Static and Dynamic Characteristics for a Pressure-Dam Bearing
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Measured rotordynamic force coefficients (stiffness, damping, and added-mass) and static characteristics (eccentricity and attitude angle) of a pressure-dam bearing are presented and compared to predictions from a Reynolds-equation model, using an isothermal and isoviscous laminar analysis. The bearings groove dimensions are close to the optimum predictions of Nicholas and Allaire (1980) and are consistent with current field applications. The bearing has a diameter of 117.1 mm (4.61 in), a length-to-diameter ratio of 0.655 and, a nominal radial clearance of 0.133 mm (5.25 mils). The upper pad of the bearing has a step located at 130 and a 0.620 mm (15.75 mils) deep dam. The bottom pad has a deep, centered relief track over 25% of the pads axial length. Test conditions include four shaft speeds (4000, 6000, 8000 and 10000 rpm) and bearing unit loads from 0 to 1034 kPa (150 psi). Laminar flow was produced for all test conditions. A finite-element algorithm was used to generate solutions to the Reynolds equation model. Excellent agreement was found between predictions and measurements for the eccentricity ratio and attitude angles. Predictions of stiffness and damping coefficients are in reasonable agreement with measurements. However, experimental results show that the bearing has significant added mass of about 60 kg at no-load conditions, versus zero mass for predictions from the Reynolds-equation model and 40 kg using Reinhardt and Lunds extended Reynolds equation model. The added mass drops quickly to zero as the load increases. Measured results also show a whirl frequency ratio near 0.36 at no-load conditions; however, a zero whirl frequency ratio was obtained at all loaded conditions, indicating an inherently stable bearing from a rotordynamics viewpoint.
