Kluitenberg, Matthew P (2015-04). Experimentally Determine the Impact of Jacking-Oil Pockets on the Rotordynamic Characteristics of a Four-Pad, LBP, Tilting-Pad Journal Bearing. Master's Thesis.
Thesis
Test results that examine the influence of jacking-oil ports on a tilting pad journal (TPJ) bearing are presented. The static and dynamic characteristics before and after adding jacking-oil ports are compared. The test bearing is a four-pad, rocker-pivot, 57% pivot offset TPJ bearing in load between pad configuration. This original bearing was modified to include jacking-oil ports on the two loaded pads and retested at the same operating conditions. The ports occupy 5% of the pad's surface area and have an approximate depth of 25 times the radial clearance. Pressurized jacking-oil was not used during testing; however, oil supply lines complete with check valves were added to emulate a typical industrial bearing assembly. This thesis compares both the static and dynamic bearing characteristics to determine the effects jacking-oil ports had. Rotordynamic coefficients are presented as dimensionless values, using a frequency-independent [K][C][M] model. The added jacking-oil ports caused a decrease in direct damping and increase in virtual-mass. Direct damping coefficients were reduced by an average of -26% in the loaded y-direction, and -9% in the orthogonal x-direction. The original bearing had all negative direct virtual-mass coefficients (stiffening with increasing frequency). The modified bearing had negative coefficients that were smaller in magnitude, and some virtual-mass coefficients that were positive. No significant changes were observed in any of the cross-coupled coefficients or the direct stiffness coefficients. The added jacking-oil ports has no significant impact on the static characteristics of the TPJ bearing. Static characteristics included the steady-state journal eccentricity, attitude angle, minimum film-thickens, and pad temperature. A predictive bearing code was modified to account for the jacking-oil port geometry but was unable to produce predictions matching the measured change in dynamic performance. Contrary to measurements, the code predicted frequency-dependent direct rotordynamic coefficients and no reduction in direct stiffness or damping for the bearing with jacking-oil ports. The influence of jacking-oil ports on a simple rotor-bearing system is presented. Rotordynamic predictions were made for a rigid rotor symmetrically supported by two TPJ bearings. Predictions showed that jacking-oil ports can significantly lower the first critical speed. This was attributed to the change in virtual-mass coefficients. There was only a small reduction in damping ratio predicted.
