Abstract:
There is disclosed a method of monitoring the health of an abradable seal located on a piston of an actuator, the method comprising the steps of: (i) measuring an initial velocity of said actuator piston while said actuator is maintained in a passive condition at an initial time; (ii) operating said actuator; (iii) measuring a subsequent velocity of said actuator piston while said actuator is maintained in a passive condition at a subsequent time; (iv) repeating steps (ii)-(iii) and either: recording or outputting the measured subsequent velocities over time; or determining a health status of said actuator when said subsequent velocity has increased above a predetermined amount.

Description:
FOREIGN PRIORITY 
       [0001]    This application claims priority to European Patent Application No. 15305773.2 filed May 22, 2015, the entire contents of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to a method for monitoring the health of a seal located on a piston of an actuator, for example an abradable seal. 
       BACKGROUND OF THE INVENTION 
       [0003]    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]    The health monitoring of such seals may carried out conventionally by replacing the seals well before the end of their potential service life. This has been done to ensure that they do not catastrophically fail suddenly during use. Methods are known for monitoring actuator health, but these typically involve manual inspection and/or are only able to detect the health of the actuator as a whole, and cannot specifically detect the health or failure of the seal that is located on the actuator piston. 
         [0005]    It is desired to provide an improved method for monitoring the health of a seal located on an actuator piston, that is able to provide an improved indication regarding its health. 
       SUMMARY 
       [0006]    The disclosure provides a method of monitoring the health of an abradable seal located on a piston of an actuator, the method comprising the steps of:
   (i) measuring an initial velocity of the actuator piston while the actuator is maintained in a passive state or condition (as defined herein) at an initial time;   (ii) operating the actuator, for example in normal use to drive a component connected to the actuator;   (iii) measuring a subsequent velocity of the actuator piston while the actuator is maintained in a passive state or condition (as defined herein) at a subsequent time;   (iv) repeating steps (i)-(iii) and either:   (v) recording or outputting the measured subsequent velocities over time; or   (vi) determining a health status of the actuator or seal when the subsequent velocity has increased above a predetermined amount.   
 
         [0013]    The seal may be an abradable seal, or may be any type of seal that is located on a piston that suffers from leakage flow across the seal. The seal may provide a sealing function between two chambers separated by the piston. 
         [0014]    The step (i) and/or (iii) may comprise setting up the actuator in a passive state or condition prior to measuring the initial velocity. A passive state or condition is defined below and essentially means that the movement of the piston is caused only by the leakage flow across the seal, or at least primarily by the leakage flow across the seal. As such, detecting this movement gives a clear indication of the health of the seal, since this is the only, or at least primary variable involved. 
         [0015]    Accordingly, the methods disclosed herein may allow the health of the seal to be monitored without manual inspection of the seal itself. Various embodiments of the present disclosure are also advantageous over conventional methods that monitor the health of a control valve, rather than specifically the seal as disclosed herein. 
         [0016]    The actuator may comprise a control valve, for example a hydraulic control valve such as a servo valve. 
         [0017]    The actuator may be part of an actuator assembly, wherein the actuator assembly may comprise said piston and optionally a cylinder within which the piston may move. The piston may separate two chambers, e.g. first and second chambers, that may vary in volume depending on the position of the piston. The actuator assembly may comprise an annular seal that optionally provides a sealing function to prevent substantial fluid transfer between the chambers. 
         [0018]    The actuator assembly may comprise a hydraulic system to control the flow of fluid into the chambers. The hydraulic system may comprise said control valve, for example a servo valve, that optionally controls the flow of fluid into the chambers. 
         [0019]    In order to extend the actuator, for example, the valve may receive hydraulic fluid from an inlet, and may pump this into the first chamber via a first line, which in turn may force hydraulic fluid out of the second chamber to an outlet via a second line and valve. 
         [0020]    In order to retract the actuator, for example, the valve may operate in reverse by receiving hydraulic fluid from the inlet, pumping this into the second chamber, which may force hydraulic fluid out of the first chamber to the outlet via first line and valve. 
         [0021]    The step of setting up the actuator in a passive state or condition may comprise setting the control valve into its null position such that there is no flow of fluid into or out of the actuator. In the passive state or condition movement of the piston may be primarily caused, or only caused by flow of fluid across the seal. 
         [0022]    The step of setting up the actuator in a passive state or condition may comprise moving the piston to its maximum or minimum extension. 
         [0023]    Measuring the initial or subsequent velocity may comprise the steps of:
   (a) moving the piston to a first position, for example its maximum or minimum extension;   (b) setting up the actuator in a passive state or condition (as defined herein);   (c) measuring the distance moved by the piston in a given time; and   (d) calculating the initial or subsequent piston velocity using the measured distance divided by the given time.   
 
