Abstract:
An abradable seal is provided, comprising an outer ring arrangement and an energiser for urging said outer ring arrangement against an opposing surface, wherein an outermost surface of said outer ring arrangement defines a sealing surface of said abradable seal; wherein said outer ring arrangement is configured such that after a first period of operation said sealing surface suddenly transitions from having a relatively large surface area to having a relatively small surface area, so as to cause a sudden increase in internal leakage across the seal at said transition.

Description:
FOREIGN PRIORITY 
       [0001]    This application claims priority to European Patent Application No. 15305772.4 filed May 22, 2015, the entire contents of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to an abradable seal. 
       BACKGROUND 
       [0003]    Abradable seals are used in a number of applications to provide a sealing function between two moving parts. For example, a piston arrangement may include a piston that is movable within a cylinder. The piston will function to separate two chambers, wherein the volume of each chamber varies depending on the position of the piston. Typically the piston and cylinder are metallic and a seal must be provided between these two components to prevent their contact whilst sealing the chambers, so as to prevent substantial amounts of fluid transferring from one chamber to the other. 
         [0004]    It is desired to provide an improved abradable seal that is able to provide an improved indication regarding its failure. 
       SUMMARY 
       [0005]    The present disclosure relates to an abradable seal comprising an outer ring arrangement and an energiser for urging the outer ring arrangement against an opposing surface, wherein an outermost surface of the outer ring arrangement defines a sealing surface of the abradable seal; wherein the outer ring arrangement is configured such that after a first period of operation the sealing surface suddenly transitions from having a relatively large surface area to having a relatively small surface area, so as to cause a sudden increase in internal leakage across the seal at the transition. 
         [0006]    A sudden transition provides a measured, controlled or predetermined and sudden increase in internal leakage during operation of the seal, namely at the transition between normal and abnormal operation. This can be used to indicate or record the moment at which the seal requires replacing. 
         [0007]    The wording “suddenly transitions” is used to indicate that the sealing surface does not transition due to, for example, a tapered outer ring. Whilst such an arrangement might transition from a relatively large surface area to a relatively small surface area, it would not do so suddenly, which is the aim of the broadest aspects of the present disclosure. A sudden transition happens due to a substantially instantaneous decrease in the surface area of the sealing surface. 
         [0008]    The outer ring arrangement may comprise an abradable outer ring and an inner ring. The inner ring may also be abradable. The inner ring may comprise voids or apertures in its outer circumferential surface, and the sealing surface may transition from having a relatively large surface area to having a relatively small surface area due to the outer ring abrading in use so as to expose the voids or apertures in the inner ring. 
         [0009]    The voids or apertures may extend between axial ends of the inner ring. The outer ring may have a uniform axial cross-section or width throughout its circumference, such that the sealing surface of the outer ring may be continuous or unbroken along its circumference. 
         [0010]    The inner ring may have a non-uniform axial cross-section throughout its circumference, for example due to the voids or apertures that may break the sealing surface at positions around its circumference. 
         [0011]    The inner ring may have a uniform axial width throughout its circumference, and may have a non-uniform radial width (e.g. due to the voids or apertures). 
         [0012]    The outer ring may comprise a base portion and may comprise two circumferentially continuous side portions extending radially inward from respective axial ends of the base portion. The inner ring may fit within the side portions of the outer ring, such that, optionally, once the base portion wears away in use, the side portions may remain to provide part of the sealing surface after the transition. 
         [0013]    The base portion may act as the sealing surface during the first period of operation. 
         [0014]    The sealing surface may have a substantially constant surface area prior to the transition. The sealing surface may have a substantially constant, albeit smaller surface area after the transition for a given period of time. 
         [0015]    The first period of time may define a period of normal operation of the seal. The transition may define a point of failure of the seal, and/or a point at which the seal requires replacement. 
         [0016]    The relatively small surface area may be at least 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% smaller than the relatively large surface area. 
         [0017]    The outer ring arrangement may be configured such that the transition does not cause catastrophic failure or damage to the abradable seal. For example, the sealing surface after the transition may still provide a sealing function, but it may be diminished slightly due to the smaller sealing surface area. The outer ring arrangement may be configured to provide a sealing function after the transition. 
         [0018]    In accordance with an aspect of the disclosure, there is provided a seal assembly comprising: a first component; a second component movable with respect to, and in sealing engagement with the first component; and an abradable seal as claimed in any preceding claim, wherein the abradable seal is located between the first component and the second component. 
         [0019]    The first component may comprise a cylinder. The second component may comprise a piston movable within the cylinder. 
