Split disk blade support

A blade support includes first and second abutting disks, with the first disk having a plurality of axial dovetail slots at one end thereof, and a circumferential dovetail first half-slot at an opposite end thereof, with the second disk having a circumferential dovetail second half-slot therein. A plurality of rotor blades have axial dovetails disposed in respective ones of the axial dovetail slots, and circumferential dovetails disposed in part in each of the first and second half-slots. The first and second disks are retained together so that the first and second half-slots collectively support the circumferential dovetails.

The present invention relates generally to gas turbine engines, and, more 
specifically, to rotor blade support arrangements. 
BACKGROUND OF THE INVENTION 
Aircraft gas turbine engines include fan, compressor, and turbine rotor 
blades which are typically removably mounted to supporting rotor disks. 
The rotor blades include various circumferential or axial dovetails which 
are supported in complementary circumferential or axial dovetail slots 
formed in the rotor disks. Axial dovetails are readily assembled to their 
respective rotor disks by merely axially sliding the dovetail into the 
rotor disk, with disassembly thereof being prevented by conventional 
forward and aft blade retainers. 
In order to install circumferential dovetails, a loading slot is required 
so that the dovetails may be initially radially inserted into the loading 
slot and then slid circumferentially into position into the dovetail slot. 
Since the loading slot is not capable of providing any radial retention 
force, the circumferential dovetails are typically sized for having 
circumferential spacing therebetween which is typically on the order of 
the circumferential extent of the loading slot itself. Upon insertion of 
the last circumferential dovetail into the loading slot, all the blades 
are circumferentially indexed so that no circumferential dovetail is 
positioned at the loading slot, with adjacent circumferential dovetails 
straddling the loading slot for ensuring the radial retention thereof and 
effective transfer of centrifugal forces from the blades to the disk 
during operation. 
In one type of engine design, fan blades may have a low radius ratio, which 
is the ratio between the inner diameter to the outer diameter of the 
flowpath typically measured at the blade platform and tip, that results in 
a relatively small perimeter of the supporting disk. This increases the 
difficulty of carrying centrifugal loads from the blades and into the disk 
with acceptably low stress. Low radius ratio blades are therefore 
typically formed integrally with the rotor disk, from a common forging for 
example, for ensuring relatively low stresses during operation and a 
suitable useful life thereof. However, individual blades are therefore not 
readily removable since cutting thereof would be required. 
It is desirable to have individually removable rotor blades in a low radius 
ratio blade configuration with acceptable retention stresses therein 
during operation. 
SUMMARY OF THE INVENTION 
A blade support includes first and second abutting disks, with the first 
disk having a plurality of axial dovetail slots at one end thereof, and a 
circumferential dovetail first half-slot at an opposite end thereof, with 
the second disk having a circumferential dovetail second half-slot 
therein. A plurality of rotor blades have axial dovetails disposed in 
respective ones of the axial dovetail slots, and circumferential dovetails 
disposed in part in each of the first and second half-slots. The first and 
second disks are retained together so that the first and second half-slots 
collectively support the circumferential dovetails.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
Illustrated in FIG. 1 is a portion of an exemplary fan rotor assembly 10 
configured for supporting individually removable low radius ratio fan 
rotor blades 12 therein. An exemplary one of the rotor blades 12 is also 
illustrated in FIG. 2 and includes an airfoil 14 and axially spaced apart 
circumferential and axial dovetails 16, 18 integrally joined thereto. The 
airfoil 14 has leading and trailing edges 14a,b over which ambient air 20 
flows downstream therebetween during operation, with the airfoil 14 also 
having a radially outer tip 14c. Each blade 12 also includes an integral 
platform 22 disposed integrally between the airfoil 14 and the axial and 
circumferential dovetails 16, 18 for providing an inner flow boundary for 
the air 20 during operation. 
In the exemplary embodiment illustrated in FIG. 1, the blade 12 has a 
relatively low radius ratio as well as a relatively low aspect ratio, 
which is the ratio of the airfoil span length to its chord length 
typically identified at the mid-span or pitchline thereof, which results 
in the platform 22 having a relatively large slope radially outwardly from 
the leading edge 14a to the trailing edge 14b of the airfoil 14. Radius 
ratio and aspect ratio are conventional terms with low values thereof 
being conventionally associated with a relatively small outer diameter of 
the rotor disk by which the centrifugal forces generated by the blade 12 
during operation may be carried. 
