Abstract:
A mechanism for adjusting the height of a chair back relative to the seat includes an arm that extends upwardly from the rear of the seat, and a carriage that is carried by the back and slidably mounted on the arm. A pinion inside the carriage engages a rack on the arm so that the pinion travels along the rack when the chair back is moved up and down on the arm. A pawl engages the pinion when the chair back is moved upwardly so as to allow incremental adjustment in the height of the back in the upward direction, while preventing downward movement of the back. When the back reaches the upper limit of its travel, an actuator spring is displaced downwardly, disengaging the pawl so that the back is free to slide on the arm. At the bottom of the limit of travel, the spring is displaced upwardly, re-engaging the pawl.

Description:
FIELD OF THE INVENTION 
     This invention relates to a mechanism for adjusting the height of a chair back relative to the seat of the chair. 
     BACKGROUND OF THE INVENTION 
     The invention has been devised in the context of back height adjustment mechanisms for office chairs. A chair of this type typically includes a seat supported on a base provided with castors. A generally L-shaped arm extends rearwardly from below the seat and then upwardly and the back is coupled to the upright limb of the arm so that the back can be adjusted height-wise on the limb to suit the comfort and preference of the user of the chair. 
     A simple form of adjustment mechanism includes a bolt having a head in the form of a large knob or knurled handle that can be turned manually by the person adjusting the chair to clamp the back to the arm. Adjustment is essentially a two-handed operation in that the bolt must be loosened while the back is supported and then moved to an adjusted position. The back is then held in that position while the bolt is re-tightened. 
     More recently, adjustment mechanisms have become available that can be operated easily using one hand. U.S. Pat. No. 5,649,741 (Beggs) and U.S. Pat. No. 5,685,609 (Miotto) both disclose adjustment mechanisms of this type in which a pawl carried by the chair back engages a rack on the upright limb of the arm. As the back is raised, the pawl ratchets along the rack until an appropriate adjusted position is reached. If the chair back is moved to the top of its travel, the pawl disengages and the back can be moved down to a bottom position, where the pawl re-engages and the adjustment process can start again. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved chair back height-adjustment mechanism. 
     The mechanism provided by the invention includes first and second structural members that are adapted to be connected to the seat and back respectively and are coupled to one another for relative sliding movement along a defined axis along which the back is to be adjusted with respect to the seat. A rack is provided on the first structural member and extends in the direction of the defined axis. A pinion carried by the second structural member engages the rack so that the pinion can rotate and move along the rack in response to relative movement between the first and second structural members along the said axis. A pawl is pivotally mounted on the second structural member for movement between (a) a position in which the pawl engages the pinion and permits rotation of the pinion in one rotational direction only corresponding to relative movement between the first and second structural members for height-wise adjustment of the chair back in a first direction along said axis, while preventing relative movement between the first and second structural members in a second, opposite direction, and (b) a disengaged position in which the pawl is clear of the pinion and the first and second structural members are free for movement in either said direction. Actuator means is carried by the second structural member for moving the pawl between its engaged and disengaged positions at respective limits of travel of the first and second structural members with respect to one another along said axis. 
     Typically, the first structural member of the mechanism comprises an arm that extends upwardly from the seat of the chair and the second structural member is a carriage that rides on the arm and is coupled to the back. While the mechanism can be designed to operate in either direction, the arrangement preferably is such that the back is moved incrementally in the upward direction to find a suitable adjusted position. When the limit of travel in that direction is reached, the actuator means disengages the pawl from the pinion so that the back can be moved freely downwardly to the bottom limit of its travel where the pawl is re-engaged so that the adjustment process can start again. 
     The first structural member (arm) may be connected directly to the seat but normally will be connected to the seat via the chair base. 
     It is believed that a rack and pinion adjustment mechanism of the form provided by the invention offers a number of significant advantages over the prior art, particularly those exemplified by the Beggs and Miotto patents referred to previously. The use of a rack and pinion allows the use of a finer tooth pitch so that finer increments of adjustment are possible. Also, the mechanism can be designed to operate more quietly. A number of other practical advantages are offered, in terms of ease of assembly and lower cost. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     In order that the invention may be more clearly understood, reference will now be made to the accompanying drawings which illustrate a particular preferred embodiment of the invention by way of example, and in which: 
     FIG. 1 is a perspective view from the rear showing a chair height-adjustment mechanism in accordance with the invention in association with other components of chair (shown in ghost outline); 
     FIG. 2 comprises two side elevational views (a) and (b) that illustrate the manner in which the height of the back can be adjusted; 
     FIG. 3 is an elevational view in the direction of arrow B in FIG. 1 with a front plate of the mechanism removed to show internal components; 
     FIG. 4 is an exploded perspective view corresponding to FIG. 3; 
     FIG. 5 is a perspective view of the front plate of the mechanism, showing its inner-face; and, 
     FIGS. 6 to  9  are elevational views similar to FIG. 3 illustrating sequential steps in the operation of the mechanism. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring first to FIG. 1, a chair back height-adjustment mechanism in accordance with the invention is generally indicated by reference numeral  20  and is shown in association with a typical office chair shown in ghost outline. The chair has a base, part of which is indicated at  22 , that supports a seat  24 , and a back denoted  26  that is coupled to the base  22  by the adjustment mechanism  20 . The mechanism includes first and second structural members, namely an arm or bracket  28  and a carriage  30 . As best seen in FIG. 2, arm  28  is generally L-shaped and includes a horizontal limb  28   a  that is coupled to the seat  24  via the chair base  22 , and a vertical limb  28   b  that extends upwardly behind the back  26 . Carriage  30  is mounted to slide up and down the vertical limb  28   b  of arm  28  along a defined axis denoted A—A and is secured to the back  26 . 
