Patent Description:
Gas turbine engines typically must pass stringent vibration tests following their production. Rotor eccentricities are a main source of engine vibration, and eccentricities can be alleviated by rotor balancing. Examples of how rotors are balanced without compromising their structural integrity include oversizing a part of the rotor disk, often referred to as a sacrificial balance appendage, rim or flange, and then either adding balancing weights or selectively removing material from that part.

Common techniques for adding balancing weights are typically employed on rotors spinning at lower speeds, rendering such techniques inappropriate for higher RPM applications due to the various stresses incurred. Common selective material removal techniques may be employed for rotors spinning at higher speeds, for instance at approximately <NUM>,<NUM> RPM or higher. However, as such techniques require material to be grinded or otherwise removed from the rotor after it is mounted for proper balancing, there are risks of metal being trapped in the surrounding components, potentially causing secondary damage to the rotor.

<CIT> discloses a trapped spring balance weight and a rotor assembly.

<CIT> discloses a radial balancing clip weight for a rotor assembly.

<CIT> discloses a balancing apparatus for a rotor assembly.

<CIT> discloses a rotor disc and a method of balancing.

<CIT> discloses a gas turbine engine rotor and a balance weight therefor.

According to an aspect of the present invention, there is provided a rotor assembly for a gas turbine engine comprising: a rotor including a rotor disc rotatable about a central axis, the rotor having a pair of opposite axially facing faces; an annular flange protruding axially from one of the opposite axially facing faces, the annular flange having a circumference disposed about the central axis, the annular flange including: a plurality of protrusions axisymmetrically disposed about the circumference of the annular flange, each protrusion extending axially from a base portion of the annular flange to a respective protrusion end, each protrusion having a mounting aperture for selectively receiving a balancing feature; and a plurality of slots axisymmetrically disposed about the circumference of the annular flange between adjacent protrusions, each slot including a pair of converging flat portions extending axially inwardly from an adjacent protrusion end, an inner flat portion at an inner end of each slot, and a pair of curved portions respectively joining each converging flat portion to the inner flat portion in each slot; wherein each slot has a slot depth extending normally from the adjacent protrusion end to the inner flat portion of the slot, said slot depth at least as great as an adjacent mounting aperture depth extending normally from the adjacent protrusion end to a far end (relative to the adjacent protrusion end) of the corresponding mounting aperture.

Optionally, and in accordance with the above, the curved portions of each slot each have a radius approximately equal to the slot depth.

Optionally, and in accordance with any of the above, each converging flat portion forms an angle ranging from twenty to forty degrees with respect to a slot longitudinal axis in line with the annular flange.

Optionally, and in accordance with any of the above, the inner flat portion of each slot has a width that is at least one tenth as great as the slot depth.

Optionally, and in accordance with any of the above, the annular flange has a radial thickness with reference to the central axis that is at least eighty-five percent as great as the slot depth of each slot.

Optionally, and in accordance with any of the above, the annular flange includes twenty-four of the plurality of protrusions axisymmetrically disposed about the circumference of the annular flange.

Optionally, and in accordance with any of the above, the rotor disc is operable to rotate at a speed of at least <NUM>,<NUM> RPM.

Optionally, and in accordance with any of the above, the annular flange protrudes axially in a direction parallel to the central axis from the rotor disc.

Optionally, and in accordance with any of the above, the rotor further includes a rotor disc cover plate, wherein the annular flange protrudes axially in a direction parallel to the central axis from the rotor disc cover plate.

<FIG> illustrates a gas turbine engine <NUM> of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan <NUM> through which ambient air is propelled, a compressor section <NUM> for pressurizing the air, a combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section <NUM> for extracting energy from the combustion gases. A shaft <NUM> interconnects the fan <NUM>, the compressor section <NUM> and the turbine section <NUM>. While <FIG> shows gas turbine engine <NUM> to be a turbofan gas turbine engine, it is understood that the present disclosure is applicable to other types of gas turbine engines as well.

