Patent Description:
Carbon seals are commonly used to seal between relatively rotating components in gas turbine engines (used in propulsion and power applications and broadly inclusive of turbojets, turboprops, turbofans, turboshafts, industrial gas turbines, and the like). These include shaft seals (i.e., where the sealing surfaces of seal and seat face radially (e.g., within <NUM>° or essentially <NUM>°) and extend axially) and face seals (i.e., where the sealing surfaces face axially (e.g., within <NUM>° or essentially <NUM>°) and extend radially). In typical face seal situations, the annular carbon seal is axially spring biased into engagement with an annular seat (typically metallic such as a steel). Often, the carbon seal is on non-rotating static structure and the seat rotates with one of the engine shafts. The sliding engagement causes frictional heating. The heat must be dissipated. With a rotating seat, it is common to use oil cooling. Generally, oil cooled carbon seals are divided into two categories: "dry face" seals wherein the oil passes through passageways in the seat without encountering the interface between seal face and seat face; and "wet face" seals wherein the oil passes through the seat to the interface so that the oil that flows through the seat cools the seat but then lubricates the interface to further reduce heat generation.

For both forms of seals, the oil may be delivered through a nozzle and slung radially outward by the rotating component and collected in a radially outwardly closed and inwardly open collection channel from which the passageways extend further radially outward.

As is discussed further below, carbon face seals can be subject to issues of dynamic instability. Permitted radial displacements can lead to coupled axial displacements. The axial displacement may be a local displacement with a maximum displacement location diametrically opposite a contact location. The contact location may rotate. In extreme situations, there may be a full <NUM>° of separation/gap. In addition to leaks when the axial gap opens, the resulting coupled radial and axial displacement can lead to rapid and advanced wear and stresses beyond the capability of the spring element resulting in premature failure of the seal.

<CIT> discloses an apparatus according to the preamble of claim <NUM> and comprising a mechanical seal suitable for a shaft sealing component.

According to an aspect of the present invention, there is provided an apparatus in accordance with claim <NUM>.

Optionally, a radial gap between the outer diameter surface and the seal is formed between: an inner diameter surface portion of the seal; and a portion of the outer diameter surface of said section having a radius ROF varying by no more than <NUM> % over a length SA1 of <NUM>% to <NUM>% of an axial span SA2 from the seat face to the rim.

Optionally, the section is along a protruding flange having: an inner diameter surface; and an axial undercut at a root of the section.

Optionally, the section has a chamfer surface between the outer diameter surface and the rim.

Optionally, the chamfer surface has a half angle of <NUM>° to <NUM>° over at least one of: a radial span RS4 of at least <NUM>% of a maximum radius of the seat section; and a length SA3 of at least <NUM>.

Optionally, the seat face and the section are of a single unitary piece or the seat face is on a first piece and the section is formed on a second piece.

Optionally, the first piece has: a contact face contacting a face of the second piece; and a channel between the seat face and the contact face.

Optionally, the seal and seat are full annular and the seal is interference fit in a seal carrier.

Optionally, the bellows spring is captured between a spring carrier and a seal carrier; and the seal carrier carries the seal.

Optionally, the seal carrier has an outer diameter sleeve section in interference fit with the seal.

Optionally, the apparatus is a gas turbine engine.

Optionally, a method for assembling the apparatus comprises: mounting the seal to the first member so as to be cantilever supported by a bellows spring; and installing the seat so that the section passes radially within the seal.

Optionally, the section has a chamfer surface between the outer diameter surface and the rim; and the installing causes the chamfer to contact the seal and radially shift the seal.

According to an aspect of the present invention, there is provided a method for using an apparatus as described above (in any of the foregoing embodiments), wherein the method comprises: rotating the shaft relative to the first member about the axis; and a dynamic radial excursion of the shaft relative to the first member causing contact between the seat section outer diameter surface and an inner diameter surface of the seal.

Optionally, the contact is effective to prevent axial separation between the seal and the seat.

