GREASE FILM THICKNESS AND FRICTION IN EHL CONTACTS

Transcription

1 GREASE FILM THICKNESS AND FRICTION IN EHL CONTACTS P. M. CANN Tribology Section, Department of Mechanical Engineering, Imperial College of Science, Technology and Medicine, London SW7 2BX, UK, SUMMARY In rolling element bearings, grease is required to provide surface protection and friction reduction over a range of lubrication conditions from the boundary to the full-film. Although there is much published work [1-4] on film thickness there is very little information on the friction behaviour. In this study, the friction response of two lithium hydroxystearate greases has been studied under fully flooded conditions. A bearing simulation device has been used to measure film thickness and friction for a range of speed and slide-roll conditions at 60 C. The film thickness measurements show that the test greases give a higher film thickness than the base oil over the speed range. Film enhancement was greatest at speeds below 0.02 m/s. The greases also reduce friction (compared to the base oil) by up to 33% at 20%SRR. Friction reduction was most marked for the low speed boundary regime (λ<0.8) and this was attributed to the formation of a thickener-rich layer. In the full-film regime grease friction was 5-10% less than for the base oil at the same λ value. Keywords: grease, EHL, rolling element bearings 1 INTRODUCTION Lubricating grease is required to fulfil many functions in rolling element bearings. One of the most important aspects is the reduction of friction over extended operating periods. There have been many papers reporting film thickness studies, both in bearing [1][2] and laboratory tests [3][4], however there is relatively little information on the friction properties of grease films. There are several sources of energy dissipation in bearings, these include cage sliding friction, lubricant churning, centrifugal forces and friction associated with the rolling element/raceway contact. Although bearings are often assumed to work under nominally pure rolling there is often significant slip associated with the contact. This can be due to differential slip in the contact or ball spin [5]. In fact, some slip is necessary in the contact to generate the friction force required to drive the rolling elements. Minimising contact friction is obviously important in reducing the energy losses in bearings. However, there are other considerations as both lubricant life and bearing fatigue life are influenced by the contact friction. The temperature rise that occurs as the surface passes through the contact zone is determined by the friction [6]. Thus, high friction increases heat generation and this will contribute to higher operating temperatures. Long term operation at elevated temperatures contributes to lubricant failure through excessive oxidation and degradation of the grease [7]. High contact friction has also been linked to a reduction in the fatigue life of bearings [8]. In this paper, the friction properties of two lithium hydroxystearate greases are studied in a bearing simulation device. Friction has been measured in a point contact under different conditions of slide-roll ratio and speed. The results have been compared to those of the base oil under the same conditions. 2 FRICTION IN GREASE-LUBRICATED CONCENTRATED CONTACTS Rolling element bearings operate in the concentrated contact regime of high pressures and relatively thin lubricant films. Film thickness is determined by the operating conditions and lubricant properties in the inlet region, whereas friction is essentially determined by the film shear strength in the contact. Friction in the rolling element/raceway contact will thus depend on degree of surface protection and the nature of surface film. The degree of surface protection is often described by the specific film thickness or lambda ratio (λ), where: λ = minimum film thickness /composite surface roughness The λ ratio can be used to define different lubrication regimes, for example boundary (λ<0.8), mixed (λ ~0.8-3) and full film (λ>3). Bearing contacts will function under a range of lambda conditions depending on operating conditions and lubricant properties. (One of the most important aspects is the lubricant supply to the contact, in this paper only fully flooded conditions are considered. The question of friction behaviour under starved conditions will be considered in a separate study.) Both bearing fatigue life and lubrication performance are related to lambda ratio [1][8]. At high lambda (λ>3) there is minimal wear and extended bearing life [1][8]. With decreasing lambda, wear takes place and fatigue or lubricant failure will occur more rapidly. The usual range quoted for bearing operation is 0.8<λ>2 [1]. Severe damage and failure will occur in the low lambda (λ<0.8) thin film regime. Thus, a bearing will experience a range of lubrication regimes ranging from the boundary to full film. At low speeds, high temperatures or at start-up generation of a fluid film does not occur and the boundary properties predominate. In many cases grease thickener systems are chemically similar to classical boundary additives

