Abstract:
A chair control providing both articulating tilt motion and dual-ratio, adjustable, synchronized tilt motion. In a first aspect, the control can be configured as either an articulating tilt control or a synchronized tilt control. In a second aspect, the angle between the seat and the backrest may be selected by the user when the control is configured for either the articulating mode or the synchronized mode. In a third aspect, when in the synchronized mode, the angle between the seat and the backrest increases during forward tilt action to avoid “clam shelling.” In a fourth aspect, a forward tilt selector enables or prohibits forward tilting of the control.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to office chairs, and more particularly to tiltable office chairs that provide ergonomic seating positions for a user. 
     Conventional office chairs are configured to allow tilting of the seat and backrest in either an articulating manner or a synchronized tilt controlled manner. With articulating chair controls, or “fixed-tilt” controls, the seat and backrest tilt as a unit with the seat and backrest at all times remaining at a predetermined angle. With synchronized tilt, or “synchro-tilt” controls, the seat and backrest tilt, but at varying rates. For example, the backrest may tilt backward, or recline, at a rate twice the rate of the seat. Put another way, for every one degree the seat tilts, the backrest tilts two degrees. While synchro-tilt control chairs provide a more ergonomically correct tilting action, many municipalities, including the city of New York, require articulating control features so that the user may set the angle of the seat with respect to the backrest at a predetermined angle specific to the user. As a practical matter, a manufacturer is required to produce and inventory both the articulating and synchro-tilt chairs. Accordingly, given the typical lag time between production and sale, manufacturers may be stuck with an excessive supply of one type of chair and a deficiency of another, depending on consumers&#39; demand. 
     Articulating chair controls also allow the user to select the angle between the seat and back; however, after selection, that angle does not change when the chair tilts. With synchro-tilt controls, the relationship between the seat and backrest, as well as the rate of tilt of the seat with respect to the backrest, is predetermined by the design of the control. For example, in the upright position, the angle between the seat and backrest is fixed. As the seat and backrest tilt to a reclined position, the angle between the seat and backrest becomes larger than the angle between the seat and the backrest in the upright position, but the angle remains pre-determined by the design of the control. Thus, with both articulating and synchro-tilt controls, the angle between the seat and the backrest is limited to a single angular relation when the chair reclines or forward tilts. 
     Conventional office chairs are also provided with a forward tilt feature. This feature allows the user to either control the seat so that it does not tilt forward past the upright position, or, alternatively, control the seat to tilt forward past the upright position. In the forward tilt position, the pressure of the forward edge of the seat acting on the underside of a user&#39;s leg is reduced while the feet of the user may remain flat on the floor in a comfortable stance. Conventional chairs come either with or without the forward tilt feature from the manufacturer. Accordingly, a distributor must produce and inventory chairs with and without the forward tilt feature. This can make ordering and inventory management difficult. 
     With conventional synchro-tilt control chairs, when the backrest and seat recline from the upright position, the angle between the seat and the backrest becomes larger. As noted above, the angle between the seat and the backrest increases at about a 2:1 ratio with respect to the angle of the seat to horizontal. Further, when the seat and backrest are tilted forward from the upright position, the angle between the seat and the back closes at the same 2:1 ratio. Accordingly, the user will typically find herself being “clam-shelled” between the seat and the backrest because the backrest is tilting forward at a rate greater than the forward tilting of the seat. This causes the user unnecessary discomfort and a potentially ergonomically incorrect position. 
     SUMMARY OF THE INVENTION 
     The aforementioned problems are overcome by the present invention wherein a chair tilt control is convertible between an articulating chair control and a synchro-tilt control, provides adjustment of the angle between a seat and backrest in synchro-tilt mode, is convertible between forward tilt and non-forward tilt; and provides a first rate of when the chair is reclined and a second rate when the chair is tilted forward. 
     In a first aspect of the invention, the chair control is convertible between an articulating, or “fixed,” tilt control or a synchro-tilt control. A chair manufacturer or distributor can convert the chair from one configuration to the other by simply configuring a lock mechanism in one of two positions. In the preferred embodiment, the control includes a housing—part of the chair base—which is pivotally attached to the seat. The backrest is pivotally attached to the seat. At a second pivot point on the backrest back lock mechanism, which is generally a mechanism of adjustable length, is attached to the backrest. One end of the bar is pivotally connected to the backrest. The other end of the bar may be attached to either a point on the seat or a point on the housing. If the bar is attached to the seat, the angle between the seat and the backrest does not change when the user reclines, thus attaining an articulating control. If the back lock bar is attached to the housing, the angle between the seat and back opens as the seat reclines, thus attaining a synchro-tilt control. 
