An Experimental Investigation of the Dynamic Performance of a Vertical Application Three Lobe Bearing

Abstract

Dynamic performance test results are provided for a verticalapplication threelobe bearing, geometrically similar to a threelobe bearing tested by Leader et al. (2010, “Evaluating and Correcting Subsynchronous Vibration in Vertical Pumps,â€‌ 26th International Pump Users Symposium, Houston, TX, March 1618) to stabilize a vertical sulfur pump. The bearing has the following specifications: 100 deg pad arc angle, 0.64 preload, 100% offset, 101.74 mm bore diameter, 0.116 mm radial pad clearance, 76.3 mm axial length, and 100 deg static load orientation from the leading edge of the loaded pad. The bearing is tested at 2000 rpm, 4400 rpm, 6750 rpm, and 9000 rpm. This bearing is tested in the noload condition and with low unit loads of 58 kPa and 117 kPa. The dynamic performance of this bearing is evaluated to determine (1) whether a fully (100%) offset threelobe bearing configuration is more stable than a standard plain journal bearing (0.5 whirlfrequency ratio (WFR)) and (2) whether a fully offset threelobe bearing provides a larger direct stiffness than a standard fixedarc bearing. Hot and cold clearances are measured for this bearing. Dynamic measurements include frequencyindependent stiffness and damping coefficients. Bearing stability characteristics are evaluated using the WFR. Test results are compared to numerical predictions obtained from a fixedarc bearing Reynolds equation solver. Dynamic tests show that the verticalapplication threelobe bearing does not improve stability over conventional fixedarc bearings. The measured WFRs for the verticalapplication bearing are approximately 0.4–0.5 for nearly all test cases. Predicted WFRs are 0.46 at all test points. The verticalapplication bearing dimensionless direct stiffness coefficients were compared to those for a 70% offset threelobe bearing. Dimensionless direct stiffness coefficients at 0 kPa are larger for the verticalapplication bearing by 45–48% in the loaded direction and larger by 15–26% in the unloaded direction. Thus, the verticalapplication bearing does impart a larger centering force to the journal relative to the 70% offset bearing, in the noload condition. Predictions using both the measured hot clearance and measured cold clearance as inputs to the code are compared to the measured dynamic data. In general, the predicted direct stiffness coefficients using both the hot and cold clearances as inputs were higher than measured direct stiffnesses. The two sets of predicted crosscoupled stiffness coefficients straddle the measured crosscoupled stiffness coefficients. Predicted direct damping coefficients using both solutions were higher than measured values in most cases, but agreement between predictions and measurements improved significantly at high speeds and when applying light loads.