Effect of Grease Filling on the Film Thickness in Deep-Groove Ball Bearings

Abstract Determining the film thickness during the bleeding phase of grease lubrication in a bearing is difficult due to the various replenishment and loss mechanisms involved. An additional factor that could influence the film thickness is the amount of grease in the bearing. In this study, film thickness is measured in deep-groove ball bearings using two different greases, each with initial grease filling percentages of 7.5%, 15%, and 30% of the bearing’s free volume. Film thickness is measured during the early stage of the bleeding phase at various loads and speeds using the electrical capacitance method. Results show that, after churning, values of temperature and film thickness are similar for the three different filling quantities. This indicates that bulk grease flow or oil bleed from the reservoirs formed in the unswept region does not impact the lubricant film thickness in the early stage of bearing operation.


Introduction
Grease is a colloid consisting of base oil, thickener, and additives (1).Grease is one of the most preferred lubricants in a bearing because of its excellent sealing properties and long-term lubrication (most deep-groove ball bearings are lubricated for life).When a bearing is filled with grease, the grease is located on the track, between the ball and cage, distributed unevenly in the bearing.Once the bearing starts to run, the grease starts to redistribute in the bearing in a process known as churning.At the end of the churning phase, most of the grease will settle on the shield and cage while the rest of the worked grease stays on the raceway track (2).
Chatra and Lugt (2) studied grease churning and showed that this phase consists of two subphases, namely, channeling and clearing.They described channeling as "a process where grease is moved from the area between the balls to the unswept area."They studied the grease left on the track during the churning process in SKF 6204 bearings and found that, within 10 min of running the bearing, only 35% of the original grease was left in the swept area of the bearing; the rest of the grease either leaked out or was located on the bearing shoulders/shields.During the clearing phase, another 5% to 10% is moved to the unswept volume, finally leaving only 20% to 25% of the total initial grease quantity in the swept area.
After churning, grease lubrication is believed to operate in the starved lubrication regime; that is, the film thickness depends on the thickness of oil layers in the inlet region of the contacts (3-5).It has been suggested that reservoirs formed during the churning phase subsequently bleed the oil and help to replenish the contacts.Along with the bled oil, a small quantity of worked grease could also lubricate the contact (6).Predicting grease film thickness during this stage-called the bleeding phase-wherein the bearing operates for most of its service life, is crucial to avoid metalto-metal contact.Estimating the grease film thickness is difficult because of the complex time-dependent grease rheology, starvation, and multiple replenishment mechanisms during grease bleeding (1).
There are three different grease lubrication mechanisms, the dominant one of which depends on the bearing geometry, grease properties, and operating conditions (7): a. Film formation is due to both thickener and base oil, including the bulk movement of grease (6,8).b.Grease reservoirs formed during churning releases the base oil in a controlled manner (9).c.Film formation is due to heavily worked grease, whereas the rest of the grease simply seals the bearing (10).
In addition to these mechanisms, there have been discussions on how the lubricant is driven toward the contact (11)(12)(13)(14)(15). Cann et al. distinguished grease lubrication mechanisms as in-contact replenishment (due to capillary action) and out-of-contact replenishment (due to replenishment between successive overrolling) (13).Of these two mechanisms, there is a clear consensus that out-of-contact replenishment within the bearing is too slow and not relevant (13,16,17).Hence, in-contact replenishment, facilitated by surface tension, is considered to be the mechanism that drives the lubricant toward the tribological contacts in rolling bearings.Understanding which mechanism governs film replenishment under which conditions remains to be studied.
Film thickness measurements at extremely low speeds studied by Morales-Espejel et al. (18), Cen et al. (19), and Concalves et al. (20) showed that the grease film is thicker than the "base oil-only film" at low speeds.In the current study, we are interested in the medium-speed range where starved lubrication conditions may be prevalent.Cann et al. studied used bearings from R2F and R0F test rigs and observed that both bearings had the same grease distribution pattern but the supply of lubricant was dependent on the bearing geometry and operating conditions (21,22).Both tests showed that only a small quantity of heavily worked grease was found on the track or on the cage, whereas most of the remaining grease was on the seals.
Cann et al. grouped the elements determining the grease lubrication mechanisms in bearings into three categories.(1) Lubrication parameters: grease rheology, base oil and thickener properties, oxidation, thermal stability, etc.; (2) Operational conditions: speed, load, temperature, and vibrations; and (3) bearing parameters: cage design, size, surface finish, and material of the bearings.
Most bearings are initially filled with grease up to 30% to 50% of the free volume (23,24).For high-speed applications, bearings are filled with grease up to about 20% of the free volume (1, 25).Past studies have shown that overfilling is adverse to bearing operation due to the higher levels of dissipative viscous drag forces overheating the bearings (24).Overfilling could result in faster thermo-mechanical grease degradation.
Grease life studies have shown that inadequate initial quantity of grease in a bearing results in shorter grease life (1).Chatra and Lugt showed that churning duration will vary in a population of bearings running under the same conditions with the same (amount of) grease, which they ascribed to small differences in the initial grease positioning during filling (2).Ward et al. found that a low initial grease volume resulted in film thickness similar to the base oil film thickness, whereas a high initial volume led to a longer runin with thicker films (26).
The question that then arises is: Does grease filling-the initial quantity of grease-in the bearing play a role in film formation during the bleed phase?If lubrication mechanism (a) is true-that is, both the thickener and bled oil lubricate the contact by the bulk movement of grease-then the film thickness would indeed depend on the quantity of grease filled.The filling quantity would also influence mechanism (b) because the total oil released via bleeding from a small quantity of grease would be low.Mechanism (c), however, will not be affected by the grease filling quantity.In this article, the impact of grease filling quantity is systematically studied using two different commonly used grease types.