     Arm  28  is essentially a flat rectangular section steel bar having a right-angled configuration. The carriage  30  is designed to embrace the upright limb  28   b  of arm  28  so that the carriage can slide up and down on the arm. The principal components of the carriage  30  are a backing member  32  that defines a recess  32   a  generally complimentary to the cross-sectional shape of the upright limb  28   b  of arm  28 , and a front plate  34  that is secured by screws  35  to the backing member  32  so as to lie in sliding contact with the outer face of limb  28   b.    
     FIG.  2 ( a ) shows the chair in a configuration in which the back  26  is in a bottom position close to the seat  24 . As will become apparent later, the height adjustment mechanism of the invention allows the back  26  to be raised in increments from the bottom position shown in FIG.  2 ( a ) to a top position shown in FIG.  2 ( b ). Once the carriage  32  reaches the top of its travel with respect to the vertical limb  28   b  of arm  28 , the mechanism releases and the carriage (and back) can be slid freely down limb  28   b  as indicated by the arrow  36  to the bottom position shown in FIG.  2 ( a ), from which the adjustment process can be resumed. 
     FIG. 3 shows the adjustment mechanism of the invention as seen in the direction of arrow B in FIG. 1 but with the plate  34  removed. FIG. 4 shows the same components as FIG. 3 with the backing member  32  of carriage  30  in an exploded position above the upright limb  28   b  of arm  28 . Limb  28   b  is formed with a vertically elongate, generally rectangular recess  38 , one vertical side edge of which is fitted with a rack  40 . Typically, the rack  40  is formed separately from the arm  28  and press-fitted into recess  38 . 
     The backing member  32  of carriage  30  is a plastic moulding, the shape of which is best seen in FIG.  4 . Recess  32   a  of the moulding has a flat back wall  32   b  which confronts the rear face of limb  28   b  and moulded formations  32   c  that slide against the side edges of limb  28   b.  Formations  32   c  also provide surfaces against which plate  34  is secured by screws (not shown) that extend through openings  34   a  (FIG. 5) in plate  34 . 
     Carriage  30  is designed for low-friction sliding movement on limb  28   b  while providing a low clearance or tolerance between the carriage and the limb. In other words, the carriage can slide freely up and down on the limb but remains stable and will not “wobble” or bind as it moves. To this end, the formations  32   c  of the backing member moulding include a pair of flexible tabs  32   d  at each side that make contact with the side edges of limb  28   b  and the back wall  32   b  of the moulding includes four flexible tabs  32   e  that provide for line contact between moulding  32  and the rear face of limb  28   b.  Similarly, the inner face of the front plate  34  of carriage  30  is embossed in corner regions to provide four raised dimples or “pips”  34   b,  as shown in FIG. 5, that make point contact with the front face of limb  28   b.    
     In other words, the flexible tabs  32   d  and  32   e  of moulding  32  and the dimples  34   b  of plate  34  take up clearance or tolerance between the carriage  30  and the limb  28   b,  while at the same time minimizing friction between the carriage and the limb. 
     Side margins of plate  34  have openings  34   c  that match similar openings in the formations  32   c  of backing member  32  to receive bolts or screws (not shown) for securing carriage  30  to the seat back  26 . 
     A pinion  42  is rotationally mounted on surface  32   b  in a position to engage the teeth of rack  40  when the carriage is in position on arm  28 . Mounted to surface  32   b  by a pivot pin  44  is a pawl  46  that has two limbs  46   a  and  46   b.  Pin  44  allows pawl  46  to pivot between a position in which limb  46   a  is engaged with pinion  42  as shown in FIG. 3 and a position in which the pawl is disengaged from the pinion as shown, for example, in FIGS. 7 and 8. In the engaged position of FIG. 3, pawl  46  allows rotation of pinion  42  in the clockwise direction in response to upward movement of carriage  30  (arrow  48 ) but prevents rotation of pinion  42  in the opposite direction. In other words, when the pawl is engaged with the pinion  42 , carriage  30  can move upwardly in increments for adjusting the vertical height of the seat back  26 , but downward movement is prevented. In the “pawl disengaged” position of the FIGS. 7 and 8, however, pinion  42  is free to turn so that the carriage  30  can move freely on the upright limb  28   b  of arm  28 . 
