Abstract:
A disc arrangement for a gas turbine engine, the disc arrangement including a rotational axis, a disc, a coverplate, and a retainer to retain association between the disc and the coverplate by interference. The retainer is displaceable in a recess of the disc or the coverplate to allow release of the interference. The recess presents the retainer at an inclined angle to the rotational axis for the disc and/or the coverplate.

Description:
BACKGROUND 
   The present invention relates to retention arrangements and more particularly to retention arrangements in the form of a split ring to retain association between components. 
   It will be understood that particularly in relation to engines where there are rotating shafts it is important to retain association between coverplate seals and other components such as a retaining disc for blades. 
   Referring to  FIG. 1 , a gas turbine engine is generally indicated at  10  and comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high pressure compressor  14 , combustion equipment  15 , a high pressure turbine  16 , an intermediate pressure turbine  17 , a low pressure turbine  18  and an exhaust nozzle  19 . 
   The gas turbine engine  10  works in a conventional manner so that air entering the intake  11  is accelerated by the fan  12  which produce two air flows: a first air flow into the intermediate pressure compressor  13  and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor  14  where further compression takes place. 
   The compressed air exhausted from the high pressure compressor  14  is directed into the combustion equipment  15  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines  16 ,  17  and  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low pressure turbine  16 ,  17  and  18  respectively drive the high and intermediate pressure compressors  14  and  13 , and the fan  12  by suitable interconnecting shafts. 
     FIG. 2  illustrates a typical prior retention arrangement for a coverplate  21  secured to a disc  22  through a retention ring  23 . As can be seen, the retention ring  23  acts between a disc retention feature  24  and an end of the coverplate  21 . 
   The coverplate  21  is fitted or removed by collapsing the ring  23  into a groove  25  between the feature  24  and the remainder of the disc  22 . It will be understood that the retention ring  23  is generally split so that the split or gap is closed when the ring  23  is collapsed into the groove  25  and allowed to open or expand into the position depicted in  FIG. 2  in use and forced into greater expansion in order to be lifted over the feature  24  for ring removal. It will be understood in order to provide retention there is a thrust rolling off-set  26  between the coverplate  21  and the feature  24  and a CF rolling off-set  27  between the ring  23  and parts of the coverplate  21 . 
   SUMMARY 
   It will be noted that other features are a discourager seal  28  which is used to facilitate sealing between parts of the arrangement. There is also a further retention ring  29  to provide association between the disc  22  and another coverplate  20 . 
   As indicated above, generally the retention arrangement will be utilised with regard to components which are rotating either relative to each other or collectively about a shaft. The ring  23  suffers an anti-clockwise couple under axial thrust loading ( 26 ,  27 ), which is opposed by a clockwise couple under centrifugal loading ( 30 , 31 ) as shown by arrowheads  102 ,  101  respectively. Generally, the clockwise and anti-clockwise couple loadings will not be equal and the ring  23  will, therefore, roll between the off-sets  26 ,  27  and provide edge loading as a result. Such rolling and edge loading may result in premature wearing of the disc  22  in particular at the feature  24 , the engagement parts of the coverplate  21  and also with respect to the ring  23  itself. Also, it will be understood that it was considered not feasible to incorporate an inner coverplate double seal as used on a bayonet type retained coverplate due to the radial access space requirements of a split ring retainer. Thus, as can be seen in  FIG. 2 , a less effective discourager seal  28  is provided. 
   In accordance with certain aspects of the present invention there is provided a retention arrangement comprising a retainer to retain association between a first member and a second member by interference, the retainer displaceable in a recess of the first member or the second member to allow release of the interference, the recess presenting the retainer at an inclined angle to a mounting axis for the first member and/or the second member. 
   Typically, the inclined angle provides a displacement path for release of the interference provided by the retainer between the first member and the second member. 
   Advantageously, the inclined angle is about 45°. 
   Typically, the interference provided by the retainer is with respective interference features of the first member and the second member. Generally, the respective interference features present portions of the first member and the second member at a mutual clearance level. 
   Generally, the retainer is a ring. Typically, the ring is a split ring. Possibly, the retainer has a rectangular cross-section. Alternatively, the retainer has a U or V shaped cross-section. 
   Possibly, the first or second member has an access aperture for a displacement tool. 
