Taking oil samples on a regular basis as part of a maintenance strategy has become state of the art. Oils are tested with regards to their condition, possible contamination and wear. Laboratory results and evaluations by experienced engineers can support the identification of upcoming component failures, prove whether maintenance actions like filtration or dehydration work properly and help establish condition-based oil drain intervals.
It’s a different story when it comes to grease. In the past, continuous trend-based grease monitoring was not a common practice even though the majority of installed bearings are grease lubricated and have a substantial impact on the reliability of the equipment. However, a change in philosophy seems to be occurring with a trend toward more routine grease analysis. This has been driven by technical issues and supported by positive experiences with oil analysis programs.
In addition, there have been many examples from the field where grease analysis has proven to provide important information about grease, including the amount of wear, contamination, consistency, bleeding behavior and condition of the base oil and additives.
Since grease properties often change significantly during operation and that the contamination and wear information is concentrated within a relatively small volume that’s not affected by filtration or diluted by a huge oil reservoir, grease analysis can be a very effective condition-monitoring tool. In many cases, grease analysis was initially performed only after damage or an accident, but trend analyses of grease samples have shown that trouble with grease or bearings can be recognized in advance with a good grease analysis program.
For a valid grease sample, the proper sampling technique is required. It obviously is much more difficult to take a representative grease sample from a bearing than to take an oil sample.
To take a grease sample, use a
syringe to pull used grease
into the sampling tube.
To take a grease sample, remove the inspection screw on a slew bearing or take off the grease nipple from a rolling-element bearing. Cut the sampling tube in a length that is appropriate to enter the bearing and reach an area for taking a meaningful sample. Mount the clean tube on a syringe and press the opening of the tube onto the corresponding greased area. Use the syringe to pull the used grease into the sampling tube (at least 1 centimeter).
For some applications, it may be necessary to repeat the procedure on different sampling points of the same bearing. Approximately 1 gram of grease is enough for analysis. Be sure to watch for any color changes to avoid taking fresh grease too close to the regreasing point.
For trend analysis, samples should always be taken at the same points. A sample of the fresh grease should also be sent as a reference sample for all future analysis.
Grease samples can be analyzed by optical emission spectroscopy (OES) according to the rotrode principle. Up to 21 elements can be evaluated to obtain information regarding wear, contamination and additives. These include:
Of special interest for diagnosing a bearing or grease condition is the amount of iron and chromium, which are present as wear particles from the bearing material. Non-ferrous materials like copper, lead and tin indicate corrosive or abrasive wear from the bearing cage. If dust (silicon or calcium) or sea water (sodium, potassium or magnesium) is present, this information can help determine the reason for the presence of wear metals. The amount of metallic soap elements or a comparison of the additive content in fresh and used grease can also reveal whether the recommended grease is in use.
The particle quantifier (PQ) index is specialized for the determination of all magnetic iron particles. An index value between O and 9,999 characterizes iron particles present in the sample independent of the particle size. Because rust particles are non-magnetic, they are not measured.
The PQ index test is based on the principle that iron (and iron wear) is magnetic and can be detected by a magnet. If a grease sample contains magnetic iron wear particles, a magnetic field is disturbed. This change in the magnetic field can be measured.
Remember, the PQ index gives the total content of magnetic wear particles. Contrary to the iron wear information determined by OES, the PQ index provides information about all iron wear particles. Also, when using OES for used grease samples, only particles up to 5 microns can be detected because larger particles are not excited.
Fourier transform infrared (FTIR) spectroscopy identifies the type of base oil and thickener of the used grease. By comparing the unused fresh grease reference to the used grease sample, additive depletion or contamination by another grease type can be determined.
In comparison to FTIR spectroscopy of oil, the measurement and interpretation of a grease spectrum are more complex. The thickener compounds especially can be very dominant within important areas of the spectrum that are normally used for the calculation of the water content or oxidation.
FTIR spectroscopy is based on the principle that the molecules present in a lubricant can absorb infrared light at corresponding wavelengths depending on its typical structure. Changes in the used grease in comparison to the fresh grease reference spectrum are calculated on the typical peaks at predefined wave numbers and interpreted as oxidation, water, etc.
An FTIR spectrum can provide information regarding contamination
and any changes in a grease sample.
A very small grease sample (less than O.1 gram) is applied to an attenuated total reflectance (ATR) cell. In the contact zone, the grease sample will be exposed to infrared light. An infrared spectrum showing the absorbance of the infrared light on the corresponding wave number will be recorded and interpreted.
The infrared spectrum of a sample provides information regarding contamination and any changes in comparison to the reference spectrum. By a spectra subtraction of used grease with reference grease, the FTIR method indicates what kind of unknown grease is in use. In addition, a mixture of different greases in many cases is revealed. The identification of the original grease and the base oil type can be found by searching a library of reference spectra and can support the cause of a failure.
The FTIR method can also show whether synthetic or mineral base oils are used. If a mineral oil is used as the base oil, FTIR can indicate whether the base oil was oxidized because too much time passed without regreasing or because the temperature was too high. If the grease contains extreme pressure (EP) additives with zinc and phosphorus, the degradation of the additives can be seen. The water content in the grease may also be provided.
Besides solid contaminants, which can be identified by the OES elements silicon, calcium or aluminum, water is a type of contamination that is often the cause of corrosion. Typically, short regreasing intervals are the result of too much water. Unfortunately, determining the amount of water in grease is not as easy as in an oil sample.
For water determination according to the Karl Fischer method, a small grease quantity (approximately 0.3 grams) is placed into a glass vial and sealed with a septic cap. In a small oven, the sample is heated to approximately 120 degrees C. The steamed-out water is transferred by nitrogen into a titration vessel in which an electrochemical reaction between the water and a Karl Fischer reagent takes place. A titration curve is recorded, and the water content is defined precisely.
Depending on the grease type and application, the water content in the grease should not exceed the recommended values. Too much water in a grease can produce a variety of adverse effects, including corrosion on bearing metals, increased oxidation of the base oil, softening of the grease, and water washout of the grease.
If the result for water content according to the Karl Fischer method is compared to the elemental analysis by OES, it can be determined whether the water in the sample is “hard” or sea water, which contains minerals like sodium or potassium, or if it is soft water like condensate or rain water. If sodium, potassium, calcium and magnesium are found in the used grease but are not in the fresh grease, the presence of “hard” water is the likely reason. Comparing these two methods, Karl Fischer titration and OES, can also indicate whether the water was already present in the fresh grease as part of the production process.
Besides the previously described methods, which should be the minimum requirement for grease analysis, there are a few other tests that can be performed. The table on the left lists most of these additional tests. Keep in mind that a failure investigation after damage has occurred often requires a more complex analysis, and not every test method is designed to be a routine analysis.
In summary, grease analysis has proven to be a useful tool to evaluate grease and bearing condition. Different situations and influencing factors for wear, contamination and grease condition have shown complex coherences between the grease analysis results and their practical meaning. This leads to the conclusion that observing and interpreting these factors with expert knowledge can enable proactive maintenance strategies to be applied in a reasonable way for grease-lubricated components.