Referring to <FIG>, a rotor assembly <NUM> which can be used in the gas turbine engine <NUM> of <FIG> or in any adequate type of gas turbine engine <NUM> is shown. The rotor assembly <NUM> is operable for rotation about a central axis <NUM>. In the shown embodiment, the rotor assembly <NUM> is a high pressure turbine (HPT) stage of a multistage turbine section <NUM> rotating at around <NUM>,<NUM> RPM or higher. However, it is understood that the present disclosure may be applicable to other rotors within a gas turbine engine, as will be discussed in further detail below.

The rotor assembly <NUM> includes a rotor having a rotor disc <NUM> to be mounted around a drive shaft <NUM> (shown in <FIG>). The rotor includes a hub portion <NUM> having a central bore <NUM> through which the drive shaft <NUM> is inserted. The rotor includes a frustoconical web portion <NUM> extending generally radially from the hub portion <NUM>. The rotor also has two opposite axially facing faces <NUM>, <NUM> with reference to a rotor disc axis Y normal to the central axis <NUM>. Opposite faces <NUM>, <NUM> may be referred to as the first opposite face <NUM> and the second opposite face <NUM>. In the shown embodiment, although not necessarily the case in all embodiments, the rotor includes a rotor disc cover plate <NUM> mounted to the first opposite face <NUM>, although other configurations may be contemplated as well. As can be seen in <FIG>, the rotor disc cover plate <NUM> is mounted in front of the first opposite face's <NUM> web portion <NUM>, for instance via fasteners 36a or other mounting hardware, while the web portion <NUM> is exposed on the second opposite face <NUM>. In other cases where the rotor disc cover plate <NUM> is not present, the web portions <NUM> are exposed on both the first opposite face <NUM> and the second opposite face <NUM>.

The rotor includes an outer periphery portion <NUM> encircling the web portion <NUM>. The hub portion <NUM>, the web portion <NUM> and the outer periphery portion <NUM> in the illustrated example are made integral with each other and form a monolithic piece, while the shown rotor disc cover plate <NUM> is mountable to the rotor, for instance via fasteners 36a. The monolithic rotor can be made of a single material. Other rotor disc constructions may be contemplated as well.

According to one or more embodiments, the rotor assembly <NUM> includes a plurality of circumferentially-disposed and radially extending blades <NUM> mounted in corresponding bladereceiving slots <NUM> provided in the outer periphery portion <NUM> for receiving roots of the blades <NUM>. The number of blades <NUM> may vary, for instance based on the type of rotor assembly <NUM> or the type of engine <NUM>. The slots <NUM> are designed to prevent the blades <NUM> from being ejected radially during rotation. Other components (not shown), such as fixing rivets, spring plates, etc., may be provided in the rotor assembly <NUM>, depending on the design. In other cases, blades <NUM> that are made integral with the rotor, i.e. forming a monolithic assembly, may be contemplated as well. In the shown case, the rotor disc cover plate <NUM> includes an inlet 36b (<FIG>) to provide a cooling flow to the blades <NUM> through an annular cooling channel 36c between the rotor disc cover plate <NUM> and the corresponding web portion <NUM>. Other methods for cooling the blades <NUM> may be contemplated as well.

The illustrated rotor assembly <NUM> has two rotor balancing assemblies each including an annular flange or circular and scalloped appendage <NUM>, one on each opposite face <NUM>, <NUM>. Each flange <NUM> is coaxially disposed with reference to the central axis <NUM> and may also be referred to as a balancing flange or rim. As such, such flanges <NUM> may be referred to as 'horizontal balancing rims', and the combination of the two flanges <NUM> may be referred to as a 'two-plane balancing system'. Although the illustrated example shows two flanges <NUM>, in other cases it is possible to provide only one instead of two. Such a sole flange <NUM> could then be on either face opposite <NUM> or <NUM>. It is also possible to provide two or more flanges <NUM> on one side and none or a different number on the other side.