Grease
Two lithium-thickened greases were selected for this study: one with base oil viscosity of 100 cSt at 40 � C and a consistency of NLGI grade 2.5, henceforth called "LiM-100-2.5" grease, and the other grease with a base oil viscosity of 474.5 cSt at 40 � C with consistency of NLGI grade 1.5, henceforth called "LiM-460-1.5" grease.Both greases have the same thickener weight percentage of 13%.Details of the grease properties are given in Table 1.

Measuring film thickness in deep-grove ball bearings
The film thickness in a deep-groove ball bearing was measured by using an electrical capacitance method in an inhouse-built bearing test rig.The detailed construction and working principle of the test rig were explained by Cen and Lugt (17).The measured capacitance is converted to film thickness using a new method described in our previous paper (27), which includes the effects of starvation on the measured capacitance.In this study, the same method is used but with a faster algorithm for the iteration loop.In our previous paper, we described a linear iteration algorithm to obtain the appropriate meniscus position.Here, we use the secant method to speed up the iteration instead of the ordinary linear iteration.SKF 6209 bearings with C3 clearance were chosen for this study.30%, 15%, and 7.5% of the free volumes of the bearings were filled with grease.The filling percentage is indicated with a prefix in the corresponding grease designation.These bearings were run for about 80 to 100 h until grease churning was finished.The bearings ran under selfinduced temperature during the churning phase, with the temperature profile used to identify when the churning phase ended.After the churning phase, the temperature of the bearing was controlled to 61 � C, using an electric heat gun and a fan.Here, the bearing temperature was measured on the outer ring of the bearings (away from the heat gun and fan).Next, the film thickness was measured for 10 min and the average value was taken for further analysis.The film thickness was measured from 400 to 4,000 rpm with 513, 700, and 900 N loads.These loads resulted in contact pressures of 1.03, 1.15, and 1.25 GPa, respectively, on the inner ring ball contact.All measurements were repeated thrice and the results are presented as averages and standard deviations.