         [0028]    The movement described in step (c) may be primarily caused, or only caused by flow of fluid across the seal. 
         [0029]    Measuring the initial or subsequent velocity may further comprise the step of:
   (e) releasing the actuator from its passive state or condition, for example prior to the step (ii) of operating the actuator as described above.   
 
         [0031]    The actuator may be a hydraulic actuator and may comprise a control valve arranged and adapted to control the distribution of hydraulic fluid in the actuator. The passive condition may be defined as operation of the actuator with the control valve set in a passive, or null condition such that hydraulic fluid is not introduced into the actuator. 
         [0032]    The step of determining a health status of the actuator may comprise determining that internal leakage has increased above a critical amount based on the increase in the velocity of the piston. The step of determining a health status of the actuator may comprise determining that the sudden increase in internal leakage has occurred on the basis of a sudden increase in piston velocity. 
         [0033]    The seal may comprise 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 seal. 
         [0034]    The outer ring arrangement may be 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. The sudden increase in internal leakage may lead to a sudden increase in piston velocity when it is set up in its passive state or condition. 
         [0035]    The method may further comprise detecting when the sudden transition has occurred by detecting a sudden increase in piston velocity. The sudden increase in piston velocity may correspond to the predetermined amount referred to above. 
         [0036]    The method may further comprise outputting said health status to an aircraft computer. In response to the health status, the aircraft computer may cease or reduce operation of the actuator, for example to avoid damage to the actuator or seal. The method may further comprise outputting said health status to a display or user interface. The actuator may have an indicator associated with it, such as a light or mechanical pointer, and the method may further comprise causing said indicator to change, for example the light to change colour or the mechanical pointer to change position, in response to the health status of the actuator or seal. 
         [0037]    The seal may be an abradable seal and/or may be an annular seal located around a circumference of the piston, and may provide a sealing function, for example to prevent substantial fluid transfer between two chambers defined or separated by the piston. 
         [0038]    The actuator may be used in an aircraft, for example to control one or more flight control surfaces of the aircraft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    Various embodiments will now be described, by way of example only, and with reference to the accompanying drawings in which: 
           [0040]      FIG. 1  shows an actuator assembly; 
           [0041]      FIG. 2  is an illustration showing a method in accordance with the present disclosure; 
           [0042]      FIG. 3  shows an exploded, perspective view of an abradable seal; 
           [0043]      FIG. 4  shows an axial cross-section through the abradable seal of  FIG. 3 ; 
           [0044]      FIG. 5  shows a circumferential cross-section of the abradable seal of  FIG. 1 ; and 
           [0045]      FIG. 6  shows an axial cross-section through the abradable seal of  FIG. 1 , once the seal has been worn in use. 
       
    
    