2 and would be expected to play an important role under thin film conditions. There is evidence that the thickener can form a deposited film (10-50 nm) at the metal surface and this has been reported for both urea [9][10] and soap-thickened greases [10]. However, compared to the many studies of EHL behaviour, the boundary properties of grease have been ignored in the research literature. One of the few papers is by Godfey [11], who studied the boundary friction properties of a range of greases in a slow-speed sliding contact. He found that the friction coefficient of soap-containing greases decreased with temperature to a value of ~ 0.1 at 100 C. This was 40% less than the corresponding value for the base oil. At high speeds, an EHL fluid film is formed which separates the surfaces. The rheological properties of the fluid in the inlet region are important for film building, whereas friction is determined by the lubricant shear strength in the contact. It is usually assumed that the bulk fluid properties dominate friction in this regime, however with grease, the presence of the thickener could also have an effect. Rasteger and Winer [12] measured traction coefficient for a range of greases in a laboratory traction device. The effects of slide/roll ratio, load and ellipticitiy ratio were studied. Generally, the traction coefficient of the grease was lower than that of the corresponding base oil although this did vary with speed. Typical values for of were obtained. Thus in both the boundary and fluid film regimes the composition of the lubricant entering the contact plays an important role in determining film thickness and friction. However, it is likely that the largest contribution from the thickener will be seen in the boundary or mixed regimes. A Fresh grease Inner raceway film Lithium hydroxystearate bands '' bands cm Figure 1: IR spectra from lubricant film on inner raceway of deep-groove ball bearing This classification of lubrication regimes was originally developed to describe base oil containing simple additive systems. The situation is more complex with grease as it is a two-phase lubricant containing base oil, thickener and often an additive package. There is considerable evidence that under fully flooded conditions thickener is entrained into the contact. The increased viscosity in the inlet generates a thicker film than for the base oil alone [3][4]. There is also evidence that at low speeds an increased concentration of thickener enters the contact [13]. However, most theories of grease lubrication in bearings concentrate on the oil-bleed models [14] and ignore any contribution from the thickener. Additional evidence of the importance of the thickener in film formation is provided by examination of lubricant films in bearings [15]. Figure 1 shows an IR spectrum taken from the film in the rolling track of a bearing inner raceway. The fresh grease spectrum is also shown. The absorption bands due to the thickener are present. The analysis clearly shows that the lubricant film in the raceway track contains a significant proportion of thickener. The foregoing review has shown that the friction behaviour depends on the properties (thickness, composition) of the lubricant film. In classical fluid film lubrication (λ >3) the friction is mainly due to fluid shear. In the boundary regime, it is determined by surface chemistry. One of the main aims of this paper was to study the friction behaviour of grease under both boundary and full film conditions. A second interest is the lubricating behaviour of the thickener. Usually this is implicitly ignored and it is assumed that the thickener plays no direct role in film formation. However, there is evidence that the thickener is present in bearing lubricating films. The role of the thickener in determining friction performance, in the both the boundary and full film regime is also studied in this paper. 3 EXPERIMENTAL 3.1 Test Lubricants The test greases were lithium hydroxystearate thickened at 5 and 15 weight percent in mineral base oil. They were additive-free. The base oil viscosity was 200 cst at 40 C. 3.2 Test Device The film thickness and friction measurements were carried out on the same ball-on-disc EHL device. This was supplied by PCS Instruments and has been described in earlier papers [4]. Although it was not possible to perform these measurements simultaneously, it is thought valid to compare the results. For the film thickness measurements a glass disc and steel ball were used. A modified form of optical interferometry was used to measure the film thickness in the centre of the contact. This technique and its application to grease studies has been extensively reported [4]. For the friction measurements the glass disc was replaced by a polished steel disc and a shaft mounted ball was used. The ball and disc are driven by separately controlled DC servomotors allowing varying slide-roll ratios to be set. The friction force on the ball is measured by a strain gauge arrangement, where the ballshaft is linked through a strain gauge beam to the ball drive motor (see Figure 2).

