Patent Document

CROSS REFERENCE TO RELATED APPLICATION 
     This claims the benefit of United Kingdom Patent Application No. 0019133.8, filed Aug. 4, 2000. 
     BACKGROUND OF THE INVENTION 
     This invention relates to a locking mechanism suitable for use in office type chairs. 
     Office type chairs are commonly provided with various adjustment possibilities, the most common being the height of the seat, the angle of the seat portion and the rake of the seat back. This adjustment is often carried out using spring biased levers, located at one side of the seat, one for each type of adjustment. The way in which this usually operates is that, to adjust a particular feature, the lever associated with that feature is raised, or sometimes lowered, against the action of the spring to unlock the adjustment means. It is necessary to hold the lever against the action of the spring during adjustment as release of the lever will allow it to return to the locked position, thus locking the feature against further movement. Because of the nature of the device being adjusted, it is necessary to have a strong spring as it is necessary to ensure that the seat feature remains locked in use. Unlocking of the seat while in use could cause accidents. 
     Difficulty, arises in carrying out adjustments because of the need to hold the lever against the spring while manoeuvring the chair feature into the desired position. 
     The present invention seeks to avoid this problem by enabling adjustments to be made without the need for holding the lever while making adjustments. 
     SUMMARY OF THE INVENTION 
     According to the invention, there is provided a locking mechanism suitable for use with office type chairs comprising an operating lever movable between a first position in which locking takes place and a second position in which unlocking takes place, spring means for biasing the lever into said first or locking position and catch means for holding the lever in said second or unlocked position on movement of the lever into said second position, the catch means being disengageable by movement of the lever past said second or unlocking position allowing the lever to return to said first or locking position under the action of the spring means. 
     Preferably, the catch means comprises a cam element rotatable with the lever and having a stop surface engageable by a lock element to prevent return of the lever to its locking position, release means being provided to disengage the lock element from the stop surface on movement of the lever past the unlocking position. 
     The stop surface may comprise a step in the cam surface and the lock element comprises a detent engageable with the step. 
     The release means may comprise a slider, slidable in a circumferential slot in the cam element, the slider having a projection engageable with the detent to move the detent away from the cam element when entrained by one end of the slot during the movement of the lever past the unlocking position. The length of the slot may be such that when the lever is in its locking position and the slider is entrained by the other end of the slot, the projection of the slider is disengaged from the detent. 
     Alternatively, the stop surface may comprise a hook located on a surface of the cam and the lock element comprises a pin element engageable in the hook. 
     In this case, the release means may comprise a second pin element constrained to move with the first pin element and movable by another surface of the cam on appropriate movement of the cam to disengage the first pin element from the hook. 
     The cam may be provided with a recessed portion having the hook at the base thereof co operating with the first pin and an Island structure located outwards of the base and co operating with the second pin. 
     An exterior slider arrangement may be provided having a slider or detent engageable with a step on an outer surface of the cam in the unlocked position of the lever. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail, by way of example, with reference to the drawings, in which 
     FIG. 1 is a side diagrammatic view of an office type chair to which one embodiment of the invention is applied; 
     FIG. 2 is a perspective view of the chair locking mechanisms in which the invention is applied to one of the mechanisms, and 
     FIGS. 3 to  7  are views of a locking mechanism in accordance with one embodiment of the invention showing various stages of its operation. 
     FIG. 8 is a perspective view of the elements of a second form of locking mechanism in accordance with the invention, and 
     FIGS. 9 to  14  are views of the locking mechanism depicted in FIG. 8 showing various stages of its operation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown in diagrammatic side view an office type chair  1 . It comprises a seat  3  mounted on a roller frame  5  by means of a telescopic height adjuster  7 . The seat  3  is adjustable so as to be pivotally adjustable so as to adjust between a generally horizontal position and a forward tilting position. The seat  1  also has a back rest  9  which is adjustable for rake. 
