Abstract:
Aircraft landing gear comprising a locking assembly including a pair of spaced-apart bearing pads and a lock member. The lock member is moveable between a locked position in which it engages the bearing pads to prevent pivotal movement of a steerable axle, and an unlocked position in which the member is disengaged from the bearing pads such that movement of the axle can occur. The lock member has a guide surface on either side for guiding the member and bearing pads into alignment during movement of the member from the unlocked to the locked position. The guide surface comprises a first inclined portion for moving the member towards alignment with the bearing pads and a second inclined portion, spaced apart from the first portion, for moving the member into alignment with the bearing pads such that the member and bearing pads are aligned in a movement comprising two stages.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention concerns aircraft landing gear. More particularly, but not exclusively, this invention concerns multi-wheel landing gears comprising at least one steerable axle. The invention also concerns a locking mechanism for a multi-wheel landing gear bogie including a steerable axle. 
         [0002]    Multi-wheel landing gears are often used on large aircraft, for example the Airbus A380. The multi-wheel landing gears are used to support the large loads exerted on the landing gears by the aircraft when taxiing, at take-off, and landing. In order to prevent excess tyre wear caused by tyre scrubbing during taxiing, the multi-wheel landing gear may include at least one pair of wheels mounted on a steerable axle. At other times, for example during take-off and landing, it is necessary to lock the steerable axle in the straight ahead position. 
         [0003]    Typically, in a multi-wheel landing gear, the steerable axle is pivotally mounted to the landing gear bogie. The steerable axle is steerable by an actuator which may extend and retract in order to change the orientation of the steerable axle. In a known arrangement, the landing gear includes a locking mechanism that comprises a wedge-shaped lock member that is pivotally mounted to the landing gear bogie. The wedge-shaped lock member is arranged to be received between two bearing pads mounted on the steerable axle in order to lock the steerable axle in a ‘straight ahead’ position for taxi, take off, and landing. The wedge-shaped lock member is secured in the locked position by two springs that act to pull the wedge-shaped lock member into engagement with the bearing pads. An actuator is arranged to act against the springs in order to move the wedge-shaped lock member from away from the bearing pads, such that the steerable axle is unlocked and pivotal movement, away from the straight ahead position, is allowed. 
         [0004]      FIG. 1  shows a schematic cross-sectional view of this known locking mechanism  1 .  FIG. 1  shows a portion of the steerable axle  2  having a recess  4  containing two bearing pads  6 . The two bearing pads  6  are located on opposite sides of the recess  4  and face each other. Each bearing pad is fixed to the axle  2  using a fixing  8 . In the locked position as shown in  FIG. 1 , a lock member  10  is located in the space between the bearing pads  6 . The far end  6   a  of each bearing pad  6  is hooked to prevent movement of the lock member  10  deeper into the space between the bearing pads  6 . Each bearing pad  6  is in contact with a corresponding side of the lock member  10 . The front corners  10   a  of the lock member  10  are bevelled giving the lock member  10  a wedge shape. 
         [0005]    Time and/or skill may be required to bring the lock member into sufficient alignment with the bearing pads so that it can move into engagement with the bearing pads. If the lock member  10  is not very closely lined up with the bearing pads  6  when attempting to lock the steerable axle  2 , the lock member  10  may not be able to move into the space between the bearing pads  6 . In that case, the steerable axle  2  would not be fully locked, and the aircraft would not be able to take off until the problem is fixed; this may cause delays to the aircraft departure time or the grounding of the aircraft. 
         [0006]    The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved landing gear mechanism. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides, according to a first aspect, an aircraft landing gear comprising a bogie, a steerable axle pivotally mounted on the bogie, and a locking assembly, the locking assembly comprising a pair of spaced-apart bearing pads associated with the axle and a lock member associated with the bogie, the lock member being moveable between a locked position in which the member is located in the space between the bearing pads such that the lock member engages the bearing pads to prevent pivotal movement of the steerable axle with respect to the bogie, and an unlocked position in which the lock member is disengaged from the bearing pads such that pivotal movement of the axle with respect to the bogie can occur, wherein the lock member has a guide surface on either side for guiding the lock member and the bearing pads into alignment during movement of the lock member from the unlocked position to the locked position, and wherein the guide surface comprises a first inclined portion for moving the lock member towards alignment with the bearing pads in a first stage, and a second inclined portion, spaced apart from the first portion, for moving the lock member into alignment with the bearing pads in a second stage, such that the lock member and bearing pads are aligned in a movement having two stages, for example a two-stage movement. 
