Gas turbine engine blade mounting arrangement

A mounting arrangement for a gas turbine engine blade having a root employs a lightweight, simple and economical leaf spring spacer which biases the blade root in a radially outward direction to minimize unwanted movement of the root within a conforming slot in a blade hub under conditions such as windmilling when centrifugal force alone is inadequate to tightly seal the root within the slot.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates generally to gas turbine engines and particularly to a mounting arrangement for gas turbine engine blades such as fan blades.

2. Background Information

Turbofan gas turbine engines such as those which power aircraft employ a multiplicity of fan blades attached to a hub mounted on the forward (upstream) end of one of the engine shafts. Typically, such fan blades are provided with a radially outer airfoil shaped portion and a radially inner root portion typically having a dovetail shape. The dovetail shaped root portion is received within a slot which conforms thereto in the fan hub. For ease in attaching the fan blade to the hub by sliding the root portion into the slot and for removal of the blade from the hub by sliding the root portion of the blade out of the slot, the slot is usually slightly larger than the dovetail root portion. This difference in dimensions between the root portion of the fan blade and the slot in the hub results in a clearance between the root portion and the slot. Under normal engine operating conditions when the engine's rotor is spinning at high speed (several thousand rpm) centrifugal force acting on the fan blade causes the blade to be held tightly in the hub slot.

However, when the engine is not in use, wind acting on the fan blades can cause the engine's rotor to slowly turn. This slow turning of the engine rotor in response to winds acting on the fan blades is referred to as windmilling. There is very little centrifugal force acting on the fan blades during such windmilling due to the low rotational speed of the engine rotor in response thereto and thus, the fan blade roots are not tightly held within the conforming slots in the fan hub, resulting in movement between the fan blade root portions and the hub slots in which they are received. This movement of the fan blade root portions within the slots, if unchecked, can result in damage to the fan blade root portions and the slots due to galling and fretting of the surfaces of the root portions and the slots. To minimize such galling and fretting, it has been a practice to employ spacers between the radially innermost end of the root portion and the adjacent portion of the slot which receives the root portion to prevent movement between the blade and the hub during windmilling of the engine's rotor. In some cases, such spacers actually resiliently bias the root portion and thus the entire blade radially outwardly to tightly secure the blade root portion within the hub slot. Such prior art blade root spacers have taken the form of relatively complex metallic configurations and elastomeric materials secured between rigid clamping members which adjustably compress the elastomeric material to control the elasticity thereof. The complexity and weight of such prior art blade root spacers adds to the cost and weight of the engine, thus detracting from the efficiency thereof. Furthermore, the complexity of such spacers detracts from the ease with which the fan blades are assembled to the hub during engine assembly and removed from the hub for engine maintenance.

Accordingly, it will be appreciated that a need exists for a simple, lightweight and economical means for minimizing movement of fan blade roots within fan hub slots under conditions such as windmilling and the like.

SUMMARY OF THE DISCLOSURE

In accordance with the present invention, a lightweight, simple and economical leaf spring spacer is disposed between a radially innermost end of a gas turbine engine blade root such as a fan blade root and an adjacent radially innermost surface of a hub slot which accommodates that root. The spacer is held in place by an interference fit between the blade root and the slot such that the spacer exerts a radially outward force on the blade root to secure the blade root within the slot thereby limiting unwanted radial movement and tilting of the blade root within the slot during such conditions as windmilling of the gas turbine engine rotor. In an exemplary embodiment, the blade comprises a fan blade. Also in the exemplary embodiment, the blade root is dovetail shaped, including a pair of radially inner and outer angularly offset longitudinally extending side surfaces, and the slot includes a pair of radially inner and outer angularly offset longitudinally extending side surfaces which are opposed to the radially inner and outer side surfaces of the blade root. The leaf spring spacer is compressed between the radially innermost surfaces of the blade root and the slot which accommodates the root exerting a radially outward force on the root which causes the radially outer side surface of the blade root to bear against the radially outer side surface of the slot, thereby preventing radial movement of the blade root within the slot during the aforementioned windmilling conditions.

According to an aspect of the present invention, a mounting arrangement for a gas turbine engine blade is provided. The mounting arrangement includes a radially directed axis and a radially inner root received within a slot in an associated blade hub. The mounting arrangement also includes an elongate resilient leaf spring spacer received within said slot between a radially innermost end of said blade root and a radially innermost surface of said slot by interference fit therebetween such that said spacer exerts a radially outward force on said blade root to reduce radial movement of said blade root within said slot.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, a turbofan gas turbine engine5has a longitudinal axis7about which the rotors8of the engine rotate. A fan10disposed at the engine inlet draws air into the engine. A low pressure compressor15located immediately downstream of fan10compresses air exhausted from fan10and a high pressure compressor20located immediately downstream of low pressure compressor15, further compresses air received therefrom and exhausts such air to combustors25disposed immediately downstream of high pressure compressor20. Combustors25receive fuel through fuel injectors30and ignite the fuel/air mixture. The burning fuel-air mixture (working medium fluid) flows axially to a high pressure turbine35which extracts energy from the working medium fluid and in so doing, rotates hollow shaft37, thereby driving the rotor of high pressure compressor20. The working medium fluid exiting the high pressure turbine35then enters low pressure turbine40, which extracts further energy from the working medium fluid. The low pressure turbine rotor provides power to drive the fan10and low pressure compressor15via low pressure shaft42, which is disposed interiorly of the hollow shaft37, coaxial thereto. Working medium fluid exiting the low pressure turbine40provides axial thrust for powering an associated aircraft (not shown) or a free turbine (also not shown).

