Abstract:
A screw ( 20 ) assembly for an actuator ( 10 ) is described comprising: a screw ( 20 ); a nut ( 22 ) threaded on said screw ( 20 ), such that rotation of said screw ( 20 ) causes axial movement of said nut ( 22 ); a stop located at an end of said screw ( 20 ) and defining an axial limit of said nut ( 22 ); a first feature located on said nut ( 22 ); and a second feature located on said stop; wherein said first and second features are configured to cooperate with one another substantially upon contact of said nut ( 22 ) with said stop so as to indicate an amount of free movement between said nut ( 22 ) and said screw ( 20 ).

Description:
BACKGROUND 
       [0001]    A horizontal stabiliser is an aircraft component that stabilises the airframe during flight. Typically, they are found at the rear of the fuselage, and incorporate a moveable surface for adjustment in response to trim signals from the pilot. The moveable surfaces are controlled by an actuator, known as a trimmable horizontal stabiliser actuator (“THSA”). This is controlled by a hydraulic and/or electric motor that is connected to the aircraft fuselage. 
         [0002]    The THSA comprises a number of moving components, including a rotating screw, rotation of which is caused by the hydraulic and/or electric motors. This rotating screw causes axial movement of a nut, which in turn drives the moveable surface of the horizontal stabiliser. 
         [0003]    The nut of the THSA moves between two opposite end stops and contacts these stops when the actuator moves to its opposite extremities. The nut will wear during use and this can be a serious issue if the wear is serious enough. Detection of the failure of the nut due to wear is essential for safety and can be critical. 
         [0004]    Furthermore, an overstroke check may be made when the actuator is installed on an aircraft. The overstroke check involves recording when the nut of the actuator reaches its opposite extremities, and noting this for tolerance and precision checks. 
         [0005]    It is desired to provide improved methods for detecting the wear of a screw assembly used in an actuator of a trimmable horizontal stabiliser actuator. 
       SUMMARY 
       [0006]    In accordance with an aspect of the disclosure, there is provided a screw assembly for an actuator, comprising: 
         [0007]    a screw, for example a ballscrew; 
         [0008]    a nut threaded on the screw, such that rotation of the screw causes axial movement of the nut; 
         [0009]    a stop located at an end of the screw and defining an axial limit of the nut; 
         [0010]    a first feature located on the nut; and 
         [0011]    a second feature located on the stop; 
         [0012]    wherein the first and second features are configured to cooperate with one another substantially upon contact of the nut with the stop so as to indicate an amount of free movement between the nut and the screw. 
         [0013]    This ensures that the free movement, which is otherwise known as backlash or clearance, can be easily determined or checked upon an inspection of the cooperation of the first and second features upon contact of the nut with the stop, or when the nut is close to contacting the stop. 
         [0014]    As used herein, “substantially upon contact” may be interpreted as close to contact or near to contact, such that the cooperation of the first and second features can be determined, for example visually. Optionally, the first and second features may be configured to cooperate with one another upon contact of the nut with the stop so as to indicate an amount of free movement between the nut and the screw. 
         [0015]    The actuator may be for use in an aircraft, and may be an actuator for driving a horizontal stabiliser of an aircraft. 
         [0016]    The first feature may comprise a first visual or tactile feature, for example a first mark, and the second feature may comprise a second visual or tactile feature, for example a second mark. The first or second tactile features may comprise a notch or protrusion. 
         [0017]    The first and second features may be positioned or configured such that their relative positions substantially upon contact of the nut with the stop provide a measure of free movement between the nut and the screw. 
         [0018]    A limit of acceptable free movement between the nut and the screw may be determined. The first and second features may be configured such that, upon contact of the nut with the stop, the first and second features substantially align with one another if the free movement between the nut and the screw is within the limit, and optionally do not substantially align with one another if the free movement between the nut and the screw is outside of the limit. 
         [0019]    A rotational position of the first and/or second feature upon contact of the nut with the stop may be adjustable, for example by adjusting the rotational position of the stop with respect to the nut. 
         [0020]    One of the nut and stop may comprise a first lug configured to contact a lug on the other of the nut and stop to define an initial axial limit of the nut. 
         [0021]    The one of the nut and stop may comprise a second lug configured to contact the lug on the other of the nut and stop before the first lug upon a predetermined increase in free movement between the nut and the screw. 
         [0022]    The contact between the second lug and the lug on the other of the nut and stop may define a subsequent, different axial limit of the nut. The subsequent axial limit may be lower or shorter, or closer to the centre of the screw than the initial axial limit. 
         [0023]    The first and second lugs may be located at the same radial position, but different circumferential positions on the one of the nut and stop. 
