The range of methods used to determine high-temperature limits varies widely, from a simple rule-of-thumb based on the grease thickener type, to static tests that measure a property like dropping point, to dynamic tests that give some indications of field performance.

Drop by Drop

When heat rises to extremes, grease can fail because of the degradation of its two key building blocks, base oil and thickener, due to oxidation. So base oil type and manufacturing methods are among the factors that determine temperature limits. Base oil can bleed (separate from the thickener) or simply evaporate as heat increases, resulting in loss of lubricity. The temperature at which the oil bleeds from the grease is called its dropping point.

Many customers rely on dropping point when selecting a grease for high-temperature applications. Dropping points seem significant, because they are listed on product data sheets and other references that grease suppliers provide their customers. Grease manufacturers also list a maximum service temperature, which is usually set at least 50 degrees below the dropping point – a margin of safety to which many ascribe. For example, the NLGIs Lubricating Grease Guide suggests that lithium complex greases have a maximum usable temperature of 177 degrees C, at least 83 degrees below their dropping point. An ExxonMobil Guidance is more conservative, suggesting 150 to 175 C as the upper range for one of its lithium complex greases.

As that example shows, its common to set recommendations solely on thickener type, without regard to how the grease is made or its components. Not all lithium complex greases are the same, of course, and complexing agents, manufacturing methods and base oil type all can influence the high-temperature performance of any grease, according to Chuck Coe, global grease technical advisor at ExxonMobil Lubricants in Fairfax, Va.

In a presentation last summer to the NLGI annual meeting in Williamsburg, Va., Coe described the confusion – and argued that grease life based on standardized dynamic bearing tests would be a more realistic way to measure grease high-temperature performance. The trick will be to pick the right test – and then reach a consensus on it.

Into the Lab

In early 2007, Coe and a team of researchers at ExxonMobil set out to find the most effective method to establish grease high-temperature limits. We started testing because we were not comfortable with our own recommended grease upper operating temperature claims we were making to our customers, Coe told the NGLI meeting. We want to have a sound scientific basis for claims we make. This aligns with our high road ethics and gives our field force the explanation they need for our claims, and helps them counter competitive claims made without a scientific basis.

Coe also stressed that the project was not a response to end-user issues with grease. There arent so many problems in the field, but there are more inquiries from our field force about our claims versus competitors claims.

The researchers examined nine commercial greases, including lithium complex (LiX) and polyurea thickener types, made with both mineral and synthetic base fluids. Each grease was tested for heat resistance by dropping point (ASTM method D2265) and PDSC (pressure differential scanning calorimeter, ASTM D5483), and also endured two dynamic life tests, the stringent FAG FE9 bearing test (DIN 51821) and the high-temperature wheel-bearing life test (ASTM D3527). Then the test results were compared against what each products data sheet had claimed.

Dropping point and PDSC, Coe explained, are static tests which calculate some specific property of the grease, but do not measure its performance in actual service conditions. Static tests are useful for comparing greases to one another, predicting grease life in storage conditions, and for controlling product quality during manufacture, he said. Results can depend on thickener type, base oil type, viscosity, antioxidant additives, and temperature.

At the other end, dynamic tests try to simulate grease performance under some type of working condition. In addition to base oil, viscosity, additives, thickener type and grease structural stability/oil release properties, such tests may describe bearing geometry, temperature, speed, load, load direction and seal design. The FAG FE9 and high-temperature wheel-bearing life tests (HTWB) were selected in the ExxonMobil test program because they are the basis for industry certifications: the FE9 for Europes DIN 51825 grease classification system, and the HTWB for NLGIs certification mark.

Disorderly Behavior

When the team looked at the results, they saw that the methods dont seem to correlate well to each other, nor to rank the greases in an orderly manner:

Grease 8 appeared to have a safe claim based on dynamic or PDSC testing, but seemed risky by dropping point.

While Grease 9 was ranked well by drop point and PDSC and near the top by HTWB life, it was one of the poorest by FE9.

While Grease 1 was ranked first by drop point, it was last by wheel-bearing life.

Grease 7 was ranked best by both dynamic tests, yet had the second-lowest drop point.

Grease 3 was ranked near-best by FE9, and near-worst by HTWB life.