<FIG> shows a seal system <NUM> having a seal <NUM> and a seat <NUM>. As is discussed further below, the seal system is used in a turbomachine such as a gas turbine engine for a purpose such as isolating a bearing compartment <NUM>. The seal is mounted to a first structure such as an engine static structure and the seat is mounted to rotate relative thereto (e.g., mounted to a shaft) about an axis A which may be the engine centerline or central longitudinal axis.

The example seal <NUM> is a carbon seal having an axially-facing/radially-extending seal surface or face <NUM>. The example seal <NUM> is formed as single-piece or segmented body of revolution about an axis normally coincident with the centerline A.

<FIG> further shows an outward radial direction <NUM> and a forward direction <NUM>. The seat <NUM> has an axially-facing/radially-extending seat surface or face <NUM> engaging the seal face <NUM>. The seal system <NUM> isolates a space or volume <NUM> from a space or volume <NUM>. The example space or volume <NUM> is a bearing compartment. The example seal system is at an aft end of the bearing compartment. A similar or other seal system (not shown) may be at a forward end of the bearing compartment. The example bearing compartment <NUM> contains a bearing <NUM> supporting the shaft for rotation relative to the static structure about the axis A. The example second space or volume <NUM> is a buffer chamber. In the example engine configuration and position, a case component <NUM> (e.g., a strut ring/frame) of the static structure is positioned radially inboard of a gas path (core flowpath) C. The seal is a dry face seal in that there are no oil passageways to the seat face <NUM>.

The faces <NUM> and <NUM> are axially-facing/radially extending faces. This engagement allows relative radial displacement of seal and seat. A dynamic component of the radial displacement is a function of the seal's dynamic response to excitation. Analytical modeling, experimental data, and engine test data has shown an excitation mode can exist which initially starts as a pure radial displacement. <FIG> shows a plot <NUM> of radial displacement versus rotational speed (frequency) from zero to maximum speed vmax (of the spool which includes the seat). This displacement increases generally steadily without any relative axial displacement until a certain value v<NUM> is reached where the radial displacement becomes coupled with a relative axial displacement <NUM> (not shown to the same scale as radial displacement in <FIG>). As noted above, the axial displacement may be measured as the axial displacement at the circumferential location of maximum axial displacement at a given time/speed. The axial displacement may increase much more rapidly with speed and the coupled motion results in catastrophic damage to structural integrity and operability of the seal. The exact extents of these conditions are functions of the seal size, material pair, engine configuration, and operating conditions.

As is discussed further below, to prevent or at least limit the coupled dynamic displacement, a constraint on the relative radial freedom of movement is imposed to limit the relative radial displacement. For example, it may be limited to less than the extent at which axial displacement initiates. In <FIG>, this displacement is associated with speed v<NUM>.

The example bearing <NUM> is a rolling element bearing have an inner diameter (ID) race <NUM> (inner race), an outer diameter (OD) race <NUM> (outer race), and a circumferential array of rolling elements <NUM> (e.g., balls). The bearing may be lubricated (e.g., via passages <NUM> delivering oil from an oil system (not shown)) so that the bearing compartment <NUM> contains oil. Thus, the seal system <NUM> may serve to contain the oil in the bearing compartment to prevent leakage out. The outer race <NUM> is carried by a bearing support <NUM> which, in turn, is carried by the case component <NUM>.

The inner race <NUM> is mounted to a shaft of the engine. An example shaft is the high speed shaft of a two-spool turbofan. The example inner race <NUM> is mounted to the sleeve <NUM> of a gear (e.g., for engaging a tower shaft for an accessory gearbox/starter) which is, in turn, mounted to a main shaft piece <NUM>.

Radially inboard of the seat face <NUM>, the seat <NUM> has an aft-facing abutment surface <NUM> abutting a forward facing surface <NUM> of a flange or shoulder <NUM> on the shaft main piece <NUM>. The flange or shoulder <NUM> has an outer diameter (OD) surface <NUM>. The seat <NUM> has a forward rim surface <NUM> abutting an aft end face <NUM> of the inner race <NUM>.

The example seal system <NUM> includes a seal housing <NUM> for mounting to the remaining static structure of the engine.

