The effect of PTFE lining on plain journal bearing characteristics L effet d un revêtement PTFE sur les caractéristiques des paliers lisses

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1 The effect of PTFE lining on plain journal bearing characteristics L effet d un revêtement PTFE sur les caractéristiques des paliers lisses Kuznetsov E, a Glavatskih S, a and Fillon M a Division of Machine Elements, Luleå University of Technology, Luleå, Sweden. b Institut Pprime, CNRS - Université de Poitiers - ENSMA, UPR 3346, Dépt Génie Mécanique et Systèmes Complexes SP2MI, Boulevard Marie et Pierre Curie, BP F86962 Futuroscope Chasseneuil Cedex. Keywords: PTFE, compliance, plain bearing, thermal effects. Mots clés : PTFE, compliance, palier lisse, effets thermiques. b One of the limitations of the modern bearing designs is caused by the use of a thin layer of white metal that protects the shaft during short periods of boundary lubrication. However, at start-up the breakaway torque can be high and unpredictable, especially after long periods of inactivity. There is also a limit in the hydrodynamic regime: white metal loses its strength at elevated temperatures, for example, caused by high shaft speeds. The focus of the current work is to show how bearing performance can be improved by using an alternative lining material. PTFE has been selected for this purpose due to its low friction and compliant properties. Operating characteristics of a compliant bearing have been analyzed using a THD model. Main bearing parameters have been computed and compared with the ones of conventional white metal bearings. The effects of thermo-mechanical properties of the PTFE lining on oil film thickness, temperature, pressure and power loss are considered. Recommendations on how to design journal bearings with PTFE linings are provided. Une des limites de la conception de paliers modernes est due à l'utilisation d'une mince couche de métal blanc qui protège l'arbre pendant de courtes périodes de lubrification limite. Toutefois, le couple au démarrage peut être élevé et imprévisible, surtout après de longues périodes d'inactivité. Il y a aussi une limite en régime hydrodynamique: le métal blanc perd sa résistance à des températures élevées, par exemple, causée par une vitesse élevée de l arbre. L'objectif des travaux en cours est de montrer comment les performances peuvent être améliorées en utilisant un matériau de revêtement de remplacement. Le PTFE a été choisi à cet effet en raison de son faible coefficient de frottement et de ses propriétés de compliance. Les caractéristiques de fonctionnement d'un palier compliant ont été analysées en utilisant un modèle THD. Les principaux paramètres du palier ont été calculés et comparés à ceux des paliers classiques (paliers régulés). Les effets des propriétés thermo-mécaniques du revêtement PTFE sur l'épaisseur du film d'huile, la température, la pression et les pertes sont considérés. Des recommandations sur la conception de paliers avec des revêtements de PTFE sont précisées. 1 Introduction A constant trend towards higher power densities in rotating machinery calls for better mechanical components capable of carrying higher loads while being the same or smaller in size without sacrificing machine safety. For hydrodynamic bearings, it is important that minimum film thickness never drops below a safety limit. Oil film becomes thinner if load increases. This also results in elevated temperatures, which reduce oil viscosity and further decrease oil film thickness. White metal or babbitt is the material of choice for the linings in conventional hydrodynamic bearings. Babbitt - L 1 -

