INVESTIGATION OF FLUID FLOW IN AXIAL HYDROSTATIC BEARING

Transcription

1 INVESTIGATION OF FLUID FLOW IN AXIAL HYDROSTATIC BEARING Michal KOZDERA VŠB Technical University of Ostrava, Faculty of Mechanical Engineering, Department of hydromechanics and hydraulic equipment, 17. listopadu 15/2172, , Ostrava, Czech Republic Phone: + (420) , michalkozdera@seznam.cz This paper describes numerical modelling of the flow in narrow gaps of axial hydrostatic bearing. These bearing produce oil layer between two sliding surfaces, where the fluid friction arises. The thickness of the lubricating film ranges from 0.05 to 0.15 mm. CFD program Ansys Fluent has been applied for the investigation of the flow field and generated forces. Keywords: axial hydrostatic bearing 1 INTRODUCTION Axial hydrostatic bearing belongs to a group of journal bearings. Their advantage is high load capacity, they are suitable for all circumferential speeds. Speed change does not have significant influence on the load capacity of the bearing. In contrast with hydrodynamically lubricated bearings, they can be operated near zero value of speed while maintaining the same properties. Bearing operating temperature depends on the type of lubricant, and can extend up to 200 C. They have very little frictional resistance, because there is fluid friction between sliding surfaces. Axial hydrostatic bearing has infinite durability, which depends on reliability of the external pressure source. Their disadvantage is very little ability to absorb vibrations. Their production and operation is more demanding than that of hydrodynamic bearings. 2 AXIAL HYDROSTATIC BEARING Axial hydrostatic bearing consists of two sliding surfaces bearing runner and bearing pad (Figure 1). On the bearing pad are created bearing recess and discharge grooves. The discharge grooves separate bearing recesses so that they do not affect each other. Figure 1 - Diagram of axial hydrostatic bearing 1

2 The lubricated fluid is supplied with the assistance of hydraulic aggregate. Basic technical parameters have been selected for the experiment: - volume discharge Q V = 50 l.min -1, - operating pressure p = 4.5 MPa. Part of the hydraulic aggregate is heating and cooling equipment. During the start up of the axial hydrostatic bearing the heating equipment is used that heats the mineral oil to the operating temperature. When running the axial hydrostatic bearing cooling equipment is used that keeps the working fluid at a constant temperature of 24 C to 28 C. Mineral oil is used for heavy-duty hydrostatic mechanisms. It contains additives against oxidation, wear, foaming and additives for viscous index increase. Classification according to ISO 6743 is HV 46 with following properties: - density ρ = 874 kg.m -3, - viscous index 170, - flash point 220 C, - congeal point -36 C. The oil is led through the distribution block from hydraulic aggregate. The fluid is divided and led to each bearing recess in the axial hydrostatic bearing. The fluid is evenly distributed with assistance butterfly throttles with stabilization of the pressure gradient to each bearing recess. Pressure fluid is fed into the lubrication grooves, which are in the fixed part of the bearing. Here, mineral oil spills evenly and starts causing hydrostatic pressure on the upper movable plate. Axial hydrostatic bearing can begin to rotate when it is lifted to the prescribed height. The fluid friction is created between two sliding surfaces. The thickness of the lubricant film ranges from 0.05 mm to 0.15 mm. The basic condition of the functionality of the axial hydrostatic bearing is to create a lubricating film, which covers all micro-roughness and macro-roughness. These deflections of the shape occur during the production process. Figure 2 : Detail of the bearing recess The fixed part of the bearing has an inner diameter of 1100 mm and outer diameter of 1450 mm. Axial hydrostatic bearing has 6 bearing recesses (Figure 2), which are separated by discharge grooves (Figure 3). Figure 3: Detail of the discharge groove 2

