Rather, the values (see Fig. 3) vary within a certain range, so that several samples (at least 3-4) are needed to form a trend. Only then is it possible to predict the range of values for the next result. It may also be the case that the same values for similar machines but differing trends lead to different recommendations. If the jump in a wear value is too large compared to the previous sample or in relation to the duration of use, an indication will appear in the OELCHECK diagnosis, even if the result of the analysis is below the absolute limit. Particularly when lubricants have been in use for a very long time, limit values can also be exceeded without the necessity of a warning (see Fig. 4). If the increase is in line with the trend and with the expected range for the duration of use, operation may continue without difficulty. The trend supplements the absolute limits by making more in-depth interpretations possible. This improves the predictive quality, and emerging problems are detected even earlier. This is because the trend only takes previous analysis values from the same machine into account, and so specific application conditions can best be considered. It works the same way with the doctor. He compares the results of a blood analysis with their general limit values in relation to the patient‘s personal attributes and life circumstances. The patient‘s medical history, i.e. the trend, is then used for the final assessment.

Trends – the tool of choice?

Trends are not always necessary for assessing analytical values. This is partly because enough previous sample data must be available, and you have to start somewhere. On the other hand, there are also parameters that can be comprehensively assessed with absolute limit values. Included among these are:

  • Increased water content – can cause corrosion and/or cavitation, different saturation limits for the different oil types.
  • Oil impurities – can lead to increased wear and affect the operation of hydraulic systems, for example.
  • Elevated silicon levels – can cause abrasive wear. When assessing silicon content, consideration must also be given to the use of silicon as an anti-foam additive.
  • Breakdown voltage – is examined, for example, for transformer and insulating oils.

The decision as to whether absolute limit values are sufficient as a basis for decision-making also depends on the situation. If an oil sample is taken at each oil change in order to detect necessary repair measures or emerging problems, often an assessment based on the absolute limit values is sufficient. If oil change intervals are to be optimised, the trends must always be considered when analyzing results that are dependent on the duration of use. However, even trends are not always reliable. This is due to the fact that the location of the sampling point and the way in which the sampling is performed can have a significant effect on the results of the analysis. Therefore, samples for trend analysis should always be taken in the same place, using the same method. Changes to the conditions of use or maintenance can also affect the trends.

The solution from OELCHECK

OELCHECK combines absolute limit values and trends in the evaluation of all analytical results. Our experienced tribologists rely on our in-house software SampleRating for sample diagnostics. The software displays limit values, trend curves, diagrams, photos and all the information from the information form pertaining to the current sample and machine at a glance. Each individual value is colour coded on the basis of often very specifi c limit values which have been validated multiple times. Our experienced tribologists can access data from the more than three million samples we have analysed, 200,000 machine-specific limit value tables and more than 10,000 fresh oil references. In this way, OELCHECK combines all the information and advantages of the various assessment methods in order to make a precise diagnosis.

OELCHECK laboratory reports do not include limit values; this is for the purpose of preventing misinterpretations, because values should never be considered individually, but always in conjunction with the other analytical values, the application and the lubricant used. Even a doctor does not simply hand the results of a blood test over to his patient. With their extensive expertise in the fields of mechanical engineering and chemistry, OELCHECK tribologists always assess the status of both the machine and the oil, taking all analysed values into account.


ÖlChecker Spring 2001, page 7
ÖlChecker Summer 2007, page 7
ÖlChecker Spring 2010, page 6/7
ÖlChecker Summer 2011, page 6
OELCHECKER Winter 2014, page 5 - 6
OELCHECKER Spring 2015, page 4 - 5