         [0008]    Aligning the lock member and the bearing pads in a staged movement has been found to reduce the risk of the lock member failing to engage with the bearing pads. Furthermore, aligning the pads in a two (or more)-stage movement using first and second inclined regions tends to facilitate an increase in the capture angle (that is to say the degree of initial misalignment between the lock member and the bearing pads that the locking assembly can accommodate while still successfully engaging). Increasing the capture angle may reduce the number of unsuccessful attempts at engaging the locking mechanism and/or reduce the time spent positioning the steerable axle prior to attempting to engage the locking mechanism. The misalignment angle may be defined as the angular difference in position between a given point on the lock member in the unlocked position and in the locked position. The capture angle may be defined as the maximum initial misalignment angle that the locking mechanism can accommodate while still engaging. 
         [0009]    When the lock member is in the locked position the lock member and the bearing pads may be said to be aligned. The degree of misalignment between the lock member and the bearing pads may be defined as the rotation of the lock member relative to the bearing pads away from this aligned position. The landing gear may be arranged such that when the lock member is in the locked position, that is to say when the lock member and the bearing pads are aligned, the steerable axle is in the straight ahead position. The bearing pads may be associated with the axle such that pivoting of the axle results in rotation of the bearing pads and vice versa. Accordingly, aligning the lock member and the bearing pads may move the steerable axle into the straight ahead position. Thus, embodiments in accordance with the presence invention may allow the steerable axis to be moved into the straight ahead position in two stages. 
         [0010]    Movement of the lock member from the unlocked position to the locked position may comprise a movement of the lock member relative to the bogie, for example a pivoting of the lock member about one end. In the case that the lock member is not precisely lined up with the bearing pads at the start of the locking movement, it may be necessary to move the lock member into alignment with the bearing pads such that the lock member can engage the bearing pads at the end of the locking movement. Moving the lock member and the bearing pads into alignment may comprise a rotation of the bearing pads (and the steerable axle associated therewith) relative to the lock member and/or the bogie. The alignment movement may be caused by the interaction between a guide surface and the respective bearing pad. Using the interaction between the guide surface and the respective bearing pad to bring the lock member and the bearing pads into alignment may provide a mechanically simple and reliable method of bringing the steerable axle into the straight ahead position. 
         [0011]    The contact between the surface of a first inclined portion and the corresponding bearing pad may produce a first relative movement of the lock member and the bearing pads which reduces the misalignment angle. The contact between the surface of a second inclined portion and the corresponding bearing pad may produce a second relative movement of the lock member and the bearing pads which brings the bearing pads and the lock member into alignment. 
         [0012]    A guide surface may guide the lock member and the bearing pads into alignment through contact between the guide surface and the bearing pads. The guide surface preferably comprises the first and second inclined portions. It will be appreciated that the profile of the guide surface may vary in dependence on the profile of the corresponding bearing pad while still providing a movement comprising two stages. 
         [0013]    The guide surface may comprise a plurality of portions, each portion having a different profile to any adjacent portion. The profile of a portion may be defined in terms of the angle of inclination of the guide surface in that region with respect to the direction of travel of the lock member relative to the bogie when the lock member travels between an unlocked position and the locked position (referred to hereafter as the direction of the locking movement). Thus, the surface of the first and/or second inclined portion may be inclined at an angle (i.e. non-parallel and non-perpendicular) to the direction of the locking movement. It may be that contact between the surface of an inclined portion and the corresponding bearing plate produces a reactive force having a component perpendicular to the direction of the locking movement. 
         [0014]    The first and/or second inclined portion may be inclined such that the width of the lock member increases with distance along the length of that portion. The profile of the two first inclined portions may result in a tapering of the lock member towards the front of the lock member at a first tapered region. The profile of the two second inclined portions may result in a tapering of the lock member towards the front of the lock member at a second tapered region. The first tapered region may be located forward of the second tapered region. 