Churning
Figure 1 shows the self-induced temperature and film thickness during the churning phase of LiM-100-2.5 grease.The temperature induced in a bearing is due to frictional heat generation in the bearing.The sources of friction in the bearing include (a) rolling friction, (b) sliding friction, (c) seal friction, and (d) friction due to viscous drag, churning, and splashing (28).In this study, noncontacting seals were used, so seal friction can be neglected.
The temperature during churning is high due to high viscous dissipation and heat generation (29,30).During the bleeding phase, the friction torque is the result of rolling and sliding friction only, leading to a lower, steady-state temperature.Except for the initial channeling subphase of churning, the instantaneous film thickness profile appears to mirror the temperature profile.This is due to a decrease in both grease and base oil viscosities at high temperatures, yielding low film thickness, and vice versa (low temperature yields high viscosity and film thickness).The initial instantaneous normalized film thickness (first 20-30 min) is very high (typically more than 1.8) during the channeling subphase of churning even though the temperature is high, which is ascribed to thickener entrapment in the contacts.During the subsequent clearing subphase wherein the grease is pushed to the sides to form reservoirs, the temperature of the bearing is still high but the film thickness is low because of lower base oil viscosity.After churning, the temperature drops to a relatively steady value, leading to a corresponding increase in film thickness.The percentage of grease filling (i.e. the initial quantity of grease) has a clear impact on the churning behavior.It can be seen that 30% filling yielded the highest peak temperature of 95 � C, whereas 15% and 7.5% fillings resulted in 73 � C and 67 � C peak temperatures, respectively.As expected, a higher filling results in higher viscous drag forces.In addition, the duration of the churning phase seems to be affected by the level of filling.Note that the final steady temperature for all filling percentages is the same.This means that rolling and sliding friction are identical (rolling friction is given by viscosity and level of starvation) and that the levels of starvation are the same at the end of churning for all of these cases.This is reflected in the bleeding phase film thickness, which is indeed also practically the same for all three different fillings.
Figure 2 shows the churning behavior of LiM-460-1.5 grease with a very high-viscosity base oil (viscosity of 474.5 cSt) and low consistency (NLGI grade 1.5).This figure shows that the channeling phase is somewhat longer and has a peak temperature that is higher than that of LiM-100-2.5 grease (with base oil viscosity of 100 cSt and consistency of NLGI grade 2.5).Note that both greases have the same thickener concentration but different base oil viscosities.The high base oil viscosity of the LiM-460-1.5grease could induce high viscous drag forces during churning.The low consistency of LiM-460-1.5 could cause it to fall back more easily onto the track, leading to more viscous heat generation.After churning-that is, during the bleeding phasethe temperature of LiM-460-1.5 grease is higher than that of LiM-100-2.5 grease.This indicates that the base oil viscosity primarily determines the bleeding phase temperature and film thickness.After churning, only a small quantity of worked grease is left on the track.Subsequently, we show that, under all of the different operating conditions, the film thicknesses in all three different fillings are the same for both greases tested.
Figure 3 shows the grease left on the seals after the churning phase.It is observed that the bearing with 7.5% filling left a small amount of grease on the seal, whereas 15% and 30% fillings deposited considerable amounts.During the churning phase, most of the grease is expelled from the swept area and deposited on the empty unswept region including the seals.Hence, a smaller grease layer is anticipated on the seals with lower fillings as these experiments verify.Table 2 shows the amount of grease left on the bearing seals after churning.It is evident that the higher the grease filling, the higher the grease layer deposited on the seals for both greases tested.Though the quantity of grease left on the seals after churning is as predicted, its effect on film thickness might be different depending on the prevalent replenishment mechanism(s).It is generally believed that, from these stationary positions, grease will release the oil by bleeding, similar to a sponge releasing water.If the quantity of grease in these stationary positions is small, the contacts will receive a low oil supply and, hence, have a low film thickness.Table 3 shows the mass of grease left inside the bearings after the tests, including the grease located on the bearing shoulders, raceways, cage bars, and cage pockets.This is measured by removing the seals from the bearings and measuring the bearing weight with grease, which is then subtracted from the bearing weight without grease.Chatra and Lugt found that, after churning in 6204 bearings (filled with grease up to 30% of the bearing's free volume), about 20% to 25% of the initial filling is left in the bearings (2).Similarly, the 30%-filled LiM-100-2.5 and 30%-filled LiM-460-1.5 greases left 22.8% and 37.8% of the initial grease amount in the bearings, respectively.With the measured mass of grease left in the bearings, we can calculate the percentage of the free volume occupied by grease; that is, the percentage of grease in the bearing relative to the free volume of the bearing.We notice that for all of the fillings, only 4.99% to 11.71% of the free space in the bearing is occupied by grease at the end of the test.
In the next section, we will discuss these effects on film thickness.We will attempt to determine whether there is indeed a bulk flow of lubricants from the unswept region to the swept region.