     DETAILED DESCRIPTION 
       [0046]    The present disclosure relates generally to a method or methods for monitoring the health of an abradable seal located on an actuator piston. Examples of such abradable seals will also be described, although the methods are applicable to other abradable seals not described in detail herein. 
         [0047]    It will be appreciated that the functional steps of the methods disclosed herein may be performed by hardware, software and/or firmware components, for example, that may be configured to perform the specified steps. For the sake of conciseness, conventional techniques that are known to the person skilled in the art by way of their common general knowledge may not be described in detail herein 
         [0048]    The methods and apparatus described herein have applications in, for example, aerospace and in particular actuators for aircraft components. Such actuators are frequently serviced and/or inspected and the methods described herein may reduce the need for servicing and/or inspecting, especially in relation to the seals of pistons used in such actuators, which are typically difficult to inspect. However, the disclosure is generally applicable to other applications outside of the aerospace industry. 
         [0049]    Taking the example of an actuator for use in an aircraft, such actuators may be employed to actuate flight control surfaces, such as elevators and ailerons. Typically a number of actuators will be employed for such purposes. 
         [0050]    An actuator assembly is shown in  FIG. 1  and may comprise a piston  10  and a cylinder  20  within which the piston  10  moves. The piston  10  may separate two chambers, e.g. first and second chambers  22 ,  23 , that vary in volume depending on the position of the piston  10 . The assembly may comprise an annular abradable seal  100  that provides a sealing function to prevent substantial fluid transfer between the chambers  22 ,  23 . 
         [0051]    The actuator assembly may comprise a hydraulic system  30  to control the flow of fluid into the chambers  22 ,  23 . The hydraulic system  30  may comprise a valve  32 , for example a servo valve, that controls the flow of fluid into the chambers  22 ,  23 . 
         [0052]    In order to extend the actuator, for example, the valve  32  may receive hydraulic fluid from an inlet  34 , pump this into the first chamber  22  via a first line  25 , which in turn forces hydraulic fluid out of the second chamber  23  to an outlet  36  via a second line  26  and valve  32 . In order to retract the actuator, for example, the valve  32  may operate in reverse by receiving hydraulic fluid from the inlet  34 , pumping this into the second chamber  23 , which forces hydraulic fluid out of the first chamber  22  to the outlet  36  via first line  25  and valve  32 . 
         [0053]    During use the abradable seal  100  will degrade and wear. An abradable seal referred to herein may generally and in broad aspects comprise an abradable outer ring or outer ring assembly, and an energiser for urging the outer ring against an opposing surface. Once the abradable outer ring is worn away this can expose the energiser to the opposing surface, which may act as a seal for a short period of time before the seal fails and the actuator may be no longer operational. An example of such an abradable seal is described in more detail below. Other types or configurations of abradable seal are possible and equally applicable to the methods disclosed herein. 
         [0054]    A computer, for example an aircraft computer or Flight Control Computer (“FCC”), may be configured to measure or monitor the velocity or speed of the piston during use. 
         [0055]    The present disclosure may relate to monitoring the piston speed in order to determine the leakage across the piston, or flow between the chambers  22 ,  23  at a particular point in time. This can enable the seal condition to be monitored throughout its life, since it may not be possible to monitor flow or leakage across the seal directly in use. 
         [0056]    The relationship between piston speed, v, piston section or area, S, and the present or instantaneous flow of the actuator, Q a  may be: 
         [0000]    
       
         
           
             
               
                 
                   v 
                   = 
                   
                     
                       Q 
                       a 
                     
                     S 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0057]    where Q a  can be made up of piston flow, Q p  (i.e. the flow caused by the piston head in, for example, cubic cm per minute) and leakage flow, Q l  across the seal, such that: 
         [0000]        Q   a   =Q   p   +Q   l    (2)
 
