Ball-on-disc experiment
In this part of the results, tests were performed on a Ball-on-disc device to determine the effect of the thickener on the film thickness. A series of experiments were carried out under the same conditions, but the grease was monitored using two optical methods, which were then compared between each other.
Fully flooded conditions: interferometry method
Figure 6 shows the experiment results for fully flooded conditions. The theoretical film thickness values for PAO8 base oil at fully flooded conditions have been added to the chart. The Hamrock & Dowson equations were used to calculate the film thickness [30]. The grease (A) produced a film thickness of about 500 nm at the beginning of the experiment and it can be seen from the images that this caused large particles of thickener to enter the contact area Fig. 5. The entry of these particles into contact results in large colour fluctuations. After about 50 revolutions, the lubricant started to continuously form a decreasing thickness and the dispersion of values was less pronounced. The contact images show the intrusion of a more homogeneous structure of the thickener. The grease (B) initially formed a thickness of about 140 nm and there was a continuous increase with increasing the number of revolutions. At the end of the experiment the thickness was around 200 nm. The contact images do not show larger thickener particles, only a gradual change in colour. The grease (C) showed an average thickness of about 240 nm at the beginning followed by a drop after 30 revolutions. The thickener formed large clumps which intruded into the contact in this region. After increasing the number of revolutions, there was a softening and a reduction in thickness. Grease (C) was the only sample that was near the values of base oil estimation of fully flooded conditions.
Fully flooded conditions: Light induced fluorescence (LIF)
The same experiment was performed and the fluorescence emission was captured from the central contact area. The trends for all samples are the same as for the optical interferometry measurements. However, the calibrated fluorescence intensity shows higher values Fig. 7
The concentration of the thickener is shown in Fig. 8. An increase in thickener concentration on contact was observed for grease (A, B) and a slightly decreasing trend was observed for grease (C). For grease (B, C) the values at the beginning of the experiment were between 15–20% but grease lubricant (A) the values at the beginning were around 45%. The increase in thickener concentration for grease (A, B) shows that the calibrated fluorescence intensity increased faster than the actual thickness measured by interferometry. For the grease (C), the values were closest to the standard concentration values for this sample, i.e., 16%, throughout the experiment.
Deposited thickener layer
Figure 9 shows a view of the rolling path behind the contact area after 10 revolutions of the experiment and after 370 revolutions. A change in the size of the thickener pieces was observed for grease (A, C). In the case of grease (A, C), the size of the thickener clusters was reduced. It was also observed that there were areas of the track at the beginning of the experiment without thickener. At the end of the experiment, small particles of thickener were observed almost everywhere along the pathway. Grease (B) showed the greatest difference between the beginning and end of the experiment, with a gradual adherence of the thickener to the track. At the beginning of the experiment there were only a few thickener particles, but the surface without these particles was predominant. At the end, most of the surface was covered with small particles, with the centre of the track showing the highest density. Figure 10 also shows the calibrated fluorescence intensity on the track at the beginning and end of the experiment. For grease (B), the intensity value at the end of the experiment was twice as large as at its beginning. For greases (A, C) there was no significant change during the experiment, however the overall difference between samples was significant.
Starved conditions: interferometry method
Figure 11 and Fig. 12 shows the experiment results under starved conditions. For grease (A), large particles of thickener were observed entering the contact at the beginning. There were greater differences between thicknesses than in the fully flooded conditions. The large particles diminished after 100 disc revolutions where a stable thickness of 120 nm prevailed. There were observed meniscus boundaries almost from the beginning of the experiment for all samples. No pieces of thickener were observed in the grease (B) during the experiment and there was a slight decrease in lubrication thickness from 55 nm to 35 nm. For grease (C), the pattern was the same as grease (B) except that a larger film around 70 nm was observed in the first 30 turns.
Starved conditions: Light induced fluorescence (LIF)
Figure 13 shows the results of a repeated experiment observing fluorescence intensity. For grease (A), there was a more significant decrease in intensity in the first 150 revolutions from 1500 to 800 values, and then the values stabilized. The shape of the curve is similar when using optical interferometry. However, the stabilization occurred at later speeds of the experiment and the stabilization occurred at a much higher value. Similar intensities were observed for grease (B) and grease (C), but larger deviations were observed for grease (B).
Figure 14 shows the evolution of thickener concentration during the experiment. For the grease samples (B, C), the concentration evolution was very similar, with values around 20% throughout the experiment. Only at the beginning higher values were observed. For the grease (A), an increasing trend was observed from 40–90% concentration and then stabilizing at a concentration value of around 80%.
Figure 15 shows the location of the thickener during starved conditions on the rolling track. During starved conditions, the situation was the same for grease (B, C) Fig. 16. The difference showed only grease (A). During the first 100 revolutions of the ratio on the raceway a more continuous thickener layer was formed. The thickener still has a particulate character. In the case of lubricant (C), rather base oil dyed with thickener was observed. A similar difference was observed for the side reservoirs. In the case of grease (C), more pronounced side reservoirs were formed. In all experiments there was a gradual reduction of the layer on the rolling path.
Fully flooded conditions for different velocities.
