Abstract:
An anti-fret assembly for a turbine engine, the assembly comprising an elongated liner having first and second walls connected by a base to provide a generally U-shaped channel, the liner having an outer surface adapted to lie against a first component and an inner surface adapted to lie against a second component at least the first wall being provided with a notch for receipt of an anti-translation pin, the assembly further comprising an anti-translation pin.

Description:
This invention claims the benefit of UK Patent Application No. 1015959.8, filed on 23 Sep. 2010, which is hereby incorporated herein in its entirety. 
     BACKGROUND 
     The invention relates to an elongate anti fret liner having a component for inhibiting movement of the liner in its direction of elongation. The liner finds particular application in a turbine engine. 
     Within a gas turbine engine some components, for example vanes, need to be restrained in a manner that resists the aerodynamic or other loads placed upon them during engine operation. The loads can be broken down into two primary components: the axial reaction load along the engine axis and the rotational reaction load about the axis. Both loads are typically transmitted from the gas washed components to a casing. 
     A gas turbine is an aggressive environment in which liquid lubricants cannot generally be used and materials with dissimilar material properties must make contact. Gas path static components tend to be constructed out of high strength alloys that make contact with casings that are generally constructed out of medium strength alloys. The different thermal coefficients of these materials and the large temperature range experienced by a turbine engine in operation means that it is not possible to rigidly join such components. Therefore the components can move relative to each other resulting in the possibility of wear, usually on the softer component. 
     To protect the two components an anti-fret liner that is made of material that is softer than either of the interfacing parts is positioned between the two components. Its purpose is to wear in preference to either of the interfacing parts and it may be considered to be a disposable part at engine overhaul. The anti-fret liner and any anti-translation features for the liner should accordingly be cheap to manufacture and easy to remove and replace during overhaul. 
     It is an object of the present invention to seek to provide an improved anti-fret assembly. 
     SUMMARY 
     In accordance with the invention there is provided an anti-fret assembly for a turbine engine, the assembly comprising an elongate liner and an anti-translation pin, the liner having first and second walls connected by a base to provide a generally U-shaped channel, the liner having an outer surface adapted to lie against a first component and an inner surface adapted to lie against a second component, at least the first wall being provided with a notch for receipt of the anti-translation pin. 
     Preferably the anti-translation pin extends between the first and second walls and protrudes through the notch in the first wall. 
     The assembly may further comprise a notch in the second wall aligned to the notch in the first wall, the anti-translation pin extending between the first and second walls and protruding through the notch in the first wall and into the notch in the second wall. The pin may protrude through the notch in the second wall. 
     The notches may be aligned such that they are located at the same distance along the length of the liner and opposed to each other. The anti-translation pin may be straight. 
     The anti-translation pin may further comprise one or more portions which extend along the channel in the liner. The portions may prevent the pin from becoming detached from the liner during operation of the machinery in which the liner is used. 
     The protrusion or protrusions of the anti-translation pin may engage an anti-translation feature or features located on the first component. The feature(s) may be a notch or keying element within which the protrusion(s) are inserted. 
     A second component may be located in the channel in the liner. The second component may have a face that abuts the pin to transfer translational forces to the first component through the pin. 
     Preferably the first component is a casing in a gas turbine and the second component is part of a vane or sealing element. The liner may be arcuate or linear. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the accompanying drawings and by way of example only in which: 
         FIG. 1  depicts a section of a gas turbine engine including an anti-fret liner according to the present invention; 
         FIG. 2  depicts an expanded version of region A of  FIG. 1 ; 
         FIG. 3  depicts a perspective view of the anti-fret liner of  FIG. 1 , further comprising an anti-translation pin; 
         FIGS. 4 and 5  depict alternative shapes for the pin of  FIG. 3 ; and 
         FIG. 6  shows a perspective view of an anti-fret liner according to the present invention assembled in a gas turbine engine. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  depicts a turbine of a gas turbine engine incorporating the present invention. The turbine has a casing  2  which contains a plurality of rotatable blades  4  and a plurality of static vanes  6 . The blades are mounted at the hub to a rotatable disc  8  and are spaced apart from the casing by a narrow gap  10  defined between the shroud at the tip of each blade and an abradable liner mounted on the casing. The blades  4  are not attached to the casing. 
     In contrast to the blades  4 , the vanes  6  are mounted to the casing  2  and are separated from the disc hub by an air gap defined between the seal and the rotatable disc  8 . The mounting of the front of the vane  6  to the casing  2  is shown in more detail in  FIG. 2 , which is an enlarged view of the region marked “A” in  FIG. 1 . It should be appreciated that the vanes  6  may be mounted to the casing  2  individually or several vanes may be grouped together and mounted as a single vane segment. Several vane segments may be needed to complete an entire circumferential array of vanes  6 . The invention will continue to be described by way of an exemplary vane segment. 
     The gas turbine is an aggressive environment where liquid lubricants cannot be used and materials with dissimilar material properties must make contact with one another. In the case of gas part statics, i.e. vanes, these components tend to be constructed out of high strength, hard, cast alloys that are by their nature segmented parts to avoid issues with thermal gradients. They make contact with casings which are made from medium strength alloys which have a lower hardness. 
