With some lubricants, gauging performance life is easy, but the task is much more difficult with greases, especially with greases used in bearings. To date, the only ways that industry has found to assess the longevity of these products are empirical tests that attempt to simulate operating conditions of the machines in which they are used – an approach that can be time-consuming and inexact.

But there is some progress. Swedish bearing manufacturer SKF recently announced an upgrade to its long-popular method, the ROF. The ROF+ allows users to test a wider range of greases and a greater variety of bearings. The company says the new method also simplifies the process of gauging grease life while at the same time improving precision.

For us, grease life is the most important grease performance parameter, Piet Lugt, a scientist at SKFs Engineering and Research Center in Nieuwegein, Netherlands, said at the European Lubricating Grease Institutes Annual General Meeting in Paris in May. The ROF+ is an excellent, simple tool to measure grease life.

In a follow-up interview, Lugt explained a variety of the factors that make it so complicated to forecast the service life of grease.

The prediction is difficult for several reasons, Lugt said. The first is that grease life theories are based on standard greases. A user will most probably have a grease that is not standard. Secondly, the life strongly depends on temperature. The service life roughly halves for every temperature increase of 10 to 15 degree C. This means that you need to know the temperature with quite high accuracy.

Then there is the difference in temperature between the outer ring and the inner ring, which is usually a heat sink. For users it is not always possible to accurately determine the expected temperature in the bearing. Lugt added that it is because of temperature that greases typically last shorter periods of time in newer equipment than in old. In general, machines are trending toward higher operating temperatures.

Predicting grease performance life is also complicated by its physical form. Oils, being fluids, generally move in consistent and predictable ways when they are lubricating machines. This even applies to the lubricating film that they create. In fact, there are formulas to predict film thickness based on oil viscosity and machine speed.

Greases, on the other hand, are semi-fluid. Rather than flow, they are pulled or pushed by the movement of the components they lubricate. Grease can also end up stranded in crevices away from contacting surfaces that they are meant to lubricate, although bearing design may limit this.

In the case of grease lubrication, [the mechanism] is different, Lugt said. No simple film thickness formulas exist.

Because of the complexity of the grease lubrication mechanism, it is not possible to use physical tests to predict service life. Instead, industry uses empirical methods that are developed through extensive testing. The most common approach is to run a greased bearing until the grease ceases to adequately lubricate it – a point often determined by excessive vibration, a rise in temperature or an increase in bearing torque.

Several tests that follow this approach have been standardized and are used widely within the industry. One of them is the ROF method, which uses deep groove ball bearings. SKF introduced it more than 40 years ago and, in view of evolution in bearings and greases, decided recently that it needed updating. Specifically, the test rig could only generate a limited range of temperature and could only be run with a narrow selection of bearing types.

The new test machine includes five cast-iron housings, each containing twin test bearings mounted on a shaft. The housings also contain heaters that can be used to elevate the temperature at which the tests are run. The housings accommodate a variety of bearing types, and controls allow users to set turning speed, temperature and both axial and radial load, Lugt said.

Users clean, fill and load the bearings, set controls and then start the machine. Each housing runs until its temperature exceeds a prescribed limit, at which point it shuts off automatically. Typically, one of the two bearings in the housing has begun to fail at that point. SKF said a good test should usually not be shorter than 500 hours. When testing a grease with life that would lead to unacceptably long test durations, the time can be shortened by increasing the temperature.

Lugt offered a few examples of tests that have been run on the ROF+. In one, a group of researchers wanted to study the impact that load can have on grease life. They loaded the test machine with 6204-2Z bearings and filled them with a standard grease formulated with lithium soap thickener and mineral base oil, then set them to run at 15,000 revolutions per minute at 130 degrees C. They also set radial loads ranging from 208 to 900 Newtons. The housing set at the lightest load ran for 2,080 hours, Lugt said, while the one with the heaviest stopped after just 322 hours.

SKF said the ROF+ has already receiving positive reactions from industry. The precision of test results is elevated by the fact that a single run actually tests 10 bearings – five pairs. Because calculations about a greases life are based on the failed half of each bearing pair, there is also an element of conservatism to results. This may lead to re-greasing intervals that are shorter than absolutely necessary but greater reliability that bearings will not fail.

Obviously, choosing a very high value of reliability means that you need to relubricate the bearing or, if it is sealed, to replace it very often, Lugt said. So there is always this compromise.

The company said new method should be helpful to several types of users – grease formulators as well as developers of bearings and machines that contain them and also academic researchers.

Despite its improvements, SKF acknowledged that the ROF+ method remains far from ideal.

A perfect test would be a test with very short duration, Lugt said. SKF has such tests, but these are only used for screening purposes. The complexity of the lubrication mechanisms in grease lubrication requires a test in a real bearing. In order to have the same failure mode as in the application, one would need to go through all phases that a grease experiences in a bearing. Therefore giving in on test time is very dangerous.

Finding more efficient ways of determining grease life remains enough of a conundrum that Lugt said he does not expect significant progress in the foreseeable future.

There are test methods in existence for testing specific physical and chemical properties of grease, such as oxidation stability, consistency and so on, he said. However, determining grease life using such tests is not possible without the proper physical and chemical models. Model development is ongoing and, for sure, one day it will be possible to predict grease life using simple and short tests.