         [0020]    The method may comprise: monitoring internal leakage across the abradable seal; and detecting a sudden increase in internal leakage due to the sealing surface transitioning from having a relatively large surface area to having a relatively small surface area. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Various embodiments will now be described, by way of example only, and with reference to the accompanying drawings in which: 
           [0022]      FIG. 1  shows an exploded, perspective view of an abradable seal; 
           [0023]      FIG. 2  shows an axial cross-section through the abradable seal of  FIG. 3 ; 
           [0024]      FIG. 3  shows a circumferential cross-section of the abradable seal of  FIG. 1 ; and 
           [0025]      FIG. 4  shows an axial cross-section through the abradable seal of  FIG. 1 , once the seal has been worn in use. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    An abradable seal in accordance with the present disclosure will now be described with reference to  FIG. 1 . 
         [0027]      FIG. 1  shows a piston  10  that is arranged to move axially inside a cylinder  20  (shown in  FIG. 3 ). The piston  10  comprises a shaft  12 , wherein the longitudinal axis  5  of the shaft  12  forms the axis of movement of the piston  10 . The piston  10  further comprises a flange  14  that extends radially from the shaft  12  to form a concentric disc extending towards the inner surface  50  of the cylinder  20  within which the piston  10  moves. The flange  14  comprises an outer peripheral surface  15  arranged to face the inner surface  50  of the cylinder and having a circumferential groove  16  therein. 
         [0028]    An abradable seal  100  (or abradable seal assembly) is configured to sit within the groove  16  to provide a sealing function between the piston  10  and the inner surface  50  of the cylinder  20 . It should be noted that  FIG. 1  shows an exploded view such that the abradable seal  100  is not shown within the groove  16  so that its components can be clearly seen. The abradable seal  100  is known as a dynamic seal, in that it provides a sealing function between two or more parts that move relative to each other. In this case the parts comprise the piston  10  and the cylinder  20 , wherein the piston  10  moves within the stationary cylinder  20 . Thus, the inner surface  50  of the cylinder  20  can otherwise be referred to as the opposing surface  50  of the abradable seal  100 . 
         [0029]    It is envisaged that the abradable seal  100  could also be provided in a groove that is within the inner surface  50  of the cylinder  20 , whilst having the same features as described herein in relation to the abradable seal  100  being within the flange  14  of the piston  10 . In this case, the outer peripheral surface  15  of the flange may be flat (i.e. may comprise substantially no grooves) and could form the opposing surface  50  of the abradable seal  100 . However, the piston  10  could only move a distance less than the axial length of the outer circumferential surface  15 . 
         [0030]    The abradable seal  100  comprises an abradable outer ring  120  that is arranged to contact the opposing surface  50  in use to form a seal between the piston  10  and the opposing surface  50 . The outer ring  120  is designed to wear (or abrade) during use due to friction between the outer ring  120  and the opposing surface  50  caused by the piston  10  moving within the cylinder. 
         [0031]    The abradable seal further comprises an energiser  140  for urging the abradable outer ring  120  away from the peripheral surface  15  of the flange  14  and against the opposing surface  50 . The energiser  140  may be a compressible material, for example an elastomer. Alternatively, the energiser  140  may be a spring, for example a metallic coil or wave spring. The outer ring  120  may be compressed when the piston  10  is initially placed within the cylinder, and act to urge the outer ring  120  away from the peripheral surface  15  of the flange  14  once it is suitably placed. 
         [0032]    In accordance with the disclosure, the abradable seal  100  further comprises an ancillary ring  150  that is located radially inward from the outer ring  120 , which is shown in more detail in  FIG. 2 . 
         [0033]      FIG. 2  shows an axial cross-section of the flange  14  and the abradable seal  100 , including the outer ring  120 , energiser  140  and ancillary ring  150 . The outer ring  120  comprises a base or outermost portion  122  extending axially to provide an initial or normal sealing surface  123 , and two side portions  124  that extend radially inward from axial ends of the base portion  122 . The base portion  122 , and thus the sealing surface  123 , have an axial length L. Thus, initially and during normal operation (defined below) the outer ring  120  has a U-shaped axial cross-section, which is uniform throughout its circumference. It will be appreciated that other shapes, for example an L-shaped axial cross-section are possible that will provide a similar uniform sealing surface throughout the outer circumference of the seal. 
         [0034]    The ancillary ring  150  sits within the U-shape of the outer ring  120 , and between the energiser  140  and the outer ring  120 . The ancillary ring  150  comprises a number of apertures  152  in its outer circumference (see also  FIG. 1 ), such that the ancillary ring  150  has a non-uniform axial cross-section throughout its circumference. In alternative arrangements the outer ring  120  and the ancillary ring  150  may be formed by the same component. The outer ring  120  and the ancillary ring  150  may be referred to as an outer ring arrangement. 
         [0035]      FIG. 3  shows a circumferential cross-section of the abradable seal  100  through the circumferential groove  16 , from which the apertures  152  in the ancillary ring  150  can be seen in more detail. In the illustrated embodiment, the apertures  152  are of uniform dimensions and are spaced apart at equal intervals in the outer circumference of the ancillary ring  150 . A plurality of projections  154  are formed in the outer circumference of the ancillary ring  150  due to the apertures  152 . Initially and during normal operation an outer surface  156  of the projections  154  contacts an inner circumferential surface  126  of the base portion  122  of the outer ring  120 . The outer surface  156  is urged against the inner circumferential surface  126  by the energiser  140 . 