     In a second aspect of the invention, the control enables the user to select the angle between the seat and the backrest even when the control is configured for synchro-tilt action. In the preferred embodiment, the adjustable synchro-tilt feature is incorporated into the above-described back lock mechanism. As noted above, the back lock is a bar of adjustable length. As a result of adjusting the length between the two points of connection of the back lock bar with the seat and the housing, the angle of the backrest changes in relation to the seat. Accordingly, a user may preselect the angular relationship between the seat and backrest of a synchro-tilt controlled chair as easily as a user may pre-select the angle in the articulating control chairs of the prior art. 
     In a third aspect of the invention, an office chair includes a forward tilt mechanism that may be enabled or disabled readily and easily, for example with the installation or removal of a lever. When the lever is removed, the forward tilt mechanism is disabled and the seat and backrest cannot tilt forward. When the lever is installed, a user may engage the forward tilt mechanism so that the seat and backrest tilt forward. 
     In a fourth aspect of the invention, the synchro-tilt chair control includes a dual-ratio action, meaning that the angle between the seat and back change at a first rate when the chair is reclined and at a second different rate when the chair is tilted forward. This feature solves the clam-shelling problem of the prior art. The geometry employed in the synchro-tilt control of the present invention is such that when the backrest and seat move from the neutral upright position into the forward tilt position, the rates of angular disposition of the backrest to the seat do not continue at the 2:1 ratio of the prior art. Rather, the closing movement reverses as the chair moves past the upright position so that the angle between the backrest and the seat increases. Accordingly, the backrest opens up with respect to the seat, therefore avoiding pinching of the user. 
    
    
     These and other objects, advantages, and features of the invention will be more readily understood and appreciated by reference to the detailed description of the preferred embodiment and the drawings. 
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a chair of the present invention; 
     FIG. 2 is a side elevational view of chair controls; 
     FIG. 3 is a perspective view of the chair in an articulating mode; 
     FIG. 4 is a perspective view of the chair in a synchro-tilt mode; 
     FIGS. 5-7 are side elevational views of the chair translating through angles in the articulating mode; 
     FIGS. 8-10 are side elevational views of the chair translating through angles in the synchro-tilt mode; 
     FIG. 11 is a side elevational view of the chair at a maximum adjustable synchro-angle; 
     FIG. 12 is a side elevational view of the chair at a minimum adjustable synchro-angle; 
     FIGS. 13 and 13 a  are sectional views of a rack assembly in a first engaged mode; 
     FIGS. 14 and 14 a  are sectional views of the rack assembly in a second engaged mode; 
     FIGS. 15 and 15 a  are sectional views of the rack assembly in a disengaged mode; 
     FIG. 16 is a perspective view of an articulating mechanism of the present invention; 
     FIG. 17 is a sectional view of a cam of the articulating mechanism; 
     FIG. 18 is a sectional view of the articulating mechanism with a lever installed; 
     FIG. 19 is a side elevational view of the articulating mechanism with the lever rotated to provide forward tilting ability; 
     FIG. 20 is a detail view of an articulating adjustment shaft installed in a bore of the articulating cam; 
     FIG. 21 is a sectional view of the articulating adjustment shaft installed in the bore of the articulating cam; 
     FIG. 22 is a detailed sectional view of the articulating cam and housing rib detents; 
     FIG. 23 is a graph illustrating adjustable synchro-angle data; 
     FIG. 24 is a cut-away detail of a chair height adjustment mechanism; 
     FIG. 25 is a side elevational view of a vertical backrest adjustment mechanism; 
     FIG. 26 is a sectional view of a tilt adjust mechanism; and 
     FIG. 27 is a top plan view of the tilt adjust mechanism. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A first embodiment of the chair of the present invention is depicted in FIGS. 1 and 2 and generally designated  10 . The chair includes a housing  30  and a top plate  50  pivotally mounted to the housing. A conventional base  20  is coupled to the housing. The housing includes a variety of controls for reconfiguring the relationship between backrest  12  and seat  14 . The synchro-angle pull sleeve  70  controls the angle between the backrest  12  and seat  14 . As can be seen, seat  14  is mounted to top plate  50  in a conventional manner. Forward tilt lever  90  may be installed into forward tilt portal  92  to allow the user to forward tilt the seat  14  and backrest  12 . The height adjust lever  100  is used to adjust the distance between the housing  30  and the floor. Seat angle adjust pull sleeve  110  allows the user to adjust the angle of the top plate  50  in relation to the housing  30 . Tilt adjuster  150  permits the user to adjust the force required to recline in the chair. Backrest height adjust  170  controls the vertical displacement of the backrest  12  in relation to the seat  14 . All of these components of the chair controls as described herein may be constructed of any suitable material including but not limited to metal, alloys, composites, or any combination thereof. The seat and backrest may be constructed of any plastic, foam, woven, natural or synthetic materials, or any combination thereof. 
