Seal for a joint or juncture – Seal between relatively movable parts – Circumferential contact seal for other than piston
Reexamination Certificate
1997-07-11
2001-11-27
Knight, Anthony (Department: 3626)
Seal for a joint or juncture
Seal between relatively movable parts
Circumferential contact seal for other than piston
C277S506000
Reexamination Certificate
active
06322081
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to circumferential seals used for sealing along rotating shafts, and in particular, to a circumferential seal having a ceramic seal rotor flexibly mounted to a rotating shaft.
BACKGROUND OF THE INVENTION
Circumferential seals are used, for example, in gas turbine engines to prevent leakage of fluid along the engine's rotating shaft where the shaft extends through a wall or partition. Referring to
FIG. 1
, a typical circumferential seal includes a rotating component called a seal rotor
20
and a non-rotating component called a seal stator
31
. The rotor
20
is made of metal and is mounted to a rotating shaft
12
. It also has a radially, outward facing sealing surface
21
. The seal stator
31
includes a metal ring
35
mounted to the housing
34
and a carbon sealing ring
36
mounted to its radial inward facing surface. The stator
31
and rotor
20
are arranged so that the carbon ring
36
circumscribes the sealing surface
21
so as to seal a leakage path represented by arrow
38
. To avoid damage to the carbon ring
36
, a small radial gap is maintained between the ring
36
and sealing surface
21
.
A common problem associated with these seals occurs as a result of variation in the radial gap between the carbon ring
36
and sealing surface
21
. This variation is due in part to the mechanical growth of the rotor
20
due to centrifugal effects, but more significantly due to a disparity in thermal growth between the metal rotor and the carbon ring in response to changes in temperature. This disparity results from the two components having different coefficients of thermal expansion. The variation in the radial gap produces undesirable effect either when the radial gap is too wide open, or if it is allowed to completely close.
If the gap becomes too large, the amount of leakage through the seal increases resulting in reduced efficiency. In addition, the increased flow can adversely affect the control of pressures in neighboring cavities and hamper the intended use of the high-pressure air therein. However, if the gap is too small then substantial contact between the carbon ring and rotor can occur which can quickly damage either or both components.
One proposal for improving seal performance is to make the seal rotor from titanium, which has one of the lowest thermal expansion coefficients of any metal, and additionally satisfies strength requirements for a seal rotor. The differential thermal growth between a titanium rotor and carbon ring is substantially less than that of a seal with a more conventional nickel or iron based alloy rotor, however, it is not reduced enough to significantly improve seal performance. This is primarily due to the fact that although the thermal expansion coefficient of titanium is low for a metal, it is still much higher than that of carbon. Further, the titanium is substantially less durable than conventional rotor alloys, and thus more susceptible to damage upon contact with the stator.
Another proposal is to actively cool the rotor. A seal rotor can be cooled by providing a flow of cooling oil over its inside surfaces. This has the beneficial effect of reducing the rotor's temperature, and correspondingly reducing its thermal growth. By actively controlling the rotor's thermal growth in this way, the differential growth between the stator and rotor can be minimized. One disadvantage to an active cooling system is the added design complexity required for providing the means to deliver the oil to the runner, and the additional costs associated with that complexity. Another disadvantage is an increased risk of contamination of the air side of the seal due to the additional supply of oil in close proximity to the seal interface.
Accordingly, a need exists for a circumferential seal having a seal rotor with adequate mechanical properties and low enough thermal and mechanical growth during engine operation so that the rotor closely tracks the thermal growth of the carbon ring without the use of external cooling, and so that damage due to contact of the carbon ring and rotor is minimized.
SUMMARY OF THE INVENTION
An object of the invention is to provide a circumferential seal in which the engagement between the carbon ring and the rotor remains substantially constant in the presence of varying temperature.
The present invention achieves the above-stated object by providing a circumferential seal having a stator with a radially inward facing carbon portion and a rotor with a ceramic sealing member having a radially outward facing surface in rubbing contact with said carbon portion. The rotor also includes a metal clamping means for mounting the ceramic sealing member to a rotating shaft. The clamping means includes an axial flexible clamping member, and a radial flexible clamping member that together clamp onto a mounting flange of the ceramic sealing member when exposed to the compressive force of a lockup assembly.
The thermal and mechanical growth characteristics of the ceramic sealing member are substantially similar to the thermal growth of the carbon seal portion. Thus, the ceramic sealing member is able to maintain rubbing contact with the carbon portion despite varying temperature. Further, the clamping means isolates the ceramic sealing member from the compressive force of the lockup assembly, thereby avoiding damage to the ceramic sealing member.
These and other objects, features and advantages of the present invention, are specifically set forth in, or will become apparent from, the following detailed description of a preferred embodiment of the invention when read in conjunction with the accompanying drawings.
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Boyd Gary
Fuller Fred
O'Brien Michael F.
Ullah M Rifat
Beres John L.
Cahill Sutton & Thomas P.L.C.
Ceramic Engineering Consulting, Inc.
Knight Anthony
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