         [0036]    The apertures  152  comprise side walls  157  and a base surface  158  and may be formed by machining the outer circumference of the ancillary ring  150 . 
         [0037]    It can be seen that, in the illustrated embodiment of  FIG. 2  and during normal operation, the apertures  152  of the ancillary ring  150  result in a plurality of chambers being formed that are enclosed by the base portion  123  and side portions  124  of the outer ring  120 , as well as the side walls  157  and base surface  158  of the ancillary ring  150 . Thus, each chamber represents a void in the outer ring arrangement, which in the illustrated embodiment comprises the outer ring  120  and the ancillary ring  150 . 
         [0038]    Initially and during normal operation, therefore, the abradable seal  100  will function by the energiser urging the outer ring  120  against the opposing surface  50 , via the ancillary ring  150 . The sealing surface  123  of the outer ring  120  is urged against the opposing surface  50  and provides a uniform circumferential sealing surface. 
         [0039]    Normal operation as defined herein corresponds to the period of time in which the initial or normal sealing surface  123  and/or base portion  122  wears down but is not worn away. The initial or normal sealing surface area is the area of the sealing surface  123  during initial set up or normal operation respectively. Since no voids, apertures etc. are present in the initial or normal sealing surface area, this area is calculated as the axial length L of the sealing surface  123  multiplied by the initial or immediate circumference of the outer ring  120 . During normal operation, therefore, the initial or normal sealing surface  123  has a constant or substantially constant surface area, known herein as a first, initial or normal sealing surface area. 
         [0040]    In use, the normal sealing surface  123  will wear down due to friction between the sealing surface  123  and the opposing surface  50 . As described above the surface area of the normal sealing surface  123  remains substantially constant whilst the base portion  122  exists during normal operation (i.e. is not worn away). That is, except for a minimal or negligible reduction in surface area due to the thickness of the seal wearing away (reducing the diameter of the seal and circumference), the first sealing surface area remains constant. 
         [0041]      FIG. 4  shows a transition point during operation of the piston  10 , in which the base portion  122  and sealing surface  123  have just worn away. This exposes the chambers referred to above, by removing their outer surface, which was formed by the now-absent base portion  122 . 
         [0042]    Since the normal sealing surface  123  is no longer present, the abradable seal  100  is no longer in normal operation. Abnormal operation may be defined herein as any operation of the seal  100  other than normal operation, or may be defined as operation of the seal  100  once the normal sealing surface  123  is worn away. 
         [0043]    In the illustrated case abnormal operation occurs once the normal sealing surface  123  wears away. At this point the sealing surface of the abradable seal  100  of  FIG. 4  is formed by the upper surfaces  125  of the two remaining side portions  124  of the outer ring  120 , as well as the outer surface  156  of the projections  154 . The sealing surface in abnormal operation has a second sealing surface area that, due to the presence of apertures  152 , is smaller than the first sealing surface area. 
         [0044]    Referring back to  FIG. 1 , it will be appreciated that the piston  10  separates two chambers which are located either side of the flange  14 , and the abradable seal  100  is configured to prevent fluid transfer between the chambers as the piston  10  moves back and forth along the longitudinal axis  5  of the shaft  12 . 
         [0045]    Typically, however, leakage exists between the two chambers and the amount of leakage is dependent in part on the surface area of the sealing surface. Leakage between chambers may be referred to as “internal leakage”. Internal leakage and sealing surface area generally have a negative correlation with respect to each other. 
         [0046]    The present disclosure provides a measured, controlled or predetermined and sudden increase in internal leakage during operation of the seal, namely at the transition between normal and abnormal operation. In the illustrated embodiment, this is achieved through a sudden reduction in the surface area of the abradable seal  100  at the transition between normal and abnormal operation. It should be noted that the outer ring arrangement still comprises a sealing surface or sealing surface area during abnormal operation, as described above. This means that, whilst the seal may not be fully operational, the transition between normal and abnormal operation does not cause catastrophic failure or damage to the seal. 
         [0047]    In the illustrated embodiment, use of the apertures  152  in the ancillary ring  150  results in a sudden or immediate drop in sealing surface area, during or immediately after the transition from normal to abnormal operation. An operator could monitor or otherwise observe this drop in internal leakage at the transition from normal to abnormal operation, and immediately determine that the seal has failed. 
         [0048]    As discussed, however, the outer ring arrangement still provides a sealing function during the transition from normal to abnormal operation, in that the side portions  124  and outer surface  156  of the outer ring arrangement provide a sealing surface. This allows the exact point at which the seal fails (or requires replacing) to be determined but without also causing damage to the seal  10 , piston  10 , cylinder  50  or other hydraulic parts associated with the seal  100 . 
         [0049]    The flow between the chambers during normal operation may be represented by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     Q 
                     l 
                   