     Articulating/Synchro Tilt 
     The chair of the present invention may readily be converted from an articulating chair to a synchro-tilt chair. FIGS. 3,  5  and  4 ,  8  illustrate the chair in an articulating and a synchro-tilt mode respectively. In the articulating mode of FIGS. 3 and 5, the backrest bracket  13  is held in fixed relation with respect to the top plate  50 , that is, the bracket cannot pivot about backrest bracket pin  34 . The top plate  50  is pivotally connected to the housing pivot pin  60  (FIG.  5 ). Seatback rivet  29  is connected to tilt pivot pin  32  by rack assembly  36  coupled to rack  38 . It will be appreciated that the rack assembly may be substituted by any mechanism adjustable in length to provide different distances between seatback rivet  29  and tilt pivot pin  32  such as driven worm gears, screw jacks, ratcheting mechanisms and the like. Alternatively, the rack assembly  36  may be replaced with a connector that is nonadjustable in length so that seatback rivet  29  and tilt pivot pin  32  are always at the same distance from one another. The preferred rack assembly illustrated in FIG. 3 is adjustable in length and will be discussed in further detail below. With the seatback rivet  29  in fixed relation to tilt pivot pin  32 , the backrest bracket  13  is non-pivotal about main backrest bracket pin  34  (FIG. 5) and maintains a consistent angular relation with respect to top plate  50 . Accordingly, the chair is in an articulating mode. 
     FIGS. 4 and 8 illustrate the chair in a synchro-tilt mode where the backrest bracket  13  may pivot in relation to top plate  50  about main backrest bracket pin  34 . Rack assembly  36  couples the housing  30  to the seatback rivet  29 . The synchro-tilt pivot pin  31  is disposed through the housing synchro-tilt bracket  33  to provide a pivot point to which rack  38  is coupled. As shown, rack  38  is slightly extended from the rack assembly  36 . It will be appreciated and further discussed below that the rack assembly  36  is variable in length so that the distance between seatback rivet  29  and synchro-tilt pivot pin  31  may be altered. Alternatively, the rack assembly  36  may be replaced with a connector that is nonadjustable in length so that seatback rivet  29  and tilt pivot pin  32 , and consequently, the synchro-tilt bracket  33 , are always at the same distance from one another. 
     Fixed/Synchro-Tilt Operation 
     The chair of the present invention may be converted from an articulating to a synchro-tilt chair. With reference to FIGS. 3,  5  and  4 ,  8 , a user may select between a fixed tilt or synchro-tilt chair by the connection of the rack assembly  36  and rack  38  to either the top plate  50  or the housing  30 . As illustrated in FIGS. 3 and 5, the chair is in an articulating configuration with the rack assembly  36  and rack  38  coupled to tilt pivot pin  32 . The tilt pivot pin  32  is removable, and the rack  38  may accordingly be connected to the housing synchro-tilt bracket  33  as illustrated in FIGS. 4 and 8. The user may insert synchro-tilt pivot pin  31  through housing synchro-tilt bracket  33  and rack  38  to convert the chair to a synchro-tilt chair. 
     Reference is made to FIGS. 5-7 to illustrate the relationship between the backrest  12  and the housing  30  in the articulating configuration. As can be seen in these figures, the backrest bracket  13  does not tilt in relation to the top plate throughout a range of tilting of the top plate  50  in relation to housing  30 . Backrest bracket  13  is held in fixed relation to the top plate  50  at pins  34 ,  29 , and  32 , and the backrest bracket  13 . Consequently, the backrest  12  remains at fixed angle Al from top plate  50 . 
     In the preferred embodiment, the top plate  50  and backrest  12  may translate through multiple angles, from 3 degrees forward tilted from horizontal, illustrated in FIG. 6, to 12 degrees reclined from horizontal, illustrated in FIG.  7 . The “neutral position” of the top plate is an angle 3 degrees reclined from horizontal as illustrated in FIG.  5 . It will be appreciated that the “neutral position” may be configured at any pre-selected angle. It will also be appreciated that the configuration of the chair may be altered so that the maximum angle of reclination of the top plate from horizontal is up to 35 degrees and the maximum angle of forward tilt from horizontal of the top plate is up to 20 degrees. 