2 provides good conformability and embeddability but loses its strength with rising temperature. Another limitation is comparatively high breakaway friction [1]. One way to avoid these limitations is to use different materials for bearing linings. It has been shown that load carrying capacity of tilting pad thrust bearings is significantly increased by using PTFE as a substitute for white metal [2]. PTFE can also withstand high temperatures and provide low breakaway friction [1] which makes it an interesting candidate to use in plain journal bearings. Application of PTFE as a bearing lining makes journal bearings compliant which may be of advantage in the field. The goal of this paper is to investigate and clarify differences in the thermohydrodynamic performance of cylindrical compliant journal bearings in comparison to conventional babbitted bearings. 2 Numerical model A thermohydrodynamic (THD) bearing model based on the algorithm described in [3] is employed. It includes the Reynolds equation with a 3D viscosity variation, 3D energy and 3D heat transfer equations. Thermal and mechanical deformations of the journal and the bearing are also considered. Mechanical deformation of the bearing lining is calculated using a plan strain hypothesis. This assumption is applicable for PTFE layers of up to 3 mm in thickness. It was verified by comparison with the results obtained using a FEM deformation model [4]. Babbitt is assumed to be rigid. We assume that the PTFE lining when heated expands inside as it is surrounded by a rigid steel housing. Zero pressure boundary condition (ambient pressure) at bearing edges is used to solve the Reynolds equation. Cavitation is taken into account by implementing a switch-function model [3]. Shaft temperature in the sliding direction is assumed to be constant by applying zero flux boundary condition. It is calculated by averaging temperature in the adjacent lubricant layer. An empirical mixing coefficient is used to represent mixing of the hot oil with the cold oil supplied to the grooves. Bearing geometry is shown in Figure 1. Bearing dimensions and material properties are given in Table 1. The numerical calculations are stopped when the point-wise difference between the consecutive iterations does not exceed 10-5 for the pressure and 10-7 for the temperature field. Results obtained by the THD model for a babbitted journal bearing have been verified against experimental data presented in [5], Figure 1. Bearing inner radius, [mm] Bearing outer radius, [mm] Bearing length, [mm] 200 Bearing cold radial clearance, [μm] 237 Bearing supply groove length, [mm] 160 Bearing supply groove width, [rad] 0.51 Supply oil temperature, [ C] 65 PTFE thickness, [mm] Journal speed, [krpm] Oil viscosity at 40 C, [mm 2 /s] Oil viscosity at 100 C, [mm 2 /s] Oil density, [kg/m 3 ] Oil specific heat, [JKg/K] Oil thermal conductivity, [W/(mK)] 0.13 PTFE thermal conductivity, [W/(mK)] 0.27 Steel thermal conductivity, [W/(mK)] 50 Steel to air convection, [W/(m 2 K)] 50 Steel to oil convection, [W/(m 2 K)] 750 PTFE thermal expansion, [K -1 ] 1.35E-4 Steel thermal expansion, [K -1 ] 1.11E-5 PTFE Young s modulus, [GPa] 0.11 PTFE Poisson s ratio, [-] 0.46 Tab. 1 Bearing and material parameters - L 2 -

3 Fig. 1 - Journal bearing geometry and comparison of the experimental and numerical pressure profiles. 3 Results and discussion The influence of lining compliancy on journal bearing steady state performance is analysed by comparing PTFE lined bearings with a conventional babbitted bearing within a range of loads and shaft speeds. The influence of mechanical and thermal deformations on bearing performance characteristics is clarified using a number of special test configurations. These configurations include a compliant bearing with a 2 mm PTFE layer without its mechanical deformation (PTFE w/o def), a compliant bearing without thermal deformation of the PTFE layer (PTFE w/o exp) and both babbitted and compliant bearings with cold clearances increased by 43 µm (Babbitt Cr and PTFE 2.0 mm Cr). In view of space limitations, we report only a part of the results, which we deem to be representative. Results are shown for 900 rpm shaft speed. The relative eccentricity is calculated based on the cold radial clearance taken at room temperature. In operation, the actual clearance is changed due to the thermal expansion of the shaft and bearing. Mechanical deformation of the compliant layer also contributes to a variation in the clearance shape. It means that babbitted and compliant bearings operating with the same relative eccentricity (based on the cold clearance) may provide significantly different oil film thicknesses. To avoid ambiguities in comparing bearing characteristics, the results obtained are plotted as a function of load that the bearing can carry. Fig. 2 - Relative eccentricity. - L 3 -

