Patent Description:
Hydrostatic seals exhibit less leakage compared to traditional knife edge seals while exhibiting a longer life than brush seals. Some hydrostatic seals may be used between a stator and a rotor within a gas turbine engine. The hydrostatic seal is mounted to the stator to maintain a desired gap dimension between the hydrostatic seal and the rotor. The hydrostatic seal has the ability to 'track' the relative movement between the stator and the rotor throughout the engine operating profile when a pressure differential is developed across the seal.

Hydrostatic seals involve motion of a spring-attached shoe whose response is based on aerodynamic forces developed between the seal shoe and a rotor surface during operation. When properly designed, the hydrostatic seal will maintain tight clearances across the operating range of the engine. The portion of the hydrostatic seal that moves is in contact with the static seal housing in order to prevent air from circumventing the primary flowpath. When the seal is in motion, a friction force develops which is proportional to the geometry of the seal (piston area) and the aerodynamic forces acting on the seal. There is potential for the seal to become "stuck" in a certain position if the friction force is higher than the aerodynamic and mechanical forces acting on the seal shoe. In maneuver conditions, the seal gap can change rapidly and the seal is expected to respond quickly to such changes. If the friction force is too high, though, the seal may not respond to gap changes and there is potential for the seal to come into undesirable contact with the rotor.

<CIT> discloses a non-contacting dynamic seal having a shoe coupled to an outer ring by an inner beam and an outer beam. <CIT> discloses a further example of a non-contacting seal for sealing the circumferential gap between a first machining component and a second machine component. <CIT> discloses a hydrostatic seal to seal between a stator and a rotor within a gas turbine engine, according to the preamble of claim <NUM>.

According to a first aspect of the invention, there is provided a hydrostatic seal as claimed by claim <NUM>.

The seal may include a beam operatively coupling the shoe to the base.

The beam may be one of a plurality of beams oriented parallel to each other.

According to a second aspect of the invention, there is provided a seal assembly as claimed by claim <NUM>.

The first component may be a stator and the second component may be a rotor.

The seal may be operatively coupled to the rotor.

According to another aspect of the invention, there is provided a gas turbine engine as claimed by claim <NUM>.

The forward end of the shoe may be moveable into contact with the seal housing to provide a friction force in a seal closed condition defined by contact between the longest tooth and a structure to be sealed, the lift force provided by the axial distance and the radial distance being greater than the friction force.

The lift force may be equal to or greater than two times the friction force.

The following descriptions are provided by way of example only and should not be considered limiting in any way.

In one disclosed embodiment, the engine <NUM> bypass ratio is greater than about ten (<NUM>:<NUM>), the fan diameter is significantly larger than that of the low pressure compressor <NUM>, and the low pressure turbine <NUM> has a pressure ratio that is greater than about five (<NUM>:<NUM>). The geared architecture <NUM> may be an epicyclic gear train, such as a planetary gear system or other gear system, with a gear reduction ratio of greater than about <NUM>:<NUM>. It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present disclosure is applicable to other gas turbine engines including direct drive turbofans.

The fan section <NUM> of the engine <NUM> is designed for a particular flight condition--typically cruise at about <NUM> Mach and about <NUM>,<NUM> feet (<NUM>,<NUM> meters). The flight condition of <NUM> Mach and <NUM>,<NUM> feet (<NUM>,<NUM> meters), with the engine at its best fuel consumption--also known as "bucket cruise Thrust Specific Fuel Consumption ('TSFC')"--is the industry standard parameter of lbm of fuel being burned divided by lbf of thrust the engine produces at that minimum point.

<FIG> illustrates a hydrostatic seal indicated generally at <NUM>. The hydrostatic seal <NUM> is intended to create a seal between two relatively rotating components, such as a fixed stator and a rotating rotor <NUM>. The hydrostatic seal <NUM> includes a base portion <NUM> and at least one, but often a plurality of circumferentially adjacent shoes <NUM> which are located in a non-contact position along the exterior surface of the rotor <NUM>. Each shoe <NUM> is formed with a sealing surface <NUM>. For purposes of the present disclosure, the term "axial" or "axially spaced" refers to a direction along the rotational axis of the rotor, whereas "radial" refers to a direction perpendicular to the rotational axis of the rotor.

Under some operating conditions, it is desirable to limit the extent of radial movement of the shoes <NUM> with respect to the rotor <NUM> to maintain tolerances, such as the spacing between the shoes <NUM> and the facing surface of the rotor. The hydrostatic seal <NUM> includes at least one spring element <NUM>. In the illustrated embodiment, each spring element <NUM> is formed with at least one beam though in practice other elements could be utilized to create the spring element. In the illustrated embodiment, two beams are shown, specifically an inner beam 116A and an outer beam 116B. The beams 116A, 116B connect the shoe <NUM> to the base portion <NUM> of the seal <NUM>. The base portion <NUM> is fixed to a carrier <NUM> that is part of a static structure.