     FIGS. 8-10 illustrate the chair of the present invention in a synchro-tilt mode. The difference between the articulating configuration of FIGS. 5-7 and the synchro-tilt configuration of FIGS. 8-10 is that the rack assembly  36  couples the backrest bracket  13  to (a) the top plate  50  at the tilt pivot pin  32  in the articulating mode and (b) the housing  30  at the synchro-tilt bracket  33  in the synchro-tilt mode. In the synchro-tilt configuration illustrated in FIGS. 8-10, the backrest bracket  13  pivots about main backrest bracket pin  34  as the chair reclines or tilts forward. For example, FIG. 8 illustrates the neutral position of the chair wherein the top plate  50  is at about an angle 3 degrees reclined from the horizontal. In this neutral position, the backrest bracket  13  and consequently the backrest  12  forms an angle A 2  of about 103 degrees with top plate  50 . It will be appreciated that depending on the geometry of the chair, angle A 2  of this neutral position may be from about 130 degrees to about 70 degrees in a first embodiment, from about 110 degrees to about 90 degrees in a second embodiment, and from about 105 degrees to about 92 degrees in a third embodiment. A detailed description of how angle A 2  may be adjusted is described below. 
     With reference to FIGS. 9 and 10, the top plate may be forward tilted as depicted in FIG. 9 or reclined as depicted in FIG.  10 . In going from the configuration of FIG. 9 to the configuration of FIG. 10, the backrest bracket  13  pivots counterclockwise about main backrest bracket pin  34  so that the angle between the backrest  12  and the top plate  50  opens up. This is due to the rack assembly  36  connecting the lower-most portion of the backrest bracket at seatback rivet  29  to the housing synchro-tilt bracket  33 . As can be seen, the angle between the backrest  12  and the top plate  50  increases the farther the top plate is reclined or forward tilted from horizontal. 
     In translating from a neutral position as illustrated in FIG. 8 to a forward tilt position as illustrated in FIG. 9, the angle between the backrest  12  and the top plate  50  opens up. As the top plate inclines in relation to the horizontal when pivoting about main pivot pin  60 , the backrest bracket  13  rotates counterclockwise about main backrest bracket pin  34  because the backrest bracket  13  is coupled to the housing synchro-tilt bracket  33  by rack assembly  36 . Accordingly, a user seated in the chair is afforded an opened seating position and is not pinched between the backrest  12  and the seat (not shown) attached to the top plate  50 . It will be appreciated that depending on the configuration of the backrest bracket and the top plate, the rotational direction of the backrest bracket may be clockwise as well. 
     Adjustable Synchro-Tilt 
     The chair of the present invention also includes an adjustable synchro-tilt mechanism whereby the angle formed between the backrest  12  and the seat  14  may be altered throughout the entire range of forward tilting and reclination when the chair is in synchro-tilt mode. It will be appreciated that the synchro-tilt mechanism may also be used to adjust the angle between the seat and the backrest in articulating mode as desired. 
     FIGS. 11 and 12 best illustrate the adjustable synchro-tilt mechanism. The rack assembly  36  of the synchro-tilt mechanism couples the backrest bracket  13  to the housing  30 . The rack assembly  36  is pinned to the backrest bracket  13  at seatback rivet  29 . The rack  38  of the rack assembly  36  is pivotally pinned to the housing synchro-tilt bracket  33  with pin  31 . 
     It will be appreciated that the rack assembly  36  may be substituted with any sort of connector that is adjustable in length so that the angle formed between the backrest  12  and the top plate  50  may be increased or decreased in the synchro-tilt or articulating mode. Alternatively, the rack assembly may be replaced with a connector of fixed length; however, the initial synchro-tilt angle in the neutral position would consequently be fixed. Additionally, the rack assembly  36  may be reversed so that the rack  38  attaches to the seatback rivet  29  and the rack assembly  36  is pinned to the synchro-tilt pivot pin. 