         [0015]    The profile of the first inclined portion may differ from the profile of the second inclined portion. The gradient of the first inclined portion may differ from the gradient of the second inclined portion. The gradient of an inclined portion may vary along the length of that portion. The gradient of the first inclined portion may vary in a different manner to the gradient of the second inclined portion. That is to say, the rate of change of the gradient with distance along the length of the portion may differ as between the first and second inclined portions. The gradient of the first and/or second inclined portion may be constant along the length of that portion. The gradient of the first and second inclined portions may be constant along the length of each portion, with each portion having a different gradient. 
         [0016]    Each guide surface may include an intermediate portion located between the first inclined portion and the second inclined portion. Said intermediate portion may be orientated such that, during movement of the lock member from the unlocked position to the locked position, the intermediate portion does not cause any substantial movement to align the lock member and the bearing pads. The intermediate portions (on each guide surface) may be substantially parallel. During the locking process oscillatory forces originating at the wheels may cause the bearing pads to move relative to the lock member such that the contact between a guide surface and the respective bearing pad produces a force that acts to push the lock member out of the space between the bearing pads (hereafter referred to as an unlocking force). The magnitude of these forces may be increased when the inclination of the guide surface increases. Including the intermediate portions described above, may therefore allow a reduction in the unlocking forces generated during the locking process in comparison with prior art lock members. 
         [0017]    The first and second inclined portions may be spaced apart by an intermediate portion. The surface of the intermediate portion may be substantially parallel to the direction of the locking movement. It will be appreciated that an intermediate portion is not necessarily parallel to the direction of motion of the lock member but may be substantially less inclined that the first and/or second inclined portion. 
         [0018]    The lock member may include a stop on either side, the stop being arranged to abut the respective bearing pad when the lock member is in the locked position such that further movement of the lock member into the space between the bearing pads is prevented. Each stop may be in the form of a protrusion on the side of the lock member. A stop may have a front surface orientated perpendicular to the direction of motion of the lock member relative to the bogie when the lock member moves from the unlocked position to the locked position. 
         [0019]    The lock member may be said to move in a forward direction when it moves relative to the bogie from the unlocked position to the locked position. The lock member may be said to move in a rearward direction when it moves relative to the bogie from the locked position to the unlocked position. The surface of the lock member facing in the forward direction may be referred to as the front of the lock member. The surface of the lock member facing in the rearward direction maybe referred to as the back of the lock member. 
         [0020]    The lock member may be arranged such that the guide surface is located forward of the respective stop on the lock member. Locating the guide surface forward of the respective stop has been found to improve load distribution within the locking assembly. It may be that the stops remain outside the space between the bearing pads when the lock member is in the locked position. 
         [0021]    It will be appreciated that the unlocked position is not a single position of the lock member relative to the bearing pads, but may be any one of a number of positions in which the axle is free to rotate depending on the angle of the steerable axle (and therefore the bearing pads) relative to the lock member and/or bogie. 
         [0022]    The locking assembly is preferably arranged such that in the locked position the lock member engages the bearing pads to prevent any significant pivotal movement of the axle with respect to the bogie in either direction (i.e. clockwise or anticlockwise). It will be appreciated that in practice there may be some play between the bearing pads and the lock member but any such movement will be limited to such an extent that the direction of motion of the aircraft is not effected and any wheel attached to the steerable axle remains in a straight ahead position. 
         [0023]    Each guide surface may include an intermediate portion located between the second inclined portion and the respective stop. Said intermediate portion may be orientated such that, during movement of the lock member from the unlocked position to the locked position, the intermediate portion does not cause any substantial movement to align the lock member and the bearing pads. Thus, it may be that each guide surface includes a first intermediate portion located between the first inclined portion and the second inclined portion, and a second intermediate portion located between the second inclined portion and the respective stop. The combination of one or more intermediate portions interspersed with the inclined portions along the length of the guide surface may facilitate designs of locking assembly that produce a two (or more)-stage movement to bring the lock member and bearing pads into alignment. 
         [0024]    It will be appreciated that the guide surface may include a transition region at the start or end of an inclined portion. In a transition region the gradient of the guide surface may change rapidly over a short distance in order to reconcile the profile of a first region (for example a first inclined region) and a second region (for example an intermediate region). The length of the transition region may be very much less than the length of an inclined region and/or an intermediate region. 
         [0025]    According to a second aspect of the invention, there is provided a lock member for use in a landing gear having a lock assembly in accordance with the first aspect. The lock assembly may comprise a guide surface on either side, each guide surface comprising a first inclined portion and a second inclined portion. 
