MEASUREMENTS VERSUS PREDICTIONS FOR THE STATIC AND DYNAMIC CHARACTERISTICS OF A FOUR-PAD, ROCKER-PIVOT, TILTING-PAD JOURNAL BEARING
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Measured and predicted static and dynamic characteristics are provided for a four-pad, rocker-pivot, tilting-pad journal bearing in the load-on-pad and load-between-pad orientations. The bearing has the following characteristics: 4 pads, .57 pad pivot offset, 0.6 L/D ratio, 60.33 mm (2.375in) pad axial length, 0.08255 mm (0.00325 in) radial clearance in the load-on-pad orientation, and 0.1189 mm (0.00468 in) radial clearance in the load-between-pad orientation. Tests were conducted on a floating test bearing design with unit loads ranging from 0 to 2903 kPa (421.1 psi) and speeds from 6.8 to 13.2 krpm.For all rotor speeds, hot-clearance measurements were taken to show the reduction in bearing clearance due to thermal expansion of the shaft and pads during testing. As the testing conditions get hotter, the rotor, pads, and bearing expand, decreasing radial bearing clearance. Hot-clearance measurements showed a 1625% decrease in clearance compared to a clearance measurement at room temperature. To look at the radial thermal gradient in the loaded pad, embedded thermocouples were inserted inside the bearing pad near the bearing housing. Results showed a 525C decrease in temperature from the rotor side of the pad to the temperatures near the bearing housing. This radial temperature gradient caused an uneven thermal deflection in the pad, changing the pads radii of curvature.For all test conditions, dynamic tests were performed over a range of excitation frequencies to obtain complex dynamic stiffness coefficients as a function of frequency. The direct real dynamic stiffness coefficients were then fitted with a quadratic function with respect to frequency. From the curve fit, the frequency dependence was captured by including a virtual-mass matrix [M] to produce a frequency independent [K][C][M] model.The direct dynamic stiffness coefficients for the load-on-pad orientation showed significant orthotropy, while the load-between-pad did not. The load-between-pad showed slight orthotropy as load increased. Experimental cross-coupled stiffness coefficients were measured in both load orientations, but were of the same sign and significantly less than direct stiffness coefficients.In both orientations the imaginary part of the measured dynamic stiffness increased linearly with increasing frequency, allowing for frequency-independent direct damping coefficients.Rotordynamic coefficients presented were compared to predictions from two Reynolds-based models. The models showed the importance of taking into account pivot contact flexibility and different pad geometries (due to the reduction in bearing clearance during testing) in predicting rotordynamic coefficients. If either of these two inputs were incorrect, then predictions for the bearings impedance coefficients were very inaccurate.