As is discussed further below, the seal <NUM> is mounted in a seal carrier <NUM> (e.g., a single-piece alloy machining) having an outer diameter collar/sleeve section <NUM> and a radial flange <NUM>. The aft portion of the seal OD face and an OD portion of the seal aft face respectively contact an ID face of the sleeve <NUM> and a forward face of the flange <NUM>. One or more springs <NUM> (e.g., a single bellows spring shown schematically encircling the centerline A) axially bias the seal <NUM> into engagement with the seat <NUM>.

The spring <NUM> extends between a forward end <NUM> and an aft end <NUM>. The spring is carried by a spring carrier <NUM> (e.g., a single-piece alloy machining) which, in turn, is mounted to the housing <NUM>. The spring carrier <NUM> has an outer diameter (OD) collar/sleeve section <NUM> and a flange <NUM>. The spring aft end abuts and is secured to the forward face of the flange <NUM> and the spring forward end <NUM> abuts and is secured to the aft face of the flange <NUM>.

<FIG> further shows the seat <NUM> as including a section <NUM> protruding axially beyond the seat face <NUM> to a rim <NUM> and having an inner diameter (ID) surface <NUM> and an outer diameter (OD) surface <NUM>. The OD surface has an aft bevel or chamfer <NUM> leading to the rim <NUM> and a main axial portion <NUM> between that and the seat face <NUM>. A junction between the OD surface <NUM> and the seat face <NUM> has an undercut <NUM> both axially and radially relieving. The undercut <NUM> provides stress relief at the junction of the surface <NUM> and the seat face <NUM> also accommodates debris and limits/prevents wear and other damage to the ID edge portion <NUM> of the seal nose at a junction sealing face <NUM> and seal ID surface <NUM>.

There is an additional axial undercut <NUM> to the ID of the flange at a proximal root of the flange. The undercut <NUM> isolates the protruding section <NUM> from contact stresses in the seat caused by the compressive engagement of the surfaces <NUM> and <NUM>. The example undercut <NUM> is axial-only; whereas the undercut <NUM> is axial/radial.

<FIG> also shows the sleeve <NUM> as having a forward rim <NUM> axially recessed relative to the seal face <NUM>. This recessing accommodates seal wear and reduces with use. A recess/rebate <NUM> in the seal's OD surface <NUM> ahead of the rim <NUM> limits contact stresses in the seal at the ID forward corner <NUM> of the sleeve cross-section. This is particularly relevant where the seal is in radial interference fit with the sleeve.

A gap <NUM> is shown between the ID face <NUM> of the seal and the OD surface <NUM> of the protruding section <NUM>. The gap <NUM> has essentially constant minimum radial span (radial height) along the main axial portion <NUM> of the surface <NUM> which has an axial span SA1. SA1 may be a substantial fraction of the span of axial protrusion of the protruding section <NUM> beyond the seat face <NUM> (shown as SA2). Example SA1 is <NUM>% to <NUM>% of SA2. An axial span of the protruding section <NUM> measured between the seat face <NUM> and the beginning (proximal end) of the chamfer <NUM> is shown as SA4.

<FIG> shows a radial span of the gap as RS1 shown centered between local seal ID radius RIS and flange OD surface portion <NUM> radius ROF. In an example of a cylindrical surface portion <NUM>, ROF varies by no more than <NUM>%, more narrowly <NUM>%, more narrowly <NUM>%. <FIG> also shows a radial span of the seal face <NUM> as RS2. As noted above, the centered RS1 is selected to correspond to the radial displacement at the <FIG> speed v<NUM>. In typical engine main shaft applications, RS1 may be <NUM> to <NUM>. Other applications (e.g., gearbox, pumps, auxiliary equipment) will typically be smaller. In a relative sense, example RS1 for such engine main shaft applications may be <NUM>% to <NUM>% of the radius ROF from the centerline A to the OD surface portion <NUM>, more particularly <NUM>% to <NUM>%. Broader applications may have RS1 <NUM>% to <NUM>% of the radius ROF (with larger percentages being associated with smaller radii). Appropriate RS1 may be determined iteratively and/or based on experience. In an iterative example, a large initial value may be progressively reduced in iterations until the axial displacement no longer is initiated. The axial displacement may be monitored using accelerometers attached to OD sleeve <NUM> or strain gauges on spring or springs <NUM>.

