Gas Bearings for Oil-Free Turbomachinery
MAJOR APPLICATION: Oil-Free Turbomachinery, Micro-Turbines, etc
Consortium (00-08), State of
Objective: To advance the technology of inexpensive gas bearings for micro gas turbines and micro power systems
Status: Hybrid flexure pivot tilting-pad gas bearings (FPTPB) can afford much higher operating speed than conventional gas bearings, i.e. as high as 100 krpm, which is the maximum speed the motor can provide. Moreover, computational model are available to predict gas bearing damping and stiffness coefficients. Measurements in progress to identify the test bearings’ dynamic force coefficients.
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FPTPBs can operate at very high speeds without instability issues due to their reduced cross-coupled stiffnesses - free tilting motion of pads supported on flexure pivots. FPTPBs offer much less friction in pivot due to their integral manufacturing utilizing wire EDM. Conventional tilting-pad bearings have major drawbacks with pivot and pad wear due to relative motion of parts. Compared with traditional gas bearings including three-lobe bearings and Rayleigh-step bearings, FPTPBs are more complex in structure and more expensive; however, their predictable rotordynamic performance and superior stability behavior at high operating speeds provide them with desirable applications in oil-free high-speed microturbomachinery.
Current research relates to the identification of dynamic damping and stiffness coefficients for the bearings. The major tasks are:
(1) Complete measurements of rotor response for increasing imbalances and compare to predictions from computational models.
(2) With e-relay deliver impact loads into test rig, measure the rotor transient response and identify bearing damping and stiffness coefficients from the displacements and forces derived from test results.
(3) Modify predictive code to include flow model for choked flow through orifices.
(4) Envision, design and implement modifications to the current test rig for future work.
Test Rig Facility & test Bearings
Measured rotor synchronous response and waterfall plot:
2005 Status: Small 100 krpm test rig continues to provide superb test data. Rotordynamic measurements conducted with Rayleigh-step gas bearings coated with Argonne‘s NFC (Near Frictionless Carbon) demonstrated the bearings’ poor static load performance and worse rotordynamic response with severe sub synchronous instabilities at low shaft speeds. The bearings could not operate at speeds above 20 krpm. This speed is too low when considering that flexure pivot hybrid bearings achieved 100 krpm without instability problems.
Gas film bearings offer unique advantages enabling successful deployment of high-speed oil-free turbomachinery. Current applications encompass micro power generators, air cycle machines and turbo expanders. The investigation will continue to advance the analysis and experimental validation of hybrid gas bearings with static and dynamic force characteristics desirable in high-speed turbomachinery.
To date hybrid (hydrostatic/hydrodynamic) flexure pivot-tilting pad bearings (FPTPBs) have demonstrated superior static and dynamic forced performance than simple three-lobe bearings and Rayleigh-step bearings tested earlier. FPTPBs are mechanically complex and more expensive; however, their enhanced stability characteristics and predictable rotordynamic performance, with verified operation to speeds as high as 100 krpm, makes them desirable for the envisioned oil-free applications in high speed micro turbomachinery.
The main objective is to advance the technology of gas film bearings for applications to oil-free turbomachinery by demonstrating their rotordynamic performance, reliability and durability. The tasks to be performed are:
a) To displace the rig to a vertical configuration and to measure the synchronous response and stability of the test rotor on flexure-pivot pad gas bearings. Stiffness and damping coefficients will be determined from measured frequency domain transfer functions (load/displacement).
b) To assess the effect of solid lubricants, namely NFC and DLC coatings, on early rotor lift-off and touchdown speeds, and to evaluate friction and wear on rotor and bearing surfaces.
c) Enhance a computational code by including the hydrostatic pressurization to predict the static and dynamic forced performance of externally pressurized gas bearings.
d) To validate predictive model with comparisons to experimental values.
TEST RIG FACILITY
Max speed = 100 krpm, rotor diameter=29 mm, weight=0.87 kg
Measured response of rotor supported on tilting pad gas bearings – Stable to 100 krpm
To be included 15 sec video clip
è Summary on computational analyses of gas bearings (past
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Micro-turbomachinery (MTM) implements gas bearings in compact units of enhanced mechanical reliability. Gas bearings, however, have little damping and are prone to wear during frequent rotor start-up and shut down conditions. Externally pressurized gas bearings provide a simple solution to overcome excessive drag and allowing rub-free operation during transient response events. Some commercial MTM currently implements gas foil bearings, a costly proprietary technology with few, if any, proven reliable predictive design models. The thrust of this research is to investigate conventional bearings of low cost, easy to manufacture (common materials) and easy to install and align.
Flexure pivot tilting pad bearings offer little or no cross-coupled stiffness with enhanced rotordynamic stability. These bearings, modified for hydrostatic pressurization, demonstrated superior rotordynamic performance over other simple gas bearing configurations. The test rig comprises of a rigid rotor, 0.825 kg and 28.6 mm in diameter, supported on two hybrid flexure pivot hybrid gas bearings, each with four pads and 60% pivot offset and 0.6 mm feeding holes. Experimental results show that external pressurization stiffens the gas bearings, increasing the system critical speed while reducing the modal damping. Most importantly, the tests demonstrate that external pressurization is not needed for super critical speed operation. In practice, the supply pressure could be shut off at high speeds with substantial savings in operational efficiency. In addition, controlling the feed pressure while the rotor passes through its critical speeds can eliminate high amplitude motions because of the bearings’ inherent little damping.
The test rig integrates an inexpensive automatic air pressure regulator to control the supply pressure into the gas bearings. The measured system dynamic response determines the regulator control scheme with a programmed schedule over a rotor speed region enclosing the system critical speeds. Rotor speed coast-down tests with controlled supply pressure into the bearings demonstrate the effective elimination of large rotor motion amplitudes while crossing the system critical speeds. The simple on-off supply pressure control, i.e. a sudden increase in pressure while approaching a critical speed, is the best since it changes abruptly the bearing stiffness coefficients and moves the system critical speed to a higher speed.
A rotordynamic analysis, integrating bearing force coefficients predicted by an existing TRC computational model, forwards critical speeds in agreement with the test results. Predicted rotor synchronous responses for the cases with controlled supply show an excellent correlation with the measured responses. The experiments validate the predictive tools and demonstrate the controllable rotordynamic characteristics of the flexure pivot hybrid gas bearings.