         [0026]    The lock member may be symmetrical about a centre line. The bearing pads may be arranged symmetrically about the lock member. In the case where the lock member is symmetrical, an inclined portion may be angled with respect to the centre line of the lock member. 
         [0027]    The profile of the guide surfaces may be such that the width of the lock member increases with distance from the front of the lock member towards the back of the lock member. The lock member may have a first width at a first location on the guide surface. The lock member may have a second width at a second location, spaced apart from the first location in a rearward direction along the guide surface. The lock member may have a third width at a third location spaced apart from the second location a rearward direction along the guide surface. The second width may be wider than the first width. The third width may be wider than the second width. There may be a change in the rate of change of width with respect to distance in the rearward direction at a location between the first and second location and at a location between the second and third locations such that the guide surface produces a two-stage movement of the lock member and the bearing pads. 
         [0028]    According to a third aspect of the invention, there is provided an aircraft including a landing gear in accordance with the first aspect. 
         [0029]    According to a fourth aspect of the invention, there is provided a method of locking an aircraft landing gear using a locking assembly, the landing gear comprising a bogie and a steerable axle pivotally mounted on the bogie, wherein the locking assembly comprises a pair of spaced-apart bearing pads associated with the axle and a lock member associated with the bogie, the lock member having a guide surface on either side, the guide surface comprising a first inclined portion and a second inclined portion, the method comprising the steps of: moving the lock member from an unlocked position in which the lock member is disengaged from the bearing pads such that pivotal movement of the axle with respect to the bogie can occur, towards a locked position in which the lock member is located in the space between the bearing pads such that the lock member engages the bearing pads to prevent pivotal movement of the steerable axle with respect to the bogie, and while the lock member is moving from the unlocked position towards the locked position, bringing the lock member and the bearing pads into alignment using the guide surface. The step of bringing the lock member and the bearing pads into alignment may comprising a first stage of alignment produced by the interaction of the bearing pads and the first inclined portions, followed by a second stage of alignment produced by the interaction of the bearing pads and the second inclined portions. 
         [0030]    It will be appreciated that the first stage of alignment reduces the misalignment angle between the lock member and the bearing pads, while the second stage of alignment finishes the alignment process by bring the lock member and the bearing pads into alignment. 
         [0031]    It may be that the method further includes a step of steering the axle such that the degree of misalignment of the lock member and the bearing pads is less than or equal to a predetermined capture angle, before moving the lock member from the unlocked position towards the locked position. The predetermined capture angle may be less than 10 degrees, for example less than 5 degrees but greater than 1 degree. 
         [0032]    It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0033]    Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 
           [0034]      FIG. 1  shows a cross-sectional schematic view of a prior art locking assembly; 
           [0035]      FIG. 2  shows a landing gear according to a first embodiment of the invention; 
           [0036]      FIG. 3  shows close-up view of a portion of the locking assembly of the landing gear of the first embodiment; 
           [0037]      FIGS. 4( a )  to (d) show a cross-sectional schematic view of a locking assembly in accordance with the first embodiment in an unlocked position, during a first alignment stage, during a second alignment stage and in the locked position respectively; and 
           [0038]      FIG. 5  shows an aircraft including a landing gear according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]      FIG. 2  shows a multi-wheel landing gear  150  having a bogie  152  with a steerable rear axle  154  and a locking mechanism  101  in accordance with an embodiment of the present invention. An actuator  156  extends between the bogie  152  and the steerable rear axle  154 . In use, the actuator  156  extends and retracts to alter the orientation of the steerable axle. 
         [0040]      FIG. 3  shows a close-up view of part of the locking mechanism  101  of the landing gear  150  of  FIG. 2 . The locking mechanism  101  comprises a lock member  110  pivotally connected at its lower end to the bogie  152  and pivotally connected at its upper end to a pair of springs  120 . When the lock member  110  is in the locked position shown in  FIG. 3 , part of the lock member  110  lies within a recess  104  integrally formed with the steerable axle  154 . Bearing pads  106  (not shown in  FIG. 3 ) are located in the recess  104  and contact the portion of the lock member  110  located in the recess on either side. 