An axial span of the chamfer is shown as SA3. Example SA3 is at least <NUM>% of SA2, more particularly, <NUM>% to <NUM>%. In typical applications, SA2 may be <NUM> to <NUM>, more particularly <NUM> to <NUM>. In typical applications, SA3 may be at least <NUM>, more particularly <NUM> to <NUM>, more particularly <NUM> to <NUM>. The example angle of the chamfer (a half angle θ of the cone of chamfering) is <NUM>° to <NUM>°, more particularly, <NUM>° to <NUM>° or <NUM>° to <NUM>°.

<FIG> shows a radial span from the radial inboard/distal end of the chamfer <NUM> to the seal ID face <NUM> as RS3 (when concentric). As is noted above, RS1 is selected based upon limiting dynamic excursions. RS3 is chosen to make the ID of the chamfer effective to partially center the seal <NUM> relative to the seat during assembly to address sag in the seal. In typical engine main shaft applications, RS3 may be <NUM>% to <NUM>% of the radius ROF from the centerline A to the OD surface portion <NUM>, more particularly <NUM>% to <NUM>%. As noted above for RS1, this may scale substantially upward for smaller scale applications.

<FIG> shows a radial span of the chamfer <NUM> as RS4. In typical applications, RS4 may be <NUM>% to <NUM>% of the radius ROF from the centerline A to the OD surface portion <NUM>, more particularly <NUM>% to <NUM>% (particularly for larger applications).

<FIG> shows a radial span of the flange as RS5. In typical applications, RS5 may be <NUM>% to <NUM>% of the radius ROF from the centerline A to the OD surface portion <NUM>, more particularly <NUM>% to <NUM>%.

<FIG> shows an alternative implementation wherein the chamfer and the radial stop surface are on a spacer piece <NUM> separate from a main seat piece <NUM>. Other than the configuration of these two pieces, the illustrated example is the same as <FIG>.

Component materials and manufacture techniques and assembly techniques may be otherwise conventional. The example bellows spring is formed of steel (e.g., stainless) and may be assembled by welding individual essentially frustoconical sections to each other. remaining components may be machined. In an example of an assembly procedure, the spring ends are welded to the associated flanges of the spring carrier and seal carrier respectively. The seal is installed to the seal carrier such as via thermal interference fit (e.g., heating the seal carrier to radial expand and then axially inserting a single-annulus or segmented annulus seal and allowing the seal carrier to cool into radial interference). This seal assembly may then, similarly, be installed to the housing <NUM>. An example installation involves a combination of thermal interference and anti-rotation keying. The housing <NUM> may be heated and the seal assembly linearly inserted and held in place while the housing is allowed to cool. Example anti-rotation /keying features (not shown) include: slot and key features; bolted flange features; and slot and groove with retention wire. A bolted flange may replace the interference fit.

The housing may similarly be installed to the case structure. In a thermal interference fit example, the inner diameter surface of the housing-receiving compartment of the case structure may be heated to radially expand. The housing/seal subassembly may be installed into case structure by linear translation while case structure remains hot. An axial load may be placed on the front of the housing and maintained until temperatures are sufficiently normalized. Bolts or other fasteners (not shown) may then be installed at various circumferential locations on mating flanges of the housing and case. Such fasteners combined with the interference fit retain the subassembly to the case. Many other methods are possible and include slot and groove engagement with retaining wire and accompanying o-ring interface.

A further subassembly comprising the bearing, seat, spacer (if present) and gear may be assembled. In an example process, the bearing, seat, spacer <NUM> (if present), are heated at their respective inner diameters (e.g., via torch) and installed onto gear sleeve <NUM> by linear translation while each remain hot. An axial load may be placed on them and maintained (e.g., until temperatures return to room temperature or within a sufficient margin to provide the thermal interference fit).