4 Figure 2 shows how load carrying capacity evolves with relative eccentricity and compliancy. Load carrying capacity of both rigid and compliant bearings grows exponentially as a function of shaft relative eccentricity. Increasing the thickness of the PTFE lining provides higher load carrying capacity for the compliant bearing compared to the babbitted bearing operating at the same relative eccentricity. It can be seen, Figure 2, that thermal expansion increases load carrying capacity while both mechanical deformation and increased clearance result in the opposite effect. Thermal expansion leads to the hot clearance reduction, which explains higher load carrying capacity of the compliant bearings compared to the babbitted bearing. The effect becomes evident if the test configuration PTFE w/o exp is compared to the compliant bearing with a 2 mm PTFE layer (PTFE 2.0 mm). Since the degree of thermal expansion depends on temperature, the difference between two curves increases with load. Influence of temperature is more pronounced if the babbitted bearing (Babbitt) is compared to the configuration PTFE w/o def. There is now a combine influence of thermal expansion and thermal insulation. As a result, the difference in expansion of the steel bearing parts further increases the gap between two configurations. Despite higher load carrying capacity, compliant bearings provide lower maximum oil film pressure. Figure 3 shows how maximum oil film pressure decreases with an increase in the PTFE layer thickness. Up to 40% lower pressure at the same load can be observed for the compliant bearing. If shaft speed is increased, the maximum pressure decreases for the same load. Fig. 3 Maximum oil film pressure. It can be seen that larger radial clearance increases maximum pressure in both babbitted and compliant bearings, Figure 3. No effect of thermal expansion on pressure can be observed since PTFE w/o exp and PTFE 2.0 mm lines coincide. Solely PTFE thermal expansion does not affect the maximum pressure since it is compensated by the lower shaft eccentricity. At the same time, a visible difference between results for the babbitted (Babbitt) and compliant PTFE w/o def bearings is due to PTFE thermal insulation that reduces thermal expansion of the steel housing. Mechanical deformation, on the contrary, provides a significant decrease in the maximum pressure and more even pressure distribution. The babbitted (Babbitt) and compliant (PTFE 2.0 mm) bearings can be compared to demonstrate the difference in the pressure fields. Circumferential pressure profiles for 122 kn load are shown in Figure 4. Pressure in the compliant bearing is higher at angles and , while much lower in the maximum pressure region. It can also be seen that cavitation in the compliant bearing starts later. The axial pressure level is similar at the edges for both bearings but it is much lower in the centre region for the compliant bearing. - L 4 -

5 Fig. 4 Oil film pressure in the circumferential and axial directions for 122kN load. A PTFE layer is deformed due to the hydrodynamic pressure resulting in a more complex geometry of the oil film. Minimum oil film thickness along the bearing centre line is shown in Figure 5. The general trend is a reduction in the oil film thickness with loading. Under heavy loads, compliant bearings provide thicker films compared to the babbitted bearing. At higher shaft speeds, oil films become slightly thicker but the difference between minimum oil film thickness in babbitted and compliant bearings remains almost the same. Fig. 5 Minimum oil film thickness at the bearing centre line and oil film profiles in the circumferential direction for 122kN load. Mechanical deformation of the PTFE layer increases minimum oil film thickness at the bearing centre line as shown in Figure 5. There is a significant increase in film thickness at A decrease in film thickness at 40 represents a reduction in clearance due to the PTFE thermal expansion. The global minimum of oil film thickness in the compliant bearing occurs at the edges as there is no deformation of the PTFE layer due to the zero pressure boundary condition and the deformation model used. Thus, oil film thickness at the bearing edges shows different behaviour, Figure 6. At lower loads oil film is the same whereas at higher loads it is thinner due to the thermal expansion of the compliant material. Figure 6 also shows thicker oil films in the centre region and thinner oil films at the edges in the complaint bearing compared to the babbitted bearing. - L 5 -

