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Abstract
This article presents a three-dimensional numerical investigation and some experimental results supporting evidence on the flow structure and the pressure distribution in a circular thrust pad bearing of hydrostatic system in machine tools. The operational conditions include the feeding Reynolds number (Rein) and low sliding Reynolds number (Res). The motion of the thrust surface is assumed to be linear. The geometric parameters include pocket depth (H), pocket radius (r1), clearance height (h), inlet hole radius (rin), and pocket shapes (circular, elliptical, square, annular, and sector), which are optimized with respect to high load capability and high fluid film stiffness (K). A flow visualization experiment using a homemade pad bearing model and particle tracking method was used to visually reconstruct the flow pattern in the pocket for stationary conditions (Res = 0). Three-dimensional Navier-Stokes equations were employed to simulate the steady-state flow in hydrostatic pad bearings with incompressible Newtonian fluid using the finite volume method. The numerical results are qualitatively compared with the experimental flow patterns, finding good agreement for static cases. The numerical results show that vortexes driven by the inlet jet and the Couette effect generated by the thrust surface exist in the pocket and the flow field structure is very complicated. Flow in the pocket is nonuniform and significantly affected by the Rein and Res. The magnitude of the static pressure is much higher than the dynamic pressure and pressure distributions in the pocket are almost uniform. Pressure increases obviously as Rein increases and decreases slightly with the increase in Res. H has an important influence on flow patterns and no obvious effect on the pressure distribution. The pressure is highest with r1 = 25 mm and the fluid film stiffness is highest with r1 = 30 mm. The pressure of a pocket with rin = 3 mm is about two times that of a pocket with rin = 1.5 mm. The annular-shaped pocket has the highest maximum pressure (Pmax), whereas the circular-shaped pocket has the highest fluid film stiffness. The results will be useful for improving the load capability and fluid film stiffness of the thrust pad bearing of hydrostatic systems in machine tools with low rotating speed.
NOMENCLATURE
| A* | = | Ratio of area (Ap/Ab) |
| Ab | = | Bearing area (m2) |
| Ap | = | Pocket area (m2) |
| dp | = | Diameter of the particle (m) |
| e | = | Dimensionless depth (H/rin) |
| ΔF | = | Variation of the carrying capacity (N) |
| K | = | Dimensionless oil film stiffness (ΔF/πr22ρVin2) |
| H | = | Pocket depth (mm) |
| h | = | Clearance height (μm) |
| k* | = | Dimensionless radius (r1/r2) |
| P | = | Dimensionless pressure (p/ρV2in) |
| Pmax | = | Maximum pressure (Pa) |
| p | = | Pressure (Pa) |
| Rein | = | Inlet Reynolds number (2ρVin rin/μ) |
| Res | = | Pocket Reynolds number (ρUh/μ) |
| r | = | Radial distance (mm) |
| r1 | = | Radius of the pocket (mm) |
| r2 | = | Radius of the oil pad bearing (mm) |
| rin | = | Supply hole radius (mm) |
| r* | = | Dimensional radial distance (r/r2) |
| U | = | Moving speed of the upper wall (m/s) |
| Uf | = | Velocity of the fluid (m/s) |
| Up | = | Velocity of the particle (m/s) |
| Us | = | Velocity lag of a particle (m/s) |
| V | = | Dimensionless velocity (v/Vin) |
| Vin | = | Inlet velocity (m/s) |
| x, y, z | = | Cartesian coordinate system |
| η | = | Dimensionless height (h/H) |
| μ | = | Dynamic viscosity (Pa·s) |
| ρ | = | Fluid density (kg/m3) |