3 Temperature controlled bath Strain gauge The constant speed levels of 0.01 and 0.5 m/s were chosen to correspond to the boundary and full film regimes. See test results in Figure 3. Steel disc Disc drive shaft Load Ball motor Figure 2: Schematic diagram of traction test method In this arrangement, the total force measured also includes losses due to seals and bearings and to remove these errors the disc speed was varied during a single measurement. Although the entrainment speed remains constant the direction of the traction force reverses so that losses on the ball drive can be estimated. Condition Friction Film thickness Temperature 60 o C 60 o C Max. Hertz pressure 1 GPa 0.5 GPa Slide-roll ratio 20% 0%, 20% Entrainment speed m/s m/s Table 1. Friction and film thickness test conditions Test specimen Specification Ball 19.05mm diameter AISI steel 10-13nm RMS roughness Steel Disc 10cm diameter AISI steel 25-30nm RMS roughness Glass disc 10cm diameter Chromium/silica coating Table 2. Test specimen specification Slide-roll ratio in these tests is defined as: %SRR = [(U 1 -U 2 )/(U 1 +U 2 )] % pure rolling, 100 % pure sliding The disadvantages of the EHL friction measurement are that due to the angle of rotation of the ball there is a small spin component within the contact, however this is estimated to be less than 0.3 %. The measurement method also assumes that the friction forces generated in either driving direction should be the same. Although this should theoretically be valid for full fluid film lubrication, the assumption is not so convincing for starved grease films. The main advantage in using the EHL device is that the friction tests can be run under controlled conditions of lubricant supply. Thus, friction measurements have been carried under fully flooded conditions in a similar fashion to the film thickness tests. Test procedure: a channelling device was used to push the over-rolled grease back into the track to ensure the lubricant supply to the inlet. A fresh sample of the grease was used for each test. The test was run initially at variable speed ( m/s) but constant %SRR (20 %). Then at constant speed (0.01, 0.5 m/s) and variable %SRR (10 50 %) 4 RESULTS 4.1 Film thickness and friction A plot of film thickness as a function of speed is shown in Figure 3. Film thickness [nm] Disc speed [m/s] Figure 3: Fully flooded film thickness results for 5 and 15 % grease, 20 %SRR. result is also shown. Over the speed range, the grease film thickness exceeds that of the base oil. The percentage film increase (relative to the base oil) increases with thickener content [4]. For example at 0.5 m/s film increases of 60 % and 150 % are obtained for the 5 % and 15 % greases respectively. In the higher speed range, the grease film increases with speed with a log-log gradient close to 0.67, as predicted by classical EHL theory [5]. At low speeds however grease often gives much thicker films than predicted, this is due to the entrainment of an increased concentration of thickener into the contact [4][13]. This effect can be seen in the film thickness results for speeds below m/s. The friction results are shown in Figures 4-6. The variable speed results are shown for both greases and the base oil in Figure Baseoil Ball speed [m/s] Figure 4: Variable speed results for 5 and 15 % greases at 60 C.. 1

4 Over the entire speed range, the greases gave lower friction than the base oil. The reduction was most marked at low speeds (<0.05 m/s) and high speeds (>0.15 m/s). Friction coefficient at constant speed and variable slideroll is shown in Figures 5 and 6. Friction increases with slide-roll ratio for both base-oil and grease. Both greases reduce friction (compared to the base oil) over the slide-roll range. The effect is most marked in the low speed (0.01 m/s) tests. The degree of friction reduction increases with thickener content Baseoil %SRR Figure 5: Friction coefficient as a function of SRR at 60 C, 0.01 m/s %SRR Figure 6: Friction coefficient as a function of SRR at 60 C, 0.5 m/s One interesting observation from these tests is the time dependence shown by the friction at slow speeds. At the end of the variable speed tests the rig was halted and then restarted at 0.01 m/s. It was noted that friction dropped during the first few minutes of running at this speed. Figure 7 plots friction coefficient as a function of time for 20 %SRR at 0.01 m/s. The coefficient of friction drops for both grease samples from 0.05 to < 0.04 over the first 120 seconds (approximately 5 disc revolutions). This is attributed to the deposition and growth of a thickener-rich film [13]. Such behaviour is not observed for the base-oil or at high speeds. The equilibrium grease friction was 33 % less than the baseoil Time [secs] Figure 7: Friction reduction with running time at 0.01 m/s. result also shown. 4.2 Discussion Film thickness and friction results have been reported for fully flooded conditions. These results are summarised in Table 3. In the slow speed region, the greases give variable film results, which is due to the thickener fluctuations. Averaged values of 20 and 40nm are quoted for the 5 and 1 thickener content respectively at 0.01 m/s. Both greases give reduced friction and increased film thickness compared to the base oil. One possible reason for the friction reduction is that the lambda (λ) ratio is higher and thus the contribution from boundary friction is reduced. To examine this the friction dependence on λ ratio was analysed. Lubricant Film thickness m/s 0.5 m/s FRICTION 0.01 m/s 0.5 m/s grease grease % SRR/0.5 GPa 2 equilibrium value after 120 secs Table 3. Summary of fully flooded film thickness(in nm) and friction results at 20% SRR. By comparing the film thickness and friction results, it was possible to estimate λ for each friction measurement. The film thickness results where curve-fitted and the equations used to calculate film thickness for each speed condition in the friction tests (20% SRR). With the greases default values of 20nm (5 %) and 40 nm (15 %) were used for speeds < m/s. λ was then calculated using a combined surface roughness of 40 nm. These results are plotted as friction coefficient against lambda ratio in Figure 8.

