Finished Lubricants

Unlock the Secrets of Your Grease


Getting used oil to give up its secrets is not easy – but experts say its a walk in the park compared to analyzing used greases. The pitfalls are numerous: Often, the few grams of grease taken from a fitting or scraped out of a bearing are not enough to do many tests on. The sample from one point in a large piece of equipment may not be representative of whats going on elsewhere in the machinery. Most standardized grease tests are meant to characterize grease during its manufacture, not to reveal what may have happened to it during use. And theres risk that the sample itself can be contaminated during handling, which makes any test result of doubtful benefit.

Only a few laboratories in the world are equipped to analyze grease based on extremely small, 1 gram to 3 gram samples. At Aprils annual meeting of the European Lubricating Grease Institute in Lisbon, engineer Steffen Bots of Oelcheck GmbH described some of the tests that are feasible, and what they may reveal about the working life of a grease.

Bots, a diagnostician and consultant with the Brannenburg, Germany-based testing laboratory, reminded the ELGI attendees that grease is not a straight-forward blend of chemicals. It contains approximately 90 percent oil and additives, held together in suspension by the other 10 percent, which is the thickener system. Most of the worlds greases are thickened with metallic soaps, especially lithium, calcium, sodium or aluminum.

Bots spent some time emphasizing the important role that customers play, since they supply the samples to be tested by Oelcheck or any other laboratory. Its important to provide samples that are large enough – not always easy when it comes to grease – and that are not contaminated, he said.

We often get very small samples, and we have to decide what type of tests we can do on such small samples to provide interesting information to our customers, Bots said. Even samples of only 1 gram or less have been probed for useful data.

The process must begin, however, with taking a good, clean sample. Usually laboratories like Oelcheck will supply kits for drawing samples; inside the kit will be a syringe to extract grease from the equipment fitting or from a bearing, a clean sample container, labels and information forms for the customer to fill out, and mailing packets. However, Bots observed that his lab has seen samples arrive in everything from bags to boxes. One of my favorites was when a customer used an empty plastic glue bottle. Surely there was some contamination in that case, he said wryly.

Once it comes into the lab, one of the first tests that may be done on a grease sample is an Optical Emissions Spectroscopy. OES can give information about up to 21 elements found in the grease, including wear particles, contaminants and additives. The method can use a sample as small as 0.3 gram of grease, which is heated in less than a minute to over 8,000 degrees C. This blast of temperature forces each of the elements in the grease to give up its own characteristic light, which can be plotted on a spectrum.

This test can generate lots of information about bearing or grease condition, Bots pointed out. For example, the presence of iron or chromium contents will suggest wear particles from the bearing itself, while nonferrous material, like copper, lead or tin, point to wear from the bearing cage. Calcium may point to contamination by dust or hard water. Also, knowing the content of elements that are supposed to be in the greases metallic soap, or comparing the additive content between the fresh and used grease, can pinpoint whether the correct grease is in place.

Another useful test is the PQ-Index, employed to detect all magnetic iron particles in the sample (but not rust, since rust particles are non-magnetic). This test uses a magnetic field to sense the total content of magnetic particles. Even though it wont tell the size and number of these particles, this test is helpful in spotting evidence of corrosion, pitting, fatigue and other wear conditions, noted Bots.

To look at how the greases base oil is doing, Fourier-Transform Infrared (FTIR) spectroscopy is helpful. I always compare FTIR to fingerprint analysis, Bots said. Each grease has a result that is very unique. FTIR is based on the principle that a greases molecules are unique, and will absorb infrared light at corresponding wavelengths depending on its structure. Tracing the peaks of the wavelengths of the fresh grease gives you the fingerprint, against which deviations in the used grease can be compared. This test can be done with as little as 0.1 gram of grease.

FTIR can show if contamination by another grease type has occurred, Bots said. It can show if synthetic or mineral base oils were used and, if the grease contains zinc-phosphorus type additives, FTIR can show if the additives have degraded. It also can help determine water content in the grease.

Water is often the culprit in corrosion and bearing damage, and can lead to shortened regreasing intervals as well. To pinpoint water content, the Karl-Fischer titration test is recommended. Again, a very small quantity of used grease may be all thats needed, about 0.3 gram.

Depending on the grease type and where its in use, the water content may suggest its reached its condemning limit and should be changed quickly. Otherwise, the water may promote corrosion of parts, oxidation of the base oil, softening of the grease and even washout of the grease from the equipment.

Grease consistency is another key property that users try to watch. For oils, viscosity is measured to tell if the oil is more thick or thin, Bots said, but for greases, the penetration or consistency indicates whether the grease is softer or more solid or stiff. Grease manufacturers usually test grease consistency using a cone penetration test. Simply put, the grease is packed into a cup, a cone-shaped weight is dropped onto the surface, and after a given number of seconds, it is measured how deep the cone has penetrated into the grease. The softer the grease, the deeper the cone weight will penetrate, of course. For used grease, though, testing this way is not as easy as it sounds, because sample sizes often are sorely limited.

Still, a used grease that is softer than the fresh sample its being compared to can yield a lot of information, Bots said, and Oelcheck can use a sample as small as 2 grams to do this test. For example, if the grease has softened, it may have been contaminated by grease of another, incompatible thickener type. It may have picked up water or another liquid, or it could have been sheared by mechanical stresses which destroyed the soap structure. If the penetration test shows the grease has hardened, its a signal that it may have lost some base oil, such as by bleed out or evaporation under high-temperature conditions.

Often, one of the most practical things that end-users seek from used grease analysis is to know whether the grease is fit to continue in service, or if regreasing intervals are timed right. In fact, most of the above tests wont tell if the grease is ageing too fast or if there is remaining useful life. But Oelcheck recommends using the RULER test to measure the content of aminic or phenolic antioxidants in the grease, and to see if their concentration is holding up. A comparison with the fresh grease can show the remaining useful lifetime of the grease, and allows a prediction of the next regreasing interval, Bots said. The RULER test can be done on a sample as small as 0.25 gram.

Being able to discern what constitutes normal wear versus more threatening wear conditions is one of the most promising aspects of used grease analysis. The good news is that even if only an extremely small sample of grease is all thats available, Bots concluded, its possible to unravel what may be going on inside it and the equipment its protecting.