Figure 17 shows the evolution of the film thicknesses for grease (B) for different velocities. The experiments were performed using optical interferometry. Different rates of increase can be observed at different speeds and for slower speeds a faster evolution was observed Fig. 18. For 25 mm/s and within 90 revolutions the increase was to almost 450 nm. This increase can also be observed in the contact images. For a speed of 25 mm/s, an inhomogeneous rise was observed, with the thickener sticking to the centre of the contact. The growing layer started to move out of the measurable range of the slow optical method after 90 revolutions. For a velocity of 50 mm/s, the increase was more spread over the contact area and there was an increase in thickness from 100 nm to 150 nm. For a speed of 100 mm/s, the film was more formed by the base oil Fig. 5 and there was an increase in thickness from 170 nm to 205 nm. For the 200 mm/s speed, there was an increase from 270 to 280 nm. With increasing speeds of 200 mm/s.
Ball-on-ring experiment
The LIF method was used in all experiments for the ball-on-ring configuration devices. First, an experiment was performed at a constant velocity of 100mm/s with all the grease samples. Then, the observation of meniscus formation at constant velocity was performed. Subsequently, experiments with grease (B) were performed at different velocities, namely (25, 50, 100, 200, 300) mm/s under natural replenishment. These results were compared with experiments on the Ball-on-disc device at the same speeds but with artificial replenishment.
Thickener mapping at constant velocity
Figure 19 shows repeated experiments on the Ball-on-ring device when LIF was used. Grease amount of 0.3ml was evenly applied to the ring using a syringe. Very similar trends to the fully flooded conditions can be observed for the Ball-on-disc device Fig. 7. It was observed for grease (A) high values at the beginning of the experiment and then a gradual decrease. For grease (B), a constant increase in thickness was observed. For grease (C), thickness was higher for the first 30 revolutions, after which there was a slight decrease and stabilization.
Connection to the meniscus formation
Figure 21 shows the evolution of the inlet meniscus before contact in a ball bearing geometry at 100 mm/s. At the beginning of the experiment (1), there is a lot of lubricant before contact and large clumps of thickener form vortices from which clumps escape into the contact area. The lubricant tends to circulate around the contact, therefore the volume of lubricant before contact is reduced. After a few revolutions, a V-shaped meniscus is formed where the lubricant tends to flow into the middle of the rolling path (2). The volume decreases and the shear stress causes the grease structure to soften. This change will wear off very quickly and subsequent changes will be much slower. Further loss of lubricant will cause the grease to form an inclined meniscus shape (3), with lubricant flowing from the side with the larger volume of lubricant to the area with the smaller volume of lubricant. At this stage, only small particles of thickener are visible. The last phase (4) occurs when there is only a small amount of lubricant in the vicinity of the contact. At this stage, major changes in the amount of lubricant or shape no longer occurred. When speed is increased, the meniscus is reduced and the lubrication layer on the track is increased, when slowing down, the meniscus is increased. Observation shows that the contact geometry retains only a small amount of lubricant in its vicinity (approximately the width of the contact). This amount is held primarily by capillary force. It can be seen that it is a mixture of oil and thickener. However, the structure of the thickener has been refined due to shear stress.
Thickener mapping at different velocity
At a velocity of 25 mm/s for the Ball-on-disc device, a steep linear increase was observed up to a rolling distance of 50 m as shown in Fig. 21. a slight deceleration in the thickness increase was observed. The Ball-on-ring device shows a more gradual linear increase and up to a rolling distance of 40 m. Thereafter, the increase in intensity slowed down. The final value on Ball-on-ring equipment was around 1980 and the final value of Ball-on-disc equipment was around 4400 and the trend indicated further growth.
At a velocity of 50 mm/s for the Ball-on-disc device, a steep linear increase was observed throughout the experiment as shown in Fig. 22. However, the increase was slower than that for the 25 mm/s velocity. The Ball-on-ring device shows good agreement in the first 35 m of the experiment. Thereafter, the readings show a slight deviation and a stabilization was observed from 60 m onwards. The final value on Ball-on-ring devices was around 520 and the final value of Ball-on-disc devices was around 980 and the trend indicated further growth.
At 100 mm/s for the Ball-on-disc device, a steep linear increase was observed throughout the experiment as shown in Fig. 23. However, the increase was slower than for both 25 mm/s and 50 mm/s The Ball-on-ring device shows a good agreement in the first 40 meters of the experiment and then a stabilization at around 380. The final value on the Ball-on-disc device was 670 and the trend showed a further increase.
At a velocity of 200 mm/s, a steep linear increase was observed for the Ball-on-disc device throughout the experiment, as shown in Fig. 24. However, this increase was slower than for the 25 mm/s, 50 mm/s and 100 mm/s devices. The Ball-on-ring device shows good agreement in the first 25 meters of the experiment and then there is a slight decrease from a value of 380 to around 220 and stabilization. The final value on Ball-on-disc equipment was 600 and the trend showed a further increase.
At a velocity of 300 mm/s, a steep linear increase was observed for the Ball-on-disc device throughout the experiment, as shown in Fig. 25. However, this increase was slower than all previous experiments. The Ball-on-ring device shows good agreement in the first 30 meters of the experiment and then there is a slight decrease from a value of 380 to around 220 and stabilization. The difference between the velocity of 200 mm/s and 300 mm/s is visible at the beginning of the experiment, when the intensity was higher at the higher velocity