Each grease has its own lubricity, complexity and characteristics that increase the demand for the modification of existing screening test methods. These performance differences must be individually assessable in order to classify the advantages or disadvantages of a product under any given operating condition.

Modern tribometric systems provide the tools and test methodology to enable a deeper understanding of lubricants behavior in the real world. However, experience in the lab has shown that because of the wide range of greases with different chemistries and consistencies, the typical reciprocating sliding friction and wear test apparatus is not the ideal equipment to measure all tribological properties. Research reveals that the use of a special rolling test adapter (see Figure 1) has, on the other hand, given promising results.

This rolling adapter converts a translator movement into a reversing rolling motion, offering users and producers of grease an additional new pre-screening test that can simulate real-world application conditions much more closely.

Film Formation

Typically, there are three phases of the formation of a protective grease film. First, an approximately constant lubricating film thickness is formed, followed by a steep reduction of this films thickness and finally a stabilization of the films thickness.

This complexity in grease tribology is well-documented and the emphasis is usually placed by testers on the effect of the thickener on a greases tribological performance. It is also accepted that full lubrication cannot be expected in highly stressed contact. The characteristics of film formation differs at the starting or during the running-in time of a test, depending on the type of thickener, then the film thickness becomes stable after some running time.

It is worth noting that film formation in both rolling and sliding motions is influenced by temperature and the roughness of the material. Furthermore, the behavior of grease tested at slow speeds is influenced by the thickener, while the base oil is more influential at high speeds. By adding solids such as graphite, molybdenum disulfide or polytetrafluoroethylene to greases to improve their tribological properties, such as adhesion, friction and wear, their rheological characteristics (i.e., flow), especially the shear stress value, are influenced too.

Investigating the Test Parameters

Modern tribometric test systems, such as high-frequency linear oscillation test machines (also known by the German acronym SRV), provide a good enough understanding of the behavior of all kinds of lubricants. And the existing standard test methods ASTM D 5706 for determining extreme pressure properties and ASTM D 5707 for measuring friction and wear have been established for many years. But testing conditions offered by a reciprocating sliding (or linear oscillation) movement are not always close enough to real-world application conditions. Therefore, numerous tests have been performed using the rolling adapter to find the adequate parameters for differentiation between grease qualities. The following parameters were varied systematically: with and without running in at lower preload; test temperature between 50 and 200C; frequency of 5 to 50 Hz; test load of 1,000 to 2,500 Newtons; roller geometries of 4×4 and 5×5 millimeters; stroke of 1 to 2 mm; and test duration of two to 50 hours.

In most cases, the best parameters were easily determined by testing with a step program in the SRV tribometer, i.e., the chosen parameters were changed automatically, step by step, after the selected measurement time. In addition to the usual coefficient of friction (the relationship between the friction force between two objects and the normal reaction between them, represented as COF), which in the SRV is normally the peak-to-peak value (represented as COF_pp), the effective coefficient of friction per period (represented as COF_eff) is considered by this parameter study as well. Figure 2 shows the differences and the mathematical formula behind these two determination methods of the COF.

While peak-to-peak values relate to static friction, the effective value is related more to the sliding and rolling fiction values, respectively.

Additionally, by using the high-resolution analysis (HRA) function of the SRV tribometer, it was possible to verify the stability of the observed system throughout the entire testing period and at each minute chosen by the operator. By using HRA, an in-depth analysis of tribological effects was captured by continuous recording of the original COF together with the instantaneous position within one oscillation, the evaluation of the friction behavior during one stroke and the evaluation of the character and origin of the peak value (e.g. at the turning points or by a wear particle being rolled over).

One example of an HRA diagram for an oscillating/sliding ball on a disk for a standard oil is shown in Figure 3, while Figure 4 presents one example of an HRA diagram for rolling motion of one grease.

Step Test Results

As mentioned, a running-in time is absolutely necessary to stabilize the greases film structure. Generally, one hour at 500 N would be enough. However, for differentiation between the greases qualities, a load of 500 N is not enough. After a running-in time of one hour, load can be increased to the required test load.

A temperature step program was applied from 50 to 200C at two hours measuring time per temperature. One result of the temperature step program is that a high temperature above 140C affected both friction values of the test grease.

A step load program was applied with running-in load at 300 N for one hour then 500 N for 1 hour with measurement at 1,000 N and 1,300 N each for four hours and 1,500 N for six hours. The systems instability starts at a higher load of 1,500 N.

The frequency was increased from 5 to 50 Hz using a step program. One example of the results is presented here for one grease measured at two different frequencies (10 and 20 Hz) for 19 hours (see Figure 5). The wear areas were measured at the end. All other parameters, e.g. temperature 200 C, stroke of 1 mm, running in for one hour at 500 N and load at 2,000 N were identical.

As can be seen, the COF values were higher at 20 Hz than at 10 Hz and the measured wear area was consequently higher. (The total wear area was 1,990 square micrometers at 10 Hz and 2,280 square micrometers at 20 Hz.) At frequencies higher than 20 Hz, the system was not stable and the repeatability of results is not given.

As mentioned in the introduction, greases are mainly used in severe application conditions. Any pre-screening or evaluating test method should closely simulate these conditions, otherwise it is impossible to differentiate between the properties of high-quality greases. Due to this fact, and based on the results of numerous tests, the suggested test parameters for this special configuration in an SRV system are temperature of 80 to 200C, frequency of 10 to 20 Hz, stroke of 1 to 1.5 mm, running-in load of one hour at 500 Newtons and test load of 1,300 to 2,000 N for a duration more than 10 hours.

As can be seen in Table 1, key parameters such as Pmean [Mpa] – the specific pressure at contact calculated based on the geometry of the specimens – and velocity are quite comparable to real working conditions. The running distance is calculated, for example, for a testing time of 50 hours and two different stroke values.

Finally, Figure 6 shows the comparison of progression of both COF values during the entire test time for three greases formulated with the same base oil and with comparable viscosity values. The test parameters in this special case were a running-in load of 500 N for one hour, test load of 2,000 N for 18 hours, stroke of 1 mm, stroke frequency of 20 Hz and temperature of 200C.

As shown, this method can differentiate the performance of greases under very severe condition quite well.

Drawing Conclusions

The measurement of tribological properties using a rolling movement in a specially adapted oscillating tribological test system showed promising results. The test configuration and its parameters for evaluating the greases presented here have potential and can simulate some aspects of real-life field applications.

Additional measured values from HRA data, such as COF_eff and stroke, increase the value of test results and their interpretation. Finally, the adjustment of the exact testing parameters should be determined by the user according to the application and the grease quality.

Dr. Ameneh Schneider is the lubricant tribology expert at Optimol Instruments Prueftechnik GmbH in Munich, Germany. She has 15 years of experience researching lubricants and tribology testing and has published more than 20 papers on the subject, and has one patent. ameneh.schneider@optimal-instruments.de