Continued from page 24
Test conditions The tests were carried out at a constant tribological normal force FN, Tribo = 1.4 N which translates to a Hertzian contact pressure pmax of about 0.5 GPa. A schematic of the test profi le is presented below in Fig. 2. In Interval I, the test load was slowly applied at the contact and the system was allowed to stabilise at the newly attained load for one minute. This was followed by the speed ramp in Interval II, wherein the ball was made to slide against the pins. The rotational speed during this step was logarithmically increased from a starting value of 10-6
rotations
rpm in Interval III. This sequence was repeated three times for each set of specimens. During the repetitions, the specimens were not separated from each other. In order to observe the infl uence of temperature on the behaviour of the greases, tests were carried out at 23°C and 60°C. Three repetitions were carried out with both greases at both temperatures.
per minute (rpm) to a peak value of 500 rpm in a span of 5 minutes. This profi le was retraced back to the starting speed of 10-6
Figure 4 Coeffi cient of friction plotted against rotational speed for data obtained from tests carried out at 60°C.
Findings The results of the tribometric tests are presented in this section in the form of Stribeck curves depicting the changes in coeffi cient of friction, µ, with regards to the rotational speed. The results have been classifi ed into two groups based on the test temperature. Only data from Interval II is presented here.
During the fi rst speed ramp, the system undergoes a break-in or running-in process wherein asperities even out either by wear or through plastic deformation. This facilitates the mating surfaces to conform with each other as much as possible. The rheological properties of grease, such as its yield strength, viscosity, etc., infl uence this running-in process. The grease physically separates the mating interfaces, especially at ambient temperatures.
Fig. 3 presents results of the tests carried out at 23°C. In this fi gure, the friction coeffi cient steeply increases during the initial phase of the speed ramp for both greases. Their peak values here represent the limiting friction coeffi cients for each of the systems. Clearly, grease #2 has a higher value of limiting friction, µ ~ 0.175, compared to grease #1 with a value of µ ~ 0.125. This implies that the force - torque in the current case - required to overcome this resistance is also higher for grease #2. Until this point the contact was defl ected by around 10 µm in both cases.
In addition to tests with the two greases, a test was also carried out under dry conditions as a reference. At lower rotational speeds, it is evident that the frictional resistance of the dry system traces the path similar to that of grease #2. However, once the ball starts its macroscopic motion at about 10-3
rpm,
the frictional resistance of the dry system drops steeply, whereas that of grease #2 takes a bit longer to reach a similar value.
Figure 5 Graph depicting the break-away friction values during the fi rst run.
In Fig. 5 the break-away coeffi cients of friction of the investigated greases are plotted as a function of temperature.
At both temperatures, grease #2 possesses a higher limiting friction compared to grease #1, with a slight increase of the coeffi cient of friction for grease #1.
Conclusion
The tests conducted in this work offer accurate, reliable and reproducible characterisation of the limiting friction of greases.
They reveal temperature-dependent effects both of static friction values and speed-dependent changes in the coeffi cient of friction.
These model-scale tests can be used for pre-screening greases prior to component or real life application testing.
Figure 3 Coeffi cient of friction plotted against rotational speed for data obtained from tests carried out at 23°C
The curves in Fig. 3 also show that the friction coeffi cient in the case of grease #2 is strongly dependent on the rotational speed as compared to grease #1.
In tests at 60°C, grease #2 displayed a higher limiting friction than grease #1, as shown in Fig. 4. Once again, the curves here illustrate that the frictional coeffi cient in tests with grease #2 is strongly dependent on the rotational speed.
Authors: Dr. Frederik Wolf1
,
Dr. Kartik Pondicherry2 1
Ostfi ldern, Germany 2
Anton Paar Germany GmbH, Department Rheology, Tribometry,
Anton Paar GmbH, Department Rheology, Tribometry, Graz, Austria
LINK
www.anton-paar.com
26
LUBE MAGAZINE NO.123 OCTOBER 2014
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