Parameters for grease analyses

10% of all lubricants analysed by OELCHECK have since become lubricating greases. Which parameters are analysed compared to oils?

How conclusive are the diagnoses? How can the results be used in practice? 

Analysis kits for lubricating grease

OELCHECK offers fi ve specially designed analysis kits for lubricating greases. Each kit contains the analysis parameters of the previous kit in the range. As a rule, we need only 3 grams of grease to carry out a comprehensive analysis. Kit 1: Wear, contaminants, additives (AES)
During every grease analysis, OELCHECK uses the AES method (according to the rotrode principle) to identify up to 26 elements that indicate wear, contaminants and the state of additives.

Kit 2: AES + FTIR + PQ Index
The use of FT infrared spectroscopy (FTIR) allows the type of grease, in particular base oils, to be checked. The age and oxidation of a grease can also be assessed using this spectrum based on too high a temperature or too long a period of use. The AES process only allows particles up to a size of approximately 25 μm to be verified. The PQ index, however, includes all magnetisable iron particles present in the sample. Using this information as well as the iron levels from the RDE element analysis, OELCHECK diagnostic engineers can draw important conclusions on the wear state of the lubricated components:

  • Normal wear

– Relatively low increase of all wear elements
– Iron (Fe) up to approx. 80 mg/kg, chromium (Cr) up to approx. 10 mg/kg
– Moderately increased PQ index, up to max. 60

  • Corrosive wear

Water, acids, alkalis, aged grease
– Relatively strong increase of all wear elements
– Iron (Fe) up to approx. 150 mg/kg, chromium (Cr) up to approx. 15 mg/kg
– PQ index more than 50% below the value for iron (Fe) (rust is only slightly magnetic)

  • Fatigue wear and scuffing

End of a bearing service life
– Moderate increase of wear elements
– Iron (Fe) lies more than 50% below the PQ index value
– PQ index far exceeds 300

  • Mechanical-abrasive wear

Hard contaminants, dust
– Relatively strong increase of all wear elements
– Iron (Fe) over 100 mg/kg, silicon (Si) over 50 mg/kg
– PQ index exceeds 150

Kit 3: AES + FTIR + PQ Index + Water
Using the Karl Fischer titrator, OELCHECK analyses the grease sample to determine the water content. Alongside ‚hard‘ water, which has penetrated the bearing position from the outside, water can also be in evidence which entered in the saponification process during grease manufacture.

Kit 4: AES + FTIR + PQ Index + Water + Bleed test
Lubricating greases generally contain more than 70% base oil, with the rest being made up of a mixture of thickeners and additives. Similar to a sponge, the thickener retains the base oil. Yet during long periods of use or with high temperatures, vibrations, water entry or even mixtures of grease, this function can be compromised. The oil separates from the thickener once again – the grease ‚bleeds out‘. Using a test developed in-house, the bleeding behaviour of used and new grease is compared. Should a grease bleed out too quickly, the oil loses its ability to act as a lubricant. If a used grease bleeds out too slowly, this usually indicates it has become too dry due to bleeding or oxidation, as it has not been suffi ciently relubricated often enough.

Kit 5: AES + FTIR + PQ Index + Water + Bleed test + Consistency
NLGI consistency grades, indicated as numbers (generally 00 to 3) after the grease name, describe the relative hardness of a grease. In order to measure the consistency, a standard cone is inserted into the grease. The further the cone enters, the softer the grease and the lower its NLGI consistency grade. The consistency or penetration number are predominantly used for fresh greases. It is then assessed whether the product meets the requirements to be used as a lubricating grease. With used grease, a consistency grade other than that used for fresh grease points to a mixture which generally renders the grease softer, for example.

Correct sampling is an absolute must
One requirement for a conclusive result with any analysis is correct sampling. To this end, OELCHECK has put together a special grease sampling kit. Every kit contains a multifunctional syringe with a connectable hose for sampling and several spatulas. You can find a detailed guide on sampling at OELCHECK engineers can advise you when it comes to setting the optimal scope of analysis, examination intervals, grease sampling locations and sample quantities.

Related information

Also read The analysis of lubricating greases


OELCHECKER Spring 2017, page 8