5 λ Figure 8: Friction coefficient plotted against lambda value (calculated from film thickness results) for fully flooded tests at 60 C. The λ calculation procedure was not totally satisfactory as central rather than minimum film thickness is used and the contact pressure in the friction tests was much higher. The effect of pressure on film thickness is relatively minor, in this case a reduction of about 12 %. Minimum film thickness however can be % less than the central value. Thus the calculated λ values are probably too high. However, the exercise does provide a rough guide to the λ dependence of friction in the tests. As expected the base-oil friction increases rapidly with decreasing λ for values of λ < 0.8. Friction reaches a minimum at 1 < λ > 2 and then increases to a value of ~ The greases reduced friction over the entire λ range. At low λ, the results show a wide variation and this is due to the time dependence found in this region. Equilibrium friction values of <0.04 were recorded after a few minutes rolling at 0.01 m/s. This is thought to be due to the deposition and growth of a thickener richlayer. The question arises as to whether the friction reduction was simply due to increased λ or whether the thickener-layer was intrinsically low friction. It was obviously difficult to assign a film thickness for the time-dependent results however it is unlikely that the film exceeded 80 nm or λ > 2. The friction levels measured were very much lower than obtained for λ > 8 with the greases. It is concluded therefore that the friction reduction is not simply due to an increased physical separation of the rubbing surfaces but that the deposited film has low shear strength. Such findings have implications for the development of greases used in machine elements operating under slow speeds and/or high temperatures. Under these conditions, it is likely that the lubrication performance will be dominated by the thickener boundary properties rather than the base oil. It is also interesting that the greases show a relative friction reduction in the full film regime and this effect increases with increasing thickener content. 5 CONCLUSIONS This paper studies the friction behaviour of grease under fully flooded conditions. Two lithium hydroxystearate greases were tested in a bearing simulation device under different conditions of speed and slide-roll ratio. The conclusions from this work are summarised as follows: Film thickness results: Grease film thickness exceeds that of the base oil over the speed range. Greatest increase seen at low speeds- due to the entrainment of thickener into the contact Friction results at 20 % SRR: At low speeds under nominally boundary conditions (λ < 1) there is deposition of a thickener-rich layer that reduces friction. At high speeds in the full film regime (λ > 3) grease gives a friction coefficient which is 5 10 % less than the base oil. Friction reduction (relative to the base oil) increases with increasing thickener content 6 REFERENCES [1] B. R. Williamson and D. Miller, Condition Monitoring of Grease Lubricated Rolling Element Bearing, NLGI Spokesman, 63, pp 8-15, (1999). [2] R. Wilson, The Relative Thickness of Grease and Oil Films in Rolling Bearings, Proc. I. Mech. E., 193, pp , (1979). [3] J. M. Palacios, A. Cameron and L. Arizmendi, Film Thickness of Grease in Rolling Contacts, ASLE Trans., 24, pp , (1981). [4] S. Hurley and P. M. Cann The Influence of Grease Composition on Film Thickness in Rolling Contacts, NLGI Spokesman, 63, pp 12-22, (1999). [5] J. Hamrock, Fundamentals of Fluid Film Lubrication, McGraw-Hill, Inc., (1994). [6] Cameron, A. N. Gordon and G. T. Symm, Contact Temperatures in Rolling/Sliding Surfaces, Proc. Roy. Soc. London, A286, pp 46-61, (1965). [7] H. Ito and T. Suzuki, Physical and Chemical Aspects of Grease deterioration in Sealed Ball Bearings, Lub. Eng., 44, pp , (1988). [8] E. Zaretsky (editor)., STLE Life Factors for Rolling Bearings, STLE Publ. SP-34, (1992). [9] T. Endo, Recent Development in Diurea Grease, NLGI Spokesman, 57, pp , (1993). [10] S. Hurley and P. M. Cann, Infrared Spectroscopic Characterisation of Grease Lubricant Films on Metal Surfaces, NLGI Spokesman, October, pp 13-21, (2000). [11] Godfrey, Friction of Greases and Grease Components during Boundary Lubrication, ASLE Trans., 7, pp 24-31, (1964). [12] F. Rastegar and W.O. Winer, On the Traction and Film Thickness Behaviour of Greases in Concentrated Contacts, NLGI Spokesman, August, pp , (1986) [13] S. Hurley, PhD Thesis, London University, (2000). [14] P. M. Cann, J. P. Doner, M.N. Webster and V. Wikstrom, Grease Degradation in Rolling Element Bearings, accepted for publication, July issue STLE Transactions. [15] P. M. Cann and A. A. Lubrecht, Analysis of Grease Lubrication in Rolling Element Bearings, Lubrication Science, 11, pp , (1999).

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