According to one or more embodiments, the two flanges <NUM> are opposed relative to the rotor disc axis Y spanning through a midpoint of the outer periphery portion <NUM>, yet are identical in size and shape. As such, they may be referred to as 'like' flanges <NUM>. In other cases, any flange <NUM> on one side may not necessarily need to be identical in size and/or in shape compared to any flange on the other side. In the shown case, the flange <NUM> on the first opposite face <NUM> protrudes or projects generally longitudinally forward or axially relative to the central axis <NUM> from the rotor disc cover plate <NUM> while the flange <NUM> on the second opposite face <NUM> protrudes or projects generally longitudinally aft or axially from the web portion <NUM>. In other cases, for instance where the rotor assembly <NUM> does not include a rotor disc cover plate <NUM>, the flange <NUM> on the first opposite face <NUM> may protrude longitudinally forward from the web portion <NUM>.

As best shown in <FIG>, each flange <NUM> comprises a base portion or root <NUM> that can be integrally connected to the web portion <NUM> or the cover plate <NUM>, respectively, thereby being part of the rotor disc <NUM> or the cover plate <NUM>. In other cases, the flange <NUM> may be positioned elsewhere on the rotor, for instance on the outer periphery portion <NUM> or on the hub portion <NUM>. Other positions may be contemplated as well. The base portion <NUM> of each flange <NUM> is circumferentially continuous. In other cases, the flanges <NUM> may be connected to the rest of the rotor without being made integral thereto. For example, a flange <NUM> could be connected by welding or brazing, by using fasteners, etc. The shown flanges <NUM> each have an inboard surface <NUM> and an outboard surface <NUM> and extend from the base portion <NUM> at the web portion <NUM> to a free or aft end <NUM>. As will be discussed in further detail below, each flange <NUM> may be operable to form a generally annular portion of the rotor where internal stresses during operation of the engine <NUM> will be below a given crack propagation threshold. Each flange <NUM> may include a raised shoulder 44a towards the base portion <NUM> on the outboard surface <NUM>, for instance to provide additional support when the weights are added.

As shown in <FIG>, each flange <NUM> includes a plurality of circumferentially spaced-apart protrusions or fingers <NUM> at their free end <NUM>. These protrusions <NUM> are the locations at which weights can be added to the rotor assembly <NUM> for balancing purposes, as will be discussed in further detail below. The protrusions <NUM> project or extend substantially axially from the base portion <NUM> of the corresponding flange <NUM> and terminate at protrusion ends 54a along the free end <NUM> of the flange <NUM>. The protrusions <NUM> are axisymmetrically disposed with reference to the central axis <NUM> and are substantially identical. As such, they may be referred to as 'like' protrusions <NUM>. The size and shape of the protrusions <NUM> and their effects on balancing the rotor assembly <NUM> will be discussed in further detail below. In the shown case, each flange <NUM> includes twenty-four protrusions <NUM>, although other numbers of protrusions <NUM> may be contemplated as well.

The protrusions <NUM> are delimited circumferentially by a plurality of axisymmetrically spaced-apart stress-relieving slots <NUM>, also referred to as scallop-shaped cutouts or recesses. These slots <NUM> are operable to relieve various stresses relating to, for instance, the weight added to the protrusions <NUM> for balancing purposes and the rotational forces acting upon the rotor assembly <NUM> in use. In the shown case, the slots <NUM> are formed on a radially-extending end face 52a at the free end <NUM> of the flange <NUM>. In the shown case, the slots <NUM> are substantially identical to each other, and thus may be referred to as 'like' slots <NUM>. Each of the slots <NUM> has an internal wall with a shape or slope minimizing the stress concentration within the slot <NUM>, as will be discussed in the further detail below.

