Stock material or miscellaneous articles – All metal or with adjacent metals – Honeycomb – or with grain orientation or elongated elements...
Reexamination Certificate
2001-04-26
2003-08-26
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Honeycomb, or with grain orientation or elongated elements...
C148S516000, C148S527000, C148S537000, C415S173400, C415S173500, C415S174400, C415S174500, C427S248100, C427S585000, C427S592000, C427S593000, C427S252000, C428S594000, C428S637000, C428S650000, C428S652000, C428S653000, C428S678000, C428S073000, C428S116000, C428S118000, C428S689000, C428S698000, C428S908800, C428S925000, C428S926000, C428S938000, C428S941000
Reexamination Certificate
active
06610416
ABSTRACT:
The present invention relates generally to rotating labyrinth seals and particularly to rotating labyrinth seals having improved machinability for use in gas turbine engines for the propulsion of aircraft.
BACKGROUND OF THE INVENTION
Rotating labyrinth seals have a wide variety of uses and one such use is to effect sealing between plenums at different pressures in gas turbine engines. Such seals generally consist of two principal elements, i.e., a rotating seal and a static seal or shroud. The rotating seal, in cross section parallel to the axial length of the engine, frequently has rows of thin tooth-like projections extending radially from a relatively thicker base toward the static seal or shroud. The static seal or shroud is normally comprised of a thin honeycomb ribbon configuration for jet engine applications. These principal elements are generally situated circumferentially about the axial (lengthwise) dimension of the engine and are positioned with a small radial gap therebetween to permit assembly of the rotating and static components. The purpose of the labyrinth seal arrangement is to minimize gas path leakage out of the primary gas path and to segregate different stages of the compressor which are at different temperatures and pressures.
To a significant extent, engine efficiency depends upon minimizing this gas leakage around rotating components by controlling the gas flow to maximize interaction between the gas stream and the components in the primary gas path. The effectiveness of the turbine engine varies directly with the proportion of gas that impinges upon the blades of the rotating member. Closer tolerances between the rotating and static seals achieve greater efficiencies. The fabrication process to obtain these close tolerances is extremely costly and time-consuming.
When the gas turbine engine is operated, the elevated temperatures of operation cause the opposed static and rotating seals, such as those in the rotating labyrinth seals, to expand in a radial direction toward each other. The rotating labyrinth seals expand radially and rub into the shroud, creating frictional contact between the thin projections of the rotating seal and the shroud. During the rub, there is high thermal compression, with resultant high residual tensile stress after the rub. This frictional contact causes elevation of seal teeth temperatures in excess of 2,000 degrees F. with resulting possible damage to one or both seal members. For example, rotating tips may crack and break off, significantly impairing the seal efficiency and operation of the engine.
The thin, honeycomb ribbon construction of the shroud is used to reduce the surface area on which the seal teeth rub while reducing the weight of the structure, and helps to minimize the heat transferred into the rotating seal, while also providing the required strength. In addition, the rotating labyrinth seal teeth tips are made thin in order to thermally isolate them from the supporting base or shell structure. However, excessive heat from deep rubs (even into honeycomb) during engine start-up and during engine excursions can damage the rotating knife edge seals, negatively affecting durability and engine efficiency and providing a leak path for the flow of gases. Furthermore, material transfer can occur which also degrades the seal characteristics. Cutting into even low-density honeycomb cells can still cause rotary seal tooth damage, leading to premature part retirement.
Various coating techniques, for example, U.S. Pat. No. 5,314,304 to Wiebe, have been employed to coat the inside diameter of the stator shroud with an abradable coating in an attempt to increase both service life and operating efficiencies. The abradable coating can be worn away by the frictional contact of the rotating seal, thereby providing a close fitting channel in which the rotating seal may travel and maintaining efficiencies. One problem with an abradable coating system is that, over time, the abradable material filling the cells of honeycomb can separate from the substrate honeycomb as a result of thermal cycling. The abradable filler can then rub against the downstream blades causing engine vibrations. Other problems include inadequate sealing, seizing of cooperating members, elevation of the temperature of the rotor teeth as a result of the frictional wear induced by contact with the abradable coating and local “hot-spots” with resulting burning of non-abradable members.
Kobayashi et al. in U.S. Pat. No. 6,039,535 also addressed the problem of performance of the labyrinth seal for a centrifugal compressor. An abradable coating is applied over the casing. The improved seal is formed by bonding an additional layer of abradable material over the substrate material of the casing. The compressor clearances are set so that the tips of the rotating seal do not contact the substrate material casing, but rather only contact the abradable coating over the casing. The overall thickness of the casing and coating is increased to fill the gap between the substrate material casing and the rotating labyrinth seal. Kobayashi et al., however, does not disclose the use of a thin, ribbon like honeycomb material for a casing, and hence does not recognize the problems associated with forming a honeycomb material from thin ductile sheets and subsequently applying an abradable coating over the honeycomb material.
Other attempts at increasing engine efficiencies have included coating the seal teeth. For example, U.S. Pat. No. 5,603,603 to Benoit et al. is directed to applying an abrasive tip coating to the seal teeth, and U.S. Pat. No. 4,884,820 to Jackson et al. is directed to bonding a ceramic or metallic coating to the seal teeth.
Another approach addressing rotating seal tooth durability has been to make the seal teeth more defect tolerant, such that cracks that form due to rubbing are benign U.S. Pat. No. 5,143,383 to Glynn et al. relates to stepping the tooth profile to act as a crack arrestor. This method has a disadvantage of being dependent on a relatively low mean stress and stress range to avoid having crack growth of critical size during the expected life of the typical gas turbine part.
U.S. Pat. No. 4,060,250 to Davis et al. is directed to non-aircraft centrifugal compressors, in which the carbon steel rotary elements are inlaid or coated with a corrosion and heat resistant alloy, such as a chromium-containing nickel-based alloy, added to protect the underlying low carbon steel from ignition. The surface of the rotatable cylindrical member is characterized by this metallurgically fused alloy protective coating.
While much effort has been directed at improving the rotating structure of the seal arrangement, There is a continuous need for improved designs for rotating labyrinth seal structures including improvements directed to the static structure to increase both service life and engine operating efficiencies. The present invention fulfills this need, and further provides related advantages.
BRIEF SUMMARY OF THE INVENTION
The present invention provides for a method to increase the machinability of the honeycomb of the stationary portion of a labyrinth seal. The increased machinability of the stationary portion of the labyrinth seal, referred to as a shroud, results in a reduction in the measured peak tooth temperature of the rotating seal teeth of the rotating portion of the labyrinth seal, while maintaining or even improving the high temperature capability of the rotating labyrinth seal, so as not to limit its operating environment. The sealing functionality of the rotating labyrinth seal is unaffected, and even improved in some instances, by the method of the present invention. The improved machinability of the shroud results in less friction between the shroud and the rotating teeth, thereby reducing damage to the teeth.
After forming a honeycomb, which will be used as a seal or shroud, from a thin ribbon of ductile substrate material such as a superalloy material, the machinability of the honeycomb is increased by selecting a lightweight diffusible el
Tholke Brent Ross
Wallace Thomas Tracy
General Electric Company
Koehler Robert R.
Maria Carmen Santa
McNees Wallace & Nurick LLC
Miller Jonathan P.
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