That bearing-seat-gear subassembly may be installed as a unit. Prior to that the bearing support <NUM> may be installed to the case structure. An inner diameter surface portion of the case structure to engage the support <NUM> may be locally heated (e.g., via torch heating). The support may then be installed (e.g., by linear translation) and an axial load applied to the support and maintained (e.g., until temperatures return to room temperature or within a sufficient margin to provide the thermal interference fit).

Bolts or other fasteners (not shown) may then be installed at various circumferential locations on mating flanges of the support and case.

The bearing-seat-gear subassembly is then heated (e.g., via torch) at the inner diameter of gear sleeve and the support <NUM> is heated (e.g., via torch) at an ID surface that mates with the bearing outer race. The bearing-seat-gear subassembly may then be installed to the shaft piece <NUM> and support <NUM> (e.g., by linear translation). During this installation, the chamfer <NUM> will pass within the seal <NUM>. As the chamfer approaches the seal, the seal, seal carrier, and bellows spring are cantilevered from the spring carrier. In a horizontal assembly situation, there will be a gravity-induced sag so that there is contact of the chamfer and seal front ID edge at <NUM> o'clock. The chamfer will guide the seal onto the main axial portion <NUM> from where the seal face passes into engagement with the seat face. Absent the chamfer, the rim <NUM> might collide with and either be stopped by or damage the seal. An axial load applied to the subassembly compresses the bellows spring and is maintained (e.g., until temperatures return to room temperature or within a sufficient margin to provide the thermal interference fit). This leaves the bellows spring precompressed.

Other components can be loaded onto the resulting stack until a stack nut ends the stack and via torque or displacement control applies a load thereby trapping components between the nut and the shaft shoulder.

Many alternate configuration exists and many alternate sequences also exist and are not limited to the methods and sequence described above. Furthermore, the gear reflects one particular location of many potential locations. The shaft may have many more members such as an oil-scoop or other bearing compartment components.

Although a stop surface and guide chamfer to the OD of the seal would also limit excursions and guide assembly, that is less advantageous from several points of view. First, debris may be flung radially outward from the seal due to centrifugal action and would be trapped by such an OD surface. Second, for typical seal configurations there is more axial space available to the ID of the seal. For example, particularly as the seal wears there is a risk that an OD structure would interfere with the seal carrier.

The function in limiting radial excursions is particularly relevant to bellows springs because the bellows springs provide sealing. Other springs (e.g., a circumferential array of coil springs) have a separate sealing sheath or sheaths directly or indirectly sealing the seal carrier to the housing. Such secondary sealing elements may damp vibration and thereby reduce the resonant coupling relative to a bellows seal of similar spring constant/compression.

In designing the protruding section of seat piece <NUM> or spacer piece <NUM>, the axial span SA1 may be selected to be sufficiently long to provide sufficient distribution of radial contact force to limit seal ID wear from excursions that are stopped. SA3 is selected to provide the desired radial span RS3 in view of RS1 and the desired angle θ. SA4 reflects the combination of these axial spans plus the desired axial span of the relief <NUM>.

The axial span SA4 may be limited in some embodiments by potential obstructions such as a seal carrier that has an ID sleeve similar to the OD sleeve <NUM> and/or a seal with an ID rebate similar to OD rebate <NUM>. In such a situation, flange axial span SA4 may accommodate the allowable wear length of the seal. The allowable wear length is inherently limited by the length SA6 (<FIG>) of the recessing of the seal carrier rim <NUM> relative to the seal face <NUM>. In typical applications, SA4 may be <NUM>% to <NUM>% of the radius ROF from the centerline A to the OD surface portion <NUM>, more particularly <NUM>% to <NUM>% or <NUM> to <NUM>.

As noted above SA1 may be selected to be sufficiently long to axially distribute forces during contact with the seal ID surface <NUM>. In typical applications, SA1 may be <NUM>% to <NUM>% of the radius ROF from the centerline A to the OD surface portion <NUM>, more particularly <NUM>% to <NUM>% (as a rough value this is given as the same as SA4 despite the presence of the relief <NUM>).