Referring to <FIG>, each slot includes a pair of converging flat portions 56a, a pair of curved portions 56b, and an inner flat portion 56c at an inner end of each slot <NUM>. The converging flat portions 56a extend axially inwardly from an adjacent protrusion end 54a. The pair of curved portions 56b respectively join each converging flat portion 56a to the inner flat portion 56c in each slot <NUM>. The slots <NUM> are designed so as to reduce various internal stresses such as hoop stresses caused by the rotation of the rotor assembly <NUM> in operation, which may extend the life expectancy of the various components. When manufacturing the rotor assembly <NUM>, the slots <NUM> may be machined in the free end <NUM> of each flange <NUM>, for instance by using a rotating tool or another technique. Such machining of the slots <NUM> thus forms the protrusions <NUM>. Each slot <NUM> in the illustrated example is oriented substantially radially with reference to the central axis <NUM>.

In an embodiment falling outside the wording of the claims, each slot <NUM> includes a pair of converging flat portions 56a extending axially inwardly from a respective protrusion end 54a and a pair of curved portions 56b meeting at an inner end of each slot <NUM>. The pair of curved portions 56b respectively join each converging flat portion 56a to the inner end of each slot <NUM>. As such, in this embodiment, although not necessarily the case in other embodiments, the slots <NUM> would not include end flat portions 56c. Other arrangements for each slot <NUM> may be contemplated as well.

As shown in <FIG>, each protrusion <NUM> includes a hole or mounting aperture <NUM> for the attachment of a balancing feature <NUM>. In the shown case, the balancing feature <NUM> includes a balancing weight 60a and a fastener 60b, illustratively a rivet, although other forms for the balancing feature <NUM> may be contemplated as well. For instance, an exemplary balancing feature <NUM> may include a weighed portion with an attached and protruding threaded portion. As such, while in the shown case the mounting apertures <NUM> are smooth and cylindrically-shaped for receiving the rivets 60b, in other cases they may be threaded for engaging with a fastener-type balancing feature. As will be discussed in further detail below, the slots <NUM> are operable to relieve various stresses within the mounting apertures <NUM> caused by, for instance, the weight of the balancing features <NUM>.

As discussed above, in the shown case each flange <NUM> includes twenty-four protrusions <NUM> and thus twenty-four mounting apertures <NUM>. For a typical balancing operation, a given flange <NUM> may be maximally rated to carry the heaviest available balancing weights 60a in one third of the mounting apertures <NUM>. In the shown case, each flange would be rated to support at most eight of the heaviest available balance weights 60a. A variety of differently-weighted balancing weights 60a may be available to accurately balance the rotor assembly <NUM>. In some cases, the various balance weights 60a may have the same cross-sectional profile and differ in their lengths.

In an exemplary balancing operation, the weight distribution of the rotor assembly <NUM> may be tested through various techniques once it is mounted to the gas turbine engine <NUM>. For instance, a computer-operated apparatus (not shown) may spin the rotor assembly <NUM>, detect and localize any imbalances, and propose remedies for the imbalances. Such remedies may include adding one or more balancing features <NUM> to the balancing flange(s) <NUM> to achieve a desired weight distribution. The balancing features <NUM> may be secured to the balancing flange via fasteners, for instance rivets 60b, to prevent detachment during engine operation. Other techniques for determining the optimal placement of the balancing features <NUM> may be contemplated as well.

As discussed above, the flanges <NUM>, in particular the protrusions <NUM> and slots <NUM>, are dimensioned to relieve various stresses incurred by various portions of each flange <NUM>. Such stresses may include stresses due to the rotation of the rotor assembly <NUM>, hoop stresses in the mounting apertures <NUM> and bending stresses at the base portion <NUM> on the outboard surface <NUM> due to the cantilevered weight of the balancing features <NUM>. As can be seen in <FIG>, the slot <NUM> is symmetric about a slot longitudinal axis L, such axis L in line with the protruding flange <NUM>. In various embodiments, a plurality of parameters may be set so that the flanges <NUM> may offer a counterweight-based balance system for the rotor assembly <NUM> rotating at high speeds, for instance around <NUM>,<NUM> RPM or higher, while reducing the overall weight of the rotor assembly <NUM> and satisfying various life requirements of the rotor. In addition, the slots <NUM> may aid in breaking up or distributing the thermal stresses incurred by the rotor assembly <NUM>. Such parameters may include, but are not limited to, the radial thickness T of the flange <NUM> with respect to the central axis <NUM>, the radius R of the curved portions 56b of each slot, the width W1 of the protrusion ends 54a, the depth W2 of each slot <NUM>, the depth W3 of each mounting aperture <NUM>, the width W4 of each inner flat portion 56c, the width W5 of each slot <NUM> taken between adjacent protrusion ends 54a, the distance W6 from the edge of each slot <NUM> to the edge of the nearest mounting aperture <NUM>, and the angle of entry θ of each converging flat portion 56a relative to the axis L. Other parameters to define the flanges <NUM> may be contemplated as well.