6 Fig. 6 Minimum oil film thickness at the bearing edges and oil film profiles in the axial (at 192 degree) direction for 122kN load A pocket shape oil film distribution is certainly favourable for bearing lubrication. But such shape also implies that oil film is slightly thinner at the narrow edge regions. In practice, this reduction is even smaller as the ridges are deformed due to the deformation of the neighbouring area. Oil film thickness at the edges can also be increased by tapers or by using a compliant material with low thermal expansion. Maximum oil film temperature in the babbitted bearing is slightly lower compared to the compliant bearings as shown in Figure 7. Temperatures increase significantly with the journal speed but the difference in bearing thermal behaviour becomes less significant. Moreover, at certain load/speed combinations compliant bearings provide lower oil film temperatures. This is a very interesting finding demonstrating how PTFE layer compliancy compensates for its higher thermal conductivity. On the other hand, a decrease in the radial clearance due to the thermal expansion increases maximum oil film temperature slightly, Figure 7. Mechanical deformation has a negligible influence leaving maximum temperature almost unchanged. Despite powerful thermal insulation temperature levels are similar in the babbitted and compliant bearings. This difference can be reduced by increasing cold clearance. Figure 6 shows that an increase in cold clearance provides a stronger impact on maximum temperature in the compliant bearing. Fig. 7 Maximum temperature. - L 6 -

7 Bearing power loss increases with loading as shown in Figure 8. There exists a small difference in power loss between the bearings. At light loads, the babbitted bearing outperforms compliant bearings whereas at heavy loads, above 150 kn, power loss is lower for the PTFE bearings. The difference in power loss at light loads is amplified when the shaft speed increases. An increase in cold clearance has a positive impact on power loss that is lower for a larger clearance, Figure 8. Thermal deformation slightly increases power loss whereas mechanical deformation reduces it. The results taken collectively show potential advantages of the compliant bearings as a substitute for the babbitted bearings. For practical applications, it is of advantage to reduce thermal expansion of the lining or/and increase its conductivity. Another efficient approach is to increase cold radial clearance. The value of the increment should be calculated using the operating condition the compliant bearing will be designed for. Fig. 8 Power loss. 4 Conclusions The effect of the PTFE lining on journal bearing performance characteristics has been investigated in terms of load carrying capacity, maximum oil film temperature and pressure, oil film thickness and power loss. A numerical three dimensional THD model including compliant liner deformation has been developed for the analysis. It is shown that application of the compliant liner provides: increased load carrying capacity, more even oil film pressure distribution with up to 40% lower maximum oil film pressure, similar or slightly higher maximum oil film temperature depending on operating conditions, more favourable, pocket shape, oil film distribution: increased minimum oil film thickness at the bearing centre line and slightly thinner at the bearing edges, similar or higher power loss depending on operating conditions. A detailed investigation on the contribution of mechanical and thermal deformations to changes in bearing characteristics shows that solely thermal expansion increases load carrying capacity, power loss and oil film thickness at the bearing edges. On the other hand, solely mechanical deformation is found to reduce load carrying capacity, maximum oil film temperature and pressure. It also increases minimum oil film thickness at the centre line of the bearing with the possibility to reduce power loss and minimum oil film thickness at the sides. From the bearing design point of view, it is recommended to increase cold clearance in the compliant bearing so that it can outperform babbitted bearing in terms of maximum oil film temperature, oil film thickness, load carrying capacity and power loss. - L 7 -

8 5 References [1] Golchin, A., Simmons, G. F., Glavatskih, S. B. (2010), Break-away friction of PTFE materials in lubricated conditions. Proc NordTrib2010. [2] Glavatskih, S. (2008): Extending performance limits of tilt pad thrust bearings: a full scale study. Proc 7 th EDF/LMS Poitiers Workshop. [3] Tanaka, M., Hatakenaka, K. (2004), Thermohydrodynamic lubrication model of journal bearings. Japanese Journal of Tribology 45, [4] Cha, M., Kuznetsov, E., Glavatskih, S., A comparative linear and nonlinear analysis of compliant cylindrical journal bearings. Proc STLE [5] Bouyer, J., Fillon, M. (2002), An experimental analysis of misalignment effects on hydrodynamic plain journal bearing performances. ASME J Tribology 124, Acknowledgments The financial support provided by the Swedish Energy Agency, ABB Automation Technologies, Alstom Hydro, Elforsk, Evonik RohMax Additives GmbH, Statoil Lubricants and Siemens Industrial Turbomachinery is gratefully acknowledged. - L 8 -