Imagine: You receive a laboratory report and the value of a certain wear metal, e.g. iron, has doubled from 2 mg/kg to 4 mg/kg. An increase of 100% – and the expert from OELCHECK does not mention any of this in his commentary!
Well, nobody is immune to mistakes. However, in lubricant analysis, even a doubling is always relative. After all, in most cases the quantities are mg/kg or ppm, (parts per million) and thus vanishingly small quantities, because 1 mg/kg or 1 ppm corresponds to only 0.0001 %.
A simple example illustrates the relationships: An ordinary metal paper clip is dissolved in a liter of oil. Afterwards, a sample of this oil is analyzed in the lab. Approximately 500 mg/kg of iron is determined. This value seems to be quite high at first glance, nevertheless, this quantity is relative. In relation to the total volume of the sample, the iron value of 500 mg/kg (0.05%) is very small.
If the measured values of a wear metal change from sample to sample in trend analyses, the following aspects must be taken into account:
- Which components of the unit consist of or contain the concerned metal?
- How big is the surface area of these components that come into contact with the lubricant?
- How big is the total amount of oil in which the wear metal is dissolved?
One of the biggest difficulties when interpreting an oil analysis is deciding and determining whether wear has occurred on all or only some surface areas of components made of the concerned metal.
Here is an example: A 20 kWh drive gear is filled with 5 liters of oil. The iron value has increased from tolerable 15 mg/kg to 281 mg/kg within 1000 hours. Was the reason for this increase in iron corrosive wear of all gear components containing iron or rather abrasive wear of tooth flanks?