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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FOREIGN PRIORITY 
       [0001]    This application claims priority to European Patent Application No. 16461523.9 filed May 17, 2016, the entire contents of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to a locking mechanism as might be used, for example, in locking a door or other movable element. 
       BACKGROUND 
       [0003]    A large number of locking mechanisms are known. In certain locking mechanisms, one or more locking elements move from a retracted position in which the mechanism allows an element such as a shaft to be moved and an extended position in which movement of the element is blocked. Such mechanisms, particularly for applications such as aircraft applications, must be reliable and, if possible lightweight. 
       SUMMARY 
       [0004]    A locking mechanism in accordance with this disclosure comprises a base defining an open mouthed recess for receiving a member to be selectively retained in the recess. The mechanism further comprises a pair of locking elements mounted for rotational movement between a retracted position in which they permit access to the recess and an extended position in which they extend at least partially across the mouth of the recess. The mechanism further comprises a drive for simultaneously moving the locking elements in opposite rotational directions. 
         [0005]    The mechanism may further comprise at least one arm, for example a pair of arms projecting from the base. The locking elements may be mounted between the pair of arms for rotational movement between the retracted and extended positions. The arm or arms may comprise respective guides to guide the movement of the locking element between the retracted and extended positions. 
         [0006]    The locking elements may comprise guide elements for engagement with the guides provided on the arms. 
         [0007]    The guides may comprise arcuate channels formed in the arm (s), and the locking elements may comprise guide pins for engagement with the channels. 
         [0008]    Each locking element may comprise at least two guide pins extending into a respective guide channel. 
         [0009]    The guides may extend over an arc of, for example, from 190-210°. In a particular example, the guides may extend 200°. 
         [0010]    The locking elements may be rotatable over an arc of, for example, about 80-100°. In a particular example, the locking elements may be rotatable over an arc of 90°. 
         [0011]    In various embodiments, tips of the locking elements may overlap one another when the locking elements are in their extended positions. 
         [0012]    The arms may each comprise a pair of elements between which a respective locking element is mounted. 
         [0013]    Each of the pair of elements may then comprise a guide channel, with the respective locking element having respective guide pins extending into said guide channels. 
         [0014]    In various embodiments, the arms are arcuate. Alternatively or additionally, the locking elements may be arcuate. 
         [0015]    In various embodiments, the locking elements may be shielded by the arms in their retracted position. 
         [0016]    In various embodiments, the drive comprises a rotary drive shaft having a right handed thread portion for driving one locking element and a left handed thread portion for driving the other locking element. 
         [0017]    The threaded portions of said drive shaft may be coupled to the respective locking elements through a drive element. The drive element may have a treaded nut portion for engaging the drive shaft and a lost motion coupling to the locking element. 
         [0018]    The lost motion coupling may be a slot, and the locking element may have a drive pin received in the slot. The slot permits linear movement of the drive pin along the slot during rotational movement of the locking elements. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]    Some embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying figures in which: 
           [0020]      FIG. 1  is a schematic sectional view of a first embodiment of the disclosure; 
           [0021]      FIG. 2  is a perspective view of the embodiment of  FIG. 1  in an unlocked condition; 
           [0022]      FIG. 3  is a perspective view of the embodiment of  FIG. 1  in a locked condition; 
           [0023]      FIG. 4  is a perspective view of a second embodiment of the disclosure in a locked condition; 
           [0024]      FIG. 5  is a perspective view of the embodiment of  FIG. 4  with some components removed for purposes of explanation; and 
           [0025]      FIG. 6  is a view corresponding to  FIG. 5 , but with some further components removed for purposes of explanation. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    With reference to  FIGS. 1 to 3 , a first locking mechanism  2  in accordance with the disclosure is illustrated. 
         [0027]    The locking mechanism  2  comprises a base  4  including a pair of arms  6 ,  8 , a pair of locking elements  10 ,  12  and a drive  14 . 
         [0028]    The base  4  comprises a pair of base plates  16  the respective arms  6 ,  8  extending upwardly from the upper surface  18  of each base plate  16 . The base plates  4 ,  6  are shown as separate elements in this embodiment, but they will be suitably attached together by suitable means not shown. Other forms of base construction may be readily envisaged. 