         [0024]    The first and second lugs may be located at different circumferential positions on the one of the nut and stop, and the rotational position of the nut and/or the stop may be configured such that a predetermined clearance is provided between the second lug and the lug on the other of the nut and stop in the final turn of the screw. 
         [0025]    The predetermined clearance may correspond to a predetermined limit of acceptable free movement between the nut and the screw, such that the first lug may contact the lug on the other of the nut and stop if the free movement between the nut and the screw is within the limit, and the second lug may contact the lug on the other of the nut and stop if the free movement between the nut and the screw is outside of the limit. 
         [0026]    The predetermined clearance, limit or increase in free movement between the nut and the screw may be due to a predetermined or predicted wear of the nut and/or screw during use. The predetermined clearance may be calculated from a predetermined or predicted wear of said nut and/or screw during use. 
         [0027]    The nut may comprises the first and second lugs, and the first and/or second lugs may constitute or comprise the first feature, and the lug on the stop may constitute or comprise the second feature; or 
         [0028]    the stop may comprise the first and second lugs, and the first and/or second lugs may constitute or comprise the second feature, and the lug on the stop may constitute or comprise the first feature. 
         [0029]    The first feature and the second feature may cooperate such that, upon contact of said nut with said stop, said first and second features present a visual or tactile confirmation or measure of free movement between the nut and the screw. 
         [0030]    In accordance with an aspect of the disclosure, there is provided an actuator for an aircraft comprising a screw assembly as claimed in any preceding claim. 
         [0031]    In accordance with an aspect of the disclosure, there is provided a method comprising: 
         [0032]    providing a screw assembly for an actuator, said screw assembly comprising:
       a screw;   a nut threaded on said screw, such that rotation of said screw causes axial movement of said nut; and   a stop located at an end of said screw and defining an axial limit of said nut;   wherein said stop or said nut comprises a first lug and a second lug;       
 
         [0037]    marking said nut and/or said stop so as to indicate an amount of free movement between said nut and said screw upon contact between said nut and said stop. 
         [0038]    In accordance with an aspect of the disclosure, there is provided a method comprising: 
         [0039]    providing a screw assembly for an actuator, the screw assembly comprising:
       a screw;   a nut threaded on the screw, such that rotation of the screw causes axial movement of the nut; and   a stop located at an end of the screw and defining an axial limit of the nut;   wherein the stop or the nut comprises a first lug and a second lug;       
 
         [0044]    locating and/or positioning the first lug and the second lug such that the nut or stop contacts the first lug upon rotation of the screw to define an initial axial limit of the nut, but does not contact the second lug upon rotation of the screw; 
         [0045]    determining an acceptable amount of free movement between the nut and the screw in an axial direction; 
         [0046]    locating and/or positioning the second lug such that the second lug contacts the nut or stop before the first lug to define a subsequent axial limit of the nut once the free movement between the nut and the screw reaches or exceeds the acceptable amount. 
         [0047]    In accordance with an aspect of the disclosure, there is provided a screw assembly for an actuator, comprising: 
         [0048]    a screw; 
         [0049]    a nut threaded on the screw such that rotation of the screw causes axial movement of the nut; and 
         [0050]    a stop located at an end of the screw and defining an axial limit of the nut; 
         [0051]    wherein one of the nut and stop comprises a first lug configured to contact a lug on the other of the nut and stop to define an initial axial limit of the nut, and the one of the nut and stop comprises a second lug configured to contact the lug on the other of the nut and stop before the first lug upon a predetermined increase in free movement between the nut and the screw. 
         [0052]    The first lug may comprise a first visual or tactile feature, for example a first mark, and the second lug may comprise a second visual or tactile feature, for example a second mark. The first or second tactile features may comprise a notch or protrusion. The lug on the other of the nut and stop may comprise a visual or tactile feature that is the same as, or different to the first visual or tactile feature and/or the second visual or tactile feature. The first visual or tactile feature may be different to the second visual or tactile feature. The second visual or tactile feature may comprise a warning sign. 
         [0053]    In accordance with an aspect of the disclosure, there is provided a screw assembly for an actuator, the screw assembly comprising a screw, a nut threaded on the screw such that rotation of the screw causes axial movement of the nut, and a plurality of stops located at or near an end of the screw, each comprising a different axial limit of the nut; 
         [0054]    wherein optionally the plurality of stops are located or configured such that the nut will contact different ones of the stops based on the amount of free movement that exists between the nut and the screw. 
         [0055]    The plurality of stops may be located circumferentially about an end of the nut, or on an object located near the axial limit of the nut, and optionally at the same radial position. The plurality of stops may be located such that the nut will contact different ones of the stops as the free movement between the nut and the screw increases over time. The plurality of stops may be differentiable from one another, for example the plurality of stops may comprise different visual or tactile features. 
