Effect of a Reduced Oil Flow Rate on the Static and Dynamic Performance of a Tilting Pad Journal Bearing Running in Both the Flooded and Evacuated Conditions
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AbstractThe mechanical efficiency of rotating machinery improves as drag power losses in bearings reduce; hence, an operation mode that reduces the supplied flow brings immediate benefits. The paper presents measurements of the static and dynamic load performance conducted with a tilting pad journal bearing configured as flooded and evacuated, and supplied with flow rates from 150% to 25% (or lesser) of a nominal magnitude, proportional to shaft speed. The lubricant is ISO VG 46 oil supplied at 60C. The four pads bearing has center pivots with single orifice feeds for the flooded condition and spray bar injection for the evacuated condition. The shaft, 102mm in diameter, operates at two speeds, 6 and 12 krpm (=64m/s surface speed), while a static load is applied between pads (LBP). The specific loads equal 345, 1034, and 2068kPa. A reduction in flowrate makes both bearings operate more eccentrically. The evacuated bearing operates at a larger eccentricity, which for the lowest flowrate does not align with the direction of the applied load, hence displaying a sizable attitude angle. Pad subsurface temperatures are similar for both bearing configurations, although the evacuated bearing is colder by a few degrees Celsius. Drag power losses derived from the oil exit temperatures show the evacuated bearing produces up to 40% lesser power loss than the flooded bearing for a nominal oil flowrate. The bearings direct stiffnesses, Kxx and Kyy, increase with applied load and show little dependency on shaft speed. The evacuated bearing has lower magnitude stiffnesses, 20% or so lesser, than the flooded bearing. Damping coefficients, CxxCyy, reduce in magnitude as the flowrate decreases. In particular for low flow rates, 35% or lower, the evacuated bearing shows rather small (though highly uncertain) damping coefficients. For sufficiently small flow rates, operation at 6 krpm shaft speed and under the smallest applied unit load (345kPa) produced subsynchronous shaft vibrations with a broad band spectrum (subsynchronous shaft vibrations (SSV) hash). The evacuated bearing produced SSV hash at flow rates equal to 30% or so of nominal, while the flooded bearing demanded very low flow rates (15% and lesser of nominal) to produce SSV hash. For both bearings, the SSV amplitude motions were rather small in amplitude. The experimental campaign demonstrates that tilting pad bearings (flooded and evacuated) can safely operate with a reduced flowrate (50% or so of nominal) to produce significant savings in drag power losses and without significant effect on the pads' metal temperatures that could affect the bearing long-term operation.
