Testing lubricating greases is challenging at the best of times. Testing them under sub-zero temperatures, more so. Kartik S. Pondicherry and Florian Rummel present their findings while developing a test in extreme cold.

With increasing demands for improved efficien cy and cost cutting across many industrial sectors, formulators are facing severe challenges when keeping ahead of mechanical lubrication needs and hence many products are undergoing drastic changes.

Under such conditions, acquiring knowledge about the characteristics of a grease beforehand is highly beneficial during the selection process. This, however, is only possible for their inherent properties, such as thixotropy – the tendency of a grease to become less viscous when under stress – density, oxidation stability and its ability to prevent rust, as well as some specific tribological properties such as extreme pressure performance and loadability.

There are, of course, various other performance parameters, such as a greases resistance toward tribo-corrosion, long-term stability and friction behavior, which all need to be characterized separately for each individual tribological system.

One crucial parameter of a grease across a variety of applications is its performance in extreme cold. What follows is a look at the tribological testing of greases for low-temperature applications such as aviation, aerospace equipment and railways – applications that can be subjected to severe cold at altitude or in winter. Here, low means temperatures down to minus 80 degrees Celsius.

Method Actions

The test methodology presented here helps to characterize the frictional properties of grease over a range of temperatures using real-life bearings. This methodology is distinctive because of its low-torque and low-speed capabilities. These are extremely important while determining the breakaway torque characteristics of a system. Breakaway torque is the torque required to overcome the static frictional resistance of the tribological system and set it into macroscopic motion.

While in most cases, a low breakaway force is desirable, it must also be noted that a certain amount of resistance is still required to inhibit involuntary movement. This parameter is also important in tribological systems with dynamic motion profiles involving start-stop events, which are often detrimental to the system.

The tests presented herein were carried out on a tribometer equipped with a convection temperature device and with a bearing test configuration. (See Figure 1)

To study the influence of temperature and the rotational speed on the greases, rotational speed ramp tests were carried out at 20 C, minus 10 C, minus 40 C and minus 80 C. The normal force was maintained at 15 newtons during the entire duration of the test. During the test, the said load was applied to the contact, and the system was allowed to stabilize for three minutes, denoted as interval one in Figure 2. This was followed by interval two, wherein the rotational speed was logarithmically increased from a very slow 0.00005 revolutions per minute to a rapid 500 rpm in a span of 6 minutes. This sequence was repeated three times for each set of specimens.

Commercially available ball bearings were used in these tests with an inner and outer diameter of 11 millimeters and 25 mm, respectively. Two greases – marked grease one and grease two – were used that had slightly different compositions. Based on data from rheological tests, grease two had relatively higher viscosity than grease one.

Bearing Witness

The results are presented in the form of extended Stribeck curves (Figure 3), wherein frictional torque is plotted as a function of rotational speed. In the first part of this section, data from tests at 20 C, minus 10 C and minus 40 C are presented. Data from the tests at minus 80 C are shown separately in the latter part of this section in Figure 5.

The general trend observed is the significant increase in frictional torque at higher rotational speeds and the shift of transition into a fluid friction regime toward lower rotational speeds with a decreasing test temperature. This is due to an increase in viscosity with a commensurate decrease in temperature, which in turn increases the fluid drag at higher rotational speeds. (See Figure 3)

At lower rotational speeds, as shown in Figure 4, the initial peak represents the transition from a static to kinetic state of motion and is known as the breakaway point, while the corresponding torque is known as the breakaway torque. The higher the value of the breakaway torque, the greater the force that needs to be applied to bring in relative motion at the contact interface.

In the case of grease one, there is no significant difference between tests carried out at minus 10 C and minus 40 C. However, with the higher viscosity grease two, the breakaway torque is seen to increase with increasing test temperature. In general, grease two had a higher breakaway torque, probably due to its higher viscosity.

Figure 5 shows that during both the accelerating and decelerating phases during the minus 80 C test, grease one has a lower frictional resistance than grease two over the entire speed range, including the breakaway torque. While the trend in the breakaway torque behavior of the greases is consistent, grease one had much lower frictional resistance than grease two even at higher rotational speeds.

Also, during the accelerating and decelerating phases, the frictional resistance of grease one did not show any significant changes with respect to the rotational speed. In contrast, during the accelerating phase, the frictional resistance of grease two increased steadily with increasing rotational speed starting from 1 rpm. Also, the fluctuations in the frictional torque of grease two at medium speeds could indicate the occurrence of stick-slip events, described as a spontaneous jerking motion when two objects slide over one another.

Practical Conclusion

The work presents tribological tests carried out on two greases at several temperatures, including sub-zero conditions. Differences were observed between the performance of the greases at different temperatures, and these differences were much pronounced at high-speed regimes. However, even in the low-speed regime, the higher-viscosity grease showed a marked dependence of the breakaway torque on test temperature.

Not only can the test methodology presented above be used to characterize frictional behavior of greases used in low-temperature applications, it can also be used as a tool for pre-screening greases or bearings prior to performing bench tests or field tests, which are extremely time and cost intensive.

It can also characterize the system over a broad range of temperatures, sliding speeds and normal forces. Combining frictional data at low speeds with rheological data can yield a more comprehensive understanding of structural changes of the grease and the breakaway behavior of the entire tribological system.