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The present invention relates to a lock mechanism. 
     BACKGROUND OF THE INVENTION 
     Locks which provide security on one side and easy and quick operation from the other side are used widely, particularly for doors providing an emergency exit from a building. 
     Commonly such locks use an engineered rack and pinion operating mechanism to operate several strong bolts simultaneously to secure the door. The door is thus secured at various points around its perimeter to provide security and prevent access from one side of the door. When such a door is an emergency exit it is necessary to comply with safety requirements to provide an operating lever inside which will operate by a single simple movement to retract the bolts and unlock the door to allow a safe and quick exit through the door from the building. Typically, a horizontal bar is provided on such doors for the operation of the lock mechanism. 
     Thus for lock mechanisms used for such doors there are conflicting requirements. One requirement is the security to prevent access from one side whilst the other requirement is a safety requirement to provide for simple and quick operation of the lock mechanism from the inside to allow the door to be unlocked quickly in an emergency. 
     The lock mechanisms currently available with a direct drive mechanism to the bolts such as a rack and pinion do not provide adequate security since they do not provide resistance to the application of pressure to the ends of the bolts. 
     GB-A-2289084 disclose a lock mechanism for use on a door hinged in a door frame at a first side. The lock mechanism comprises bolt members moveable between an engaged position in the door frame and a disengaged position. The bolt members are arranged to engage said door frame from at least two of the sides of said door which are not hinged. Latch means are arranged to prevent movement of at least one of the bolt members from the engaged position to the disengaged position. A latch release arrangement is used to disengage the latch means and allow movement of the bolt member. The latch release arrangement and the bolt members are arranged to be operable in response to a single movement of an operating member to allow the latch means to be disengaged and the bolt members to be disengaged from said engaged position. The bolt members are arranged to be directly driven between said engaged and disengaged positions by movement of the operating member. 
     The lock mechanism of GB-A-2289084 utilises a direct drive mechanism between the operating member, e.g. a handle, and the bolt members providing for positive and visible locking since the position of the operating member will indicate whether the bolt members are properly engaged or not. Such a direct driven bolting arrangement provides for a heavy duty lock mechanism which together with the latching arrangement, which operates when the bolt members are engaged, provides for high security. 
     Typically, such lock mechanisms include three bolt members—a horizontal bolt member, an upper bolt member and a lower bolt member. A European Standard has made the requirement that application of pressure to the end of the lower bolt member which is able to move the lower bolt member from the engaged position to the disengaged position should not be able to effect movement of the remaining bolt members from the engaged position to the disengaged position. 
     The present invention therefore seeks to provide a lock mechanism which complies with the European Standard. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided a lock mechanism for use on a door hinged at a first side, said lock mechanism comprising a first bolt member and at least one further bolt member, each bolt member being movable between an engaged position extending from the door and a disengaged position, each bolt member being arranged to be directly driveable between said engaged and disengaged positions by movement of a drive member, the first bolt member having a first end for engagement with the drive member and a second end remote from the drive member, the first bolt member being adapted to be reduced in length on application of a pressure towards said first end at said second end. 
     Advantageously the first bolt member is compressible. 
     Advantageously the first bolt member comprises a first portion and a second portion coupled together so that said second portion moves towards said first portion on application of a pressure towards said first end at said second end. Preferably said second portion is slidably movable with respect to said first portion. Advantageously said first portion and said second portion are coupled together by a compressible link. 
     Advantageously the lock mechanism further comprises stop means for preventing movement of said second end with respect to said first end beyond a predetermined displacement. Preferably said stop means comprises a stop member for engagement with a fixed part of the lock mechanism which is fixed with respect to the axis of rotation or oscillation of the drive member, the stop member being mounted on the first bolt member. Preferably the stop member is movable between free and stopped positions with respect to the first bolt member, said free position not enabling engagement of the stop member with said fixed part of the lock mechanism and said stopped position enabling engagement of the stop member with said fixed part of the lock mechanism. 
