Static characteristics and rotordynamic coefficients of a four-pad tilting-pad journal bearing with ball-in-socket pivots in load-between-pad configuration

摘要

Static characteristics and rotordynamic coefficients were experimentallydetermined for a four-pad tilting-pad journal bearing with ball-in-socket pivots in loadbetween-pad configuration. A frequency-independent [M]-[C]-[K] model fit themeasurements reasonably well, except for the cross-coupled damping coefficients. Testconditions included speeds from 4,000 to 12,000 rpm and unit loads from 0 to 1896 kPa(0 to 275 psi).The test bearing was manufactured by Rotating Machinery Technology (RMT),Inc. Though it has a nominal diameter of 101.78 mm (4.0070 in.), measurementsindicated significant bearing crush with radial bearing clearances of 99.6 ?m (3.92 mils)and 54.6 ?m (2.15 mils) in the axes 45? counterclockwise and 45? clockwise from theloaded axis, respectively. The pad length is 101.6 mm (4.00 in.), giving L/D = 1.00.The pad arc angle is 73?, and the pivot offset ratio is 65%. The preloads of the loadedand unloaded pads are 0.37 and 0.58, respectively.A bulk-flow Navier-Stokes model was used for predictions, using adiabaticconditions for the bearing fluid. Because the model assumes constant nominalclearances at all pads, the average of the measured clearances was used as an estimate.Eccentricities and attitude angles were markedly under predicted while power loss wasunder predicted at low speeds and very well predicted at high speeds. The maximum detected pad temperature was 71?C (160?F) and the rise from inlet to maximum bearingtemperature was over predicted by 10-40%.Multiple-frequency force inputs were used to excite the bearing. Direct stiffnessand damping coefficients were significantly over predicted, but addition of a simplestiffness-in-series model substantially improved the agreement between theory andexperiment. Direct added masses were zero or negative at low speeds and increasedwith speed up to a maximum of about 50 kg; they were normally greater in the unloadeddirection. Although significant cross-coupled stiffness terms were present, they alwayshad the same sign. The bearing had zero whirl frequency ratio netting unconditionalstability over all test conditions. Static stiffness in the y direction (obtained from steadystateloading) matched the rotordynamic stiffness Kyy (obtained from multiple-frequencyexcitation) reasonably at low loads but poorly at the maximum test load.