In one or more embodiments, such parameters may be defined by measurement ranges with various tolerances. Such ranges offer a balance between minimizing the hoop stresses within the mounting apertures <NUM>, thus extending the life expectancy of the rotor, without adding an excessive amount of cantilevered stress in creating the slots <NUM>. In addition, the removal of material to create the slots <NUM> lowers the overall weight of the rotor assembly <NUM>. Such measurements may be applied to a flange <NUM> having a radius of approximately <NUM> inches (<NUM>) with reference to the central axis <NUM>. In such an embodiment, the thickness T of each flange <NUM> measures approximately <NUM> inches (<NUM>). The radius R of the curved portions 56b measures between <NUM> and <NUM> inches (<NUM> and <NUM>). The width W1 measures approximately <NUM> inches (<NUM>). According to the invention, depth W2 of each slot <NUM> corresponds to at least the depth W3 of each mounting aperture <NUM> (i.e. the distance between the protrusion end 54a and the furthest point on the corresponding in-line mounting aperture <NUM>) and should measure between <NUM> and <NUM> inches (<NUM> and <NUM>). The width W5 of each slot <NUM> measures between <NUM> and <NUM> inches (<NUM> and <NUM>. The minimum distance W6 between the edge of each slot <NUM> to the edge of the nearest mounting aperture <NUM> is approximately <NUM> inches (<NUM>). The angle of entry θ of each converging flat portion 56a relative to the axis L is approximately <NUM> degrees, with a tolerance of plus or minus <NUM> degrees. The angle of entry θ of each converging flat portion 56a relative to the axis L is approximately <NUM> degrees, with a tolerance of plus or minus <NUM> degrees. Preferably, the angle of entry θ of each converging flat portion 56a relative to the axis L is approximately <NUM> degrees, with a tolerance of plus or minus <NUM> degree. Such an angle allows the width W1 to be sufficient to support the various sizes of balancing weights 60a that may be mounted to each protrusion <NUM>. In various cases, the protrusion end 54a may reduce the concentrated stresses at the inner ends of the cutouts <NUM>. In addition, the angle of entry θ allows more material to be removed when forming each slot <NUM> (relative to a zero degree entry relative to the slot longitudinal axis L), further reducing the overall weight of the rotor assembly <NUM>. Unless otherwise stated, the above-listed measurements have a tolerance within <NUM> inches (<NUM>). Other parameters may be contemplated as well.

In another embodiment, the relationship between a number of the above-listed parameters may be described as ratios. Such ratios may complement the measurements listed in the above embodiment. In other cases such ratios may be utilized to scale up or down the design of the flanges <NUM> for a different-sized rotor operating under similar conditions, i.e. rotating at high speeds in the order of <NUM>,<NUM> RPM. In such an embodiment, the depth W2 of each slot <NUM> corresponds to at least the depth W3 of each mounting aperture, and the radius R of each curved portion 56b should approximately correspond to the depth W2 of each slot <NUM>. The angle of entry θ of each converging flat portion 56a relative to the axis L is approximately <NUM> degrees, with a tolerance of plus or minus <NUM> degrees. Preferably, the angle of entry θ of each converging flat portion 56a relative to the axis L is approximately <NUM> degrees, with a tolerance of plus or minus <NUM> degree. The width W4 of each inner flat portion 56c is at least one tenth of the depth W2 of each slot <NUM>. The thickness T is at least eighty-five percent as great as the depth W2 of each slot <NUM>. In some cases, the width W1 of the protrusion ends 54a may vary, for instance to increase or decrease the number of protrusions <NUM> and slots <NUM>. Other relationships between the various parameters may be contemplated as well.