In other words, the results were all over the place, just as the product claims had been. I was not surprised to find out that there were discrepancies between the PDSC and dynamic tests, but I was hoping that the FE9 and wheel-bearing test would tell a similar story, Coe reported. Turns out there was no correlation between the dynamic tests.

He explained that any test – dynamic or static – will rank greases differently because of variances in test parameters, such as the temperatures a particular test is set to or the operating cycles it runs. What isnt known at this point, he said, is why the two dynamic tests provide different information.

More work needs to be done to understand why different dynamic test methods rank greases differently, so that the best method can be chosen for a given grease, based perhaps on its thickener type or based on its intended application, Coe said.

Industry Responds

I see in what direction Coe is pushing, and I think it is a very good idea, said Carl Kernizan, technical director at Jesco Resources, a grease manufacturer based in North Kansas City, Mo. But I think one test would not be able to meet all your needs. It would depend on different operating conditions like high/low speeds, water ingress and the application it is used in. The application should dictate.

Bill Kersey, product manager for greases and food-grade products at Fuchs Lubricants in Kansas City, Mo., concurs. One test method may not be applicable to every application, he said. Whats needed is a test that correlates to specific field parameters that end users are comfortable with.

Kerseys colleague Nael Zaki, Fuchss grease research and development manager, also raised concerns about a new industrywide standard. Older, well-established companies have already made greases that perform at high upper-operating temperatures. We would be telling them that they have to now go back and retest all of their 180 grease products through a new way. Thats a lot of money spent, he said, but newcomers to the industry will like it.

At bearing manufacturer SKF USA, application engineer and quality director Daniel Snyder would like to see a proper grease standard – correlated to performance – to help customers make better lubricant selections. His hope is that this would stop implicating the bearing in an application failure. In application, improper grease selection results in early bearing failure, he said. Since the grease failed, the bearing is blamed for a shut-down even though improper lubrication caused it.

But Snyder, who is based in Kulpsville, Pa., seems less hopeful about the timeframe for a grease high-temperature standard. It would be difficult as it needs to be correlated to actual field performance, and proper baselines established that everyone agreed with, users and manufacturers. This will take a long time.

A Better Way?

If, as Coe proposes, a dynamic test method is the way to go, what criteria or components would an industry accepted standard need to include?

One key will be the temperature limits each application is based on, said Ken Hope, senior program leader for polyalphaolefins at Chevron Phillips Chemical in Kingwood, Texas. The duty cycle of the temperature history is very important, such as continuous service at a certain temperature, or brief excursions to very high or low temperatures, or both.

Hope believes that an industry standard would be very beneficial. Such a test should encompass both the temperature and time exposed to that temperature, since thermal degradation is a chemical process where both temperature and time are driving forces. Relying on rule-of-thumb guidelines may give rise to apples-and-oranges comparisons.

Although we do not have any issues, temperature is important, offered grease end-user David Keister, a plant engineer at Jersey Shore Steel in Jersey Shore, Pa. JSS uses lithium complex grease to get higher drop-point properties relative to a lithium grease.

A standardized test method would be superior to the rule-of-thumb guidances, but it would require the agreement of all the companies in the industry, said Christopher Horvath, a technical service chemist at FedChem LLC, Bethlehem, Pa. FedChem makes and markets aluminum complex grease thickeners. If the process of establishing the test and its criteria was backed industrywide and executed properly, ASTM would be equipped to shepherd the testing development process.

Test development can be arduous. A test or test set would have to be decided upon and then any necessary round-robin data would have to be generated, Horvath related. Data generation can be helped or hindered depending upon how prevalent the testing equipment is in the industry currently. New equipment would probably require capital investments for companies and could also hinder the development if there is not buy in that the test is necessary. I would venture that the method could take several years to complete.

Coe, whose team finished compiling its grease high-temperature data early last year, knows that it will certainly take time, more research and a wide consensus before any standard is adopted.

The way we want it to play out is for the method to become the basis of an NLGI document guidance, and then be put into ASTM test methods as a standard, Coe said.

Our goal is to be able to say that you can use this test method for these greases, so that it benefits the customer and that they can rely on it. Data sheets are confusing the customer, and what were really trying to do is take the witchcraft out of it.