         [0029]    The arms  6 ,  8  are somewhat hook-like in shape. They are, in this embodiment arcuate in shape and extend in a circular arc from the upper surfaces  18  of the base plates  16  over an exemplary arc of approximately 65°. Of course, the arc covered may be different, for example in the range 60-70°. Although each of the arms  6 ,  8  is shown as having a generally constant width W, in some embodiments, the base end  20  thereof may be wider, as illustrated by dotted lines in  FIG. 1 , to facilitate manufacture and for strength. In other embodiments, the arms  6 ,  8  may simply be formed as an upper part of the base  4  as illustrated schematically in double dotted nines. 
         [0030]    In this embodiment, the inner edge surfaces  22  of the arms  6 ,  8  are arcuate in order to receive a circular section member  24  such as a shaft  24  which is to be retained by the locking mechanism  2 . The profile of the inner edge surfaces  22  may be chosen to match that of the element  24 , should the element  24  have a non-circular profile. The upper surface  18  of each base plate  16  is provided with a shallow groove  26  which smoothly continues the profile of the inner edge surfaces  22  of the arms  6 ,  8 . The arms  6 ,  8  and base plates  16  define an open mouthed recess  28  to receive the member  24 . 
         [0031]    The inner wall  30  of each arm  6 ,  8  comprises an arcuate guide  32  in the form of a channel  32  which extends downwardly into the respective base plates  16 ,  18 . In this embodiment, the channel  32  is formed as a groove, but it may also be formed as a through slot in other embodiments. The channel  32  may typically extend around an arc of approximately 190-210°, for example around 200°. As will be explained further below, the channel  32  will guide the movement of the locking elements  10 ,  12 . 
         [0032]    The locking elements  10 ,  12  are sandwiched between the arms  6 ,  8 . In the views of  FIGS. 2 and 3 , the base plates  16  have been moved apart for purposes of explanation, but in practice, the base plates  16  will be closer together, thereby retaining the locking elements  10 ,  12  in position. 
         [0033]    Each locking element  10 ,  12  is arcuate in shape having opposed inner and outer planar faces  34 ,  36 , and inner and outer edges  38 ,  40 . The inner edges  38  are arcuate in shape and generally match the contour of the shaft  24  and the inner edge surface  22  of the arms  6 ,  8 . A pair of guide pins  42  extends outwardly from the outer planar surface  36  of each locking element  10 ,  12 . The guide pins  42  are received in the channel  32  provided on the inner wall  30  of the adjacent arm  6 ,  8 . The guide pins  42  are separated by an arc of about 90-120°, for example 110°. Not only will the guide pins  42  guide the movement of the locking elements  10 ,  12 , they will also abut the ends of the channels  32  to provide stops for the locking elements  10 ,  12 . The separation of the guide pins  42  therefore determines the maximum range of movement of the locking elements  10 ,  12 . 
         [0034]    The locking elements  10 ,  12  each comprise a tip  44  and a root  46 . The tip  44  of a first locking element  10  is, as illustrated in  FIG. 2 , located adjacent the base plate  16  when it is retracted. The root  46  of the first locking element  10  is located adjacent the adjacent arm  6 . In contrast, the tip  44  of the second locking element  12  is located adjacent the arm  8 , with its root  46  located adjacent the adjacent base plate  16 . 
         [0035]    As can be seen from  FIG. 2 , in the retracted position, the locking elements  10 ,  12  are substantially shielded or covered by the arms  6 ,  8  which may prevent inadvertent damage thereto. 
         [0036]    One guide pin  42  of the first locking element  10  is positioned at the root  46  of that element  10 . The other guide pin  42  is located approximately half way between the root  46  and the tip  44 . 
         [0037]    One guide pin  42  of the second locking element  12  is positioned at the tip  44  of that element  12 . The other guide pin  42  is located approximately half way between the root  46  and the tip  44 . 
