Scan DeCamillo, Manager, Research and Development
Kingsbury, Inc.,Philadelphia, Pennsylvania
Keith Brockwell, Senior Research Officer
National Research Council of Canada, Institute for Aerospace Research, Ottawa, Ontario, Canada
Journal bearing pad temperatures, oil flow requirements, and power losses can impose limitations on the design and operation of high-speed turbomachinery. Over the past few years, the authors have conducted extensive tests and studies of parameters that affect pivoted shoe journal bearing performance. A special, high-speed test rig is described, which was designed and built for the purpose of measuring bearing steady-state performance characteristics under light to moderately heavy loads, and to very high operating speeds that are being approached in new turbine and compressor designs. Instrumentation includes a detailed array of pad temperature detectors and the direct measurement of frictional torque.
Data are presented that compare the effects of pivot offset, oil flow, load orientation, method of lubrication, and oil discharge configuration on 6 inch diameter pivoted shoe journal bearing performance. The parameters are shown to significantly influence bearing pad temperature and power loss, particularly at high loads and speeds. Pad temperature profiles, isotherms, torque, and oil outlet temperatures are compared and evaluated. Discussions address the prediction and application of these parameters, and how they may be used to improve the capacity and performance of high-speed turbomachinery. The data and discussions are intended to provide useful information to engineers, programmers, and personnel involved with the study or operation of pivoted shoe journal bearings.
Turbomachinery operating speeds and loads have increased over time resulting in increased bearing temperatures and power losses. Gardner and Ulschmid (1973) addressed the concern of journal bearing limitations at turbulent operation, documenting a dramatic increase in pad temperature and power loss of a 17 inch pivoted shoe bearing at 3600 rpm (267 fps surface speed). The pad was a center pivot design, which has the advantage of being able to operate in either direction of rotation. There are many technical papers that study center pivot journal bearings. Offset pivots improve journal bearing pad temperature limitations, but are not as well documented in literature.
Large steam and gas turbine designers are presently considering 22 inch and larger diameter journal bearings for power generation where surface speeds at 3600 rpm exceed 330 fps. Designers of steam turbines and compressors have intentions for operating speeds approaching 400 fps. Such conditions are well into the turbulent regime where conventional bearing losses and temperatures become so high that options must be considered to address limitations. Direct lubrication is one solution that has been successfully applied in thrust bearings for many years, and also in special journal bearing applications dating back to the mid-sixties. It is only recently that direct lube journal bearings have been seriously considered for general turbomachinery applications because high surface speeds now warrant such a consideration.
As in the case of offset pivots, technical papers on direct lube journal bearings are sparse. Data are published by Tanaka (1991) and Tanaka and Mishima (1989) comparing 100 mm (3.94 inch) diameter designs to 137 fps; Harangozo, et a1. (1991), tested 5 inch diameter bearings to 152 fps; and Fillon, et a1. (1993), report on 100 mm (3.94 inch) diameter data to 70 fps. The authors' (Brockwell, et aI., 1992, 1994; Dmochowski, et aI., 1993) tested 3.88 inch diameter, leading-edge-groove (LEG) designs to 270 fps; DeCamillo and Clayton (1997) provide data on an 18 inch LEG generator bearing at 283 fps; and Edney, et a1. (1996), report on 5 inch diameter LEG steam turbine bearings running to 312 fps.
In assessing bearing limitations, there are many parameters that affect results including geometry, operating conditions, and even instrument location. For example, DeChoudhury and Barth (1981) show that the drop in oil outlet temperature between the bearing and drain line can lead to significant differences in thermal balance calculation of power loss. Pinkus (1990) describes peculiar behavior between laminar, transitional, and high-speed turbulent regimes of bearing operation. Pettinato and DeChoudhury (1999) note high edge temperatures from misalignment in ball-in-socket pivot geometry. Wygant, et a1. (1999), show differences in steadystate and dynamic performance attributed to restriction of pad motion by friction in sliding contact pivots. Conventional flooded journal bearings were used in these references. The flooded journal bearing design has been studied for many years, and information is available for a wide range of operating loads and speeds.
In contrast, the few published papers on direct lube journal bearings cited earlier mostly report on low speed operation, in the laminar to transitional range of operation. There is some disagreement regarding the magnitude of the benefits of direct lubrication. This may be due to laminar/transitional influences, but there are also differences in the method of direct lubrication, as well as the type of pivot, instrument location, etc. In general, all references agree that direct lubrication provides pad temperature benefIts that appear to improve with load and speed. There is also agreement that direct lube power loss is lower, although most authors report that thermal balance methods are of insufficient precision to allow an accurate assessment. Benefits are typically attributed to eliminating seal losses and reducing churning losses, but these are hypothetical because there are little data available for confirmation.
In order to address such issues, a special test rig was designed and built to measure steady-state performance under light to moderately high loads, and to very high operating speeds that are being approached in new turbine and compressor designs. An important feature of the rig is the direct measurement of frictional torque, which provides a more precise measurement of power loss than thermal balance techniques. Over the past few years, the authors have conducted extensive tests and studies of parameters that affect the performance of pivoted shoe journal bearings. The purpose of this work is to improve the capacity and performance of high-speed turbomachinery by extending bearing speed and/or load limitations and improving efficiency.
Read full PDF