Abstract:
A safety switch operating mechanism having an engagement mechanism mechanically linked to a plunger and arranged to receive an actuator such that insertion of the actuator into the engagement mechanism moves the plunger to a first position and removal of the actuator from the engagement mechanism moves the plunger to a second position. The safety switch operating mechanism having a resilient member which engages with the plunger and resiliently resists movement of the plunger from the first position to the second position. The engagement of the resilient member with the plunger resists inadvertent removal of the actuator from the engagement mechanism.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to British Patent Application GB0515583.3 filed on Jul. 29, 2005 and the entirety of which is incorporated herein. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a safety switch operating mechanism. 
     Safety switches are well known, and are typically used to prevent access to for example dangerous electromechanical machinery when that machinery is in operation. In an conventional arrangement the safety switch is mounted on a door post of a machinery guard, and an actuator for the safety switch is mounted on a corresponding door. When the door is closed the actuator engages with the safety switch, which in turn closes an electrical contact that allows power to be supplied to the machinery. This arrangement ensures that power can only be supplied to the machinery when the guard door is shut. When the guard door is opened, the actuator disengages from the safety switch, thereby opening the electrical contact and cutting off the supply of power to the machinery. 
     In some instances a problem has arisen in that an operating mechanism of the safety switch may allow an actuator to be too easily removable from the safety switch. In one situation, vibration of the electromechanical machinery may be sufficient to cause the actuator to jump out of the safety switch, allowing the door to swing open and interrupting the supply of power to the electromechanical machinery. Since this immediately interrupts operation of the electromechanical machinery, it will be appreciated that it reduces the efficiency of the operation of the machinery. An engineer or other operator must close the door of the housing, so that the actuator engages with the safety switch, thereby allowing power to be supplied to the electromechanical machinery before it can resume operation. 
     The present invention is directed to overcome or substantially mitigate the above disadvantage. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a safety switch operating mechanism comprising an engagement mechanism mechanically linked to a plunger, the engagement mechanism being arranged to receive an actuator such that insertion of the actuator into the engagement mechanism moves the plunger to a first position and removal of the actuator from the engagement mechanism moves the plunger to a second position, wherein the safety switch operating mechanism further comprises a resilient member which engages with the plunger, and which resiliently resists movement of the plunger from the first position to the second position and thereby resiliently resists removal of the actuator from the engagement mechanism. 
     The invention is advantageous because it reduces the likelihood of the actuator accidentally being removed from the engagement mechanism. 
     Preferably, the resilient member comprises a planar member formed from a resilient material. 
     Preferably, the planar member is configured such that it may flex about a fulcrum point, the fulcrum point being located partway along the planar member. 
     Preferably, the location of the fulcrum point is adjustable using an adjustment member. 
     Preferably, the adjustment member comprises a block, the block being configured to provide an abutment point which presses against the planar member, thereby establishing the fulcrum point. 
     Preferably, the orientation of the block is adjustable to allow the abutment point to be located at different positions on the planar member. 
     Preferably, the block is provided with a plurality of faces, at least some of which provide different abutment points. 
     Preferably, the block is provided with four or more faces. 
     Preferably, the block may be rotated to allow the different abutment points to press against the planar member. 
     Preferably, the block is rotatably mounted and is connected to an adjustment device. 
     Preferably, the block may be inverted, to allow a given abutment point to press against a different position on the planar member. 
     Preferably, the planar member is L-shaped. 
     Preferably, the resilient member is provided with a recess which engages with the plunger. 
     Preferably, the engagement mechanism is a rotatably mounted cam member. 
     Preferably, the cam member is provided with a cam surface which pushes the plunger against the resilient member during removal of the actuator from the engagement mechanism. 
     Preferably, the plunger is one of a plurality of plungers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A specific embodiment of the invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a safety switch operating mechanism which embodies the invention, with an actuator in situ; 
         FIG. 2  shows components of the safety switch operating mechanism of  FIG. 1 ; 
         FIG. 3  shows the safety switch operating mechanism of  FIG. 1  with the actuator being removed; 
         FIG. 4  shows a variation to the safety switch operating mechanism of  FIG. 1 ; and 
         FIGS. 5 and 6  show part of an alternative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a safety switch  10  for use on a door or gate of a guard for electromechanical machinery. The safety switch  10  requires insertion of an actuator  12  for electrical contacts to be made to allow the machinery to operate. The electrical contacts, which are not shown in  FIG. 1 , are included in the power supply circuit for the machinery such that opening the electrical contacts will interrupt the supply of power to the machinery. The safety switch  10  may be, for example, mounted on a guard door post and the actuator  12  mounted on the guard door, so that closing of the guard door inserts the actuator into the safety switch. 