Particularly when the hydrostatic seal <NUM> is used in applications such as gas turbine engines, pressures are developed which apply an aerodynamic force to the shoe <NUM>, which is counter-balanced by the spring <NUM>, causing it to move radially with respect to the rotor <NUM>. The initial assembly point has a defined radial gap between the shoe <NUM> and the rotating surface, with no aerodynamic forces acting upon the shoe <NUM>. In operation, the hydrostatic seal <NUM> is used to restrict flow between a high pressure region and a lower pressure region. To assist with the flow restriction, a plurality of teeth <NUM> are included on the sealing surface <NUM> of the shoe <NUM>. The pressure drop across the shoe <NUM> results in a radial force on the shoe <NUM> which is counter balanced by the spring <NUM> with spring force. In operation, when the air flow between the shoe <NUM> and rotor <NUM> increases, the pressures on the shoe <NUM> generally decrease. The reduction in pressures along the shoe <NUM> reduces the radial force acting on the shoe <NUM> such that the force balance between the overall aerodynamic forces on the seal shoe and the spring force S causes the shoe <NUM> to be pushed radially inwardly toward the rotor <NUM>, thus decreasing the gap, until the seal reaches an equilibrium position considering the spring force of the displaced beam(s). Conversely, in operation, when the air flow between the shoe <NUM> and rotor <NUM> decreases, the pressures on the shoe <NUM> generally increase. The increase of radial force on the shoe <NUM>, and its overall impact with the net aerodynamic forces on the seal shoe <NUM> considering the spring force S, causes the shoe <NUM> to move radially outwardly from the rotor <NUM> until the seal reaches an equilibrium position considering the spring force of the displaced beam(s).

Referring now to <FIG>, a portion of the sealing surface <NUM> of the shoe <NUM> is shown. For illustration purposes, only one of the teeth 118A is shown. In the illustrated condition, the tooth 118A is in contact with the rotor <NUM> to define a limit of zero clearance between the shoe <NUM> and the rotor <NUM>. In this condition, the tooth 118A divides the region into a high pressure region axially forward of the tooth 118A and a low pressure region axially rearward of the tooth 118A. This condition represents a "closed" condition of the seal <NUM>, which requires movement of the seal away from the rotor <NUM>. A frictional force caused by contact between the shoe <NUM> that is to be moved and any static portion of the overall seal housing resists the required radial movement of the shoe <NUM>. The frictional force is a function of the normal load on the shoe <NUM>, the area in contact with the static structure and the friction coefficient. These variables depend upon the particular application of use.

The force that moves the shoe <NUM> and overcomes the friction force is referred to herein as an opening force. The opening force is determined by the axial position of the tooth (referred to herein as "longest tooth 118A") that projects furthest radially away from the sealing surface <NUM> of the shoe <NUM>. Two axial distances are referenced in <FIG>, <FIG>. In particular, an axial region of the shoe <NUM> referenced with l represents the upstream (high) pressure acting on both the upper and lower surfaces of the seal in the limit of zero clearance. The lift force on the lower surface cancels with the force on the upper surface over the axial region l near <NUM> gap and this region thus does not contribute to the effective aerodynamic lift force that is a function of the axial length represented with character L, which extends from the axially rearward location of length l to the axial forward side of the longest tooth. The radial distance from the radial end of the longest tooth to the sealing surface <NUM> is referenced with H in <FIG>, <FIG>. Dimensions L and H determine the aerodynamic lift force that provides the opening force during a "lock up" or "closed" condition with the tooth contacting the rotor <NUM>. In the illustrated embodiments, the longest tooth 118A is the most axial forward tooth. The embodiments disclosed herein provide a seal opening force that is equal or greater than two times the friction force to prevent seal lock-up. Therefore, the opening force assuredly overcomes the friction force, with margin. In some embodiments, the axial distance L is greater than the radial distance H.

<FIG> and <FIG> illustrate a ramp <NUM> proximate the leading edge of the shoe <NUM>. The ramp <NUM> may be oriented at different angles in various embodiments. For purposes of example, <FIG> illustrates a steeper angle that leads to the longest tooth 118A over a shorter axial distance, when compared to the ramp of <FIG>. The specific dimensions will be dictated by the coefficient of friction, as described above in detail. The ramp <NUM> maintains the desired opening force behavior of the seal <NUM> for situations where the seal <NUM> has experienced tooth wear which may occur in operation during contact with the rotor, which without the ramp <NUM> may result in degradation of the maximum opening force that can be generated. However, it is to be appreciated that alternative geometries within the scope of claim <NUM> may be suitable. Regardless of the particular geometry at the leading edge of the shoe <NUM>, the length L and height H must be optimized to provide the sufficient opening force that exceeds the frictional force.

The embodiments described herein improve the opening force characteristics of the seal <NUM>, such that the seal will always open (i.e., not get stuck by friction). This protects the seal <NUM> under maneuver conditions, which is particularly important for certain applications, such as military engines.

While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the appended claims.

In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the scope of the appended claims.

Claim 1:
A hydrostatic seal (<NUM>) configured to be disposed between relatively rotatable components, the seal comprising:
a base (<NUM>);
a seal housing;
a shoe (<NUM>) operatively coupled to the base and extending axially from a forward end to an aft end; and
a plurality of teeth (<NUM>) extending radially from a sealing surface (<NUM>) of the shoe, one of the teeth (118A) being a longest tooth that extends furthest radially from the sealing surface and having a radial tooth tip, and a ramp extending from the forward end of the shoe to the radial tooth tip,
characterized by:
an axial distance from a forward end of the shoe to the longest tooth being greater than a radial distance from the radial tooth tip to the sealing surface.