         [0056]    The features of any of the above aspects of the disclosure may be combined with the features of any of the other aspects of the disclosure described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0057]    Various embodiments will now be described, by way of example only, and with reference to the accompanying drawings in which: 
           [0058]      FIG. 1  shows a horizontal stabiliser actuator; 
           [0059]      FIGS. 2A-2C  show cross sections of the screw/nut arrangement of the actuator; 
           [0060]      FIG. 3  shows a flow diagram of an overstroke check; 
           [0061]      FIG. 4  shows the screw/nut arrangement of the actuator; 
           [0062]      FIGS. 5A and 5B  show the backlash between the nut and the screw of the actuator. 
       
    
    
     DETAILED DESCRIPTION 
       [0063]      FIG. 1  shows a horizontal stabiliser actuator  10 . 
         [0064]    The actuator  10  is connected to a fuselage structure  12  of an aircraft (not shown). A moveable component  14  is driven by the actuator  10 . Specifically, the actuator  10  comprises a mechanical screw assembly comprising a screw  20 , as well as a nut  22  threaded to the screw  20  and coupled to the moveable component  14 . The screw  20  may be fixed in its longitudinal, or axial direction, such that rotation of said screw  20  causes the nut  22  to move up and down its length in the longitudinal, or axial direction. This in turn drives the moveable component  14  as required. 
         [0065]    The screw  20  may be a ballscrew such that movement of said nut  22  is caused due to the presence of balls (not shown) between the nut  22  and the shaft of the screw  20 . 
         [0066]    The screw  20  is rotated by means of one or more motors  16  that drive a gear assembly  18 , which gears cause said screw  20  to rotate. Upon rotation of the screw  20 , the nut  22  moves between an upper stop  24  and a lower stop  26 . The upper stop  24  and the lower stop  26  define the limits of the nut  22  and are rotatable with the screw  20 . 
         [0067]      FIGS. 2A-2C  show the mechanical screw arrangement of the actuator  10 . 
         [0068]      FIG. 2A  shows the nut  22  in its maximum retracted position, i.e. as it is retracted against the upper stop  24 , and a retracted distance  30  is defined between the nut  22  and its attachment  28  to the fuselage  12 . 
         [0069]      FIG. 2B  shows the nut  22  in its maximum extended position, i.e. as it is retracted against the lower stop  26 , and an extended distance  32  is defined between the nut  22  and its attachment  28  to the fuselage  12 . 
         [0070]      FIG. 2C  illustrates a differential distance  34  that the nut  22  moves as it travels from its maximum retracted position to its maximum extended position. 
         [0071]    The differential distance  34  may be known as the maximum stroke of the nut  22  and/or the actuator  10 . The maximum stroke includes what is known as an “overstroke”, which is a portion of the stroke just preceding the contact between the nut  22  and the upper stop  24  and/or lower stop  26 . During normal operation, the nut  22  is not extended into the overstroke. Thus, the overstroke is present to avoid contact between the nut  22  and the upper stop  24  and/or lower stop  26 . 
         [0072]    The overstroke is calculated to take into account a given amount of free movement between the nut  22  and the screw  20 . This free movement is otherwise known as backlash, or clearance, and is due to, for example, the free axial movement of the nut relative to the screw shaft. This might be caused in part by the movement of balls that may be present between the nut and the shaft of the screw (in the form of a ballscrew). Thus, the overstroke value is chosen such that during normal operation the nut  22  cannot contact the upper stop  24  and/or lower stop  26  due to this free movement. 
         [0073]      FIG. 3  shows a flow diagram corresponding to the steps an aircraft computer might take to check and verify the overstroke. 
         [0074]    To calculate the overstroke at a given point in time (“test overstroke”), a first initiated built-in test (“IBIT”) may be performed by the aircraft computer. A first step  3012  orders extension of the actuator  10  until the nut  22  contacts the lower stop  26  at  3013 , and the position of the nut  22  in this maximum extended position is stored in memory  3014 . The actuator  10  may then be retracted until the nut  22  contacts the upper stop  24  at  3015 , and the position of the nut  22  in this maximum retracted position may be stored in memory  3017 . An overstroke value is calculated at  3018  and output as the test overstroke. 
         [0075]    A procedure  3021  may be carried out to provide a reference value for overstroke when an actuator  10  is fitted (“reference overstroke”). The reference overstroke may be calculated at  3022  using the same procedure as in steps  3012 - 3018 . This reference overstroke may be stored in memory at  3023 . 
         [0076]    An overstroke comparison  3050  may be made between the reference overstroke and the test overstroke at a given point in time. This comparison outputs the difference between the reference overstroke and the test overstroke, and outputs state corresponding to the outcome of this comparison. For example, if this difference is lower than a given value then the outcome of the test may be positive, indicating that the current overstroke is within predefined limits. 
