Abstract:
A lockable switch mechanism for a machine guard includes a switch plunger and locking and switch mechanisms. The plunger moves, upon insertion of an actuator, between two positions to actuate the switch mechanism and is locked by the locking mechanism, which has one locking member biased against a surface of the plunger and another locking member that is displaceable between locked and released positions. The plunger has an annular shoulder that displaces the first locking member when the plunger is moved. The second locking member prevents displacement of the first locking member by the plunger to thereby prevent movement of the plunger. Thus removal of the actuator is prevented unless the second locking member has been moved to the unlocked position when the machine is in a safe condition.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to United Kingdom appl. Ser. No. 0214205, filed on Jun. 19, 2002 and published as GB 0214205D DO on Jul. 31, 2002. Related applications are published as EP 1376632 A1, published on Jan. 2, 2004, and JP 2004022549 A, published on Jan. 22, 2004. 
     STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a lockable switch mechanism which may be used in for example a machine guard to prevent the opening of a door of the machine guard until predetermined conditions have been established. 
     A lockable switch mechanism is described in U.S. Pat. No. 5,777,284, hereby incorporated by reference as though fully set forth herein. That mechanism comprises a switch plunger which is mounted in a housing and is displaceable relative to the housing along a predetermined axis between a first unlocked position and a second locked position. A locking mechanism is provided for locking the switch plunger in the second position and the switch plunger actuates a switch mechanism as a result of movement of the switch plunger between the first and second positions. The locking mechanism comprises two pivotally mounted latches which are normally biased against the switch plunger so as to engage behind an axially facing surface defined by the switch plunger when the plunger has been moved to the second position. The latches can only be withdrawn so as to permit axial displacement of the switch plunger if a plate extending transversely of the switch plunger is displaced to a latch release position. The latch releasing plate is driven by a lever mechanism the position of which is controlled by a solenoid arranged to one side of the switch mechanism housing. This arrangement works well but is relatively bulky and complex. 
     It is an object of the present invention to provide an improved lockable switch mechanism. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a lockable switch mechanism comprising a switch plunger which is mounted in a housing and is displaceable relative to the housing along a predetermined axis between a first unlocked position and a second position, a locking mechanism for locking the switch plunger in the second position, and a switch mechanism which is actuated by movements of the switch plunger between the first and second positions, wherein the locking mechanism comprises at least one first locking member which is biased against a surface of the switch plunger and at least one second locking member which is displaceable between locked and released positions, the surface of the switch plunger against which the first locking member is biased defining a profile arranged such that movement of the switch plunger from the second to the first position causes the profile to displace the first locking member, and the second locking member when in the locked position preventing displacement of the first locking member by the profile to thereby prevent movement of the plunger from the second to the first position. 
     In contrast to the mechanism described in U.S. Pat. No. 5,777,284, the mechanism in accordance with the present invention relies upon a first locking member which does not prevent axial displacement of the switch plunger unless a second locking member is moved into a locked position. This means that rather than providing a relatively complex mechanism to release a latch a relatively simple and compact mechanism can be provided which is positionable either so as to maintain the first locking member in a position in which axial displacement of the switch plunger is not permitted or in a position in which the first locking member can be simply displaced by axial movement of the switch plunger. All of the necessary components can be arranged along a common axis with the switch plunger axis in a compact and reliable assembly. 
     Preferably, the or each first locking member comprises a locking pin extending transversely relative to the axis of displacement of the switch plunger, the locking pin being spring biased towards the switch plunger in a direction perpendicular to the switch plunger axis. Two locking pins may be provided on opposite sides of the switch plunger. The locking pins may be mounted in a housing assembly defining an aperture through which the switch plunger extends, the locking pins being spring-biased towards each other from opposite sides of the aperture by springs supported in the housing assembly. The housing assembly may comprise a frame which receives the locking pins and springs and a cover plate which retains the locking pins and springs within the assembly. 
     The profile may be defined by an annular shoulder extending around the switch plunger. That shoulder may be tapered so as to readily lift the locking pins away from the switch plunger if the mechanism is not in the locked condition. The or each locking member may comprise a locking arm which is displaceable in a direction parallel to the switch plunger axis and, when in the locked position, extends on the side of the first locking member remote from the switch plunger to prevent displacement of the first locking member in a direction away from the switch plunger axis. Two locking arms may be provided to lock respective locking pins against displacement relative to the switch plunger axis. The locking arms may extend from one end of a solenoid plunger which is arranged at one end of the switch plunger and is displaceable along the switch plunger axis by a solenoid winding within a solenoid housing. The solenoid may be arranged so that, when energised, the locking arms are displaced from the locked position, or alternatively may be arranged so that, when energised, the locking arms are displaced to the locked position. 