     Advantageously the stop member is mounted on one of said first and second portions, the other of said first and second portions including maintaining means for maintaining the stop member in said stopped position on application of a pressure towards said first end at said second end. Preferably said maintaining means comprises a projection slidable with respect to said one of said first and second portions. Preferably the projection is slidable in a slot provided in said one of said first and second portions. 
     Advantageously the stop member is mounted on said first portion. 
     Advantageously the stop member is pivotable about a pivot point between said free and said stopped positions. 
     Advantageously the stop member is rotatable about a mounting point between said free and said stopped positions. 
     Advantageously the stop member is biased towards said free position. 
     Advantageously said second portion is biased to move away from said first portion. 
     According to a second aspect of the present invention there is provided a bolt member for a lock mechanism for use on a door hinged at a first side, the bolt member having a first end for engagement with a drive member of the lock mechanism and a second end remote from the first end, the bolt member being adapted to be reduced in length on application of a pressure towards said first end at said second end. Preferably the bolt member is compressible. 
     Advantageously the bolt member comprises a first portion and a second portion coupled together so that said second portion moves towards said first portion on application of a pressure towards said first end at said second end. Preferably said second portion is slidably movable with respect to said first portion. 
     Advantageously said first portion and said second portion are coupled together by a compressible link. 
     Advantageously the bolt member further comprises a stop member for engagement with a fixed part of the lock mechanism which is fixed with respect to the axis of rotation or oscillation of the drive member and for preventing movement of said second end with respect to said first end beyond a predetermined displacement. Preferably the stop member is moveable between free and stopped positions with respect to the first bolt member, said free position not enabling engagement of the stop member with said fixed part of the lock mechanism and said stopped position enabling engagement of the stop member with said fixed part of the lock mechanism. 
     Advantageously the stop member is mounted on one of said first and second portions, the other of said first and second portions including maintaining means for maintaining the stop member in said stopped position on application of a pressure towards said first end at said second end. Preferably said maintaining means comprise a projection slidable with respect to said one of said first and second portions. Preferably the projection is slidable in a slot provided in said one of said first and second portions. 
     Advantageously the stop member is mounted on said first portion. 
     Advantageously the stop member is pivotable about a pivot point between said free and said stopped positions. 
     Advantageously the stop member is rotatable about a mounting point between said free and said stopped positions. 
     Advantageously the stop member is biased towards said free position. 
     Advantageously said second portion is biased to move away from said first portion. 
     According to a third aspect of the present invention there is provided a lock mechanism for use on a door hinged at a first side, said lock mechanism comprising a first bolt member and at least one further bolt member, each bolt member being movable along a path between an engaged position extending from the door and a disengaged position, each bolt member being arranged to be directly driveable between said engaged and disengaged positions by movement of a drive member, the first bolt member having a first end for engagement with the drive member and a second end remote from the drive member, blocking means for preventing movement of the first bolt member from said engaged to said disengaged position, said blocking means being moveable between a blocking portion in which said blocking means blocks the path of the first bolt member to prevent movement of the first bolt member from said engaged position to said disengaged position and a non-blocking position in which said blocking means does not block the path of the first bolt member, the drive member including a blocking drive member for moving said blocking means from the blocking position to the non-blocking position, the drive member including the blocking drive member being arranged to be operable in response to a single movement of an operating member to allow said blocking means to be moved from the blocking position to the non-blocking position and the bolt members to be driven from said engaged position to said disengaged position. Preferably said blocking means in said blocking position abuts the first bolt member. Preferably said blocking means in said blocking position abuts said first end of the first bolt member. 
     Advantageously said blocking means is arranged to be responsive to an initial movement of the operating member. Preferably the first bolt member is not responsive to said initial movement of said first operating member. 
     Advantageously said drive member comprises at least one drive gear for directly driving each bolt member between said engaged and disengaged positions, said at least one drive gear and the blocking drive member being coupled such that initial rotation of the operating member causes rotation solely of the blocking drive member, further rotation of the operating member causing rotation of said at least one drive gear to drive each bolt member between said engaged and disengaged positions. Preferably said at least one drive gear comprises a first drive gear arranged on a common shaft with the blocking drive member, the bore of the first drive gear through which the common shaft extends being shaped to permit initial rotation of the common shaft and the blocking drive member without rotation of the first drive gear. 