     The safety switch  10  has a body (not shown in  FIG. 1 ) provided with an aperture or a pair of apertures through which the actuator  12  is insertable to act on an engagement mechanism comprising a rotatably mounted cam member  14 . The cam member  14  is shaped to cause linear movement of a plunger  16  to operate the electrical contacts (not shown). The plunger  16  passes into an inner housing  18  which contains the electrical contacts. An end of the plunger  16  which is furthest from the cam assembly  14  is connected by a helical spring  20  to a wall of the inner housing  18 . The helical spring  20  resiliently biases the plunger  16  towards the cam member  14 . Although the plunger is illustrated as comprising two parts  16   a ,  16   b , it will be appreciated that the plunger may alternatively be formed as a single entity (or may have any other suitable form). 
     In  FIG. 1  the actuator  12  has been inserted into the safety switch  10  and has moved the cam member  14  to a first rest position. When the cam member  14  is in the first rest position, a recess  22  in the cam member is aligned with the plunger  16 . This allows the helical spring  20  to push the plunger  16  outwards, to the position shown in  FIG. 1 . When the plunger  16  is in this position, the electrical contacts which allow power to be supplied to the electromechanical machinery are closed. 
     Above the plunger  16 , a resilient member  24  is mounted on a housing  26  of the safety switch  10 . The resilient member  24  is planar, and is arranged in an L-shape. One limb of the resilient member  24  is fixed to the housing  26  by means of a bolt  28  which passes through a block  30 . The other limb of the resilient member  24  depends from the housing  26 , a free end of the resilient member locating in a neck of the plunger  16 . 
       FIG. 2  is a perspective exploded view which shows the resilient member  24 , the bolt  28 , the block  30 , and a lid  26   a  of the housing  26 . From  FIG. 2  it can be seen that the resilient member  24  is provided with a recess  32  at the end of one limb, the recess being positioned such that it locates over the plunger  16  when the safety switch  10  is assembled. The other limb of the resilient member  24  is provided with an aperture  34  which aligns with a corresponding aperture  36  in the block  30 , thereby allowing the resilient member and the block to be securely fixed to the lid  26   a  of the housing  26  using the bolt  28 . 
     The construction of the resilient member  24  is such that when it is in an equilibrium configuration (i.e. when no forces are being applied to it), it depends directly downwards as shown in  FIG. 1 . When the resilient member  24  is in this configuration it does not apply any force to the plunger  16 . 
       FIG. 3  illustrates the removal of the actuator  12  from the cam member  14 . The cam member  14  must be rotated through approximately 90 degrees before the actuator  12  can be removed from the safety switch  10 . The recess  22  provided in the cam member  14  is curved such that during rotation of the cam member the plunger  16  is pushed towards the inner housing  18  by the cam member. Movement of the plunger  16  in this direction opens the electrical contacts (not shown) of the safety switch. The plunger  16  pushes against the resilient member  24 , which resiliently bends as shown in  FIG. 3 . The resilient member  24 , when bent in this manner, applies force to the plunger  16  which pushes the plunger  16  towards the cam member  14 . The plunger  16 , by pushing against the cam member  14 , resists rotation of the cam member in the clockwise direction, which in turn resists removal of the actuator  12  from the safety switch  10 . The resilient member  24  thus provides a resistive force which acts against the withdrawal of the actuator  12  from the safety switch  10 . This is advantageous because it reduces the likelihood of the actuator  12  being accidentally removed from the safety switch  10  (for example due to vibration of the guard upon which the safety switch and actuator are mounted). 
     The force applied by the resilient member  24  which acts against removal of the actuator  12  depends upon the material properties of the resilient member, its thickness, and also the length of that part of the resilient member which generates the force. The resilient member  24  may for example be formed from stainless steel or some other suitable metal or other material. The resilient member  24  may be for example between 0.25 and 0.4 millimetres thick. 
     Referring to  FIG. 3 , it can be seen that an upper portion  24   a  of the resilient member  24  remains static when the plunger  16  is pushed towards the inner housing  18 , whereas a lower portion  24   b  of the resilient member bends towards the inner housing. The length of the lower portion  24   b  of the resilient member is dictated by the block  30 . The block  30  is provided with a tapered face  38 , a lowermost end of the tapered face  38  providing an abutment point  39  which presses the resilient member  24  against an inner surface of the housing  26 . The block  30  thereby provides a fulcrum  40  below which the resilient member  24  is allowed to bend (i.e. the lower portion of  24   b  of the resilient member). 