     A compression spring may be arranged between the switch and solenoid plungers to bias the plungers apart, and a compression spring may also be arranged between the solenoid plunger and the solenoid housing to bias the solenoid plunger towards the switch plunger. The switch plunger may be axially displaced by rotation of a cam from a datum position by insertion of an actuator into the mechanism, withdrawal of the actuator being prevented unless the cam is rotated back to the datum position, and such rotation being prevented by the locking mechanism if the or each second locking member is in the locked position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic cut-away view of a locking switch mechanism in accordance with the present invention with the switch in an unlocked condition; 
         FIG. 2  illustrates the mechanism of  FIG. 1  after the insertion of an actuator to switch the mechanism and locking of the mechanism; 
         FIG. 3  is a partial perspective view of some of the components of the mechanism of  FIGS. 1 and 2  showing those components in the positions adopted when the switch is unlocked as shown in  FIG. 1 ; 
         FIG. 4  is a side view of the components of  FIG. 3 ; 
         FIG. 5  is a partial perspective view of the components shown in  FIGS. 3 and 4  with those components in the switch locked position corresponding to  FIG. 2 ; 
         FIG. 6  is a side view of the components shown in  FIG. 5 ; 
         FIG. 7  shows the mechanism of  FIGS. 1  to  6  after insertion of an actuator but before locking of the mechanism; 
         FIG. 8  illustrates the application of a force to withdraw the actuator when the mechanism is locked; 
         FIG. 9  illustrates the mechanism after unlocking of the mechanism and partial withdrawal of the actuator; 
         FIG. 10  is a perspective view of assembled components of the locking mechanism and  FIG. 11  is an exploded view of the components making up the assembly of  FIG. 10 ; 
         FIG. 12  is a sectional view through a solenoid plunger incorporated in the mechanism of  FIGS. 1  to  11 ; 
         FIG. 13  is a perspective view of a solenoid locking fork incorporated in the mechanism of  FIGS. 1  to  12 ; 
         FIG. 14  is a sectional view through the solenoid locking fork of  FIG. 13 ; 
         FIG. 15  is a schematic cut-away view of a second locking switch mechanism in accordance with the present invention with the switch in an unlocked condition; 
         FIG. 16  illustrates the mechanism of  FIG. 15  after the insertion of an actuator and locking of the mechanism; and 
         FIG. 17  is a perspective view of a locking fork incorporated in the mechanism of FIGS.  15  and  16 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , the illustrated lockable switch mechanism comprises a housing  1  in which a plunger  2  is slidable and which supports a head assembly  3  supporting a rotatable cam  4 , the cam  4  being rotatable about a pin  5 . The plunger  2  comprises a metal core supporting an outer casing  6  which is slidably received in a sealing cap  7 . The plunger  2  is symmetrical about its longitudinal axis and is slidable relative to the housing  1  along that axis. 
     The end of the plunger  2  remote from the cam  4  is received in a bore  8 , a compression spring  9  being located within the bore  8  so as to bias the plunger  2  in the direction indicated by arrow  10 . The bore  8  is formed in the end of a solenoid plunger  11  which is received within a solenoid housing  12 . Energisation of a solenoid winding (not shown) in the solenoid housing  12  drives the solenoid plunger  11  to the right in FIG.  1 . Denergisation of the solenoid results in the solenoid plunger  11  being moved to the left in  FIG. 1  by a compression spring  13  ( FIG. 2 ) which is located between the solenoid housing  12  and a locking fork  14  which is engaged in a groove extending around the end of the solenoid plunger  11  in which the bore  8  is formed. 
     Two locking pins  15  are positioned on either side of the plunger  2 , the locking pins  15  being biased by springs  16  against the plunger  2 . The locking pins  15  and springs  16  are retained within a housing assembly made up from a frame  17  and a cover plate  18 . It will be seen that with the plunger  2  in the position shown in  FIG. 1  the pins  15  are held at a distance from the axis of the plunger  2  such that they obstruct the passage of arms  19  supported by the locking fork  14  in the direction of the arrow  10 . 