     Embodiments of the present invention will now be described with reference to the drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of a lock mechanism disclosed in GB-A-2289084 with the cover plate removed; 
     FIG. 2 is a cross-section A—A through FIG. 1; 
     FIG. 3 a view of a prior art latch gear wheel and latch member disclosed in GB-A-2289084; 
     FIG. 4 shows an elevational view of a lock mechanism according to a first embodiment of the present invention; 
     FIG. 5 shows a modification of the embodiment of FIG. 4; 
     FIG. 6 a  shows a schematic view of a bolt member for a lock mechanism according to a second embodiment of the present invention; 
     FIGS. 6 b  and  6   c  show schematic views of the bolt member of FIG. 6 a  in different configurations; 
     FIG. 6 d  shows a schematic view of the bolt member of FIG. 6 a  in a further configuration and in relation to the housing of the lock mechanism (shown in part) according to the second embodiment of the present invention; 
     FIG. 7 shows a view of the bolt member of FIG. 6 a  along the line VII—VII of FIG. 6 a;    
     FIGS. 8 a  and  8   b  shows a schematic view of a bolt member in different configurations for a lock mechanism according to a third embodiment of the present invention; 
     FIG. 8 c  shows a schematic view of the bolt member of FIGS. 8 a  and  8   b  in a further configuration and in relation to the housing of a lock mechanism (shown in part) according to the third embodiment of the present invention; 
     FIG. 9 shows a view of the bolt member of FIGS. 8 a  and  8   b  along the line IX—IX of FIG. 8 a;    
     FIG. 10 shows an elevational view of a lock mechanism according to a fourth embodiment of the present invention; 
     FIG. 11 shows a part cross-section through FIG. 10; 
     and FIGS. 12 a  and  12   b  show gear wheels for use with the embodiment of FIGS. 10 and 11; 
     FIG. 13 a  is a cross section of a fifth embodiment of the invention; and FIG. 13 b  is a cross section along line b—b in FIG. 13 a.   
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, FIGS. 1,  2  and  3  illustrate a lock mechanism which can be operated from one side using an operating member to unlatch the bolts when they are in the engaged position and to withdraw the bolts to a disengaged position. The release of the latch mechanism and the retraction of the bolts occurs by a single simple motion of the operating member. 
     This arrangement can be used for a door providing an emergency exit wherein an operating member is provided only on the inside of the door and no operating member is provided on the outside. From the inside the lock mechanism can be operated simply and quickly by for instance a single push of a release bar, whilst on the outside the lock mechanism is not accessible. Thus from the outside the lock mechanism provides for security since not only is the lock mechanism inaccessible, but also the bolt members which engage the door frame are resistant to end pressure to retract them from the engaged position. 
     The specific construction of the lock mechanism will now be described with reference to FIGS. 1,  2  and  3 . 
     In FIG. 1 the cover plate  1  which is shown in FIG. 2 is removed to show the construction of the lock mechanism in detail. Three bolt members  2 ,  3  and  4  project from the housing  5  and are capable of translational motion in and out of the housing  5 . Two of the bolt members  2  and  4  move in opposite directions whilst the third bolt member  3  moves in a direction which is generally perpendicular to the opposed directions. 
     The bolt members  2 ,  3  and  4  are shown in FIG. 1 to have a fairly short length. However, the lengths of the bolt members  2 ,  3  and  4  will depend on the door to which the lock mechanism is designed to fit. Normally, the bolt members  2 ,  3  and  4  will be much longer and will be arranged to engage the top and bottom and one side of the door frame. The remaining side of the door frame is the side on which the door is hinged. Such an arrangement of bolts is a conventional method of providing security and provides a high resistance to a physical attack. 
     Within the housing  5  there is provided a first drive gear wheel  6  which has a portion  6   a  which fits into a hole in the cover  1  to allow rotation of the first drive gear wheel. Cogs of the first drive gear wheel engage cogs provided along one side of a length of the bolt members  3  and  4  which are arranged generally perpendicularly. 