     Control of the three types of adjustment is carried out by a three lever arrangement  11  of which the first lever  13  provides for seat portion tilt, the second lever  15  provides for seat height and the third lever  17  provides for back rake. The present chair is provided with a locking arrangement in accordance with the invention in connection with the first lever  13  although it will be understood that the invention could equally be applied to any of the levers or to all of them. The lever  13  is selected in the present situation as this is believed to be the operation which is most difficult to perform adequately of the three. 
     FIG. 2 shows a perspective view of the three lever mechanism  11  of which only the mechanism of the lever  13  will be considered. It is to be observed that neither the connection between the mechanism and the feature being controlled, usually a bowden cable or the like, nor the spring for biasing the lever  13  are shown for the sake of clarity. 
     As can be seen, the lever  13  is connected to pivot about an axle  21  with which it moves and which carries a rotary cam element  23 , also tied to the movement of the lever  13 . The cam element  23  has a cam surface  25  provide with a step  27  which co-operates with a spring urged detent  29  as will be described hereafter. The cam element  23  also has a circumferentially extending slot  31  in which is located a slider  33  freely slidable therein. The slider  33  has a projection  35  which also interacts with the detent  29 . 
     The operation of the mechanism will now be describe with reference to FIGS. 3 to  7 : 
     FIG. 3 shows the lever  13  in the locked position. In this position, the cam element  23  is positioned with its step  27  above the detent  29  so that the detent rides on the high part of the cam  23 . The lever  13  is retained in this position by a biasing spring (not shown). The slider  33  is located in the clockwise end of the slot  31 . 
     In order, to unlock the seat tilting mechanism, the lever  13  is raised through the position shown in FIG. 4 where it will be seen that the cam element  23  has rotated clockwise and the projection  35  of the slider  33  has engaged the detent  29  and is starting to move anticlockwise in the slot  31 . 
     In FIG. 5, the lever has reached its unlocking position. In this position, the detent  29  has passed over the step  27  so as to engage behind it, thus preventing a return of the lever  13  to its locking position. At this time also, the slider  33  has been pushed back to the anticlockwise end of the slot  31 . 
     Having attained this position, the lever  13  can be released and the position of the chair seat  3  can be adjusted. Once the desired adjustment of the seat  3  has been achieved, the lever  13  is released to return to its locking position as shown in FIG.  3 . To achieve this, the lever  13  is raised still further, again rotating the cam element  23  clockwise. The projection  35  of the slider  33  remains in contact with the detent  29  but the slider  33  cannot be moved any further anticlockwise. Instead, it is moved clockwise by the end of the slot  31 , forcing the detent  29  outwards away from the cam element  23 , releasing it and permitting anticlockwise movement of the lever  13  under the action of its spring to its locking position shown in FIG.  6 . In passing to this position, the engagement of the projection  35  of the slider on the detent  29  causes the slider  33  to remain stationary and thus be repositioned at the clockwise end of the slot  31  (FIG.  7 ). 
     At this point, the slider  33  is again entrained by the slot  31  to move anticlockwise and the projection  35  disengages from the detent  29 , allowing the detent  29  to reengage the surface  25  of the cam element  23 . 
     FIGS. 8 to  14  show a second embodiment of the invention. 
     FIG. 8 shows a perspective view of the elements of a second locking mechanism in accordance with the invention. here only the parts actually involved in the locking are shown. Thus the mechanism basically comprises three parts viz: a cam  51  pivoted about a pivot bore  41 , a spring urged pin assembly  43  pivoted about a bore  45  and a pivoted slider arrangement  47  pivoted about a bore  49 . 
     The lever  13  (FIGS. 9 to  14 ) is constrained to pivot about the same axis as the cam  51  and to move therewith. 
     The cam has an outer surface having a detent recess  81  thereon. It also has a recessed portion  46  in which are located a cam island  67  with upper and lower edges  65  and  85  respectively. At the right hand end of the cam island  67  is a hook recess  83 . The base portion of the cam recess also has a hook recess  77 . The cam, recess has a left hand side wall  57  and a right hand side wall  75 . The cam also has a bore  48  to receive the end of a Bowden cable  79  as can best be seen in FIGS. 9 to  14 . This Bowden cable  79  acts to provide locking and unlocking of the chair feature to be adjusted in known manner. 