   Possibly, the recess and retainer are configured relative to one another to provide a desired load transfer distribution under load in use. Possibly, the desired load transfer distribution equalises load between axial load and any centrifugal load presented to the retention arrangement. In essence, the angled axial contact faces are opposite to each other, giving zero thrust rolling couple. In essence, the angled CF contacts are balanced about the ring CG, giving zero CF rolling couple. The ring is this insensitive to the relative sizes of the thrust and CF forces. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of certain aspects of the present invention will now be described by way of example only with reference to the accompanying drawings in which 
       FIG. 1  illustrates a gas turbine engine; 
       FIG. 2  illustrates a typical prior retention arrangement for a coverplate secured to a disc through a retention ring; 
       FIG. 3  is a cross-section of a retention arrangement in accordance with first aspects of the present invention; 
       FIG. 4  is a cross-section of a retention arrangement in accordance with second aspects of the present invention; and, 
       FIG. 5  is a cross-section of a retention arrangement in accordance with third aspects of the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   Providing a retention arrangement which is both sufficiently robust for operational use as well as convenient to allow assembly and disassembly is important but, as indicated above, it is also important to avoid problems with unequalisation in load distribution in the arrangement and the potential for rolling and uneven loading of the arrangement. 
     FIG. 3  illustrates a retention arrangement in accordance with first aspects of the present invention. Thus, a coverplate  31  is retained relative to a disc  32  by a retainer  33  in the form of a split ring. For the avoidance of doubt the aspects of the invention as depicted in  FIG. 3  are described relative to a coverplate  31  and a disc  32  but it will be understood that the arrangement is particularly related to associating a first member with a second member through an interference engagement by the retainer  33 . 
   The retainer  33  is located within a recess  35  to allow movement of the retainer  33  generally in a displacement direction to facilitate assembly and disassembly. In such circumstances it will be understood that the retainer  33  can be moved in the direction of arrowhead A either into or out of the recess  35  as a result of positive action using a displacement tool or to take account of shrinkage or other distortions of the retainer during assembly and through environmental factors such as temperature, etc. 
   The recess  35  is defined in the disc  32  to present an interference retention feature  36  against which a part of the retainer  33  acts. In such circumstances it will be understood that the interference between the retainer  33  and the feature  36  will be subject to axial load across the major axis of the arrangement defined as a mounting axis X-X. 
   The coverplate  31  will also provide an interference retention feature  37  which in accordance with these first aspects of the invention depicted in  FIG. 3  takes the form of an angular heel which is engaged by the retainer  33 . It will also be understood that a rear part of the retainer  33  is in close proximity to a part  38  of the recess  35  to provide a disc guidance feature for shrinkage of retainer  33  prior to coverplate  31  fitting or removal. 
   In the above circumstances it will be appreciated that the disc  32  in terms of the recess  35  is modified to accommodate an inclined presentation of the retainer to interference retention features  36 ,  37 ,  38  for load distribution. 
   In terms of assembly it will be understood that respective parts of the coverplate  31  and retention feature  36  will be presented towards a clearance level  39  which allows the components to pass over each other for removal or installation when the interference retention provided by the retainer  33  is removed through general displacement into the recess  35 . 
   As indicated above, a retainer  33  is generally in the form of a ring which extends around a disc and is displaced in the direction of arrowheads A into the recess  35  in order to allow fitting and removal. In such circumstances, the retainer  33  will be pushed downwardly into the recess  35  such that it is below the clearance level  39 . The retainer  33  as indicated is generally in the form of a split ring such that the gap in the split ring is reduced to allow reduction in the circumference of the retainer  33  as it moves into the recess  35 . In its normal state, the retainer  33  illustrated will provide an interference association with the coverplate  31  and interference portion  36  of the disc  32 . Removal of the retainer  33  will be achieved in a split ring format by expanding the diameter of the retainer  33  to allow removal over an upper edge  40  of the feature  36 . 
   As indicated above, achievement of a desired load distribution in a retention arrangement is possible in accordance with the present invention due to the inclined angle of the retainer. Thus, as illustrated in  FIG. 3  axial thrust load will be presented in the direction of arrowheads B and centrifugal force loading in the direction of arrowheads C. As can be seen, these loads B, C may be in opposite directions and typically the inclined angle of the retainer  33  within the recess  35  will be such that there is balance in the loadings provided. 
   In such circumstances, a retention arrangement may eliminate roll couples in all conditions with the thrust loading and centrifugal force loading independently balanced within the arrangement. Dynamic balance in the axial plane given by section symmetry within the retainer  33  whilst in the rotational plane by flattening the section inner convex radius opposite to the split gap in the retainer  33 . 