Film thickness
In our previous paper (27), we showed that film thickness is almost independent of speed during the bleeding phase of the grease.In this article, we present film thicknesses measured during the bleeding phase for different initial grease filling percentages.Here, the measurements are taken at a constant temperature of 61 � C.This temperature selection is arbitrary.After the churning phase of grease lubrication, the film thickness depends only on the operating conditions; that is, load, speed, and temperature.During the speed sweep, in the first few minutes after changing the speed, the film thickness is transient.Subsequently, after a few more minutes, the film thickness becomes steady at that speed and load, varying with temperature only (which here is also held constant).The instantaneous film thickness is then recorded for 10 min in the steady region and averaged to give one discrete data point for further analysis.A sample film thickness measurement is shown in Figure 7.It is evident in Fig. 4 that for the three different fillings of LiM-100-2.5 grease, film thickness remains the same at low and high speeds.It can also be seen that film thickness becomes almost constant at higher speeds for all three fillings.This accords with our previous observation (27).The film thickness in a severely starved contact is governed by the side flow of the lubricant from the contact.This side flow is determined by the residence time and frequency of over-rolling.As the speed increases, these two mechanisms balance out making film thickness almost independent of speed (14,27,31,32).
We repeated the measurements at three different loads to check whether the film thickness would change.Under fully flooded conditions, film thickness weakly depends on the load by an exponent of only −0.067 for point contacts (5).Cen and Lugt showed that the normalized film thickness during the bleeding phase of grease lubrication is related to the contact width (contact width changes with load) (14).This indicates that the relationship between film thickness and load under starved conditions is different from that during fully flooded conditions.In this study, we investigate whether this relationship is impacted by different initial grease fillings.Figure 4 shows the three different loading conditions.The contact widths for these three conditions are 1.154 mm, 1.281 mm, and 1.392 mm for the inner ringball contact across the running direction.
Cann et al. (13) studied the effect of starvation on lubricant film thickness using a ball-on-disc machine.They showed that the normalized film thickness can be related to the product: viscosity (g) � speed (u)� load 1=3 � lubrication level -1 � surface tension -1 , or viscosity �speed � contact width (b) � lubrication level -1 � surface tension -1 .On a deep groove ball bearing, Cen and Lugt (17) showed that in grease-lubricated bearings, with greases having very different bleeding rates, base oil viscosities and thickener types, the level of starvation was determined by base oil viscosity � contact width � speed-gbu-indicating that bleeding did not play a major role during this phase of the lubrication.Recently Lugt et al. (33) found that grease does not bleed before oxidation starts, and oxidation starts only after all the anti-oxidants are consumed from the grease.It was observed in their experiments that until 500 h, grease did not bleed.Their experiments were conducted at higher temperatures (110 � C).It can therefore be expected that in our tests bleed will not take place for even much longer times.Figures 4 and 5 show that the filling does not change the film thickness in any of the loading conditions.These results indicate that immediately after the churning phase, the level of lubricant remaining on the track does not depend on the initial filling quantity.The excess grease will The volume percentage is the fraction of the bearing's free volume occupied by the grease.be cleared from the track during churning.The reservoirs formed near the contact also do not play a role in lubricating the contact.If the reservoirs were contributing to the replenishment via either increased bleed or assisting the cage scraping/lubricant reflow between overrollings, an increase in film thickness would have been observed.