         [0041]    In use, when pivotal movement of the steerable axle  154  is required, a locking actuator (not shown) is arranged to counteract the action of the springs  120  and to rotate the lock member  110  away from the recess  104  such that the steerable rear axle  154  can move relative to the lock member  110  and the bogie  152  on which the lock member  110  is mounted. When the axle  154  is to be locked in the straight ahead position the force exerted by the locking actuator on the lock member  110  is reversed such that the springs  120  and actuator act together to rotate the lock member  110  forwards until part of the lock member  110  is located in the recess  104  and is engaged with the bearing pads  106 . 
         [0042]      FIGS. 4( a ) to 4( d )  shows a series of schematic cross-sectional view of a locking mechanism  101  in accordance with the first embodiment of the invention, with the lock member  110  in (a) an unlocked position, (b) during a first alignment stage, (c) during a second alignment stage and (d) in a locked position. 
         [0043]      FIG. 4( a )  shows the lock member  110  which is symmetrical about a centre line C. The front surface of the lock member faces towards the recess  104  which is formed in the body of the steerable axle  102  and which appears u-shaped when viewed in plan in  FIG. 4( a ) . A guide surface  112  extends rearwards along each side of the lock member  101  from the front of the lock member  101 . The profile of the guide surface is stepped, and changes with distance from the front of the lock member  101 . In order from front to back along the side of the lock member  110 , the guide surface  112  includes a first inclined portion  112   a,  inclined at an angle to the direction of travel of the lock member, a first intermediate portion  112   b  having a very much reduced angle of inclination in comparison to the first inclined portion  112   a,  a second inclined portion  112   c  inclined at a different angle to the first inclined portion  112   a  and a second intermediate portion  112   d  extending in a direction substantially parallel to the centre line C of the lock member  110 . Thus, each guide surface  112  has a stepped profile when viewed in cross section. The gradient of each inclined portion  112   a,    112   c  is substantially constant along the length of that portion. To the rear of each guide surface  112  the body of the lock member  110  protrudes outwards to form a stop  114  having an abutment surface  114   a  that extends substantially perpendicular to the direction of travel. Accordingly, when viewed in cross-section in  FIG. 4( a ) , the front side of the lock member  110  (i.e. the end nearest to the recess  104 ) is narrower than the rear side of the lock member  110 . 
         [0044]    Two bearing pads  106  are located opposite each other on either side of the recess  104 . The width of each bearing pad  106  increases in with distance into the recess  104 . The surface of each bearing pad  106  is shaped so as to be complementary to the first intermediate portion  112   b,  the second inclined portion  112   c  and the second intermediate portion  112   d.  In  FIG. 4( d )  a dashed line A is used to denote the angle of the centre line C of the lock member  110  relative to the bearing pads  106  when the lock member  110  is in the locked position (i.e. when the lock member  110  is aligned with the bearing pads  106 ). In the unlocked position shown in  FIG. 4( a )  the centre line C is angled with respect to A. 
         [0045]    In use, the lock member  110  moves from the unlocked position to the locked position by pivoting about its lower end. If the steerable axle is in the straight ahead position when this pivoting occurs then the pivoting movement of the lock member  110  will bring the lock member  110  into engagement with the bearing pads  106 . However, if, as shown in  FIG. 4( a ) , the lock member and bearing pads are initially misaligned such that the centreline C of the lock member  110  is offset from the axis A between the pads  106 , it is necessary to ensure that the lock member  110  and bearing pads  106  become aligned during the locking process. If the lock member were to fail to engage the bearing pads  106 , it may, for example, jam against the sides of the recess  104 . If the locking mechanism  101  fails to engage this may delay the departure of a flight, or in some cases necessitate the flight returning to stand in order for the mechanism to be locked. When starting from a misaligned unlocked position as shown in  FIG. 4( a )  the lock member  110  moves forward towards the recess  104  under the action of the springs  120  until the first inclined portion  112   a  of the left-hand guide surface  112  is in contact with the left-hand bearing pad (as shown in  FIG. 4( b ) ). 