   It will be noted that in the embodiment depicted  FIG. 3  a discourager seal (see  28  in  FIG. 2 ) is removed. 
   It will be noted that the coverplate  31  has a hooked cross-section  29  for seal static member clearance. 
   Double-angle pre-swirl holes  27  towards inward facing seals  28  are provided but they require displacement relative to situations as depicted in  FIG. 2 . The holes  27  are provided to allow air flow cooling. 
   The retainer  33  as described above is typically of a split ring form. The cross-section of this retainer  33  is relatively simple and compact in comparison with the previous split ring (ring  23  in  FIG. 2 ) having better crack shape factors, and allowing the retainer to be manufactured more cheaply. By achieving a balanced even loading of forces B, C stresses within the arrangement depicted in  FIG. 3  are generally lower in comparison with previous retention arrangements and contact surfaces are more uniformly loaded reducing frettage concerns. There is much less frettage movement. It will be understood previous split ring retainer arrangements may have failed as a result of excessive rolling under high axial loads. Typically, the inclined angle for the retainer  33  is about 45°. Such angling may cause hoop growth of the coverplate  31  and disc in the area of the interference feature  36 . Generally there should be sufficient free contact area provided in the arrangement to allow for deflexions in the interference contact between the retainer  33  and the feature  36  and interference features of the coverplate  31  without loss of interference contact area. 
   It will be understood that the embodiment depicted in  FIG. 3  has a retainer which has a U or V cross-section. A symmetrical section as depicted cannot be fitted the wrong way around within the arrangement. 
   The retainer as indicated is self-centering under the heel part  37  of the coverplate  31  and, therefore, is self-centering under centrifugal loading. It has low vertical shear CF stresses. 
   By dynamic balancing as described above the arrangement is less sensitive to angular positioning or gradual rotational movement of the arrangement in service. If it is desired to reduce the retainer weight then the bore flat could be extended around the retainer to provide balance by only leaving a full section around the gap of the retainer rather than by removing material opposite to the gap. Nevertheless, in such circumstances, it is important to ensure that the possibility of centrifugal crack initiation or propagation is avoided. 
   As described above, the retainer  33  is displaceable in a displacement direction which is typically consistent with the inclined angle to allow fitting and removal. In such circumstances, in the embodiment depicted in  FIG. 3  it will be seen that an upper face  25  of the retainer  33  is exposed to allow access by a manual action through a displacement tool. 
   Typically, the retainer  33  will include a small corner chamfer to ease coverplate  31  fitting and removal, improve crack shape factors and remove the free surface from the most intense Poisson&#39;s ratio tension from the bulk material axial load compression. The actual size of the corner chamfers will be dependent upon operational requirements. It will also be understood that the surface between these chamfers may be rendered convex or concave. 
   As indicated above, most conveniently the retainer  33  is in the form of a split ring. In such circumstances holes can be drilled into the surfaces adjacent to the split gap in the ring to allow the use or circlip pliers for ring manipulation. Alternatively, the ring can be shrunk by direct external pressure on the retainer surfaces to allow coverplate removal. Subsequently, the retainer ring may be expanded by hooking L shaped “tyre lever” type displacement tools under the ends of the split gap and pulling the ring upwards and forwards over the edge  40 . It will be understood the arrangement as depicted in  FIG. 3  may require less expansion to allow removal of the retainer if the retainer bore balancing flat is extended around the circumference as suggested above for weight reduction. 
   As indicated above, the arrangement as depicted in  FIG. 3  has no discourager seal. In such circumstances it will be understood that potential disc damage as a result of contact with this discourager seal is eliminated from the arrangement depicted in  FIG. 3 . 
   As indicated above, forward axial movement is prevented by contact with the interference retention part  36  and, in particular, upper part  40  of the disc  32 . Rearwards axial movement of the coverplate is limited in the embodiment depicted in  FIG. 3  by the U shaped nature of the retainer  33  and engagement with a part of the recess  35 . The recess  35  provides a guide surface as indicated above for guided displacement of the retainer. This guide surface would contact the retainer and restrain this movement, typically aided by centrifugal loading on the retainer. Furthermore, the coverplate  31  can be arranged to contact the disc just forward of the radial Coverplate spigot to avoid contact and disc diaphragm damage by the rear of the coverplate engaging the disc. 