     Due to thermal expansion it is not possible to rigidly attach the gas path statics to their containment casings. Therefore both components have the ability to move against each other. This leads to the possibility of wear (or fretting) occurring, most usually on the softer component. 
     In the embodiment of  FIG. 2 , the casing  2  is provided with a hook  12  and a channel  14  into which a hook  16  on the vane segment  18  is located. An anti-fret liner  20  is provided between the casing  2  and the hook  16  on the vane segment  18 . The liner  20  is made from a softer material than either of the interfacing parts. Its purpose is to wear in preference to either of the interfacing parts and thus is considered a disposable part at engine overhaul. 
     The anti-fret liner  20  is inserted into and removed from the channel  14  in an axial direction denoted by arrow  22  and is clipped to the hook  12  using clip  24 . Preferably the liner  20  is a continuous circumferential hoop but it may be formed from a plurality of parts assembled adjacently in the channel  14  to form a circumference. Beneficially the liner  20  combines the function of preventing rotation of a gas path static while also preventing wear between a gas path static (including, but not limited to, turbine or compressor vanes or seal segments) and a support structure such as the casing. 
       FIG. 3  is a perspective view of a portion of the liner  20  and an anti-translation pin  30 . The liner  20  provides an elongate, generally “U” shaped channel defined by first and second elongate walls  32 ,  34  and a base  36  connecting the two walls  32 ,  34 . In the embodiment shown in  FIG. 3  the liner  20  also has an optional stop face  38  which provides a location feature inhibiting axial movement of the liner  20  towards the hook  12  beyond its intended position, and the optional clip  24  which holds the liner  20  onto the casing  2 . 
     The anti-translation pin  30  extends across the channel between the first and second walls  32 ,  34  and is positioned within notches  37  formed in each wall  32 ,  34 . The pin  30  is secured to the liner  20  by welding, soldering, brazing or any other suitable method including an interference fit. A portion of the pin  30  projects beyond either one or both of the first and second walls  32 ,  34  and the portion is used to engage the casing  2  as shown and described later with regard to  FIG. 6 . Where notches are provided in both of the walls  32 ,  34  of the liner  20  it is possible to align them so that they are at the same circumferential position on the liner  20 . 
     In this arrangement the pin  30  is straight but advantages may be received by using a pin  30  with one or, as shown, two arms that extend along the channel of the liner  20 . For example if, in service, the weld holding the pin in place fails, a straight pin may slide from the liner  20  and be caught into the gas flow through the engine which could cause potential damage further downstream in the engine. Where at least one arm is provided the pin  30  cannot drop from the liner  20  during use. 
     Where the notches  37  are not aligned to the same circumferential point it is possible to use pins  30   a  of other shapes such as the one shown in  FIG. 4  which has a central portion  32   a  which extends circumferentially within the channel of the liner  20  and radial portions  32   b ,  32   c  which extend radially outwards and inwards respectively. If it is only desirable to put notches in one wall of the liner  20  then a pin  30   b  of the form shown in  FIG. 5  may be used, which has an elongate central portion  32   a  and two ends  32   d  that each project radially outwards. It will be appreciated that these pin shapes are exemplary and that other shapes as appropriate may be used. 
       FIG. 6  depicts a perspective view of the liner  20  in situ within the casing  2  and supporting a vane segment hook  16 . The casing  2  is provided with a notch  40  arranged so that when the pin  30  is located in the liner  20  the portion of the pin  30  protruding from the liner  20  engages with the notch  40  in the casing  2 . The vane segment hook  16  is located to engage the inner surfaces of the U shaped channel of the liner  20 . Although not shown in  FIG. 6  the vane segment hook  16  can extend over at least a portion of the pin  30  to butt up to the rear of the pin  30  and prevents it moving rearwards in use should the joint between the pin  30  and the liner  20  fail. Beneficially, the pin  30  and liner  20  assembly is not reliant on the joint strength between the pin  30  and liner  20  to keep the pin  30  in position. 
     The vane segment hook  16  has a face  42  which engages an end face of the pin  30 . The face  42 , which may be located towards the end of the vane hook  16 , or machined in a more central location, transfers sideways or circumferential translational force from the vane segment  18  or the liner  20  to the pin  30 . As the pin  30  is constrained by the notch  40  in the casing  2 , sideways or translational movement of the vane segment  18  or liner  20  in the direction of arrow  46  is prohibited. 
     It is desirable that there is at least one anti-translation pin  30  per vane segment  18  to avoid excessive pressure build up on a selected pin  30 . By using multiple anti-translation pins  30  it is possible to divide the liner  20  into multiple discrete sections with each section having at least one pin  30 . Beneficially, using multiple anti-fret liners about a circumference reduces the cost of the overall component and reduces the risk of relative movement between gas path components buckling or tearing the anti-fret liner  20 . 
     Whilst the invention has been described with regard to a circumferential arrangement and thus with anti-fret liners  20  that are arcuate and which singularly or together form a circumference, the invention also finds application where the anti-fret liners  20  are linear. The invention has also been described with regard to gas turbines but will also find application in other machinery where there is relative movement between two components and it is desired to mitigate damage with the presence of an anti-fret liner and also to minimise translational movement between the two components.