Influence of Oil Viscosity Grade on Thrust Pad Bearing Operation

S B Glavatskih^1* and S DeCamillo^2
1Division of Machine Elements, Luleå University of Technology, Luleå, Sweden
²Kingsbury Inc., Philadelphia, Pennsylvania, USA

Abstract

The effect of oil viscosity grade on the performance of tilting pad thrust bearings is examined in a wide range of shaft speeds and specific bearing loads. Data being discussed were obtained in tests carried out with 228 mm outer diameter (o.d.) and 267 mm o.d. bearings lubricated with ISO VG32 and ISO VG68 mineral oils in a fully flooded mode. In a low-speed range (below 40 m/s), the performance of the 228 mm bearing is analysed in terms of pad and collar temperatures, power loss and oil film thickness. Pad temperature and power loss are employed in the analyses of the 267 mm o.d. bearing operated at high sliding speeds of up to 115 m/s. The results show that there is a significant effect of oil viscosity grade on bearing main operating parameters. The substitution of ISO VG32 oil for ISO VG68 oil results in considerably reduced pad temperatures, lower power loss and thinner oil film thickness. However, no measurable difference in power loss was observed after the onset of turbulence. The influence of oil viscosity grade on pad temperature pattern is analysed. The thermal effect of oil viscosity grade on pad temperature pattern and power loss is also compared with the effect of offset pivot.

Introduction

The performance of a tilting pad thrust bearing can be best quantified in terms of developed power loss, operating temperature and oil film thickness. These parameters are all affected by oil viscosity. Designers and the end-users of rotating machinery are often confronted with the question as to which oil viscosity grade should be chosen to provide the most efficient bearing operation. It is commonly supposed that thinner oils provide lower energy consumption and lower operating temperatures. At the same time, low-viscosity oils develop thinner oil films, which can jeopardize bearing safety. Hence, thicker oils are usually used at low speed and thinner ones at high speed. However, to what extent will the main operating parameters be affected if the thrust bearing operates with an oil of higher or alternatively lower viscosity grade? Typical mineral oils used for bearing lubrication in turbomachinery are ISO VG32, VG46 and VG68. Most of the data published are for light turbine oil ISO VG32, and only very few articles have presented data on the influence of oil viscosity on bearing performance. The effect of the grade of oil used on the limits of safe operation for a 196 mm mean diameter eight-pad bearing was presented in reference [1] through a bearing duty diagram. The approximate viscosities of the oils used in that analysis were 15.5, 27.0 and 53.0 mm2 /s at 60 8C. The first and the second oils apparently correspond to VG32 and VG68 oils. It follows from the bearing duty diagram that the substitution of the VG68 oil for the VG32 oil significantly extends the limit of safe bearing operation at high and low speeds. As an example, the limit is extended by 50 per cent at a mean sliding speed of 3 m/s and by 40 per cent at a speed of 100 m/s.

Test data for a 267 mm o.d. six-pad bearing were presented in reference [2]. A heavy oil (94 mm2 /s at 37.8 8C) was replaced with light turbine oil (VG32). Test data showed that the lighter oil cut power losses by 25 per cent. It should be noted that the bearing lubricated by the lighter oil required lower supply oil flowrate so that some of the reduction in loss is due to less oil.

The influence of oil viscosity on power loss of a spring-supported bearing was studied in reference [3]. Substitution of ISO VG32 oil for ISO VG68 oil reduced power loss by 30 per cent at a specific bearing load of 4 MPa (specific bearing load is defined as the loading force divided by the area of the bearing load-carrying surface). The temperature, measured at the mid-point of pad thickness at a location 25 per cent circumferentially from the trailing edge and 62 per cent radially from the inner edge, was also reduced by 3–6 K. The calculated reduction in minimum oil film thickness was 18–20 per cent. It was concluded that bearing performance could be significantly improved by substitution of the thinner oil and simultaneous increase in oil pot temperature. The minimum film thickness obtained, 10–15 mm, was considered adequate to prevent wiping.

Pad maximum temperature data from 12 test bearings combined with previously published results of other researchers were analysed in reference [4] at sliding speeds of up to 80 m/s and a mean pad pressure of up to 4.1 MPa. The position of the maximum temperature for 12 test bearings was at the centre of the trailing outboard quadrant. It was concluded that the effect of oil viscosity grade was ‘of minor importance over the range of typical oils used in tilting pad bearings (ISO grades 32 to 68)’.

Thus, when it comes to typical mineral oils used for bearing lubrication in turbomachinery, none of the published information taken collectively provides a definite answer to the question of the importance of viscosity grade influence on bearing operating characteristics. The purpose of the present paper is therefore to clarify this ambiguity and to provide detailed information necessary to quantify the effect of oil viscosity grade on bearing main operating parameters: temperature, power loss and oil film thickness.

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