     The spring urged pin assembly comprises a wire element having a spring part  91 . two arms  93  and  95  ending in right angle bends to form two pins  53  and  55  which co-operate with the various parts of the cam recess  46  as will be described hereafter. 
     The pivoted slider arrangement  47  comprises an arm  97  pivoted about a bore  49  and urged against the surface of the cam  51  by spring means (not shown). It carries a stop  69  adjacent to the bore  49  and a slider  59  at its free end. 
     The operation of the mechanism will now be describe with reference to FIGS. 9 to  14 : 
     FIG. 9 shows the lever  13  in the locked position. In this position, the cam element  51  is in its most clockwise position with both pins  53  and  55  against the left hand wall  57  thereof and the slider  59  is sliding on the high part  61  of the cam element  51 . The cam element  51  from is then in its most clockwise position. The lever  13  is retained in this position by a biasing spring (not shown). 
     In order to unlock the seat tilting mechanism, the lever  13  is raised through the position shown in FIG. 10 where it will be seen that the cam element  51  has rotated anticlockwise thereby and the second pin  55  has slid rightwards along the surface  63  on the lower surface of the cam recess  46  and caused the upper pin  53  to be raised upwards and passed forward into the recess  83  in the cam island. The lower pin  55  has moved past the end of the surface  63 . In this position, the slider  59  of the slider arrangement  47  will have dropped into the detent recess  81  and will prevent any significant return movement of the lever  13 . 
     Release of the lever  13  will cause it to move clockwise causing the lower pin  55  to enter the hook recess  77  and prevent further clockwise movement, locking the mechanism in this position as shown in FIG.  11 . The upper pin  53  will also drop out of the recess  83 , leaving it free. At the same time, the slider  59  will also be brought up against the shoulder of the detent recess  81  so as to assist the prevention of clockwise movement of the cam  61   
     With the lever  13  in this position, adjustment of the chair can take place. 
     To move the lever  13  back to its locking position once adjustment of the chair has taken place, the lever  13  is again raised slightly moving the lower pin  55  out of the hook recess  77  and, as a result, moving the upper pin  53  along the underside of the cam island  67  so as to clear it. Further upward movement of the lever  13  moves the lower pin  55  along the bottom surface of the cam recess  46  raising the upper pin  53  around the end of the cam island  67  so as to engage the upper surface thereof (FIG.  12 ). At the same time, the slider  59  moves out of the detent recess  81 . 
     Release of the lever  13  at this point will cause it to move down under the action of its spring and rotate the cam  61  clockwise. The pin  53  moves along the upper surface of the cam island  67  engaging the stop  69  and raising the slider  59  into a position (FIG. 13) in which it is clear of the detent recess  81 . 
     The released lever  13  continues to move downwards, further rotating the cam  61  clockwise. The upper pin  53  moves along the upper surface of the cam island, still maintaining the stop  69  in its raised position so that the slider  59  again engages the outer surface of the cam  61  beyond the detent recess  81  as shown in FIG.  14 . 
     Finally, the lever returns to its starting position as shown in FIG. 9 with the two pins  53  and  55  engaging the leftward wall  57  of the cam recess  46 , thus preventing any further clockwise movement of the cam  61  and thus also the lever  13 . 
     It will be appreciated that various modifications or additions may be made to the above described embodiment without departing from the scope of the invention. For example, the operation of the mechanism could be reversed, with the lever operation being downwards instead of upwards. 
     In the second embodiment, the pins  53  and  55  could be mounted on a pivotally arranged biased plate. Also, it will be appreciated that the slider arrangement  47  is not essential to the operation of the mechanism and merely provides additional strength. It can therefore be omitted in some circumstances. 
     Furthermore, while the mechanism has been described for use with an office type chair, it will be understood that the mechanism could be applied in other situations where a locking mechanism of this type would be useful.

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