     The height-adjustment mechanism of the invention includes actuator means for pawl  46  comprising a spring  50  that acts on pawl  46  to, at appropriate times, either bias limb  46   a  into engagement with pinion  42  or bias the limb  46   b  of pawl  46  downwardly to retract the pawl from engagement with pinion  48 . The actuator spring  50  is displaceable in the direction of the defined axis A—A to appropriately actuate the pawl  46 , by abutment with end stops provided by top and bottom surfaces respectively of the opening  38  in which the rack  40  is formed. In this way, the pawl is automatically disengaged from pinion  42  when the carriage  30  reaches the upper limit of its travel, so that the carriage can then move freely down on limb  28   b,  and is re-engaged with the pinion  42  at the lower limit of its travel. 
     In more detail, it will be seen from FIG. 3 that spring  50  is shaped to provide a generally hook-shaped upper portion having a limb  50   a  that bears against limb  46   a  of pawl  46 , and a lower angled limb  50   b  that cooperates with the bottom limb  46   b  of pawl  46 . The spring is constrained for sliding movement by three formations that project from surface  32   b  of backing member  32 . These projections comprise a rear tab  52  which provides an upright sliding surface for the spring, a tab  54  that constrains the upper hook-shaped portion of the spring, and a pin  56  that locates the bottom limb  50   b  of the spring. 
     When carriage  30  reaches the upper limit of its travel, the top end of spring  50  abuts against an end stop provided by a top inner surface  38   a  of opening  38  in limb  28   b.  The spring is pushed downwardly with respect to the projection  52 ,  54  and  56 . Immediately above projection  54  in the position FIG. 3, is a slightly outwardly curved portion  50   c  of spring  50 . Spring portion  50   c  is pressed inwardly by projection  54  as the spring moves down, as best shown in FIG.  7 . This causes the hook-shaped upper portion of the spring to be compressed somewhat, reducing the biassing effect of spring limb  50   a  of pawl limb  46   a.  At the same time, the whole spring is displaced downwardly so that the bottom limb  50   b  of the spring acts on pawl limb  46   b  to disengaged the pawl from the pinion. 
     FIGS. 6 and 7 best show this sequence of events. In FIG. 6, carriage  30  is moving up towards the upper limit of its travel. In FIG. 7, the spring has contacted the end stop  38   a  and been displaced downwardly so that the pawl is retracted from pinion  42 . The carriage can then move freely down on arm  28 . 
     FIG. 8 shows the carriage approaching the bottom limit of its travel with the pawl  46  still out of engagement with pinon  42 . In this condition, the carriage is free to move down (or up) on limb  28   b.  When the carriage reaches the bottom limit of its travel, stop surface  38   b  of recess  38  is contacted by the bottom limb  46   b  of pawl  46  so that the spring  50  is displaced upwardly, causing its upper end portion  50   a  to urge the pawl  46  back into engagement with pinion  42 . 
     Vertical adjustment of the height of the back can then proceed in increments determined by the pitch of the teeth on rack  40  and pinion  42 . In other words, the user moves the back upwardly in increments until the desired adjusted position is reached. The back will then remain in that position (unless further adjustments is made) and is prevented from moving down by engagement of the pawl  46  with pinion  42  and engagement of the pinion  42  with rack  40 . 
     It will of course be appreciated that the preceding description relates to a particular preferred embodiment of the invention and that many modifications are possible. 
     For example, a different form of actuator could be provided for moving the pawl between its engaged and disengaged positions. The pawl  46  could be provided with detents that positively define its engaged and disengaged positions and the pawl could be displaced between those positions by contact with end stops provided on limb  28   b.    
     More generally, the arrangement could be reversed so that the back is adjusted in the downward direction by movement of carriage  30 . Also, the structural design could be reversed so that the rack is provided on the carriage and the pinion and pawl on the arm  28 . It might even be possible for the carriage ( 30 ) to be connected to the chair seat and the arm ( 28 ) to the back. For example, the arm could extend downwardly into engagement with a carriage supported by a bracket on the seat base. 
     Having said that, it is believed that the design of the preferred embodiment combines the virtues of simplicity and low cost with functionality. The mechanism is found to operate efficiently and quietly. Fine incremental adjustment can be provided by providing a fine pitch rack and pinion. Variations in tolerance between arm  28  and carriage  30 , for example, due to variations in the width of arm  28  are controlled by means of moulded plastic tabs in the backing member of carriage  30 . At the same time, the backing member and front plate  34  make point contact with the arm, for minimizing frictional resistance. 
     In an alternative embodiment, carriage  30  could of course be made as two complimentary mouldings that embrace the arm, rather than as a backing member and front plate.