     In the preferred embodiment, the rack assembly is a conventional pawl rack assembly as illustrated in FIGS. 13-15. The rack assembly includes a body  40 , a rack guide  41 , a panel  42  and a pawl guide  43 . Spring  45  biases the pawl  42  so that pawl teeth  44  may engage the rack teeth  37  of rack  38 . Cable  46  is operably connected to the pawl  42  to move the pawl  42  within the pawl guide  43 . Cable guide  47  connects to body  40  in a conventional manner to provide an outlet for the cable  46 . Cable  46  may be encapsulated within a sleeve  48  to prevent unnecessary interference with the function of the cable  46 . The pawl  42  of the present invention is preferably outfitted with a Y-ramp system to allow multi-positioning of the pawl within the pawl guide  43 . As depicted in FIGS. 13 a - 15   a , a conventional Y-ramp system  143  is included in a face of the pawl  42 . The Y-ramp  143  interacts with pin  144  that is coupled to the exterior of the body  40 . 
     Operation of Adjustable Synchro-Tilt 
     To adjust the angle of the backrest  12  in relation to the top plate  50 , the rack  38  is extended and retracted from the rack assembly  36  to alter the distance D between the rack assembly  36  and the housing synchro-tilt bracket  33 . This adjustment in the distance is illustrated in comparing FIGS. 11 and 12. From FIG. 11 to FIG. 12, the rack  38  has been extended from the rack assembly  36  so that the angle formed between backrest  12  and top plate  50  is decreased, that is, angle A 5  closes to angle A 6 . Conversely, in going from FIG. 12 to FIG. 11, the rack  38  is retracted into the rack assembly  36  so that the angle formed between the backrest  12  and the top plate  50  increases, that is, angle A 6  opens up to angle A 5 . It will be appreciated that this extension and retraction of a rack  38  from the rack assembly  36  also may be used to alter the angle between the backrest  12  and the top plate  50  when the chair is in the articulating mode, that is, when the rack  38  is attached to the top plate  50  at pin  32  as illustrated in FIGS. 4 and 5. 
     When the pawl  42  engages rack  38  as illustrated in FIGS. 13 and 13 a , pin  144  is in an engaged position  145 . As illustrated in FIGS. 15 and 15 a , when the pawl teeth  44  are disengaged from the rack teeth  39 , the pin  144  is in a disengaged position  146  as illustrated in FIG. 15 a . Accordingly, the rack  38  may extend and retract freely from the body  40 . 
     With reference to FIGS. 13-15 and  13   a - 15   a , the operation of the rack assembly  36  used in the present invention will now be described. As shown in FIGS. 13 and 13 a , the pawl  42  is in an engaged position so that the pawl teeth  44  engage rack teeth  37  whereby the rack  38  cannot retract or extend from rack slide  41 . The spring pushes the pawl  42  into engagement with the rack  38 . With further reference to FIG. 13 a , the pin  144  is in the pawl engaged position  145  in the Y-ramp  143 . 
     To disengage the pawl, the cable  46  is pulled outward through cable guide  47  whereby the spring  45  is compressed and pawl  42  slides within pawl guide  43 . The cable  46  is actuated remotely by a pull sleeve disposed in the housing of the chair  30 , as will be discussed below in further detail. It will be appreciated by those skilled that any mechanism capable of extending and retracting the cable  46  may be used. Pawl teeth  44  are disengaged from rack teeth  37  whereby the rack  38  is free to slide within rack guide  41 . With reference to FIGS. 14 and 14 a , the pin  144  slides in the Y-ramp  143  as indicated until the pawl teeth  44  are fully disengaged from the rack teeth  39 . At this point, illustrated in FIG. 15 a , the pin comes to rest in the pawl disengaged position  146  of the Y-ramp  143 . With the pawl  42  locked in this position by the pin  144 , the rack  38  is free to slide within the rack guide  41  to extended or retracted positions. 
     To re-engage the pawl teeth  44  with the rack teeth  37 , the cable  46  must be momentarily pulled once again through the cable guide  47  to remove the pin  144  from the pawl disengaged position  146 . After the pull of the pawl removes the pin  144  from the pawl disengaged position  146 , the spring  45  pushes the pawl within the pawl guide  43  so that the pawl teeth  44  re-engage the rack teeth  37  as illustrated in FIGS. 13 and 13 a . Accordingly, the pin  144  returns to the pawl engaged position  145 . With this mechanism, the rack  38  may be selectively retracted and extended from the body  40  of the rack assembly  36 . Further; the distance between the two pivot points to which the rack  38  and body  40  are attached may be selectively altered. Consequently, the angle between the top plate  50  and the backrest  12  may be selected by a user. 
