Fan blade axial retention device

Abstract: The fan blade retention device includes a collar (24) inserted into a circumferential groove (29) cut into the hub (30) of a turbofan engine. The outward facing groove (29) is positioned aft of the fan blade fixing channel. When the fan blade root (16) is completely received within the blade fixing channel, the blade root (16) contacts the collar (24) and is prevented from traveling further aft. If the collar (24) is segmented, an annular rear cover plate (25) holds the segmented collar (24) radially within the groove (29).

Description: Technical Field This invention relates to fan blade axial retention devices and more particularly to fan blade retention devices oriented at an angle to the centerline of the engine. Background Art In turbofan engines, fan blade fixing commonly takes the form of a dovetail joint. In conventional configurations, the base of the joint lies in a plane aligned parallel to the centerline of the engine. Because of gas path aerodynamics, conventional engines require that the blade platform be positioned to maintain the gas path profile established by the nose cone. In engines with aligned blade fixing, this means that a significant distance exists between the outer hub diameter and the aft blade platform. This distance, combined with the mass of the blade root comprising the distance, causes an additional load on the hub due to the centrifugal forces which act on the blade as the hub assembly rotates. To compensate for this additional load, it is necessary to make the blade root "stockier" than the blade, which compounds the problem. These additional loads result in undesired hub strain and bearing stress within the hub. A significant amount of the loading can be eliminated, however. A fan blade fixing means configuration at an inclined angle relative to the engine centerline reduces the distance from the blade platform to the outer hub diameter. As a result, blade mass is reduced as well as the centrifugal force associated with the blade mass. There is a tradeoff, however. In conventional configurations, the load on the fan blade due to centrifugal forces, is almost entirely handled by the dovetail joint. This is possible because the centrifugal force appears solely as a radial force acting up through the fan blade. With the blade fixing at an incline, however, the centrifugal force translates into both a radial force and a axial force and it becomes necessary to secure the blade against both forces. Existing designs secure the blade axially by a "hook" formed on the forward edge of the blade root. The hook contacts the hub, or a locking ring, and physically prevents the blade from traveling up the blade fixing incline. US-A-4,470,756 on which the two-part form of independent claim 1 is based discloses a blade retaining device of this type. A disadvantage of this configuration is the required size of the hook, especially for larger engines. Gains in the reduction of blade mass produced by the inclined configuration, are lost as the hook is necessarily designed larger and larger with increasing engine size. Moreover, at some point hooks are no longer feasible from a physical constraint standpoint. Another disadvantage of this design is that the entire axial component of the centrifugal force is localized to the contact area between the hook and the hub. Since that contact area is relatively small, the load is analogous to a point load and as such produces a concentration in stress. Reference is also made to US-A-3,744,930 which discloses a blade retaining device oriented parallel to the engine centerline. A segmented sealing ring is mounted on each side of the blade in an outwardly facing circumferential groove cut into the exterior surface of a rotor disc rim portion to block movement of the blade relative to the rotor disc. Disclosure of Invention The object of the invention is the provision of an axial retention device for a turbofan blade fixed to the fan hub at an incline relative to the engine centerline which eliminates the need for a retention hook on the blade root. To achieve this, there is provided according to the present invention a device for retaining a blade for an axial fan having a dovetail root received within a corresponding dovetail channel in an annular hub, wherein the dovetail channel slopes radially outward with respect to the fan axis, from a front end to an aft end of the dovetail channel, and a radially outward facing circumferential groove is cut into the exterior surface of the annular hub, characterized in that the groove is adjacent to the aft end of the dovetail channel; that a plurality of collar segments are received within said groove, said segments each having a radial dimension sufficient to block aftward movement of the blade root beyond the end of the dovetail channel; and that an annular rear plate cover is secured to the hub and disposed circumferentially around the hub, and closely fits around said collar segments, thereby holding said segments radially within said groove. Advantages of this design include the nature of the relatively small blade root "footprint" contacting the collar segments. When the blade root contacts the collar segments, the axial component of the centrifugal force acting on the blade transfers to the collar segments and is distributed circumferentially over the collar's length. Consequently, the load is distributed along the length of the groove, thereby reducing local hub stress concentrations which might be associated with, a point load. Preferably, the cover plate attaches to the hub aft of the circumferential groove, in an area outside of the high stress zone produced by the centrifugal loading on the hub. The cover plate would be, for instance, riveted to a flange at the rear of the hub. The flange would be positioned such that under load, none of the rivet holes produce any appreciable stress to the hub. One of the advantages of the present invention's segmented collar is that no holes are required in the body of the hub to secure the radial retaining device. A hole in the hub, especially in close proximity to a highly stressed region such as the groove, would create a considerable stress concentration in the hub under load. To overcome these stress concentrations, hub design must be larger and less economical. In the present invention, however, hub stress is minimized because the collar segments themselves require no attachment holes and the segmented collar cover plate employs rivets which attach outside the stress area of the hub. Therefore hub design can be smaller and more economical. According to another advantageous feature of the present invention, a front ring is provided which prevents the fan blades from sliding forward, down the incline. Brief Description of the Drawings FIG. 1 is a perspective view detailing the segmented collar hub assembly. The figure shows a breakaway of the segmented collar in the groove and the rear cover plate holding the collar within the groove. FIG. 2 is a cross-sectional view of the hub with the blade positioned in the dovetail. This view shows a cross-sectional view of the segmented collar, the rear cover plate, and the front ring attached to the hub. FIG. 3 is a sectional view of the dovetail joint between the fan blade and the hub. Best Mode for Carrying Out the Invention Referring to FIG.1, FIG.2 and FIG.3, a fan blade 10 is attached to the rotating hub 30 of a turbofan engine. The fan blade 10 is attached to the hub 30 by a dovetail joint 12 which is sloped at an angle 13 relative to the engine centerline 14. The dovetail channel 15 is broached in the hub 30 and the base of the blade root 16 is formed into the corresponding male dovetail 17. Once fully received within the dovetail channel 15, the blade root 16 is prevented from moving aftward, up the incline 13, by an annular segmented collar 24. The segmented collar 24 sits in an outward facing circumferential groove 29, cut into the hub 30, that abuts the aft end of the dovetail channel 15. In addition to preventing the blade root 16 from moving aft, the collar 24 also reduces stress in the hub 30 by distributing the aftward component of the centrifugal force load acting on the hub 30 through the blade 10. Stress on the hub 30 is further reduced by a fillet 22 positioned on the aft inside corner 21 of the groove 29. The fillet 22 on the aft inside corner 21 is a large radius fillet that undercuts the corner. Since the aft wall 23 of the groove 29 directly receives the axial force transmitted by the blade 10 and collar 24, the aft corner fillet 22 dimensions are such that the stress is minimized in that corner 21 of the groove 29. The collar 24 may be placed within the groove 19 following assembly of the hub 30 and blade 10. The radial dimension of the collar 24 is sufficient to block aftward movement of the blade root 16. In addition, the forward inside corner 33 may also include a stress reducing fillet 34. The segmented collar 24 is held radially within the groove 29 by an annular rear cover plate 25. The cover plate 25 slides over the outer diameter 26 of the assembled segmented collar 24 and is secured by rivets 31,for example, to the hub 30 on a flange 27 extending aftward of the hub 30. Rivet holes 28 in the hub flange 27 are located outside the high stress zone around the groove 29 which is created by the axial force on the groove 29. In this configuration, these holes 28 are the only holes necessary for axially retaining the fan blade 10 in the aft direction 20. At the other end of the dovetail channel 12, the smaller diameter end 32, of the segmented collar hub 30, the blade 10 may be prevented from sliding out the dovetail channel 15 by a front ring 36, or any other retention device known in the art.