         [0058]    Therefore, piston speed and the internal leakage or flow across the seal may be directly correlated. As such, by monitoring piston speed it can be possible to monitor the internal leakage of the abradable seal  100 . The actuator can be set up in a passive state to keep the other variables constant. For example, the control valve  32  may be set into a null position (Q p =0) such that, for example, there is no additional flow into the chambers  22 ,  23 . The load on the actuator can be set to be the passive mass of the component that the actuator is connected to, or it may be (somehow) released from the component. The actuator can be set to be at its extension stop (e.g. to the extreme left or right in  FIG. 1 ). It is possible to set the actuator up with some or all of these conditions at any point in its life cycle. 
         [0059]    A passive state or condition as disclosed herein may be one in which the flow of fluid in the actuator is caused by leakage flow across the abradable seal (e.g. Q p =0). As such, any movement experienced by the piston may be due only to the leakage flow across the seal. 
         [0060]    In accordance with the disclosure, therefore, the actuator can be periodically set up in a passive state and the piston speed may be measured, so as to monitor the internal leakage across the abradable seal over time. This can give an efficient and accurate indication of the internal leakage throughout the life cycle of the seal. Moreover, once the piston speed increases above a predetermined amount then a health status, for example a warning can be outputted, and/or recorded by a computer, for example an aircraft computer or FCC. 
         [0061]    This method can have advantages over methods that monitor, for example, valve travel since it may focus specifically on the leakage across the abradable seal  100 . In other words, the only variable that may change when setting up the actuator in its passive state may be the leakage flow across the seal, as discussed above. Leakage variations in respect of the control valve  32 , for example, may not affect the health status of the abradable seal  100 . As such, the method of the present disclosure may give a specific health status of leakage across the abradable seal  100 , that may not be affected by other leakage within the actuator. 
         [0062]      FIG. 2  is an illustration of the process  200  according to the disclosure. Process  200  may begin by measuring an initial velocity of the actuator piston while the actuator is optionally maintained in a passive state or condition at an initial time (step  202 ). 
         [0063]    The actuator may then be operated for a period of time (step  204 ). At a subsequent time, a subsequent velocity of the actuator piston may be measured, again while the actuator is optionally maintained in a passive condition (step  206 ). 
         [0064]    Steps  204  and  206  may be repeated and the measured subsequent velocities may be recorded over a predetermined period of time, and/or a health status of the actuator may be determined, for example when the subsequent velocity has increased above a predetermined amount (step  208 ). 
         [0065]    An example of an abradable seal  100  that may be used in the method described herein will now be described with reference to  FIG. 3 . 
         [0066]      FIG. 3  shows the piston  10  that is arranged to move axially inside the cylinder  20  (shown in  FIGS. 1 and 5 ). 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. 
         [0067]    The 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. 3  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 . 
         [0068]    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 . 
         [0069]    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. 
         [0070]    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. 
         [0071]    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  FIGS. 4, 5 and 6 . 
         [0072]      FIG. 4  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. 
         [0073]    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. 5 ), 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. 
         [0074]      FIG. 5  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 . 
         [0075]    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 . 
         [0076]    It can be seen that, in the illustrated embodiment of  FIG. 4  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  153  represents a void in the outer ring arrangement, which in the illustrated embodiment comprises the outer ring  120  and the ancillary ring  150 . 
         [0077]    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. 
         [0078]    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. 
         [0079]    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. 
         [0080]      FIG. 6  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 . 
         [0081]    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. 
         [0082]    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. 6  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. 
         [0083]    Referring back to  FIGS. 1 and 3 , it will be appreciated that the piston  10  separates two chambers  22 ,  23  which are located either side of the flange  14 , and the abradable seal  100  is configured to prevent fluid transfer between the chambers  22 ,  23  as the piston  10  moves back and forth along the longitudinal axis  5  of the shaft  12 . 
         [0084]    Typically, however, leakage exists between the two chambers  22 ,  23  and the amount of leakage is dependent in part on the surface area of the sealing surface. Leakage between chambers  22 ,  23  may be referred to as “internal leakage”. Internal leakage and sealing surface area generally have a negative correlation with respect to each other. 
         [0085]    In this embodiment, there is provided 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. 
         [0086]    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. 
         [0087]    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 . 
         [0088]    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 
                   ) 
                 
               
             
           
         
       
     
         [0089]    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. 
         [0090]    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 
                         a 
                       
                       ≈ 
                       
                         
                           
                             ( 
                             
                               1 
                               - 
                               α 
                             
                             ) 
                           
                            
                           
                             
                               
                                 K 
                                  
                                 
                                   ( 
                                   oil 
                                   ) 
                                 
                               
                                
                               
                                 j 
                                 3 
                               
                             
                             L 
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           P 
                         
                         + 
                         
                           α 
                            
                           
                             
                               
                                 K 
                                  
                                 
                                   ( 
                                   oil 
                                   ) 
                                 
                               
                                
                               
                                 j 
                                 3 
                               
                             
                             
                               2 
                                
                               
                                 L 
                                 B 
                               
                             
                           
                            
                           Δ 
                            
                           
                               
                           
                            
                           P 
                         
                       
                     
                     = 
                     
                       
                         [ 
                         
                           1 
                           + 
                           
                             α 
                              
                             
                               ( 
                               
                                 
                                   L 
                                   
                                     2 
                                      
                                     
                                       L 
                                       B 
                                     
                                   
                                 
                                 - 
                                 1 
                               
                               ) 
                             
                           
                         
                         ] 
                       
                        
                       
                         Q 
                         n 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                       
                   
                    
                   where 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   
                       
                   
                    
                   
                     α 
                     = 
                     
                       ne 
                       
                         π 
                          
                         
                             
                         
                          
                         D 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    and where Q represents the flow between the chambers, Q n  represents the flow during normal operation (see (1)), 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 . 
         [0091]    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 . 
         [0092]    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. 
         [0093]    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. 
         [0094]    The present disclosure allows an instant discovery of the transition between normal and abnormal operation. 
         [0095]    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. 
         [0096]    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.