Typical low radius ratio airfoils are integrally formed with a supporting 
disk in a one-piece rotor known conventionally as a blisk. However, such 
airfoils are not readily removable therefrom without cutting thereof, and 
it is desirable to have individually removable blades in a low radius 
ratio arrangement. This is accomplished in accordance with the present 
invention by providing both dovetails 16, 18 in a compact and efficient 
supporting arrangement with annular first and second rotor disks 24, 26 as 
illustrated in FIG. 1. 
In the exemplary embodiment of the invention illustrated in FIGS. 1 and 2, 
the circumferential dovetail 16 is preferably disposed upstream of or 
forward of the axial dovetail 18 adjacent to the leading edge 14a, with 
the axial dovetail 18 being disposed adjacent to the trailing edge 14b. 
Correspondingly, the first disk 24 illustrated in. FIGS. 1, 3, and 4 
includes a plurality of circumferentially spaced apart, axial dovetail 
slots 28 around the perimeter thereof which are defined by 
circumferentially adjacent axial dovetail posts 30 at an aft end of the 
first disk 24. The first disk 24 has an axial centerline axis, with the 
axial dovetail slots 28 extending generally parallel thereto. The axial 
dovetail 18, complementary axial dovetail slots 28, and the axial dovetail 
posts 30 may take any conventional form such as that illustrated in the 
Figures, with the axial dovetail 18 being symmetrical with two opposing 
lobes or tangs for example. 
As shown in FIGS. 1 and 3, the first disk 24 also includes a 
circumferential dovetail first half-slot 32 in an opposite or forward end 
thereof which is continuous around the-circumference of the first disk 24. 
The second disk 26 as illustrated in FIG. 1 has a complementary 
circumferential dovetail second half-slot 34 at its aft end, with the 
second disk 26 and half-slot 34 also being continuous around the 
circumference thereof. 
Each of the fan blades 12 illustrated in FIG. 1 may be initially installed 
in the first disk 24 by axially sliding the respective axial dovetail 18 
in a respective one of the axial dovetail slots 28. The posts 30 radially 
support the fan blades 12 therein in restraint against centrifugal force 
during operation. The corresponding circumferential dovetail 16 of each 
blade 12 is firstly disposed in part in the first half-slot 32 in abutment 
therewith which prevents further axially rearward movement of the axial 
dovetail 18. In this way all of the plurality of fan blades 12 may be 
firstly installed in their respective axial dovetail slots 28. The second 
disk 26 is then disposed in abutting contact with all of the blade 
circumferential dovetails 16, with the second half-slot 34 receiving the 
remaining parts of the circumferential dovetails 16. 
The first and second disks 24, 26 abut together at a circumferentially 
extending splitline 36 which is preferably disposed symmetrically and 
radially below the blade circumferential dovetails 16, with the first and 
second half-slots 32, 34 collectively defining a complete circumferential 
dovetail slot for supporting the blade circumferential dovetails 16 
therein. The individual circumferential dovetails 16 and the complementary 
dovetail slot 32, 34 may take any conventional form such as that 
illustrated in FIG. 1 including symmetrically opposite lobes or tangs, 
with the splitline 36 being centered therebetween. The splitline 36 may 
have alternate configurations including a rabbet. 
Both the first and second disks 24, 26 at the circumferential dovetail slot 
32, 34 are circumferentially continuous for providing hoop stress load 
carrying capability. In this way, the circumferential dovetails 16 are 
retained not only by radial shear force in the corresponding posts 
defining the half-slots 32, 34, but also by the circumferential hoop load 
carrying capability of the mating half-slots 32, 34. Since no loading slot 
is required in the first and second disks 24, 26 for radially inserting 
the individual circumferential dovetails 16, the load path interruption 
created thereby is avoided. Furthermore, by removing the entire second 
disk 26, any one of the individual fan blades 12 may be removed and 
replaced as desired which is not possible in a conventional 
circumferential dovetail arrangement having a single loading slot. In that 
case all blades between the loading slot and the desired blade must be 
removed in order to reach it. 
