Once the reason for the oil analysis has been established, the experienced tribologist knows what he needs to look out for during the analysis. With this in mind, he first of all considers the fresh oil values, then the limit and warning values compiled in-house for the relevant system.
Fresh oil values and oil changes compared
The values that are characteristic of the fresh oil are considered for every sample. In addition to the additive elements present, particular attention is given to the IR index, IR spectrum and viscosity. For this purpose, the reference values of over 8,000 fresh oils are available in the OELCHECK database. If the wrong oil has been poured in, then this is exactly where the first indications of that will become visible. The examination of how closely the fresh oil values correspond to those of the used oil sample are taken into account for the diagnosis, particularly where there are noticeable deviations. Where there are questions regarding the inspection of fresh oils or oil changes that have been carried out, comparing with fresh oil values is essential.
Limit and warning values
Defining limit values for wear and contaminants is the biggest challenge in assessing the analysis values of used lubricants. Generally applicable limit values are only available from machine/lubricant manufacturers or their associations in a small number of cases. But which value counts as too high or unexpectedly low? Which value is still tolerable for the time being and at what point will it become critical? Is it even possible to have fixed limits for wear that is directly linked to the service life of the lubricant? The more analysis results with different usage times for a particular type of machine or engine there are available, the better the limit values for wear based on usage time that can be generated. When reviewing the laboratory results, the experienced OELCHECK tribologists use limit values that have been calculated based on many thousands of analyses. In the company‘s internal database, which is growing daily, there are now, after 25 years, over 2 million records of a wide range of machines, lubricants and applications available. In order to assess the values for wear metals, it is important to know where the elements might come from and in which combination and concentration they could potentially cause problems. Elements which in one application indicate wear could in another case be a component of oil additives or even contaminants. A number of individual values are taken into account when diagnosing a sample. However these must never be viewed in isolation, but in terms of how they relate to one another.
Here‘s an example: the viscosity of an engine oil is higher than expected. This could be the result of oil oxidation, nitration, soot ingress or even cooling water ingress. But it is also possible that the viscosity class stated is simply incorrect, or that there is fuel in the oil which is disguising the result. The abnormally high viscosity value can therefore only be interpreted in relation to other individual values.
The individual trend
For wear metals, limit values have only limited significance, particularly for very long oil usage times. If results from previous tests are available, a continual comparison is made of how the values have developed over time for the oil charge of the machine. This means that the different operating and environmental conditions can be considered individually. Detailed prognoses can therefore be made for continued low-wear operation of the machine or the extension of an oil change interval. The following example shows just how crucial this can be: an industrial gear oil CLP 220 is monitored at six-month intervals using trend analyses. There are now five oil analyses saved in the database. The copper value was always between 2 and 3 mg/kg in all five of the previous analyses. The limit value for copper for the gears being analysed is set by OELCHECK at 15 mg/kg. In the sixth analysis, the copper value suddenly reaches an unexpected 9 mg/kg. – No problem though, since the defined limit value of 15 mg/kg is still far from being reached? No, watch out! The increased copper value could indicate the early stages of a wear process in a copper-containing component, like a roller bearing with brass cage. The tribologist advises in his diagnosis that the gear oil be analysed again after three months at the latest so that countermeasures can be implemented promptly or the cause can be established by other methods.