Gas Bearings for
Oil-Free Turbomachinery
MAJOR APPLICATION: Oil-Free Turbomachinery, Micro-Turbines,
etc
Sponsors:
Turbomachinery Research
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.
è To learn more,
order/read our PUBLICATIONS
2007-08
Work
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.
Current work: Hybrid Tilting Pad Gas Bearings: Analysis and
Tests
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
VIRTUAL TOUR
To be included 15
sec video clip
è Summary on computational analyses of gas bearings (past
research)
è To learn more,
order/read our PUBLICATIONS
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.