Means for reducing stress or fretting in clamped assemblies

In an assembly including components clamped together and in which one or both of the components is subject to a relatively high and variable tensile stress, fretting due to vibration at abutting surfaces of the components can start cracks in the surfaces which is then accentuated in the stressed surface. FIG. 1 of the specification shows a fan turbine rotor disc 4 clamped on a shaft 2 by nut 12 and interposed between confronting surfaces 14 and 16 on the disc and an adjacent bearing is a stress-reducing collar 6 which is slotted at 18 to define struts 20 extending between the surfaces. The struts 20 transmit the clamping load while allowing torsional flexibility to reduce the shear stress and hence fretting between contacting surfaces. In an alternative embodiment the surfaces themselves may be slotted which gives the added advantage that the tensile stress in the surfaces can be reduced or eliminated.

The present invention relates to assemblies including components clamped 
together, and in which one or more of the components is subject to a 
relatively high and variable tensile stress. 
Such a situation arises, for example, in a gas turbine engine when a rotor 
disc, or an assembly of rotor discs, is clamped onto a shaft by means of a 
nut. The rotor disc is subject during engine operation to varying 
centrifugal loading which creates a tensile stress in the disc. 
The problem which arises is that, although relative gross rotational 
sliding of the rotor disc and the nut, or other adjacent component, is 
prevented by splines between the disc and the shaft, torsional vibrations 
normally present in rotating turbo-machinery may cause slight relative 
movements between abutting clamped faces of the disc and the other 
components. These movements produce fretting of the surfaces and can lead 
to cracking of the disc, or wear which releases the clamping load. 
The object of the present invention is to provide means whereby the 
fretting between the two adjacent joint faces is reduced or eliminated. 
The invention as claimed significantly reduces or even eliminates the 
fretting between two adjacent joint faces by the provision of spaced 
struts or pillars between the joint faces which are capable of 
transmitting the clamping loads between the two components but which are 
sufficiently flexible to reduce any shear forces between the joint faces. 
The struts may be formed on one or more of the confronting faces of the two 
components, or may be provided on a member interposed between the two 
components.

Referring now to FIGS. 1, 1a and 1b of the drawings, there is shown part of 
a main shaft 2 of a gas turbine engine on which are assembled a rotor disc 
4, a stress-reducing collar 6 and a bearing 8. The assembly is retained in 
position axially on the shaft by a shoulder 10 against which all of the 
elements are clamped by a nut 12, and relative rotation of the rotor disc 
4 and the shaft 2 is prevented by splines 13. When tightened, the nut 
exerts a compressive load on the assembly. The stress-reducing collar 6 is 
interposed between axially confronting surfaces 14, 16 respectively on the 
rotor disc 4 and the bearing 8. 
The stress reducing collar 6 comprises a slotted annulus in which the slots 
18 extend both axially of the shaft and radially completely through the 
collar so as to define a plurality of spaced struts 20. The struts 20 must 
be dimensioned to be capable of transmitting the compressive clamping load 
between the confronting surfaces of bearing 8 and the disc 4, which, in 
this example, constitute the two components of the assembly, but at the 
same time must provide sufficient flexibility to ensure that shear forces 
arising in operation between the rotor disc 4 and the bearing 8 are 
reduced beyond the level at which relative sliding would take place 
between the surface 14 of the disc and the contacting surface 22 of the 
collar 6. 
In order to achieve different flexibility in the stress-reducing collar 6 
as may be required in different embodiments of the invention, or to 
simplify manufacture of the collars, the numbers and positions of the 
slots may be varied as shown by the examples given in FIGS. 2 to 4. 
In the FIG. 2 embodiment, for example, the collar is split radially and the 
slots 18 are machined into an axial end of each part. The collar is 
assembled on the shaft in the engine with the two slotted axial ends in 
abutment. FIG. 3 shows an alternative in which the spacer has slots 18 
only in one axial end, and FIG. 4 shows an alternative in which alternate 
slots are cut in opposite axial ends. 
In each of the above described examples the collar 6 is adapted for the 
assembly shown in FIG. 1 in which the rotor disc 4 is subject to 
significant centrifugal loading during operation, and the tensile stress 
thus generated is circumferential so that the axis of the principle shear 
stress between abutting surfaces of the assembly is radial. 
In other assemblies however, the axis of the principle shear stress may be 
non-radial, or may vary in direction during operation. In the first case 
the slots should then be angled so that their mid-planes lie in the 
direction of the axis of the shear stress, and the struts extend normal to 
the confronting surfaces of the components. In the second case two series 
of slots should be provided having their axes at a large angle to each 
other, preferably at right angles, and normal to the planes of the 
confronting faces. 
In another embodiment of the invention, the abutting faces themselves are 
slotted to produce the struts and the stress-reducing collar of FIGS. 1 to 
4 is removed. An example of this embodiment is given in FIGS. 5 to 7. 
In this example, two rotor discs 30, 32 are assembled on a shaft 34, and 
each disc carries an axially-extending cylindrical extension 36, 38 which 
forms the rotary part of a labyrinth seal. Each of the extensions 36, 38 
carry sealing fins 40 which run in close co-operation with a static 
sealing surface 42 on static structure 44 of the engine. 
The cylindrical extensions 36, 38 each have a respective surface 48, 50 
which are in abutment and a compressive clamping load is transmitted 
across the surfaces 48, 50 when the discs are clamped between a shoulder 
52 on the shaft and a nut 54. Relative rotation between the shaft and the 
rotor discs is prevented by pins 56. 
The surfaces 48, 50 are slotted at a plurality of circumferentially-shaped 
positions 58 thereby defining projections 60 which abut in the assembly 
forming the required struss for transmitting the compressive clamping 
loads. As described hereinbefore the struts are dimensioned to be flexible 
enough to reduce the shear forces between the surfaces 48 and 50. An 
additional benefit of this embodiment is that because the slots at 58 
interrupt the material of the disc near the abutting surfaces, the tensile 
stress now acts adjacent the roots 62 of the slots. The effect of this is 
to reduce the stress at the surfaces, and by suitable adjustment of the 
height/width ratio of the struts 60, the tensile stress at the surfaces 
can be arranged to be zero or even negative (i.e. compressive). If a zero 
stress level or compressive stress can be achieved then the avoidance of 
fretting is not important so that the strut design can be a compromise 
between providing the flexibility for minimising the fretting and the 
height/width ratio to minimise the tensile stress.