Measurements of the Static and Dynamic Load Performance of a Carbon-Graphite Porous Surface Tilting-Pad Gas Lubricated Journal Bearing
Abstract
Hydrostatic gas journal bearings with porous tilting pads enable shaft support with minute drag power losses. Yet, there is little archival information on the static and dynamic load performance of this bearing type. Thus, the report characterizes the static and dynamic load performance of a gas journal bearing (diameter D = 101.6 mm) with four porous carbon-graphite tilting pads, each L=76 mm in length. Each pad is supported by two nested Belleville washers (BW) acting on spherical pivots. First, experiments characterize the stiffness from the washers as a pivot support with KP=20.6 MN/m during loading and 9.3 MN/m during unloading. Dry friction at the pivot-washer interface causes the difference in stiffness.
With the bearing installed in an existing high-speed facility, measurements from static load tests include bearing eccentricity and attitude angle, pad temperatures and pad tilt displacements. As the air supply pressure into the bearing pads increases, so does the bearing hydrostatic specific load capacity that equals 58% of the pressure difference (supply-ambient). Upon operation with shaft speed, and to avoid contact, there is a significant decrease in load capacity to ~43% of the hydrostatic load. Supplied with air at 7.8 bara, the bearing supported static loads up to ~172 kPa (300 lbf) at 6 krpm (36 m/s) and ~117 kPa (203 lbf) at 9 krpm (48 m/s). At ambient temperature 31°C -41°C, the bearing static stiffness KS ~13.1 MN/m (75 klbf/in) and independent of either shaft speed or static load.
Results of dynamic load tests acting on the bearing during operation with shaft speed at 6 krpm and 9 krpm, and for specific static loads up to 115 kPa and 101 kPa respectively, show frequency-independent stiffness and damping coefficients (K, C) model best the bearing dynamic forced response. Bearing stiffnesses (KXX ~ KYY) increase with static load and range from 13.6 MN/m to 32.7 MN/m (186 klbf/in). The bearing damping coefficients (CXX ~CYY) are as large as 5.8 kN∙s/m (34 lbf∙s/in), though with a large experimental uncertainty.
Subject
Gas bearingsreliability
rotordynamic force coefficients
oil-free turbomachinery
hydrostatic lubrication
Citation
Bolen, Rachel (2020). Measurements of the Static and Dynamic Load Performance of a Carbon-Graphite Porous Surface Tilting-Pad Gas Lubricated Journal Bearing. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /192687.