Bearings43,45,50and53radially support the concentric high pressure and low pressure turbine shafts from separate frame structures52,54,55and56respectively, attached to engine case57, which defines the outer boundary of the engine's stator which circumscribes rotors8. However, it will be appreciated that the present invention is also well suited for mid-turbine frame engine architectures wherein the upstream bearings for the low and high pressure turbines are mounted on a common frame structure disposed longitudinally (axially) between the high and low pressure turbines.

Referring toFIGS. 1-4, the fan10comprises a hub60mounted on the forward end of low pressure shaft42as by bolts65and a plurality of blades70mounted on hub60about the periphery thereof. As best seen inFIG. 2, each of the blades70has a radial central axis77and comprises a radially outer airfoil shaped portion75which in a manner well known in the art draws air into the engine and a radially inner root portion80which is received within a conforming slot85in the periphery of hub60. The blades70may be formed from a metallic material such as titanium or an alloy thereof, a composite such as a glass-epoxy composite or any combination thereof as is well known in the art. As best seen inFIG. 4, root portion80is generally dovetail shaped in cross-section, being defined by the radially innermost end of the airfoil portion75, a longitudinally extending radially innermost surface90, and a pair of longitudinally extending angularly offset side surfaces95and100. As set forth hereinabove, each slot85in hub60which receives a dovetail root of one of the fan blades70conforms to that root, and includes a longitudinally extending radially innermost surface105facing (opposed to) surface90of blade root80as well as angularly offset radially inner and outer side surfaces110and115which face (are opposed to) side surfaces95and100of root portion80.

It will be appreciated that to enable root portion80to be inserted into slot85for assembly and removed therefrom for disassembly of the fan blades from the hub, slot85must be dimensionally larger than dovetail root80thereby defining a clearance (e.g., a radial clearance)120therebetween. As set forth hereinabove, under windmilling conditions, when the engine is not running and the fan is turned slowly by wind entering the engine, clearance120would allow limited radial movement and tilting of blade70within slot85, thereby causing unwanted wear between the root portion80of the blade70and the slot side surfaces95,100due to frictional galling and/or fretting between the dovetail root portion80and the slot85. To prevent such wear of the root portion and slot during such windmilling conditions, a leaf spring spacer125(seeFIG. 3) is disposed within clearance120between radially innermost surface90of blade root80and radially innermost surface105of slot85.

Referring to FIGS.3and5-7, the leaf spring spacer125includes a longitudinal axis130which, when the spacer is assembled with the blade70and the hub60, is generally parallel to the longitudinal axis7of the engine itself. The spacer may be formed from any material having the requisite strength and elasticity such as any of various thermoplastic materials such as Vespel TP-3985 manufactured and sold by E.I. DuPont de Nemours and Company. The leaf spring spacer125is bowed in a radial direction and is retained between root portion80and slot85by an interference fit therebetween. The leaf spring spacer125includes first and second opposed longitudinally extending major surfaces135and140and a pair of opposed longitudinally extending lateral side edge portions145and150. The side edge portions145,150may be smooth as shown inFIG. 5or notched along the lengths thereof to define tabs155as shown inFIG. 6. The lateral side edge portions may be generally coplanar to the main body of the spacer or may be radially bent to fill any clearance between the side surfaces95,100of the dovetail root80and slot85. Furthermore, the leaf spring spacer125may be of a uniform thickness or may include thickened portions thereof either along substantial portions of edges145and150or along the edges of tabs155(seeFIG. 6). Such thickened portions145,150of leaf spring spacer125may conveniently fill any clearances between the root portion80of the blade70and the disk slot85defined by beveled edges of the blade root80between adjacent surfaces thereof (seeFIG. 7). Also, leaf spring spacer125may include a groove162for cooperation with a suitable removal tool (not shown) used in removing the spacer in the disassembly of the fan blade from the hub. As shown inFIG. 6, to minimize the weight which the leaf spring spacer125adds to the fan rotor, the leaf spring spacer125may be apertured at one or more locations160along length thereof. The apertures160are also useful to accommodate balance weights (not shown) which may be necessary to properly balance the fan rotor in the assembly thereof.

A fan blade70is mounted on the hub60by first positioning the hub60such that the slot85in which a fan blade70is to be inserted is located at a bottom dead center position and the aft retaining ring is then installed. The blade70is then slid into the slot85and released so that the radially outer side surface95,100of the root80rests on the opposed surface of the slot85. Leaf spring spacer125is then inserted into the slot. Referring toFIGS. 2 and 3, the forward retaining ring170is then installed in mating grooves175in the periphery of the hub60to longitudinally restrain the blade root80within the slot85.

Accordingly, it will be appreciated that the blade mounting arrangement of the present invention provides a lightweight, economical and effective arrangement for securing a fan blade70to a hub60to minimize movement therebetween under windmilling conditions. The simple compact shape of the leaf spring spacer125adds minimally to the weight of the fan rotor. No complex and/or heavy mechanisms are necessary to effectively hold the blade root80within the hub60.

While the present invention has been described within the context of a gas turbine engine fan blade mounting arrangement, it will be appreciated that the present invention may be employed in the mounting of various other blades to a gas turbine engine rotor such compressor blades or turbine blades. While specific shapes and materials for the blade spacers employed in the present invention have been discussed, it will be appreciated that various modifications thereto may be made without departing from the present invention and it is intended by the appended claims to cover such modifications as may fall within the true spirit and scope of this invention.