         [0077]    If the difference is higher that a given value then the outcome may be negative, indicating that the nut  22  travel is shorter than previously. This may be due, for example, an increase in the free movement between the nut  22  and the screw  20  that causes the nut  22  to contact the upper stop  24  and/or lower stop  26  earlier in the stroke. In such a case, an inspection of the actuator may be required to check for wear. 
         [0078]      FIG. 4  shows in more detail the nut  22  of the present disclosure. The nut  22  comprises a primary nut body  221 , which forms the main part of the nut  22 . A secondary nut body  222  is provided to act as a failsafe and provide a force to the moveable component  14  in case the primary nut body  221  fails, for example due to failure of the thread between the nut  22  and the primary nut body  221 . Fuse pins  223  are provided between the primary nut body  221  and the secondary nut body  222 . It is important to be able to provide a measure of the wear of the screw assembly during use, and this is the aim of the present disclosure. 
         [0079]      FIG. 5A  shows a schematic of the lower end stop  26  and the nut  22 . It will be appreciated that the upper end stop  24  may have the same features as described below for the lower end stop  26 . The nut  22  is shown in solid line at the start of the overstroke, and in a dotted line at the end of the overstroke. The screw (not shown), and hence the lower end stop  26 , rotates a given number of times, for example in the illustrated case 1.5 times, between the start of the overstroke and the end of the overstroke. 
         [0080]    The lower end stop  26  comprises a lug  50  that provides the contact surface for the nut  22 . The axial travel of the nut  22  corresponding to a full turn of the screw may be less than the height of the lug  50 , ensuring that the lug  50  is optionally the only contact surface of the lower end stop  26  with the nut  22 . 
         [0081]    The nut  22  comprises a first lug  52  and a second lug  54 . The first lug  52  comprises a mark  53  and is configured to contact the lug  50  of the lower end stop  26  to define the initial axial limit of the nut  22 . The second lug  54  does not comprise a mark, or may be marked differently, such that the first lug  52  and the second lug  54  are visually differentiable from one another. 
         [0082]      FIG. 5B  shows a schematic of the lower end stop  26  and the nut  22  in the middle of the overstroke, that is after 1 turn from the start. The dimensions are exaggerated and the schematic is not to scale. At this point in the rotation of the screw and lower end stop  26  the second lug  54  is travelling past the lug  50  of the lower end stop  26  and will not contact it due to the axial clearance  55  between the lug  50  and the second lug  54 . It is apparent that if this axial clearance  55  is reduced then the second lug  54  will contact the lug  50  on the lower end stop  26  instead of, and prior to the first lug  52 . 
         [0083]    Upon an increase in free movement, or backlash between the nut  22  and the screw the axial clearance  55  shown in  FIG. 5B  will decrease. Therefore, the axial clearance  55  may be configured such that it corresponds to an acceptable amount of free movement between the nut  22  and the screw. Once the free movement between the nut  22  and the screw falls below this acceptable amount, the second lug  54  will contact the lug  50  on the lower end stop  26  instead of, and prior to the first lug  52 . 
         [0084]    This means that instead of the mark  53  on the first lug  52  showing next to the mark  51  on the lug of the lower end stop  26 , there will be no mark showing. This allows a very easy indication of the amount of free movement between the nut and the screw. For example, an inspector can run the screw assembly to the end of the overstroke, and check whether the marks are aligned, or not. 
         [0085]    The axial clearance  55  may be adjustable by adjusting the rotational position of the nut  22 , or by adjusting the rotational position of the lower end stop  26 . For example, the lower end stop  26  could be fixed to the screw by means of splines, and the lower end stop  26  could be removed from the screw assembly, rotated, and then inserted back into the screw assembly at a different rotational position. 
         [0086]    It is possible to vary the structure described above whilst achieving the result of providing an indication of the amount of free movement between the nut and the screw using features provided on the nut and the stop. 
         [0087]    It will be appreciated that the first and second lugs could be provided on the stop, rather than the nut, and a cooperating lug could be provided on the nut. 
         [0088]    Instead of providing two lugs on the nut, for example, a single lug having a grading could be provided. A corresponding or cooperating mark could be provided on the lug on the stop such that the amount of free movement is indicated by the point at which the mark stops on the grading. 
         [0089]    A plurality of lugs could be provided on the nut and/or stop, wherein the amount of free movement between the nut and the screw could be indicated by which of the plurality of lugs contacts a cooperating lug on the other of the nut and/or stop. 
         [0090]    It will be appreciated that further arrangements are contemplated that achieve the effect of indicating an amount of free movement between said nut and said screw using features provided on the nut and the stop, and 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 without departing from the scope of the accompanying claims.