TB20: On The Experimental Static and Dynamic Load Performance of a Water Lubricated Hybrid Thrust Bearing
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Date
2021
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Turbomachinery Laboratory, Texas A&M Engineering Experiment Station
Abstract
Fluid film thrust bearings (TBs) carry axial loads and control the position of rotating machinery thus enabling tight clearances forthe efficient aerodynamic operation of impellers in pumps and compressors Prediction of thrust forces is largely empirical, emphasizingthe need to test TBs for use in reliable rotating machinery equipment This paper presents a water lubricated test rig housing a pair ofhydrostatic TBs and describes the experimental quantification of their static and dynamic forced performance The test rig comprises ofa drive motor, a coupling and quill shaft, and a rigid rotor with two end collars facing the TB elements Water lubricated radial bearingssupport the test rotor One slave TB rigidly attaches to the rig casing while the test TB connects to a load shaft supported on aerostaticbearings The TBs, each with ID = 406 mm and OD = 762 mm, have eight pockets, 20° in arc length and 81 mm in radial length, withdepth and feed orifice diameter equal to 051 mm and 180 mm for the test TB, and 044 mm and 155 mm for the slave TB, respectively Copyright© 2020 by Turbomachinery Laboratory, Texas A&M Engineering Experiment Station 2During operation, while the rotor spins to a maximum speed of 6 krpm (TB OD surface speed = 24 m/s), cold water at a maximumsupply pressure (Ps) of 483 bar(g) lubricates the TBs A calibrated taut string mechanism applies a static load (W) on the movable shaftand which pushes the test TB against one end of the rotor collar The rotor in turns transmits the same static load onto the slave TB Fordynamic load tests, a calibrated gun delivers impacts onto the load shaft and which excites both TBs and the rotor The measurementsinclude rotor axial displacements, load shaft acceleration, and the applied loads, static and dynamic Form these measurements, amechanical system model aids to estimate the TB force coefficients; namely, stiffness (Kz), damping (Cz), and added mass (Mz) Fortests with a constant axial load (W), the axial clearance (Co) increases as the TB lubricant supply pressure (Ps) increases Under a constantwater supply pressure (Ps) into the bearings, the test TB clearance (Co) decreases as the applied specific axial load (W) increases to amaximum of 16 bar The measured static load performance, agreeing with predictions as well as with earlier test results, demonstratesthe TBs operate mainly as hydrostatic ones since hydrodynamic features derived from surface speed are small Form the multiple impacttests, for the TBs operating with a fixed supply pressure, the test TB experimental stiffness (Kz) and damping (Cz) increase as W increases,meanwhile the added mass (Mz) varies slightly Operation with a constant W and with an increase in water Ps yields an increase in Kzand Cz without a change of its added mass Predictions agree well with the experimentally derived force coefficients The test results arerelevant to the pump industry, in particular for vertical lift processes
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