Additionally, a relationship can be identified between the thickness T of each flange <NUM>, a middle radius Rm of each flange <NUM> (i.e. the distance between the central axis <NUM> and the midpoint of the each flange <NUM> along the thickness T), and the rotational operation speed ω of the rotor assembly <NUM>: <MAT>.

Through this relationship, the parameters of the flange <NUM> may be scaled up or down to accommodate varying sizes of rotor assemblies <NUM> rotating at different speeds. Other relationships may be contemplated as well. For instance, there is an inverse relationship between the number of protrusions <NUM> and the radius R of the curved portion 56b of each slot <NUM>. In the shown case, the flanges <NUM> have a lower number of protrusions <NUM> (twenty-four) compared to a balance flange of a similarly-sized rotor rotating at slower speeds which does not incur the same stresses. However, by reducing the number of protrusions <NUM>, the radius R can be increased, thus reducing the overall stresses incurred in the flange <NUM>.

In various cases, the above-described ratios, dimensions and relationships may be used independently of each other or used to complement each other. In addition, in different embodiments, a flange <NUM> may respect certain ratios, dimensions and/or relationships and not others. Various combinations of the above may be contemplated.

Various materials and machining and assembly techniques may be employed in forming the rotor assembly <NUM>. The rotor disc <NUM>, flanges <NUM> and cover plate <NUM> may be made from various nickel alloys, although other materials may be contemplated as well. The flanges <NUM>, and in particular the slots <NUM> and mounting apertures <NUM> may be formed through various processes such as peening, milling, turning and drilling, although other techniques may be contemplated as well. In the shown case, the balancing weights 60a are mounted to the flanges <NUM> via rivets passing through the mounting apertures <NUM> in the protrusions <NUM>, although other fastening techniques may be contemplated as well.

Claim 1:
A rotor assembly (<NUM>) for a gas turbine engine (<NUM>) comprising:
a rotor including a rotor disc (<NUM>) rotatable about a central axis (<NUM>), the rotor having a pair of opposite axially facing faces (<NUM>, <NUM>); and
an annular flange (<NUM>) protruding axially from one of the opposite axially facing faces (<NUM>, <NUM>), the annular flange (<NUM>) having a circumference disposed about the central axis (<NUM>), the annular flange (<NUM>) including:
a plurality of protrusions (<NUM>) axisymmetrically disposed about the circumference of the annular flange (<NUM>), each protrusion (<NUM>) extending axially from a base portion (<NUM>) of the annular flange (<NUM>) to a respective protrusion end (54a), each protrusion (<NUM>) having a mounting aperture (<NUM>) for selectively receiving a balancing feature (<NUM>); and
a plurality of slots (<NUM>) axisymmetrically disposed about the circumference of the annular flange (<NUM>) between adjacent protrusions (<NUM>), each slot (<NUM>) including a pair of converging flat portions (56a) extending axially inwardly from an adjacent protrusion end (54a),
characterised in that each slot (<NUM>) further includes:
an inner flat portion (56c) at an inner end of each slot (<NUM>), and a pair of curved portions (56b) respectively joining each converging flat portion (56a) to the inner flat portion (56c) in each slot (<NUM>), wherein each slot (<NUM>) has a slot depth (W2) extending normally from the adjacent protrusion end (54a) to the inner flat portion (56c) of the slot (<NUM>), said slot depth (W2) at least as great as an adjacent mounting aperture depth (W3) extending normally from the adjacent protrusion end (54a) to a far end of the corresponding mounting aperture (<NUM>).