     A second drive gear wheel  7  is provided spaced from the first drive gear wheel  6  and has a portion  7   a  which fits in a hole in the cover  1  and a portion  7   b  which fits in a hole in the housing  5  to allow rotation of the second drive gear wheel  7 . Cogs of the second drive gear wheel  7  are engaged with cogs provided along a length of the second side of the bolt member  3  and along a side of the bolt member  2 . Thus the first and second drive gear wheels are arranged to rotate in opposite directions and the rotation thereof causes translational motion of the bolt members  2 ,  3  and  4 . 
     The bolt members  2 ,  3  and  4  are biased in an engaged position by a spring  8  and a spring link member  9  which has cogs provided along a length to engage with the cogs of the first drive gear wheel  6 . 
     The first drive gear wheel  6  is provided with offset slots  10  to receive pins  11  of a latch gear wheel  12 . The latch gear wheel  12  has a portion  12   a  which fits into a hole in the housing  5  to allow rotation thereof. The first drive gear wheel  6  and the latch gear wheel  12  are mounted to have the same axis of rotation and support each other in the centre. 
     In the engaged position shown in FIG. 1 the pins  11  of the latch gear wheel  12  engage the slots  10  in the first drive gear wheel and reside at a position near the anticlockwise sides of the slots. 
     The latch gear wheel  12  is provided with a splined recess  13  which is designed to receive an operating member such as a handle for rotation of the latch gear wheel  12 . 
     On the outside of the housing  5  there is provided a decorative plate  14  together with a rose plate  15  to support the handle and allow its rotation and operation of the lock mechanism. 
     The pins  11  of the latch gear wheel  12  are held in their position against the anticlockwise side of the slot  10  in the first drive gear wheel  6  by bias means provided in the form of springs  16  interconnecting the first drive gear wheel  6  and the latch gear wheel  12 . 
     Cogs of the latch gear wheel  12  engage cogs provided on a latch member  17 . In the engaged and latch position shown in FIG. 1 the latch member  17  has been extended to the right to prevent movement of the spring link member  9 . Before the bolt members  2 ,  3  and  4  can be retracted to the disengaged position it is necessary to move the latch member  17  to the left to allow the spring link member  9  and hence the drive gear wheels  6  and  7  and the bolt members  2 ,  3  and  4  to move. 
     The movement of the latch member is achieved by rotation of the operating member and thus the rotation of the latch gear wheel  12 . As the latch gear wheel rotates clockwise the latch gear wheel  17  is moved to the left. The pins  11  of latch gear wheel  12  move clockwise within the slots  10  of the first drive gear wheel until they make contact with the sides of the slots in the clockwise direction. When this occurs the latch member  17  has moved far enough left to be out of the way of the spring link member  9 , i.e. it is in the unlatched position, and thus further rotation of the operating member causes the latch gear wheel  12  to drive the first drive gear wheel  6  in the clockwise direction against the spring force of the spring  8  to retract the bolt members  2 ,  3  and  4 . 
     It can thus be seen that by rotation of an operating member inserted in the splined recess  13  of the latch gear wheel  12 , by a single turning motion of the operating member the latch mechanism which holds the bolt members  2 ,  3  and  4  in the extended position is unlatched and the bolt members  2 ,  3  and  4  are retracted to release the lock mechanism. Thus during an initial rotation of an operating member the lost motion between the latch gear wheel  12  and the first drive gear wheel  6  is taken up against the biasing action of the spring means  16 . Further rotation of the operating member will retract the bolt members  2 ,  3  and  4  against the biasing of the spring  8 . Once the operating member is released the spring  8  will return the bolt members  2 ,  3  and  4  to the engaged position and as the operating member is further released to its original position the springs  16  will return the lost motion between the latch gear wheel and the first drive gear wheel. 
     The lock mechanism of FIGS. 1,  2  and  3  provides for a simple lock mechanism whereby no access is required to the mechanism from one side, i.e. outside. Such a lock mechanism can be used for a simple door forming an emergency exit. 