       FIG. 2  shows the assembly of  FIG. 1  after the insertion of an actuator  20  into the head assembly  3  so as to cause rotation of the cam  4 . Such rotation of the cam  4  enables the plunger  2  to move towards the pin  5 . As a result a profile  21  in the form of an annular shoulder on the plunger  2  is moved to the left of the locking pins  15 . The locking pins  15  are biased towards each other so as to remain in contact with the plunger  2 , thereby enabling the arms  19  of the locking fork  14  to pass the locking pins  15 . 
     The actuator  20  and cam  4  are shaped such that insertion of the actuator into the head assembly  3  causes the cam to rotate from a datum position, that is the position of the cam  4  as shown in FIG.  1 . In known manner, the actuator defines projections (not shown) which engage in recesses defined by the cam  4  (as shown in  FIG. 2 ) so that once the cam  4  has been rotated from the datum position the actuator  20  cannot be withdrawn from the head assembly  3  unless the cam  4  has been rotated back to the datum position. An actuator and cam mechanism of this general type is described in the abovementioned U.S. Pat. No. 5,777,284. 
       FIGS. 3 and 4  show the assembly in the unlocked condition. In  FIG. 3 , the solenoid plunger  11  has been moved to the position it assumes when the solenoid is energised and the plunger  2  is in the position in which it is displaced by the cam  4  as far as possible towards the solenoid housing  12 . As a result the spacing between the pins  15  is such that even if the solenoid is then deenergised the arms  19  cannot move past the pins  15 . The pins  15  therefore impose no restraint on the axial displacement of the plunger  2 . In contrast, as shown in  FIGS. 5 and 6 , if the cam  4  is then rotated to displace the plunger  2  so that the pins  15  can drop down the profiled shoulder  21  defined by the plunger  2 , the springs  16  urge the locking pins  15  towards each other so as to engage behind the shoulder  21 . Deenergisation of the solenoid then results in the arms  19  being extended past the pins  15 , restraining the pins  15  against movement away from each other. Any attempt therefore to drive the plunger  2  towards the solenoid housing  12  will be resisted as a result of the pins  15  jamming between the profile  21  and the arms  19 . 
       FIG. 7  shows the assembly after displacement of the plunger  2  towards the cam pin  5 . Unless the solenoid is energised, the arms  19  of the locking fork  14  will engage around the pins  15  as shown in  FIGS. 5 and 6 . In the configuration shown in  FIG. 7  however the solenoid has been energised, displacing the arms  19  to the right. There is then nothing to stop the locking pins  15  being moved apart against the biasing force provided by the springs  16 . Thus if the actuator  20  was to be withdrawn from the head assembly  3  this would result in the displacement of the plunger  2  to the right in  FIG. 7 , such movement being permitted as the tapered surface of the shoulder  21  would push against and force apart the two locking pins  15 . 
     Referring to  FIG. 8 , this shows the assembly if an attempt is made to withdraw the actuator  21  when the assembly is in the configuration shown in  FIG. 2 , that is with the pins  15  locked in position by the arms  19 . Pulling on the actuator  21  causes the cam  4  to rotate in the clockwise direction in  FIG. 8 , thereby applying an axial force to the plunger  2  and causing the plunger to move in the direction indicated by arrow  22 . Such displacement is however resisted by the locking pins  15  which bear against the profile  21 . The arms  19  prevent the pins  15  moving apart and thus further axial displacement of the plunger  2  is prevented. 
     In contrast, if the solenoid is energised so as to displace the arms  19  to the position shown in  FIG. 7 , and the actuator  20  is pulled out of the head assembly  3 , rotation of the cam  4  is not resisted by contact between the pins  15  and the profile  21  and as a result the plunger  2  can be displaced in the direction of arrow  23  as shown in FIG.  9 . 
       FIG. 10  illustrates the housing assembly for the locking pins  15  and springs  16  and  FIG. 11  shows the components of the assembly of  FIG. 10  in exploded form. 
       FIG. 12  is a sectional view through the solenoid plunger  11  showing the bore  8  and the groove extending around the end of the plunger  11  in which the bore  8  is provided, that groove being engaged by the locking fork  14  shown in  FIGS. 13 and 14 . 