Replenishment
A classical explanation of grease lubrication says that during the churning phase, grease is moved to the side of the rolling elements, where it remains due to its stiffness and from where it bleeds the oil behind the contact and replenishes the next rolling element (11,34) and 2) show that the level of lubricant remaining on the track is the same for all fillings for both greases.Because the level of lubricant on the track is the same and replenishment is due to interaction between the oil and the contact, a change in the filling will not alter the film thickness.
As mentioned above, the replenishment rate of a lubricant depends on the viscosity of the base oil, irrespective of the replenishment mechanism; that is, in-contact replenishment or out-of-contact replenishment (14).Point contact results and deep-groove ball bearing film thickness results from Cann et al. and Cen et al. have shown that the higher the viscosity, the higher the starvation level.Figure 5 shows that LiM-460-1.5 grease with a high-viscosity base oil also shows similar behavior as LiM-100-2.5 grease.It was originally speculated that low-consistency greases might have lumps that continue to fall back onto the track, so film thickness might also depend on the filling quantity.However, to our surprise, experimental results show that this is not the case.Figure 5 shows that for all three loading conditions and speeds, 7.5%, 15%, and 30% grease fillings give the same film thickness.From this result, we rule out the bulk reflow of grease lumps onto the track as the predominant replenishment mechanism.
In the case of oil lubrication, if the inlet of the contact is filled with abundant lubricant-that is, under fully flooded conditions-the film thickness developed will not change with a further increase in oil level (3).It can be argued that if the quantity of worked grease left on the track is high enough even with 7.5% and 15% fillings, then the film thickness would be independent of fillings.To check whether this is the case, the relative film thickness was calculated.Relative film thickness is the ratio of measured grease film thickness to the film thickness calculated using base oil properties.We use Hamrock and Dowson's equations to calculate the base oil film thickness using the bearing's outer ring temperature.If this ratio is greater than or equal to 1, the contacts are fully flooded.If the ratio is less than 1, the contacts are starved.Figure 6 shows that both greases were under starved lubrication at higher gbu values, indicating that even during starved conditions, the film thickness is independent of grease filling in deep-groove ball bearings at moderate speeds.These results only show the lubrication mechanism in the deep-groove ball bearings in the early operating hours.One should not conclude that lower grease filling is indeed better.It was shown earlier that grease life is proportional to the volume of grease filled (36)(37)(38).Hence, lower or inadequate grease filling would result in shorter grease life.

Conclusion
� Bearings filled with different initial quantities of grease showed different temperature and film thickness profiles only during the churning phase.� After the churning phase, the self-induced temperature was the same in all three cases (30%, 15%, and 7.5% filling).This indicates that the oil level left on the track is the same irrespective of the filling.� The film thickness at different speeds and loads was the same for the various fillings.� Similar results were obtained for both greases tested, indicating that the results are applicable for greases with various base oil viscosities and consistencies.
� Bulk reflow of grease lumps or by the bleeding of base oil from the reservoirs formed on the shoulder and seals is not significant during the early stages of bearing operation.Only the oil left on the track after churning appears to lubricate the contacts.� The thickness of the layers of oil on the tracks is independent of the filling.� We showed here that the early film thickness is independent of filling.This means that clearing (the last phase in the churning process) leaves oil layers with thicknesses determined solely by the dynamics of oil flow in and around the contact (side flow and replenishment) and not by bleed from the grease reservoirs.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 6 .
Figure 6.Relative film thickness vs. product of viscosity, speed, and contact width.

Figure 7 .
Figure 7. Sample film thickness measurement showing the initially transient instantaneous film thickness stabilizing after a few minutes, at a set speed.The instantaneous film thickness values in the steady region are averaged to yield each discrete data point plotted in Figs.4-6.Grease: LiM-100-2.5,speed: 3000 rpm, load: 900 N, filling percentage: 30%.

Table 1 .
Properties of greases used in this study.

Table 2 .
Mass of grease left on the seals, measured after the experiments.

Table 3 .
Mass and volume percentage of grease left in the bearings, measured after the experiments.
(36)udies byBaly et al. (35), Gershuni et al.(16), and Cen et al.(14)showed that this is indeed not the relevant replenishment mechanism.Cann et al.(13)and Jacod et al.(36)suggested that replenishment is a local phenomenon caused by the interaction between the oil and the contact itself.The different filling results in this study show the same.Both tested greases under all three different loads and at various speeds showed that film thickness does not change with different fillings.If replenishment was not local, the higher filling would have resulted in higher film thickness.The churning results (refer to Figs. 1