Brush seals have been incorporated in the latest turbine engines
to reduce leakage and improve efficiency. However, the life of
these seals is limited by wear. Studies have shown that optimal
sealing characteristics for a brush seal occur before the interference
fit between the brush and shaft is excessively worn. Research
to develop improved tribopairs (brush and coating) with reduced
wear and lower friction has been hindered by the lack of an accurate,
low-cost, efficient test methodology. Estimated costs for evaluating
a new material combination in an engine company seal test program
are on the order of $100,000.
To address this need, the NASA Lewis Research Center designed,
built, and validated a unique, innovative brush seal tuft tester
that slides a single tuft of brush seal wire against a rotating
shaft under controlled loads, speeds, and temperatures comparable
to those in turbine engines. As an initial screening tool, the
brush seal tuft tester can tribologicaly evaluate candidate seal
materials for
1/10th the cost of full-scale seal tests
.
Previous to the development of the brush seal tuft tester facility,
most relevant tribological data had been obtained from full-scale
seal tests conducted primarily to determine seal leakage characteristics.
However, from a tribological point of view, these tests included
the confounding effects of varying contact pressures, bristle
flaring, high-temperature oxidation, and varying bristle contact
angles. These confounding effects are overcome in tuft testing.
The interface contact pressures can be either constant or varying
depending on the tuft mounting device, and bristle wear can be
measured optically with inscribed witness marks.
In a recent cooperative program with a U.S. turbine engine manufacturer,
five metallic wire candidates were tested against a plasma-sprayed
Nichrome-bonded chrome carbide. The wire materials used during
this collaboration were either nickel-chrome- or cobalt-chrome-based
superalloys. These tests corroborated full-scale seal test results
and provided insight into previously untested combinations.
As the cycle temperature for improved efficiency turbine engines
increases, new brush seal materials combinations must be considered.
Future brush seal tuft testing will include both metallic and
ceramic bristles versus commercial and NASA-developed shaft coatings.
The ultimate goal of this work is to expand the current data base
so that seal designers can tailor brush seal materials to specific
applications.
Fellenstein, J.A., et al.: High Temperature Brush Seal Tuft Testing of Metallic Bristles Versus Chrome Carbide. NASA TM-107238, 1996.
Fellenstein, J.A.; and DellaCorte, C.: A New Tribological Test for Candidate Brush Seal Materials Evaluation. NASA TM-106753, 1995.
Previous articleLast updated May 5, 1997
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