     Referring to  FIG. 4 , the block  30  may be inverted such that the abutment point  39 , and hence the fulcrum  40   b  below which the resilient member  24  is allowed to bend, is located further away from the plunger  16 . The lengthening of the resilient member  24  which results has the effect of reducing the force that is generated by the resilient member when the plunger  16  pushes against it. This is, turn reduces the amount of force that is required in order to remove the actuator  12  from the safety switch  10 . 
     The orientation of the block may be selected to be as shown in  FIG. 3  or as shown in  FIG. 4 , depending upon the specific requirements of the application for which the safety switch  10  is used. 
     In order to invert the block  30 , the lid  26   a  of the housing  26  is removed by unbolting lid securing bolts  42 . The bolt  28  is then unbolted from the lid  26  to allow the block  30  and the resilient member  24  to be disassembled, as shown in  FIG. 2 . The block is positioned in the desired orientation, and is secured together with the resilient member  24  using the bolt  28 . The lid  26  is then replaced and secured using the securing bolts  42 . 
     It will be appreciated that in addition to the block  30  being inverted, the block may also be rotated. Referring to  FIG. 3 , it can be seen that a right hand side of the block  30  is curved such that it provides an abutment point  39   b  halfway between an uppermost and a lowermost surface of the block. Rotating the block through 180 degrees, for example following disassembly as shown in  FIG. 2 , will result in the abutment point  39   b  pushing against the resilient member  24 . The part of the resilient member  24  which is allowed to bend will thus be midway between the lengths shown in  FIGS. 3 and 4 , with the result that an intermediate force is applied by the resilient member when the actuator  12  is removed from the safety switch  10 . 
     The resilient member  24  may be arranged to apply a restraining force, which resists removal of the actuator  12  from the safety switch  10 , of for example between 10 and 100 Newtons, depending upon the orientation of the block  30 . 
     An alternative embodiment of the invention is shown in  FIGS. 5 and 6 . Like reference numerals are used in  FIGS. 5 and 6  for elements which correspond with those shown in  FIGS. 1 to 4 .  FIG. 5   a  shows in section a perspective view of a lid  26   a  of a housing of a safety switch. A resilient member  24  and a block  30  are secured to the lid  26   a  by a bolt (not visible) which passes through the lid and is secured in a selecting knob  42 . The selecting knob  42  is rotatable, and is arranged such that when it rotates it causes the block  30  to rotate with it. This is advantageous because it allows different abutment points  39  to be pushed against the resilient member  24 . For example, in  FIG. 5   a  an abutment point  39   c , which is part way down the block  30  is pushed against the resilient member  24 , whereas in  FIG. 5   b  an abutment point  39   d  which is at the bottom of the block  30  is pushed against the resilient member  24 . 
     Alternative abutment points may be provided at different heights on other faces of the block  30 . If desired, the block may be provided with more faces, for example the block may be hexagonal in cross-section. 
     Corners between faces of the block  30  may be rounded off, to allow the block to be easily rotated using the selecting knob  42 . 
       FIG. 6  is a perspective view of the lid  26   a  and the selecting knob  42 . As shown in  FIG. 6 , the lid  26   a  may be provided with indicators, and the selecting knob  42  may be pointed at one side, such that a user can easily determine which face of the block  30  is pushed against the resilient member  24 . 
     It will be appreciated that the plunger  16  referred to above may be one of a pair (or more) of plungers that act in unison. 
     Although the resilient member  24  is illustrated as an L-shaped member in the described embodiments, it will be appreciated that it may take other suitable forms. For example, the resilient member may be straight rather than L-shaped. An L-shape is preferred because this allows more convenient attachment of the resilient member  24  to the housing  26  of the safety switch  10 . 
     The electrical contacts provided in the safety switch  10  may be any suitable type of mechanically actuated contacts. One form of safety switch to which the embodiment of the invention could be applied is the MTGD2 switch (proprietary trademark) sold by EJA Engineering of Wigan, United Kingdom. 
     Although the description of the safety switch  10  refers to it being provided on a guard of electromechanical machinery, it will be appreciated that the safety switch may be used for any other suitable purpose. For example, the safety switch  10  may be provided on a guard of an electrical circuit or circuits. 
     Although the actuator  12  has been described as being provided on a guard door, it will be appreciated that the actuator  12  may be provided in any other suitable location. For example, the actuator  12  may be located on a chain near to the safety switch  10 . Where this is the case the safety switch  10  may be arranged to lock the guard door when the actuator  12  is inserted into the safety switch  10 .