DYNAMIC CHARACTERIZATION OF A LOW DRAG POWER LOSS TILTING PAD JOURNAL BEARING

High performance turbomachinery relies on tilting pad journal bearings (TPJB) for high-speed operation while maintaining acceptable stability margins. However, TPJBs can produce large drag power losses that increase with machine size and surface speed. In a conventional configuration, the unloaded pads will drag on the surface of the shaft while, presumably, having a minimum impact on the bearing dynamic performance. The effects of removing unloaded pads from a TPJB has been studied before but research has yet to describe the force coefficients of this bearing type. The present work aims to evaluate the effect on dynamic forced performance and power loss of a 4-pad TPJB configured as load on pad (LOP) and having the unloaded pad, opposite to the loaded pad, replaced by a fixed insert with a large clearance. The replacement insert has a significantly larger clearance (similar to 14.4 x nominal clearance), effectively making this a 3-pad bearing. The original 4-pad bearing was previously tested, hence allowing for an accurate comparison before and after the pad exchange. The experiments include operation at both 6 and 12 krpm (shaft surface speeds of 32 m/s and 64 m/s, respectively) with unit loads equaling 345 kPa, 862 kPa and 1379 kPa. The bearing is supplied with ISO VG 46 oil (inlet T=50 degrees C) and flow equaling 50%, 100%, and 150% of a nominal flow set for the original 4-pad bearing. For the nominal flow and a unit load of 345 kPa, the modified bearing shows a substantial decrease in power loss, similar to 38% at 6 krpm and similar to 10% at 12 krpm. At the largest flow rate, however, the drag power loss reduction is less apparent, just similar to 2% at 12 krpm. The 3-pad bearing shows signs of lubricant starvation due to the large volume created by replacing the unloaded pad with the large clearance fixed pad. Incipient sub-synchronous vibrations (SSV) appeared at 100% nominal flow, at both 6 and 12 krpm and a unit load of 1379 kPa. SSV hash is a common occurrence in bearings supplied with a low flow rate and low loads. During tests with the lowest flowrate, the modified bearing produced very small stiffness (similar to 75% reduction) and damping (similar to 57% reduction) coefficients along the unloaded direction, most likely due to lubricant starvation.