     FIG. 4 shows an elevational view of a lock mechanism according to a first embodiment of the present invention. The lock mechanism comprises a housing  20  with a lower bolt member  22 , upper and horizontal bolt members (not shown) and a drive mechanism within the housing  20  similar to the drive mechanism of FIGS. 1 to  3 . However, the embodiment of the present invention shown in FIGS. 4 and 5 may equally be applied to other lock mechanisms having a direct drive mechanism to the bolt members and so the drive mechanism within the housing  20  is not being described in detail. 
     Essentially, the bolt member  22  is formed as first and second portions  24 ,  26  coupled together by a compressible portion  28 , provided in the specific embodiment as a compressible spring. A guide tube  30  is attached to the housing  20  and guides movement of the first and second portions  24 ,  26  and the spring  28  in the door when the lock mechanism is being used. The upper end of the bolt member  22  is attached to a rack  32  which engages with a drive gear  34  within the housing  20 . 
     When pressure is applied to the end of the bolt member  22 , the second portion  26  is caused to move towards the housing  20  in the sense indicated by the arrow B. Because the first and second portions  24 ,  26  are coupled together by a compressible spring  28 , the pressure applied to the second portion  26  is absorbed by the compressible spring  28  and so the pressure applied to the first portion  24  and the rack  32  is reduced. Advantageously, all of the pressure applied to the end of the bolt member  22  is absorbed by the spring  28  so that the rack  32  does not move at all and so no force is applied to the drive gear  34  which might effect movement of the other bolt members in the lock mechanism. 
     Alternatively, the drive mechanism within the housing  20  is arranged to resist movement of the bolt member  22  from the engaged position to the disengaged position and so the compression spring  28  is sufficiently compressible so that the pressure transmitted to the rack  32  is insufficient to overcome the resistance to movement provided by the drive mechanism itself. In FIG. 4, this resistance to movement of the bolt member from the engaged position to the disengaged position is provided by a spring link member  36  to which a bias spring (not shown) is attached at an attachment point  38 . Cogs provided along a length of the spring link member  36  engage with the cogs of the drive gear  34 . The bias spring and spring link member  36  are arranged to resist motion of the drive gear  34  in the sense C and so resist movement of the rack  32  to move the bolt member  22  from the engaged to the disengaged position. 
     During normal operation of the lock mechanism, an operator turns an operating member (not shown) to rotate the drive gear  34  in the sense indicated by the arrow C and so move the rack  32  further within the housing  20 . In the embodiment of FIG. 4, the rack  32  is shown attached to the first portion  24  and so movement of the rack  32  causes the first and second portions  24 ,  26  and the spring  28  to move within the guide tube  30  and so move the bolt member  22  from the engaged to the disengaged position. 
     FIG. 5 a  shows a modification of the embodiment of FIG. 4 in which the compressible spring  28   a  is provided further away from the housing  20  than in the embodiment of FIG.  4 . Thus, the first portion  24   a  of FIG. 5 is longer than the first portion  24  of FIG.  4  and the second portion  26   a  of FIG. 5 is shorter than the second portion  26  of FIG.  4 . Also shown in FIG. 5 is a guide pin  40  for resisting rotation of the bolt member  22  about its longitudinal axis when the bolt member  22  slides within brackets  42   a ,  42   b . The length of the bolt member  22 , as a whole, is adjustable by means of a screw adjustment  44 . Also shown in FIG. 5 is a bolt hole guard  46  comprising a cylinder  48  and a cover plate  50  into which the bolt member  22  projects in the engaged position. Depending on the locality of the door to which the lock mechanism is affixed, the bolt hole guard  46  may be provided in a door frame or in the floor. 
     A disadvantage of the embodiments of FIGS. 4 and 5 is that the compression spring  28  reduces the force transmitted from the end of the bolt member to the rack  32  but may, depending on the characteristics of the compression spring  28  and the clearance of the door from the bolt hole guard, still allow the end of the bolt member  22  to be withdrawn from the bolt hole guard into a disengaged position. Thus, the embodiment of FIGS. 4 and 5 prevents movement of the end of the bolt member  22  from being transmitted to the other bolt members in the lock mechanism but does not necessarily prevent disengagement of the bolt member  22  to which pressure is applied. 