Claims: Device for retaining a blade (10) for an axial fan having a dovetail root (16) received within a corresponding dovetail channel (15) in an annular hub (30), wherein the dovetail channel (15) slopes radially outward with respect to the fan axis (14), from a front end to an aft end of the dovetail channel (15), and a radially outward facing circumferential groove (29) is cut into the exterior surface of the annular hub (30), characterized in that the groove (29) is adjacent to the aft end of the dovetail channel (15); that a plurality of collar segments (24) are received within said groove (29), said segments (24) each having a radial dimension sufficient to block aftward movement of the blade root (16) beyond the end of the dovetail channel (15); and that an annular rear plate cover (25) is secured to the hub (30) and disposed circumferentially around the hub (30), and closely fits around said collar segments (24), thereby holding said segments (24) radially within said groove (29). Device for retaining a blade for an axial fan according to claim 1, characterized in that said rear cover plate (25) is attached to the hub (30) by rivets (31) on a flange (27) which extends aft, outside of the stress zone produced by the axial loading on the circumferential groove (29). Device for retaining a blade for an axial fan according to claim 1, characterized in that said circumferential groove (29) comprises two inside corners (21, 33), wherein both corners (21, 33) contain stress reducing fillets (22, 34). Device for retaining a blade for an axial fan according to claim 3, characterized in that said aft inside corner (21) contains a stress reducing fillet (22) which undercuts the corner (21). Device for retaining a blade for an axial fan according to any one of claims 1 to 4, characterized in that at the front end of the dovetail channel (15) a front ring (36) is provided to prevent the blade (10) from sliding forward, down the incline.

Inventors: CHIJIIWA HARUHISA, DOWHAN MICHAEL JOHN, GREER JOHN, CHIJIIWA, HARUHISA, DOWHAN, MICHAEL JOHN, GREER, JOHN
Applicants: PRATT & WHITNEY CANADA, PRATT & WHITNEY CANADA INC.
Application year: 1991
Main classification: F01D5
IPC codes: F01D5