Static and Dynamic Performance of Pressure Dam Bearings With Dam Steps That Are (I) Square and (II) Filleted
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Measured rotordynamic force coefficients (stiffness, damping, and added-mass) and static characteristics (eccentricity and attitude angle) are presented for two nearly identical pressure-dam bearings. One bearing has a square step at the dam; the other has a filleted step. Because of reduced manufacturing costs, the filleted-step design is used widely. The bearings groove dimensions are close to the optimum predictions of Nicholas and Allaire [2] and are consistent with current field applications. The bearings have 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 bottom pad has a deep, centered relief track over 25% of the pads axial length. The upper pad for both bearings has a step located at 130 from the horizontal and a 0.620 mm (15.75 mils) deep dam. The dam on the upper pad of one bearing has a square step; the other bearing has a filleted step. 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 within the bearing lands. For the same operating conditions, the filleted step bearing operates at a lower eccentricity ratio (has a larger minimum film thickness). The filleted step design has higher direct stiffness coefficients. Both cross-coupled stiffness coefficients are positive (favorable for stability) for both designs but the filleted design produces higher values. In regard to direct damping, the filleted-step design has higher damping in the load direction and comparable values in the unloaded direction. Hence, for the same operating conditions, a filleted step design should produce reduced amplitudes at or near a critical speed. With respect to stability as defined by WFR, the filleted design is consistently better (lower value) than the square step design, resulting in an elevated onset speed of instability for the filleted-step design.