     FIGS. 6 a ,  6   b ,  6   c ,  6   d  and  7  show a bolt member for a lock mechanism according to a second embodiment of the present invention in which some resistance is provided to movement of the end of the bolt member  60 . 
     FIG. 6 a  shows the bolt member  60  in a stable configuration in which no pressure is applied to the end of the bolt member  60 . A sleeve portion  66 , which may be attached to the rack portion  64  or which may form part of the rack portion  64 , includes a slit  68  having a closed end  69  and an open end  70 . Within the sleeve portion  66  is a cam  72  rotatably mounted on a pin  74 . A further portion  76  of the bolt member  60  includes a slide pin  78  which is slidable within a closed slot  80  in the sleeve portion  66 . The further portion  76  is therefore able to slide in and out of the sleeve portion  66  with respect to the slide pin  78 . 
     Bias means, comprising a spring  82  and a ball  84 , mounted within the rack portion  64  are effective to bias the position of the cam  72  to the positions shown in FIGS. 6 a  and  6   d.    
     FIG. 6 d  shows the bolt member  60  in a stable configuration in which no pressure is applied to the end of the bolt member  60 . Pressure is applied by the spring  82  to maintain the cam  72  in a position with all of its edges within the sleeve portion  66 . The bolt member  60  can therefore be easily moved between the engaged and disengaged positions using a drive gear  34 . 
     In the configuration of FIG. 6 c , as can be seen from the position of the pin  78  within the slot  80 , pressure has been applied to the bottom end of the bolt member  60 . With the upward movement of the further portion  76 , an end  76   a  of the further portion  76  pushes against a side  72   a  of the cam  72 , thus rotating the cam  72  about the pin  74 . A corner  72   b  of the cam is thereby caused to protrude out of the sleeve portion  66  through the slit  68 . Further upward movement of the cam  72 , and therefore of the further portion  76  is prevented because the corner  72   b  of the cam  72  cannot move further against the cover panel  86  of the housing  20 . As movement of the bolt member  60  relative to the housing  20  is prevented, the rack portion  62  is not caused to move upward by application of pressure at the end of the bolt member  60  and so the other bolt members in the lock mechanism are not affected. 
     When the end of the bolt member  60  is released, the cam  72  may return to the position shown in FIG. 6 d.    
     It is conceivable that the cam  72  may be caused to move into the configuration shown in FIG. 6 b . With this configuration also, a corner  72   b  of the cam projects out of the sleeve portion  66  and so excessive upward motion of the further portion  76  is prevented. However, should the bolt member  60  be caused to move upward by the drive wheel  34 , then it is possible for the cam  72  to rotate in the slit  68  so that the corner  72   b  is no longer protruding from the sleeve portion  66 . The bolt member  60  can therefore be retracted (moved from the engaged position to the disengaged position) from the configurations shown in either FIGS. 6 a  or  6   b.    
     FIGS. 8 a  and  8   b  show a schematic view of a bolt member  100  for a lock mechanism according to a third embodiment of the present invention. The bolt member  100  is shown in relation to the housing  102  of a lock mechanism in FIG. 8 c . The lock mechanism further comprises upper and horizontal bolt members (not shown) and a drive mechanism within the housing  102  similar to the drive mechanism of FIGS. 1 to  3 . However, the embodiment of the present invention shown in FIGS. 8 a ,  8   b ,  8   c  and  9  may equally be applied to other lock mechanisms having a direct drive mechanism to the bolt members and so the drive mechanism within the housing  102  is not being described in detail. The upper end of the bolt member  100  includes a rack portion  104  which engages with a drive gear  106  within the housing  102 . 
     The bolt member  100  is recessed to accommodate a rocking cam  108  which rocks or pivots about a pin  110  extending through the bolt member  100 . 