                   = 
                   
                     
                       K 
                        
                       
                         ( 
                         oil 
                         ) 
                       
                     
                     × 
                     
                       
                         j 
                         3 
                       
                       L 
                     
                     × 
                     Δ 
                      
                     
                         
                     
                      
                     P 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0050]    where Q l  represents the internal leakage or flow between chambers of the piston, K represents the frictional coefficient of the oil, j represents the amount of radial movement of the seal, L is the axial length of the sealing surface (see L in  FIG. 2 ) and ΔP is the pressure difference between the chambers. As is evident, the internal leakage has a negative correlation with the sealing surface area. This is represented by a length in equation (1) since the sealing surface in normal operation is uniform and continuous throughout its circumference. 
         [0051]    Once the base portion  122  of the outer ring  120  is worn away, the internal leakage between the chambers suddenly increases, and the pressure difference suddenly decreases, due to the smaller surface area of the second sealing surface. The seal will still function, due to the remaining parts of the outer ring  120  (i.e. the side portions  124 ) and the ancillary ring  150 . However, the flow between the chambers during abnormal operation may now be defined by the following relationship: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         Q 
                         ab 
                       
                       ≈ 
                       
                         
                           
                             ( 
                             
                               1 
                               - 
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                            
                           
                               
                           
                            
                           
                             
                               
                                 K 
                                  
                                 
                                   ( 
                                   oil 
                                   ) 
                                 
                               
                                
                               
                                 j 
                                 3 
                               
                             
                             L 
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           P 
                         
                         + 
                         
                           α 
                            
                           
                               
                           
                            
                           
                             
                               
                                 K 
                                  
                                 
                                   ( 
                                   oil 
                                   ) 
                                 
                               
                                
                               
                                 j 
                                 3 
                               
                             
                             
                               2 
                                
                               
                                 L 
                                 B 
                               
                             
                           
                            
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                     = 
                     
                       
                         [ 
                         
                           1 
                           + 
                           
                             α 
                              
                             
                               ( 
                               
                                 
                                   L 
                                   
                                     2 
                                      
                                     
                                       L 
                                       B 
                                     
                                   
                                 
                                 - 
                                 1 
                               
                               ) 
                             
                           
                         
                         ] 
                       
                        
                       
                         Q 
                         n 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   where 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   α 
                   = 
                   
                     ne 
                     
                       π 
                        
                       
                           
                       
                        
                       D 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0052]    and where Q ab  represents the flow between the chambers in abnormal operation, Q l  represents the flow during normal operation (see (1)), K represents the frictional coefficient of the oil, j represents the amount of radial movement of the seal, ΔP is the pressure difference between the chambers, L is the axial length of the sealing surface (as in (1)), L B  is the axial length of the ancillary ring  150 , n is the number of apertures  152  and e is the circumferential length of the apertures  152 . 
         [0053]    Equations (1) and (2) show that there will be a clear difference in the measured flow or pressure between the chambers separated by the abradable seal  100 . 
         [0054]    An abradable seal  100  according to the above-described embodiment may be used in many applications. In particular, the above abradable seal  100  may be used in an aircraft hydraulic actuator. The piston  10  as described above may be used to actuate a specific aircraft component, for example a flap or rudder. 
         [0055]    Whilst a flight control computer (“FCC”) may monitor internal leakage or pressure, it is not possible in conventional aircraft actuators to detect failure of the seal, other than to operate the seal until catastrophic failure. This is clearly undesirable, and means that most seals have to be manually inspected and usually replaced before the end of their service life. Using an abradable seal  100  according to the present disclosure allows an operator to detect failure of the seal in, for example, a pre-flight check, by monitoring internal leakage or a pressure drop in the actuator using, for example, a flight control computer. This reduces the need for manual inspections and allows the seal to be used to the full extent of its service life. 
         [0056]    Although the present disclosure has been described with reference to the embodiments described above, it will be understood by those skilled in the art that various changes in form and detail may be made. 
         [0057]    For example, in its broadest aspects the abradable seal of the present disclosure may have uses in any application where it would be beneficial to detect certain points in the service life of the seal without having to manually inspect it. For example, a plurality of sudden reductions in the surface area of the sealing surface could be provided, corresponding to 50%, 20%, 10% etc. of the remaining service life of the seal. This is beneficial for certain seals which, for example, may be embedded in pumping equipment and could be very difficult to inspect.