<FIG> schematically illustrates a gas turbine engine <NUM> as one of many examples of an engine in which the seal system <NUM> may be used. The fan section <NUM> may include a single-stage fan <NUM> having a plurality of fan blades <NUM>. The fan blades <NUM> may have a fixed stagger angle or may have a variable pitch to direct incoming airflow from an engine inlet. The fan <NUM> drives air along a bypass flow path B in a bypass duct <NUM> defined within a housing <NUM> such as a fan case or nacelle, and also drives air along a core flow path C for compression and communication into the combustor section <NUM> then expansion through the turbine section <NUM>. A splitter <NUM> aft of the fan <NUM> divides the air between the bypass flow path B and the core flow path C. The housing <NUM> may surround the fan <NUM> to establish an outer diameter of the bypass duct <NUM>. The splitter <NUM> may establish an inner diameter of the bypass duct <NUM>.

The example engine <NUM> generally includes a low speed spool <NUM> and a high speed spool <NUM> mounted for rotation about an engine central longitudinal axis A (forming the axis <NUM>) relative to an engine static structure <NUM> via several bearing systems <NUM>.

The inner shaft <NUM> is connected to the fan <NUM> through a speed change mechanism, which in the example gas turbine engine <NUM> is illustrated as a geared architecture <NUM> to drive the fan <NUM> at a lower speed than the low speed spool <NUM>. The inner shaft <NUM> may interconnect the low pressure compressor (LPC) <NUM> and low pressure turbine (LPT) <NUM> such that the low pressure compressor <NUM> and low pressure turbine <NUM> are rotatable at a common speed and in a common direction. In other embodiments, the low pressure turbine <NUM> drives both the fan <NUM> and low pressure compressor <NUM> through the geared architecture <NUM> such that the fan <NUM> and low pressure compressor <NUM> are rotatable at a common speed. Although this application discloses geared architecture <NUM>, its teaching may benefit direct drive engines having no geared architecture. The high speed spool <NUM> includes an outer shaft <NUM> that interconnects a second (or high) pressure compressor (HPC) <NUM> and a second (or high) pressure turbine (HPT) <NUM>. A combustor <NUM> is arranged in the example gas turbine <NUM> between the high pressure compressor <NUM> and the high pressure turbine <NUM>.

The low pressure compressor <NUM>, high pressure compressor <NUM>, high pressure turbine <NUM> and low pressure turbine <NUM> each include one or more stages having a row of rotatable airfoils. Each stage may include a row of static vanes adjacent the rotatable airfoils. The rotatable airfoils and vanes are schematically indicated at <NUM> and <NUM>.

Claim 1:
An apparatus, for example a gas turbine engine (<NUM>), comprising:
a first member (<NUM>);
a shaft (<NUM>, <NUM>) rotatable relative to the first member (<NUM>) about an axis (A); and
a seal system (<NUM>) comprising:
a seal (<NUM>) carried by the first member (<NUM>) and having a seal face (<NUM>); and
a seat (<NUM>) carried by the shaft (<NUM>, <NUM>) and having a seat face (<NUM>) in sliding sealing engagement with the seal face (<NUM>);
wherein:
the seal system (<NUM>) is a dry face seal system;
the seat (<NUM>) further comprises a seat section (<NUM>) having:
an outer diameter surface (<NUM>) having a main axial portion (<NUM>) encircled by the seal (<NUM>); and
a rim (<NUM>);
a radial gap (<NUM>) is present between the seal (<NUM>) and the outer diameter surface (<NUM>)
of the seat section (<NUM>), the gap (<NUM>) having constant minimum radial span (RS1) along the main axial portion (<NUM>) of the outer diameter surface (<NUM>), characterised in that a bellows spring (<NUM>) axially biases the seal (<NUM>) into engagement with the seat (<NUM>), wherein in use an inner diameter surface (<NUM>) of the seal is configured to contact the seat section outer diameter surface (<NUM>); and the radial span (RS1) of the radial gap (<NUM>) is sized to limit relative radial dynamic displacement of the seat (<NUM>) and the seal (<NUM>).