     The rack portion  104  is coupled to a further portion  112  of the bolt member  100  by a compressible link  114 . The compressible link  114  comprises a link member  116  with one end of the link member  116  fixedly attached to the further portion  112 . The other end of the link member  116  is shaped to be received in a blind recess  118  in the rack portion  104 . The link member  116  is mounted in the rack portion  104  by a pin  120  which is slidable within a slot  122  in the rack portion  104 . 
     FIGS. 8 a  and  8   b  show the bolt member  100  respectively in the engaged position and in the disengaged position. In the configuration shown in FIG. 8 c , pressure has been applied to the bottom end of the bolt member  100  to move the further portion  112  towards the housing  102 . This pressure is transmitted through the link member  116  to move the pin  120  of the link member  116  towards the housing  102  relative to the rack portion  104 . The pin head  120   a  of the pin  120  co-operates with a face  108   a  of the cam  108  to hold the cam  108  in the position shown in FIGS. 8 a  and  8   c . In this configuration, a stop portion  108   b  of the cam  108  protrudes from the rack  104  to abut against the cover panel  124  of the housing  102 . Abutment of the stop portion  108   b  against the cover panel  124  prevents further pressure applied to the end of the bolt member  100  from being transmitted through the rack portion  104  to move the drive gear  106 . In this way, the other bolt members in the locking mechanism are unaffected by the application of pressure to the end of the bolt member  100 . 
     As can be seen in FIGS. 8 a  and  8   b , when pressure is not applied to the end of the bolt member  100 , there is sufficient clearance between the cam  108  and the pin head  120   a  for the cam  108  to rock between the position shown in FIGS. 8 a  and  8   b.    
     In FIGS. 10 and 11, the cover plate (which would be on the left-hand side of the section of FIG. 11) is removed to show the construction of the lock mechanism in detail. Three bolt members  152 ,  153  and  154  project from the housing  155  and are capable of translational motion in and out of the housing  155 . Two of the bolt members  152  and  154  move in opposite directions whilst the third bolt member  153  moves in a direction which is generally perpendicular to the opposed directions. 
     The bolt members  152 ,  153  and  154  are shown in FIG. 10 to have a fairly short length. However, the lengths of the bolt members  152 ,  153  and  154  will depend on the door to which the lock mechanism is designed to fit. Normally, the bolt members  152 ,  153  and  154  will be much longer and will be arranged to engage the top and bottom and one side of the door frame. The remaining side of the door frame is the side on which the door is hinged. Such an arrangement of bolts is a conventional method of providing security and provides a high resistance to a physical attack. 
     Within the housing  155  there is provided a first drive gear wheel  156 . Cogs of the first drive gear wheel  156  engage cogs provided along one side of a length of the bolt members  152  and  153  which are arranged generally perpendicularly. 
     A second drive gear wheel  157  is provided spaced from the first drive gear wheel  156 . Cogs of the second drive gear wheel  157  are coupled to be engaged with cogs provided along a length of the second side of the bolt member  153  and along a side of the bolt member  154 . Thus the first and second drive gear wheels are arranged to rotate in opposite directions and the rotation thereof causes translational motion of the bolt members  152 ,  153  and  154 . 
     The bolt members  152 ,  153  and  154  are biased in an engaged position by a spring  158  and a spring link member  159  which has cogs provided along a length to engage with the cogs of the first drive gear wheel  156 . 
     The lock mechanism further includes a restraint gear wheel  160  on which is mounted a pin  162 . A restraint cam  164  includes a slot  166  through which the pin  162  projects. As the restraint gear wheel  160  is rotated (clockwise in FIG.  10 ), the pin  162  causes the restraint cam  164  to move. 
     In the configuration shown in FIG. 10, the bolt members  152 ,  153  and  154  are in the engaged position. The restraint cam  164  is effective to prevent movement of the lower bolt member  152  from the engaged position to the disengaged position against the application of a force at the end of the lower bolt member  152 . 