3 3 MATHEMATICAL MODEL OF THE AXIAL HYDROSTATIC BEARING Journal of applied science in the thermodynamics and fluid mechanics Solution of the flow in the narrow gap of the axial hydrostatic bearing comes out from the assumption of continuous, isotropic environment. Fluid flow is associated with the modelling of the problem, which is defined as: - three-dimensional flow, - laminar flow (fluid moves in very thin layers), - incompressible flow (ρ = constant), - isothermal flow (T = constant), t - stationary flow ( = 0 ), - flow without considering transmission of additions. Figure 4: Boundary conditions of the model For the numerical simulation CFD program Ansys Fluent was applied, which uses finite volume method. Since the periodic conditions can be used, definition of geometry was simplified to the 1/6 of the bearing geometry. Boundary conditions were chosen as shown in Figure 4 and 5. On inlet of the bearing, the velocity condition (flow condition) was defined and on the outlet zero gradients was assumed. For repetitive rotational geometry periodic boundary condition was used. The lower wall was set as a fixed and upper wall as rotating, where rotational speed was defined. Figure 5: Definition of the type of bearing walls 4 APPLICATION OF A MATHEMATICAL MODEL ON THE AXIAL HYDROSTATIC BEARING Evaluation was performed for the fluid layer thickness of 0.13 mm. Load capacity of the bearing and size of the static pressure were determined: - load capacity of the axial hydrostatic bearing N, - average static pressure 4.47 MPa, - maximum static pressure 5.41 MPa. With decreasing thickness of fluid layer the load capacity and static pressure of the axial hydrostatic bearing significantly increases. Conversely, the load capacity is decreasing with the increase of the oil thickness. 3

4 Figure 6: Velocity vectors in the axial hydrostatic bearing To evaluate the results, the graphic output was created on the evaluation plain, which is located in the middle of the gap thickness. Figure 6 and 7 show the size of the velocity vectors of the axial hydrostatic bearing. The maximum of the velocity magnitude is 4.16 m.s -1. Mineral oil is fed into the bearing recesses and is flowing between the sliding surfaces around the bearing. The fluid can flow in all directions and chooses the path of lowest resistance. Figure 7: Detailed view of the size of the velocity vectors Velocity field on the evaluation plain at the outlet of the axial hydrostatic bearing is shown in Figure 8. The maximum magnitude of the velocity is located in the bearing recesses on the outlet from the bearing (shown in red) and decreases evenly towards the bearing discharge grooves. The magnitude of the velocity approaches zero in the field of the discharge grooves (shown in blue). 4

5 Figure 8: Velocity field on the outlet of the axial hydrostatic bearing Figure 9 shows the pressure field in the gap thickness of 0.13 mm in the evaluation plain of the hydrostatic axial bearing. The maximum magnitude of the static pressure is located in the bearing recesses (shown in red). Static pressure drops gradually to discharge grooves and reaches the value close to zero (shown in blue). Figure 9: Pressure field in the evaluation plain of the axial hydrostatic bearing 5

6 All bearing recesses exhibit the same distribution of pressure and velocity, as they are separated by bearing discharge grooves. In this design variant of axial hydrostatic bearing constant rotational speed of 40 min -1 was defined on the bearing runner. Speed, however, can be arbitrarily changed. Reduction of the speed to 20 min -1 did not change the load capacity or pressure conditions. It can be concluded that the speed change does not have significant influence on the axial bearing function. Further simulations could be carried out for design modifications of axial hydrostatic bearing, focused on different number of the bearing recesses and their shapes or changing the cross-sectional area of the discharge grooves. REFERENCES [1] BEČKA, J.: Tribologie. Praha: Vydavatelství ČVUT, [2] BLÁHA, J. - BRADA, K.: Hydraulické stroje. Praha: Státní nakladatelství technické literatury, [3] BLAŠKOVIC, P. - BALLA, J. - DZIMKO, M.: Tribológia. Bratislava: Alfa nakladatelství, [4] BOJKO, M.: BOJKO M.: Návody do cvičení "Modelování Proudění" - FLUENT. Ostrava: Vysoká škola báňská Technická univerzita Ostrava, [5] DRÁBKOVÁ, S.: Mechanika tekutin. Ostrava: Vysoká škola báňská Technická univerzita Ostrava, [6] KOZDERA, M.: Projekční návrhy axiálních hydrostatických ložisek. Ostrava: Vysoká škola báňská Technická univerzita Ostrava, [7] KOZUBKOVÁ, M.: Modelování proudění Fluent.. Ostrava: Vysoká škola báňská Technická univerzita Ostrava, [8] VINŠ, J.: Kluzná ložiska. Praha: Státní nakladatelství technické literatury,