Patent Application
20120275921
This invention relates generally to gas turbine engines and more specifically to bearing assemblies and load reduction devices for gas turbine engines.
A gas turbine engine includes a turbomachinery core having a high pressure compressor, combustor, and high pressure turbine in serial flow relationship. The core is operable in a known manner to generate a primary flow of propulsive gas. A typical turbofan engine adds a low pressure turbine driven by the core exhaust gases which in turn drives a fan rotor through a shaft to generate a bypass flow of propulsive gas. In the case of a high bypass engine this provides the majority of the total engine thrust.
The fan rotor includes a fan that includes an array of fan blades extending radially outward from fan disk. The fan shaft transfers power and rotary motion from the low pressure turbine to the fan disk and is supported in several rolling-element bearing assemblies spaced along its length. The bearings are commonly referred to as no. 1, no. 2, and no. 5 bearings, identifying their sequential position in the engine.
During operation of the engine, a fragment of a fan blade may become separated from the remainder of the blade as a result of impact with a foreign object. Accordingly, a substantial rotary unbalance load may be created within the damaged fan and carried by the bearings, bearing supports, and the fan support frames.
To minimize the effects of potentially damaging abnormal imbalance loads, known engines include support components for the fan rotor support system that are sized to provide additional strength for the fan support system. However, increasing the strength of the support components undesirably increases an overall weight of the engine and decreases an overall efficiency of the engine when the engine is operated without substantial rotor imbalances.
Other known engines include a bearing support that includes a mechanically weakened section, or primary fuse, that decouples the fan rotor from the fan support system. During such events, the fan shaft seeks a new center of rotation that approximates that of its unbalanced center of gravity. This fuse section, in combination with a rotor clearance allowance, is referred to as a load reduction device, or “LRD”. The LRD reduces the rotating dynamic loads to the fan support system.
For the LRD to operate successfully, it is often desirable to have a specific ratio of the axial distance from the fan disk to the #5 bearing, divided by the axial distance from the #2 bearing to the #5 bearing. However, newer engine designs with long cores and short forward overhangs do not provide sufficient axial length for this configuration.
Accordingly, there is a need for a fan rotor load reduction device which is effective in a limited axial space.
These and other shortcomings of the prior art are addressed by the present invention, which provides a forward fan shaft with increased flexibility in a given space.
According to one aspect, the invention provides a fan rotor apparatus including: a rotatable fan disk defining a central aperture and carrying an array of airfoil-shaped fan blades around its periphery, the disk having a forward end and an aft end; an annular, generally axially-extending forward fan shaft extending through the aperture and coupled to the fan disk for rotation therewith, where the forward fan shaft joins the fan disk at or near the forward end.
According to another aspect of the invention, a turbofan engine includes: a turbomachinery core operable to produce a flow of pressurized combustion gases; a turbine disposed aft of the core; a rotatable fan disk mounted forward of the core, the fan disk defining a central aperture and carrying an array of airfoil-shaped fan blades around its periphery, the disk having a forward end and an aft end; and an annular, generally axially-extending forward fan shaft mechanically coupled to the turbine, the forward fan shaft extending through the aperture and coupled to the fan disk for rotation therewith, where the forward fan shaft joins the fan disk at or near the forward end.
The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:
Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,
The inner shaft 18 comprises a forward fan shaft 28 and a rear fan shaft 30 coupled together and mounted for rotation in several rolling-element bearings. The forward fan shaft 28 is carried by a first bearing 32 (commonly referred to as a “no. 1 bearing”) and a second bearing 34 (commonly referred to as a “no. 2 bearing”). The rear fan shaft 30 is carried by a bearing 36 (commonly referred to as a “#5 bearing”).
The fan 12 of the engine 10 shown in
A no. 1 bearing 132 is mounted to a surrounding structural support frame 146 by an annular, generally axially-extending fuse 148. In accordance with known principles, the size, material, and mechanical design of the fuse 148 is selected to fail at a predetermined radial load, such as a load that might occur after separation of a fan blade 140. Failure of the fuse 148 allows the fan disk 138 to rotate about a new axis of rotation without imposing excessive radial loads on the surrounding structure. Other types of fuse structures are known, such as bolted joints or fuse pins designed to fail in tension or in shear, or collapsible member(s) in a frame designed to crush at designated loads. The specific type of fuse structure is not critical to the present invention.
In contrast with prior art designs, the forward fan shaft 128 extends axially forward past the aft end 144 of the fan disk 138, traversing the longitudinal extent of the fan disk 138, and is coupled to the fan disk 138 at a point at or near the forward end 142 of the fan disk 138. As used herein, the term “coupled to the fan disk at or near the forward end” means that torque is transferred from the forward fan shaft 128 to the fan disk 138 through a load path passing at or through the disk's forward end. It does not necessarily imply any specific type of mechanical connection between the forward fan shaft 128 and the fan disk 138, or require any specific location of a mechanical joint between the two components. In the example shown in
The disk arm 150 shown in
In operation, the forward fan shaft design described herein permits the fan rotor to safely windmill after a blade release event while limiting the bending loads applied to the core. This can be achieved without the need for any specific engine length or bearing position requirements.
The foregoing has described load reduction device for a gas turbine engine. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.