     To unlock the lock mechanism, an operating member having a handle with a cross-section to fit through the hole  166  in the restraint gear wheel  160  is rotated. Rotation of the restraint gear wheel  160  in the clockwise sense (as shown in FIG. 10) causes the pin  162  to move from the position  162   a  shown in FIG. 10 to the position  162   b  in FIG. 10 thereby moving the restraint cam  164  so that it does not prevent movement of the lower bolt member  152 . 
     Because of the relative shapes of the hole  166  in the restraint gear  160  and the hole  168  in the first drive gear wheel  156  as shown in FIGS. 12 a  and  12   b , initial rotation of the restraint gear wheel  160  to move the pin  162  from  162   a  to  162   b  has no effect on the first drive gear wheel  156 . As the pin  162  and restraint cam  164  are moved to allow movement of the lower bolt member  152 , the handle of the operating member picks up the first drive gear wheel  156  effecting rotation of the first gear wheel  156  with corresponding movement of the lower bolt member  152 , the other bolt members  153 ,  154  and the second drive gear wheel  157  to effect disengagement of the bolt members  152 ,  153 ,  154 . 
     It can thus be seen that by rotation of an operating member inserted in the hole  166  of the restraint gear wheel  160 , by a single turning motion of the operating member the restraint cam  164  which holds the bolt member  152  in the extended position is released and the bolt members  152 ,  153  and  154  are retracted to release the lock mechanism. Thus during an initial rotation of an operating member the lost motion between the restraint gear wheel  160  and the first drive gear wheel  156  is taken up. Further rotation of the operating member will retract the bolt members  152 ,  153  and  154  against the biasing of the spring  158 . Once the operating member is released the spring  158  will return the bolt members  152 ,  153  and  154  to the engaged position and as the operating member is further released to its original position the lost motion between the restraint gear wheel  160  and the first drive gear wheel  156  will be returned. 
     A fifth embodiment of the invention is shown in FIGS. 13 a  and  13   b . This embodiment is similar in principle to the first embodiment in that it has a bolt member  170  formed as first and second portion  172 ,  174  coupled together by a compressible portion  176 , provided in the specific embodiment as a compressible spring. In this case, the first bolt portion  174  consists of a head  178  to which a stem  180  is fixed by a pin  182 . The second bolt member  172  comprises a tubular member  184  and a cylindrical insert  186 . The insert  186  has a peripheral groove  188  and a through bore  190 . 
     To assemble the bolt member  170  the stem  180  is fitted through the through bore  190  of the insert  186 . The stem  180  has an enlarged head  192  which prevents it from sliding right through the through bore  190  of the insert. The spring  176  is then placed over the stem  180  and abuts the insert  186 . The head  178  is then inserted through the lower end of housing  194  with its upper end protruding. The upper end is pinned to the stem  180  by pin  182  so that the spring  176  is sandwiched between the insert  186  and head  178 . The tubular member  184  is then fitted over this and crimped adjacent to the groove  188  in the insert so as to fix the insert  186  and tubular member  184  against relative sliding movement. The bolt member  170  can thus be slideably retained on a door by the housing  194  which is fixed to the door by fasteners through fastening holes  196 . 
     The head  178  of the bolt member  170  is moved into the engaged position shown in FIGS. 13 a  and  13   b  by movement of the tubular member  184  by a drive gear similar to that described in earlier examples. The spring  176  has sufficient strength to transmit this movement to the second portion  174 . On the other hand, when upward pressure is applied to the head  178 , the first member  170  moved upwardly compressing the spring  176  which is sized to absorb the upward movement. 
     This embodiment is cheap to assemble and is reliable to operate as the spring  176  is retained between the tubular member  184  and the stem  180 .

Summary:
There is described a lock mechanism for a leaf hinged within a frame, the lock mechanism comprising a first bolt and at least one further bolt, each bolt being movable between an engaged position in which a distal end of the bolt projects from the leaf and a disengaged position, the bolts being directly driveable together between the engaged and disengaged positions by movement of a common drive member, the first bolt having a proximal end engaged by the drive member and isolating means for ensuring that, when pressure is applied to the distal end of the first bolt tending to move it to the disengaged position, the or each further bolt is not moved to the disengaged position.