In the exemplary configuration illustrated in FIG. 1, the platforms 22 of 
the blades 12 slope substantially outwardly in a downstream direction. By 
positioning the circumferential dovetails 16 upstream of the axial 
dovetails 18, a compact and efficient load carrying arrangement is 
created. Since the diameter of the platform 22 at the circumferential 
dovetails 16 is substantially less than the diameter of the platform 22 at 
the axial dovetails 18, the circumferential dovetails 16 and supporting 
dovetail half-slots 32, 34 provide a maximum amount of circumferential 
load retaining structure in a relatively small area without interruption. 
Although the axial placement of the circumferential and axial dovetails 
16, 18 could be reversed in other embodiments if desired, the resulting 
arrangement would not enjoy this benefit. 
Referring again to FIG. 1, each of the first and second disks 24, 26 has a 
respective coextensive, central bore 24a, 26a through which a low pressure 
rotor shaft 38 extends axially. The shaft 38 includes an annular retaining 
flange 38a disposed at an aft end thereof in abutting, retaining contact 
with an aft end of the first disk 24, and also includes conventional screw 
threads 38b at an opposite, forward end thereof disposed adjacent to the 
forward end of the second disk 26. A conventional spanner nut 40 is 
threadingly joined to the shaft threads 38b for axially compressing 
together the first and second disks 24, 26 at the splitline 36, and 
between the nut 40 and the retaining flange 38a. 
As shown in FIG. 1, the second disk 26 is relatively compact in section for 
efficiently carrying centrifugal loads from the circumferential dovetails 
16, and the forward end of the first disk 24 is similarly compact and 
extends axially coextensively with the second disk 26. Furthermore, the 
axial dovetail slots 28 are disposed at an elevation radially above the 
first half-slot 32, with the blade axial dovetails 18 being 
correspondingly disposed at an elevation radially above the blade 
circumferential dovetails 16. In this way, the first disk 24 is solid 
below the axial dovetail slots 28 and axially aft of the first half-slot 
32 for effectively carrying centrifugal loads from both the 
circumferential and axial dovetails 16, 18 at suitably low stress levels. 
As shown in FIG. 3, each of the circumferential dovetails 16 preferably 
extends the full circumferential width of the blade platform 22, with the 
circumferential dovetails 16 circumferential abutting each other 
completely around the circumferential dovetail slot 32, 34. In this way, 
the entire circumferential extent of the first and second half-slots 32, 
34 may be fully utilized for carrying the centrifugal loads from the 
blades 12 which can not be done in a conventional loading slot 
configuration. 
As shown in FIG. 4, the axial dovetail posts 30 have a circumferential 
pitch P.sub.1 measured center-to-center which is preferably equal to the 
circumferential pitch P.sub.2 of adjacent ones of the axial dovetails 18 
and their slots 28 measured center-to-center. In this exemplary 
embodiment, there are eighteen fan blades 12 with the respective pitches 
P.sub.1, P.sub.2 being each 20.degree.. 
As shown in FIGS. 1 and 3, the second disk 26, further includes an integral 
annular band 26b around the perimeter thereof, with the band 26b being 
coextensive with the upstream ends of the blade platform 22 for providing 
an aerodynamically smooth transition therewith. 
Accordingly, the fan rotor assembly 10 described above allows for 
individually replaceable fan blades 12 in a low radius ratio application 
while effectively carrying centrifugal loads from the blades 12 through 
the respective dovetails 16, 18 into the clamped together first and second 
disks 24, 26. The circumferential dovetail slot defined by the half-slots 
32, 34 extends a full 360.degree. which provides hoop load carrying 
capability not found in conventional circumferential dovetail 
arrangements. 
While there have been described herein what are considered to be preferred 
and exemplary embodiments of the present invention, other modifications of 
the invention shall be apparent to those skilled in the art from the 
teachings herein, and it is, therefore, desired to be secured in the 
appended claims all such modifications as fall within the true spirit and 
scope of the invention. 
Accordingly, what is desired to be secured by Letters Patent of the United 
States is the invention as defined and differentiated in the following 
claims.