         [0046]      FIG. 4( b )  shows the lock member  110  during a first alignment stage. A portion of the lock member  110  is located within the recess  104 . The first inclined portion  112   a  of the left-hand side guide surface  112  is in contact with the rearmost corner of the corresponding bearing pad  106 . By virtue of the relative sliding movement between the first inclined portion  112   a  and the pad  106  during movement of the lock member  110  towards the locked position, the lock member  110  and the pads begin to be brought into alignment (as evidenced by the angle of inclination of the line C with respect to the line A being reduced in comparison to  FIG. 4( a ) ). As the forward movement of the lock member  110  continues the point of contact between the bearing pad  106  and the lock member  110  moves rearwards along the surface of the first inclined portion  112   a  of the lock member  110  until it reaches the start of the first intermediate portion  112   b.  The angle of inclination of the first intermediate portion  1120   b  is very much reduced in comparison to that of the first inclined portion  112   a,  and accordingly while the rearmost point of contact between the bearing pad  106  and the lock member  110  is located on the intermediate portion  112   b  the lock member  110  moves forward without experiencing any significant movement of the bearing pads  106  further into alignment with the lock member  110 . Further forward movement of the lock member  110  results in the bearing pads  106  coming into contact with the second inclined portion  112   c  as shown in  FIG. 4( c ) . 
         [0047]      FIG. 4( c )  shows the lock member  110  during a second, subsequent, alignment stage. The lock member  110  is located further into the recess  104  than in  FIG. 4( b ) . The second inclined portion  112   c  of the left-hand side guide surface  112  is in contact with the rearmost corner of the corresponding bearing pad  106 . By virtue of the relative sliding movement between the second inclined portion  112   c  and the pad  106  during movement of the lock member  110  towards the locked position, the lock member  110  and the pads begin to be brought further into alignment (as evidenced by the angle of inclination of the line C with respect to the line A is reduced in comparison to  FIG. 4( b ) ) in a second significant period of movement. The inclination of the first and second inclined portions  112   a ,  112   c  is such that by the end of this second stage of movement the bearing pads  106  have rotated relative to the lock member  110  until the bearing pads  10  and the lock members  110  are aligned and the steerable axle  154  is in the straight ahead position. After the second stage of alignment the lock member  110  then continues to move forward without further substantial rotation of the bearing pads  106  relative to the lock member  110  as the second intermediate surface  112   d  is substantially parallel to the direction of travel. This forward motion continues until the upper surface  114   a  of the stop  114  contacts the bearing pad  106  on either side of the lock member  110 . The lock member  110  is then engaged with the bearing pads  106  as shown in  FIG. 4( d ) . 
         [0048]      FIG. 4( d )  shows the lock member  110  in the locked position. On both sides of the lock member  110  the first intermediate portion  112   b,  second inclined portion  112   c  and second intermediate portion  112   d  of the guide surface  112  are in contact with the corresponding bearing pad  106 . The upper surface  114   a  of each stop  114  is in contact with the rear surface of the corresponding bearing pad  106 . The line C is aligned with the line A. 
         [0049]    The landing gear  150  and lock member  101  are arranged such that when the lock member is in the locked position as shown in  FIG. 4( d )  the steerable axle is in the straight ahead position. Accordingly, the two-stage movement that brings the lock member and the bearing pads into alignment also brings the steerable axle into the straight ahead position. 
         [0050]    Locking mechanisms in accordance with the present embodiment may be advantageous as the profile of the guide surfaces of the lock member results in the bearing pads and lock member being guided into alignment in two stages, thereby reducing the risk of the lock member failing to engage. The two-stage profile of the lock member may also allow locking mechanisms in accordance with the present embodiment to cope with a greater degree of misalignment between the bearing pads and the lock member, at the start of the movement between the unlocked position and the locked position than prior art mechanisms (i.e. embodiments of the invention have a relatively large ‘capture angle’). This may reduce the probability of the locking mechanism failing to successfully engage and/or reduce the need to so closely align the lock member before beginning the locking process. Locking mechanisms in accordance with the present embodiment may also be advantageous because the profile of the intermediate portions may result in relatively low unlocking forces (see above) being generated, thereby further reducing the probability of the locking mechanism failing to successfully engage. 
         [0051]    Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described. 
         [0052]    Each inclined portion of the embodiment described above has a gradient that is constant along the length of the portion. It will be appreciated that the gradient of an inclined portion may vary along its length. It will also be appreciated that the geometry of the bearing pads and the consequently the profile of the inclined sections may vary while still producing a two-stage movement. It will be appreciated that the alignment movement may comprise more than two stages. It will be appreciated that the guide surfaces may include further inclined portions arranged to produce a movement including further stages. 
         [0053]    Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.