         [0038]    Respective drive pins  48  extend inwardly from the inner planar surface  34  of the respective locking element  10 ,  12 . The drive pin  48  of the first locking element  10  may be located generally opposite the guide pin  42  located intermediate the ends of the first locking element  10 . The drive pin  48  of the second locking element ( 12 ) may be located generally opposite the guide pin  42  located adjacent the root  46  of the second locking element ( 12 ). Other positions of the drive pins  48  will of course be possible, depending on the particular layout and range of motion required of the locking elements  10 ,  12 . 
         [0039]    The drive pins  48  are coupled to the drive  14  to drive the respective locking elements  10 ,  12 . 
         [0040]    The drive  14  comprises a rotary drive shaft  50  which is driven, in this embodiment, by gear  52 . The drive gear  52  may, for example, be a pinion gear or a worm gear driven by suitable means. The drive shaft  50  has a first threaded portion  54  and a second threaded portion  56 . The first threaded portion  54  has a right hand thread and the second threaded portion  56  has a left hand thread in this embodiment. Of course the thread directions may be reversed. What is important is that the two threaded portions  54 ,  56  have an opposite thread. 
         [0041]    First and second drive elements  58 ,  60  are mounted to the first and second threaded portions  54 ,  56  respectively. Drive element  58  comprises a nut portion  62   a  and a link portion  64   a  extending from the nut portion  62   a  towards the gear  52 . Drive element  60  comprises a nut portion  62   b  and a link portion  64   b  extending from the nut portion  62   b  towards the gear  52 . The nut portion  62   a  has a right handed internal thread which matches the thread of the threaded portion  54  of the shaft  50  to which it is mounted. The nut portion  62   b  has a left-handed internal thread which matches the thread of the threaded portion  56  of the shaft  50  to which it is mounted. The link portions  64   a,    64   b  are mounted to the respective nut portions  62   a,    62   b  at a proximal end  66  and each is formed with a slot coupling  68  at its distal end  70 . The drive pin  48  of the adjacent locking element is received in the slot  68 . The slot  68  acts as a lost motion mechanism as will be discussed further below. 
         [0042]    Having described the structure of the locking mechanism  2 , its operation will now be described. 
         [0043]    In the open position shown in  FIG. 2 , the recess  28  defined between the arms  6 ,  8  and the base  4  is fully open, the locking elements  10 ,  12  being fully retracted to lie adjacent the arms  6 ,  8 . In this condition, the respective link portions  64   a,    64   b  of the drive  14  overlap one another as can be seen from  FIG. 2 . In this open position, a member to be retained, such as the shaft  24  may be located within the recess  28 , as illustrated schematically by the arrow A in  FIG. 2 . The shaft  24  may, for example, be attached to a pivotally mounted member such as a door or cover and may act to hold the member in a closed position. The shaft  24  is received in the recess  28  and may contact the inner surface  22  of the arms  6 ,  8 . 
         [0044]    When it is desired to lock the shaft  24  in position such that it cannot be withdrawn from the recess  28 , the drive  14  is operated. The gear  52  is rotated in one direction and, due to the different threading on the respective threaded portions  54 ,  56  of the drive shaft  50  and the nut portions  62   a,    62   b  of the drive elements  58 ,  60 , the respective drive elements  58 ,  60  move in opposite directions away from one another along the drive shaft  50 . 
         [0045]    The motion of the drive elements  58 ,  60  is transmitted to the locking elements  10 ,  12  via the slots  68  in the link portions  66  and the drive pins  48  attached to the locking elements  10 ,  12 . The drive pins  48  move the respective locking elements  10 ,  12  along the guide channels  32  in the arms  6 ,  8 . The guide pins  42  guide the locking elements  10 ,  12  to move in an arcuate path along the channels  32 . It will be appreciated that due to this arcuate path, the drive pins  48  will not only move horizontally relative to the base  4 , but will be forced to move vertically as well. However, the slots  68  accommodate this movement as a lost motion mechanism, permitting linear movement of the drive pins  42  along the slots  68  during angular movement of the locking elements  10 ,  12 . 
