Patent Application
20120027601
1. Technical Field
This invention relates generally to gas turbine engines and particularly to an arrangement for mounting a rotatable component on the rotor of such a gas turbine engine.
2. Background Information
Gas turbine engines, such as those which power aircraft, employ a stator which supports stationary components of the engine, such as vanes which direct the flow of air and combustion gases through the engine, and a rotor of the stator on which rotatable components such as fan, compressor and turbine blades are mounted. Such blades are ordinarily mounted on hubs therefore which are fixed to one or more rotor shafts which extend through the interior of the stator. It is a common practice to mount such hubs on mounting flanges or bases which are either fixed to the rotor shaft or integrally formed therewith. Such hubs are typically fixed to the associated mounting flanges or bases in arrangements wherein elongate fasteners such as bolts extend through overlying apertures in the hubs and associated mounting flanges. Consistent with known manufacturing techniques, it is a common practice to provide the mounting holes in the hubs and flanges that are slightly larger than the cross-sectional areas of the bolts which extend therethrough to allow the bolts to be inserted in the apertures without binding thereon. This arrangement defines a clearance between the bolts and the mounting apertures. Under operating conditions such as surge events wherein the engine rotor experiences a radial imbalance of working fluid flow, the presence of such clearances between the bolts and mounting apertures allow a radial shift of the hub on the mounting flange, inducing a radial imbalance in the rotor, resulting in whirl which can damage the rotor by a bending of the shaft or a mechanical failure of the bearings on which the shaft is mounted. Accordingly, it is imperative that such radial imbalances in the rotor be avoided as much as possible. One known method for avoiding such radial imbalances caused by a shifting of the hub on the mounting flange is to entirely eliminate the clearances between the mounting bolts in the apertures and the hub and flange through which the bolts extend. Such clearances may be eliminated by forming the apertures with precisely the same area as the bolt shanks. However, such arrangements add substantially to engine rotor engine rotor manufacturing efforts quality control problems and therefore costs, requiring extreme precision in the formation of the mounting apertures and difficulty in insertion of the bolts into such apertures due to the bolts binding on the interior surfaces of the apertures when inserted therethrough.
Accordingly, an arrangement for mounting a rotatable component on a gas turbine engine rotor which minimizes the risk of any radial imbalance of the rotor due to radial shifting of the component on a mounting flange or base therefor without requiring excessive precision in the formation of mounting apertures and increase costs associated with the assembly of such a mounting arrangement due to a lack of clearance between the mounting bolts and the apertures within which such bolts are received, is sought.
In accordance with the present invention, a rotatable component such a blade hub is mounted on a mounting flange or base disposed on a rotor shaft of a gas turbine engine by elongate fasteners such as bolts received within an arrangement of overlying apertures in the component and base wherein the apertures in one of the component and base are slightly radially offset from the underlying apertures in the other of the component and base to partially radially close the underlying apertures in the other of the component and base (i.e., reduce the aligned area between the apertures in the component and those in the base) such that the fasteners are disposed in a radial interference fit within the apertures. As used herein, “radial interference fit” shall mean that the radially inner and outer surfaces of the fasteners are disposed in generally surface-to-surface contact with the radially inner and outer interior surfaces of the apertures within which the fasteners are received to eliminate radial clearances between the fasteners and the apertures therefor. Since the radial clearances between the fasteners and apertures within which the fasteners are received are eliminated, radial shifting of the component in response to radially imbalanced loads on the engine's rotor blades due to, for example, engine surge, are minimized, thereby minimizing the risk of damage to the engine's rotor from such conditions. Elimination of the radial clearances between the fasteners and apertures is achieved by radially offsetting the apertures in the rotatable component from the apertures in the mounting flange or base therefor. In a preferred embodiment, the apertures and one of the rotatable component and base are disposed in a circular array having a radius R1 while the apertures in the other of said component and base are staggered around opposite sides of a circular line of radius R1 such that a first set of apertures is disposed in a circular array disposed at a radius R2 which is slightly less than R1 and a second set of apertures in the other of said component and base are disposed in a circular array at a radius R3 from the axis of rotation of the engine's rotor wherein R3 is slightly greater than R1. The first set of apertures alternate circumferentially with the second set of apertures so that the radial loads on the fasteners received within the apertures are generally evenly distributed around the circumference of the rotatable component and underlying flange.
The radial component may comprise any of the components normally mounted on the engine's shaft such as any of various bladed hubs (either integrally bladed or with separate, attached blades) in the engine's fan compressor or turbine. The mounting arrangement of the present invention is conveniently implemented by aligning the rotatable component with the underlying mounting flange or base such that the mounting apertures are in radial alignment with one another fixturing the rotatable component and then sequentially heating and cooling the rotatable component to achieve the radial offset of the apertures in that component with those in the underlying mounting flange or base.
Referring to
Bearings 43, 45, 50 and 53 radially support the concentric high pressure and low pressure turbine shafts from separate frame structures 52, 54, 55 and 56 respectively, attached to engine case 57, which defines the outer boundary of the engine's stator 9 which circumscribes rotors 8. 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 to
While the present invention has been described within the context of mounting a bladed hub for a fan compressor or turbine stage on mounting flange disposed on gas turbine engine shaft, it will be appreciated that the present invention may be employed with equal efficacy for mounting any rotatable component on a gas turbine engine shaft. While the invention has been described and illustrated with twelve pairs of overlying apertures in the flange and hub, it will be appreciated that the exact number of apertures and size thereof will be determined by the size of the hub and mounting flange which will in turn be determined by the performance requirements of the engine in which the present invention is implemented. While the elongate fasteners 85 have been described as bolts, it will be appreciated that equivalent fasteners, such as rivets, pins or other elongate fasteners, may be employed. Accordingly, it will be understood that various modifications to the preferred embodiment described herein may be made without departing from the present invention and it is intended by the appended claims to cover such modifications as fall within the true spirit and scope of the invention.