     Referring to  FIGS. 13 and 14 , the locking fork which supports the locking arms  19  has a C-shaped body defining an inwardly projecting edge  24 , that edge being received in the slot formed around the end of the solenoid plunger  11  shown in FIG.  12 . The inner faces of the fork arms  19  are tapered such that, on energisation of the solenoid, the arms  19  are released easily from engagement with the pins  15 . 
     Given the structure of the plunger and locking fork combination, it is a relatively easy matter to assemble the combination. In an alternative arrangement it would of course be possible to fabricate the plunger  11  and the locking fork  14  including the locking fork arms  19  as a single piece component. 
     In the embodiment of  FIGS. 1  to  14 , energisation of the solenoid is necessary to release the locking mechanism. The solenoid is not energised accept when it is desired to release the locking mechanism. In the event of a power failure when the mechanism is locked, it is not possible to unlock the mechanism and therefore it is not possible to release the actuator from the cam. The actuator can only be released after the supply of power is restored. In some applications, this can be a significant disadvantage.  FIGS. 15  to  17  illustrate a second embodiment of the invention in which this disadvantage is avoided by relying upon a solenoid which is energised when the switch is locked and de-energised when the switch locking mechanism is released. 
     Referring to  FIGS. 15  to  17 , components of the second embodiment which are equivalent to components of the first embodiment shown in  FIGS. 1  to  14  are identified by the same reference numerals. Thus, in the second embodiment a plunger  2  is biased against a cam  4  by a compression spring  9 . The plunger  2  is located between a pair of locking pins  15  which are biased against the sides of the plunger  2  by springs  16 . The plunger  2  defines a shoulder  21  behind which the locking pins  15  engage when the plunger  2  is displaced towards a pin  5  about which the cam rotates.  FIG. 15  shows the locking mechanism before insertion of an actuator into the assembly so as to rotate the cam. In this configuration the locking pins  15  cannot engage behind the shoulder  21 .  FIG. 16  shows the mechanism after displacement of the plunger  2  as a result of rotation of the cam  4 . In this configuration the pins  15  are biased inwards by the springs  16  so as to engage behind the shoulder  21 .  FIG. 16  shows the locking pins  15  after displacement of a locking fork  14  so that locking arms  19  extend outside the locking pins  15 , thereby preventing the locking pins  15  from moving outwards. In the condition shown in  FIG. 16 , the plunger  2  cannot therefore be moved to the right in  FIG. 16  as such movement would be prevented by inter-engagement between the shoulder  21  and the locking pins  15 . 
     The locking fork  14  is mounted on solenoid plunger  11  and is biased towards the cam  4  by a compression spring  13 . If the solenoid is de-energised, the spring  13  ensures that the locking arms  19  are displaced away from the locking pins  15 . The mechanism is therefore unlocked in that axial movement of the plunger  2  is not obstructed. If the solenoid is energised, the plunger  11  is driven to the right in  FIG. 16  such that, providing the plunger  2  is in the position shown in  FIG. 16 , the locking arms  19  can engage outside the locking pins  15 , thereby locking the mechanism. 
     With the arrangement illustrated in  FIGS. 15 and 16 , the switch will remain locked only so long as the solenoid is energised. When it is desired to unlock the mechanism, the solenoid is simply de-energised. With such an arrangement it will be appreciated that, in the event of a power failure, the mechanism is automatically unlocked. In some applications this is a significant advantage. In contrast, with the mechanism illustrated in  FIGS. 1  to  14 , unlocking of the mechanism requires energisation of the solenoid and therefore in the event of a power failure it would not be possible to release the actuator  20  from the cam  4 . 
       FIG. 17  illustrates the structure of the locking fork  14  of the embodiment of  FIGS. 15 and 16  in greater detail. It will be noted that the locking arms  19  are mounted on an L-shaped extension  25  of the locking fork  14 , the locking fork  14  defining a C-shaped body defining an inwardly projecting edge that is received in a slot formed around the end of the solenoid plunger  11 . 
     It should be appreciated that merely preferred embodiments of the invention have been described above. However, many modifications and variations to the preferred embodiments will be apparent to those skilled in the art, which will be within the spirit and scope of the invention. Therefore, the invention should not be limited to the described embodiments. To ascertain the full scope of the invention, the following claims should be referenced.