Written by Amy Rishell and John Sander
Testing
Testing by Observation
Whether testing in the field or in the lab, these initial observations should be made and recorded first.
Color/Appearance – Taking note of color changes of a grease while in-service is important. For example, if new grease is red or blue, but when sampled the grease appears green it would be significant. A visible change of this sort might suggest that the wrong grease was installed or that it has changed over time. To avoid installing the wrong grease, some users employ clear grease guns that allow the user to see the grease before installing it into the lubrication point. Common visible contaminants such as dirt, sand or wear particles can be small, while others can be large, such as pieces of rags or organic material like bark or grass. Water is another very common contaminant. When it is present, it might look like beads of sweat dripping from the grease. Water can also cause the used grease to look cloudy, runny or much lighter colored than when the grease was new.
Odor – Another thing to note is the odor of the sample. Does it smell burnt? Does it smell like fuel? Does it have a chemical odor or food odor? Depending upon where the sample was collected, caution should be practiced when smelling the sample, as certain chemical contaminants may be noxious or dangerous to inhale.
Consistency – A change in consistency is frequently observed when comparing sampled in-service grease with a new sample. For example, it may be gritty, tacky, smooth or runny. Grit can indicate particle contamination or wear particle generation. A grease changing from tacky to smooth or smooth to tacky can indicate contamination or chemical transition resultant of the end of useful life.
Environment – Seeing exactly where the grease sample was collected and taking note of the environment around the application can be helpful in any evaluation. This can be accomplished by physically sending in the part containing the lubricant or a photo of the part. Photos of the surrounding environment are also very helpful as they may provide clues about contamination sources or operating conditions.
Field Testing
The SKF Grease Test Kit is a portable kit – about the size of a briefcase – that requires only a small amount of sample. The kit is designed to evaluate consistency between batches of purchased grease and to verify whether a particular grease is acceptable for a specific application per oil bleed characteristics. It can also be useful for root cause analysis.
Lab Testing
Physical
A penetration test (F) can be used to check consistency in a grease sample. There are three different sizes of penetration tests: full size, ½ scale and ¼ scale. The ½ and ¼ scale penetration tests (ASTM D1403) are listed by their ASTM repeatability as less precise when compared to the full scale test (ASTM D217). As such, they should only be used when the sample size is limited.
Looking at the ferrous content of the sample (G) is most commonly done with analytical ferrography and ferrous density testing. This is accomplished using solvents to separate the grease components from the contaminants and then using a magnetic field to achieve further separation. The ferrous density test gives a ferrous metal concentration. Analytical ferrography involves microscopic examination of contaminants on a glass slide, allowing identification and categorizing of ferrous and nonferrous contaminants.
ASTM Subcommittee G is currently developing a rheometry test (H) for greases. A real advantage is that this new test uses an even smaller amount of sample than that of the ¼ scale penetration. Preliminary results look very promising for this technique, and it is possible that in the future this test may provide better data than the currently accepted standard tests.
The dropping point method (I), ASTM D2265, is useful for testing used greases for a couple different reasons. First, if the sample drops earlier than the published dropping point, then there typically is a contamination issue, such as lube mixing. Second, if the grease fails to drop, the test shows that the grease may have reached the end of its useful life.
Using a lightbox to examine the grease sample's appearance can be very useful. Foreign debris and inconsistencies can be seen when the sample is thinly spread out on the lightbox. This technique makes it easier to identify the type of debris. Pairing the lightbox up with a magnet can allow you to determine if the debris is ferrous.
Chemical
Fourier Transform Infrared (FTIR) analysis (J) is a great test to be run on every grease sample. It becomes even more useful if there is a sample of the new grease that can be run and have its spectrum overlaid on the used sample's spectrum. This overlay allows the technician to immediately see organic contamination and lube mixing, as well as be able to determine the base fluid characteristics, thickener type, additive degradation (oxidation), and sometimes the ratio of thickener to base fluid.
In recent years, companies such as Spectro Inc. have introduced portable spectrometers, which have been certified by ASTM D7889 Standard Test Method for Field Determination of In-Service Fluid properties using IR Spectroscopy. When immediate onsite oil condition information is needed, these handheld devices make it possible for FTIR to be performed as part of field testing.
Elemental analysis (K) on a grease sample can be accomplished with many different methods. These methods include X-ray spectroscopy, inductively coupled plasma (ICP), spark emission spectroscopy, atomic absorption (AA), and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS). In choosing the best analysis method for a given sample, considerations include particle size, sample size and sample degradation. Chosen correctly, the analysis method can provide very helpful results. Environmental contaminants, additives, thickeners and wear metals can be detected in the sample. In addition, it will be possible to identify if any lube mixing has occurred, and if so, what type.
Determining the water concentration of grease can be very important. Some of the environments that greases are used in are wet, but even when that is not the case, water can be a "surprise" contaminant that is detected in a sample. Some greases contain a minimal amount of water due to the cooking process, and some are even considered water-free. The simplest way to determine the water content in a grease sample is with Volumetric Karl Fisher. Other methods are more subjective, such as the crackle test, or require a larger sample size, such as ASTM D95 Standard Test Method for Water in Petroleum Products and Bituminous Materials by Distillation.
Pressure Differential Scanning Calorimetry (PDSC) is a useful tool for measuring the oxidation stability of lubricating grease when run according to ASTM D5483 Standard Test Method for Oxidation Induction Time of Lubricating Greases by Pressure Differential Scanning Calorimetry. However, a sample of unused grease will need to be available for comparison purposes. If an unused sample is unavailable, this test loses most of its value. The used and new samples are run under severe conditions that include elevated temperatures and pressurized oxygen.
A recently developed grease test is the RULER test (L) ASTM D7527 Remaining Antioxidant by Linear Sweep Voltammetry, which measures the remaining phenolic and aminic antioxidants in a grease sample. This test also needs a new sample to run and use as a baseline for the used sample. If a new grease sample is unavailable, the value of this test is also greatly diminished.
Reporting
Collecting the sample and having the testing done on a sample of grease is important, but completely worthless if you do not use the resulting data. When the report is received, some action needs to be taken, even if that action is to continue the normal sampling and testing.
Grease is sampled and tested typically because a problem is suspected. If that is the case, one or more of the following actions should be taken: Increase lube frequency, decrease lube amount, replace seals, remove contamination sources, or change grease. In addition, sampling and testing of the grease should continue.