         [0046]    The first locking element  10  rotates counter-clockwise in the sense of  FIG. 1  such that its tip  44  extends above the base plate  16 . In contrast, the second locking element  12  rotates counter-clockwise such that its tip  44  extends beyond the adjacent arm  8 . Both locking elements  10 ,  12  follow a circular path and thus wrap around the shaft  24  from different directions, as shown in  FIG. 3 . It will be seen that the tips  44  of the locking elements  10 ,  12  overlap such that the shaft  24  is engaged over a full 360° providing improved retention of the shaft  24  in the recess  28 . 
         [0047]    The locking elements  10 ,  12  will stop when the drive shaft  50  ceases to rotate. Suitable means such as sensors etc. may be provided to achieve stoppage in the correct position. Over-rotation of the locking elements  10 ,  12  is prevented by the guide pins  42  which will abut the ends of the guide channel  32 . A suitable holding mechanism such as a brake may be provided in the drive  14  to avoid inadvertent movement of the drive shaft  50 . 
         [0048]    In this condition, the shaft  24  is firmly retained within the recess  28  and cannot be withdrawn therefrom. To permit withdrawal, the drive shaft  50  must be rotated in the opposite direction, which causes the rotation of the locking elements  10 ,  12  in the opposite direction, thereby returning them to their original positions, whereupon the shaft  24  can be withdrawn from the recess  28 . 
         [0049]    In the embodiment above, the locking elements  10 ,  12  are retained laterally in position by the drive elements  58 ,  60 . These elements may therefore be provided with a low friction surface to allow sliding of the locking elements  10 ,  12  relative thereto. 
         [0050]      FIGS. 4 to 6  show a second embodiment of locking mechanism  2 ′ in accordance with the disclosure in which the locking elements  10 ,  12  are laterally located in an alternative manner. The basic construction of the second embodiment is similar to that of the first embodiment so only the differences therebetween will be described in detail. 
         [0051]    In this embodiment, each arm  6 ,  8  comprises a first part  6   a,    8   a  and a second part  6   b,    8   b  spaced laterally from the first part  6   a,    8   a.  Similarly each base plate  16  comprises a first part  16   a  and a second part  16   b  spaced laterally from the first part  16   a.  The first and second locking elements  10 ,  12  are located between the first and second parts. 
         [0052]    The first parts  6   a,    8   a,    16   a  of the locking elements  6 ,  8  and the base plates  16  are similar in construction to the locking elements  6 ,  8  and base plates  16  of the first embodiment. The second parts  6   b,    8   b,    16   b  are formed with a through slot  80  aligned with the channels  32  in the first parts  6   a,    8   a,    16   a.    
         [0053]    Each movable locking member  10 ,  12  still comprises guide pins  42  on their outer surface  36  as in the earlier embodiment. However, each locking element comprises an additional guide pin  82 , extending from its inner surface  34 . This guide pin  82 , together with the drive pin  48  pass through the slot  80 . In this manner, the drive pin  48  also acts as a guide pin. 
         [0054]    The operation of the locking mechanism of the second embodiment is the same as that of the first embodiment and need not therefore be described again. 
         [0055]    The locking mechanism of the disclosed embodiments may be advantageous in providing a lightweight reliable mechanism using multiple locking elements to provide a locking effect. 
         [0056]    It will be appreciated that the above embodiments are only exemplary and that various modifications may be made thereto without departing from the scope of the disclosure. 
         [0057]    For example, other drive mechanisms may be provided, as long as they produce movement of the locking elements in opposite directions.

Summary:
A locking mechanism comprises a base comprising a pair of arms projecting therefrom and defining an open mouthed recess for receiving a member to be selectively retained in the recess. The mechanism further comprises a pair of locking elements mounted between said pair of arms for rotational movement between a retracted position in which they permit access to the recess and an extended position in which they extend at least partially across the mouth of the recess. The arms comprise respective guides to guide the movement of said locking elements between said retracted and extended positions. The mechanism further comprises a drive for simultaneously moving the locking elements in opposite rotational directions. The drive may comprise a drive shaft having left handed and right handed threaded portions.