cylindrical pins measured 6 mm both in height and diameter. The test surface of the pins was polished to an average roughness (Ra) of 0.02 µm. The specimens were rinsed in isopropanol just before starting the test to remove dirt and other organic substances.


Three lubricants with slightly different compositions were tested during these investigations. Here, they are referred to as Oils A, B, and C.


In addition to the Stribeck tests, the wear behaviour of the system with each of the lubricants was studied by running a test at a constant rotational speed of 213 rpm (100 mm s-1) for 90 minutes. At the end of the test, the wear scar diameter on the pins was also measured with the help of a light microscope.


Findings The results of the Stribeck tests are discussed in the first part of this section. The second part deals with the assessment of the data obtained from the wear tests.


Stribeck Curves During the first speed ramp (1st run) of the Stribeck test, the


mating surfaces undergo a breaking-in or running-in process, wherein surface asperities even out, either by wear, or through plastic deformation, or both. This facilitates the mating surfaces’ conforming to each other as far as possible. Also, if the lubricant contains any film-forming additives, such films could form during the 1st


run, thus influencing the behaviour of the system in the


consecutive runs. Results from the 1st


run of the Stribeck tests presented in


Figure 2. Schematic of the ball-on-three-pins setup along with the images of the measuring shaft and the sample holder.


Test conditions


The operational speed in engine components is not constant and changes dynamically. The engine oils are therefore expected to function seamlessly over the entire range of speeds. This can be simulated in a model test with the help of a Stribeck test. Herein, the rotational speed is increased logarithmically over a specified range to observe the friction response of the system.


The Stribeck tests carried out during the current investigations comprise three steps, see Table 1. In the first step, the measuring shaft is lowered until the ball comes into contact with the pins. The maximum load attained during this phase is restricted to 0.1 N in order to avoid shock loading or sudden impact on the surfaces. In the second step, the load is gradually increased until the tribological normal force (FN, Tribo) reaches the predetermined value of 15 N. The system is then held at the test load for 15 minutes. The stresses produced at the contact by the applied load are partially relaxed during this step. In the third step, the rotational speed is increased logarithmically from 10-6 to 2400 rotations per minute (rpm). This step is performed twice during each test. In the first repetition, the running-in characteristics of the system are observed.


Fig. 3 show significant differences in the friction behaviour of the oils at speeds below 1 rpm. The peak value of friction here represents the limiting friction of the system. Oil A has a distinct transition from a static to kinetic state of motion, with the value of limiting friction being around 0.48, which is significantly higher than the other two oils. The system with Oil B displays moderate resistance to initiation of motion, while Oil C shows almost no difference between its static and kinetic states of friction. At speeds greater than 1 rpm, the difference in the friction behaviour of the oils is not so significant.


Figure 3. Coefficient of friction plotted as a function of rotational speed from the 1st Run of the Stribeck tests.


During the 2nd


run, the friction behaviour of the systems depends on the extent of running-in during the 1st


a real life application.


In the current investigations, reaction films, otherwise known as tribofilms, were observed on the surface of the pins at test end. The effect of these films and the running-in process on the friction behaviour of the oils is clearly seen in Fig. 5. Here, unlike in Fig. 4, neither Oil A nor Oil B exhibits prominent peaks in its static friction regime. On the other hand, the behaviour of Oil C did not change significantly between the 1st


and 2nd Table 1. Test profile Continued on page 18 runs. run, and also


on the formation of reaction films on the mating surfaces. The 2nd


run usually offers insight into the actual running conditions in


LUBE MAGAZINE NO.130 DECEMBER 2015


17


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