   It will be understood that generally either the coverplate or the retaining ring, will typically incorporate some form of entrant section to axially locate the two together under a CF field. In  FIG. 3  this is achieved through a U or V shape in the retainer  33 . Alternatively, as depicted in  FIG. 4  the coverplate includes a female entrant channel or portion  140  which is entered by a retainer  43 . Thus, the approach of providing a retainer  43  with thrust load and centrifugal load presented at an inclined angle is still retained. This inclined angle is about 45° but as will be appreciated could be at differing angles if an alternative design load distribution is required. The embodiment depicted in  FIG. 4  still retains thrust load distribution by the inclined angle, 45° and centrifugal load balance by a symmetrical section for the retainer  43 . 
   A mounting disc  42  still defines a recess in the form of a groove into which the retainer  43  can be displaced in a displacement direction given by the direction of arrowhead AA. Force to provide for this displacement of the retainer  43  will typically be provided through an access aperture  49  by a displacement tool. 
   As will be noted, the retainer again is preferably in the form of a split ring which in the aspect of the invention depicted in  FIG. 4  has a substantially rectangular and approximately square cross-section with chamfered corners. Such a retainer  43  can generally have a smaller, lighter and simpler construction in comparison with previous rings. 
   In order to release association between the coverplate  41  and the disc  42  the retainer  43  is moved from its locking or interference position as depicted in solid line in  FIG. 4  to a release position depicted by a broken line  43   a . This displacement can be achieved by forcing a cylindrical tool through the annulus  49   a  or direct pressing through the aperture  49 . The aperture  49  may include a screw thread to allow for insertion of a bolt to cause retainer  43  displacement to release position  43   a.    
   In the locked position  43  as previously thrust or axial load components are presented in the direction of arrowheads BB whilst centrifugal loads are presented in the direction of arrowheads CC. In such circumstances as previously the inclined angle of the interference between the retainer  43  and engaged parts  140 ,  50  of the coverplate  41  and the disc  42  act to provide balancing of the axial thrust load whilst the symmetrical cross-section of the retainer  43  which is also symmetrically supported similarly acts to balance the centrifugal force load to create the desired load distribution. As described previously, with an inclined angle of 45° this will generally equalise load distribution although other specific inclined angles as well as shaping of the retainer may allow adjustment of such load distribution if desired. 
   It will be appreciated above aspects of the present invention have been described relative to a first member and a second member in the form of a disc and coverplate retained by interference through a retainer.  FIG. 5  illustrates third aspects of the present invention in the form of a hub  51  secured about a shaft  52 . In such circumstances it will be appreciated that axial movement along the shaft  52  by the hub  51  should be prevented. In accordance with the embodiment depicted in  FIG. 5  a retention arrangement comprises two retainers  53   a ,  53   b  located in a recess  55 . The retainers  53  are again presented at an inclined angle to a mounting axis of the arrangement. In the embodiment depicted in  FIG. 5  this mounting axis will generally be the axis of rotation M-M for the shaft  52 . In such circumstances, thrust loads in the direction of arrowheads BBB are balanced by engagement between parts of the retainers  53  and the hub  51  and shaft  52 . Similarly, through the symmetrical or otherwise shaping of the retainers  53  it will be understood that centrifugal force loading is appropriately distributed for a desired load distribution. 
   The retainers  53  are displaceable in the displacement path which is generally consistent with an inclined angle in order to assume a release position shown by broken lines  53   a ′,  53   b′.    
   It will be noted the hub  51  is secured in both directions and that both retainers  53  in the form of split rings will require movement to the release position  53   a ′,  53   b ′ in order to remove the hub  51  from the shaft  52 . In such circumstances position of centering spigots may provide a problem and so require a central feature or release clearance castellation. 
   It will be understood that provision of a single sided retention arrangement in accordance with certain aspects of the present invention may be easier than the embodiment depicted in  FIG. 5 . In such circumstances, the hub  51  may be axially secured upon the shaft against a fixed shoulder in one direction and against the displaceable retainer in the opposite direction. Radial location may be provided on the shouldered side by a spigot, and circumferential location by a spline. On the retainer side, an annular gap between the hub and the shaft will then provide access for entry of a thin cylinder as at CCC for retainer displacement for hub removal. 
   As indicated above, it will be understood that a deep release groove or recess  55  is required. For a static assembly, release could be into the hub with the retainer in the form of a split ring with spring stiffness acting inwards. 
   Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.