     The rack assembly  36  may be remotely actuated by the synchro angle pull sleeve  70  as depicted in FIG.  27 . To cause the rack  38  to extend or retract from the rack assembly  36 , and consequently alter the angle between the backrest and the seat, the rack must be disengaged. To disengage the rack assembly  36  and tilt the backrest bracket  13  and consequently the backrest in relation to the top plate  50 , a user pulls the synchro angle pull sleeve  70  in a direction outward from the housing  30  a predetermined distance, indicated in broken lines. This action pulls cable  46  to disengage the rack assembly  36 . Once in this disengaged mode, the rack  38  is free to extend and retract from the rack body  40  as described in detail above. After the user selects a desired backrest angle, she again pulls outward on the pull sleeve  70  to re-engage the rack assembly  36 . Accordingly, the rack  38  is locked in relation to the rack body  40 . It will be appreciated that the pull sleeve  70  does not impede the rotation of the tilt adjustment shaft  152  about which it is circumferentially disposed. As further will be appreciated, any remote actuating mechanism may be substituted for the pull sleeve  70  to engage and disengage the rack assembly  36  to promote tilting of the backrest  12  with respect to the top plate  50 . 
     Forward Tilt 
     With reference to FIG. 16, the selectable forward tilt mechanism  79  generally includes a forward tilt cam  94  rotatably retained in the housing  30  by housing ribs  86 . The cam  94  is positioned to selectively engage top plate flange  88 . Removable forward tilt shaft  80  is disposable in the forward tilt portal  92  to rotate forward tilt cam  94 . As best depicted in FIGS. 17 and 18, cam  94  includes first cam surface  95  and second cam surface  96 . First cam surface  95  is disposable in the path of travel of top plate flange  88  to restrict rotation of top plate  50  about fixed housing pivot pin  60  in direction F further than as depicted. With reference to FIG. 19 in contrast, second cam surface is disposable in the path of travel of top flange plate  88  to allow rotation of top plate  50  about housing pivot pin  60  in direction F a predetermined angle to allow the chair to forward tilt. It will be appreciated that the top plate flange  88  may be replaced with any sort of conventional protrusion from the top plate that would sufficiently engage the cam to provide forward tilt control. 
     With reference to FIG. 17, forward tilt cam  94  further includes cam positioning protrusions  102  which selectively engage rib detent  87 . As shown in FIG. 17, the cam is prevented from rotating from the position shown due to engagement of the rib detent  87  with the cam positioning protrusion  102 . Similarly in FIG. 19, the cam  94  is prevented from rotational translation from the position as shown due to engagement of the rib detent  87  with the cam positioning protrusion  102 . Additional cam positioning protrusions and rib detents may be added to the cam and ribs as desired to provide a plurality of forward tilt positions. 
     With particular reference to FIG. 20, the cam  94  includes a partially squared bore  98  for receiving the squared portion  81  of forward tilt shaft  80  to prevent rotation of the shaft  80  in relation to the cam  94 . As illustrated in FIGS. 16,  20  and  21 , the cam also includes finger  97 . When the forward tilt shaft  80  is inserted into the partially squared bore  98  of cam  94 , finger  97  engages key groove  84  to hold forward tilt shaft  80  in locked relation with respect to the forward tilt cam  94 . The squared portion of  81  of the forward tilt shaft  80  engages the partially squared internal bore  98  of the cam to further prevent rotational translation of the forward tilt shaft  80  with respect to the partially squared bore  98 . Forward tilt lever  90  is attached to the forward tilt shaft  80  to provide the user with a grasping surface to rotate forward tilt shaft  80  and consequently the cam  94 . It will be appreciated that any configuration of the internal bore of the camshaft and the forward tilt shaft may be used to prevent rotational translation of the shaft with relation to the cam so that the cam may be rotated. 
     With reference to FIG. 22, the cam  94  is retained in the housing by cam main tube  85  being held in place by housing ribs  86 . The housing ribs include slight protrusions  89  that hold the cam main tube  85  in fixed relation to the housing rib  86 , while still allowing the cam main tube  85  and forward tilt cam  94  to rotate. As will be appreciated by those skilled in the art, the cam main tube and consequently the cam  94  may be retained in fixed relation within the housing  30  by other conventional means. 
     Forward Tilt Operation 
     A chair including the forward tilt mechanism of the present invention is delivered to a distributor of such chairs with the forward tilt shaft  80  removed from the cam  94 . Accordingly, the distributor may then distribute the chair to consumers with the forward tilt capability of the chair disengaged, that is, without the forward tilt shaft  80 . Alternatively, the distributor may distribute the chair to consumers with the forward tilt shaft  80  installed. Accordingly, the end user of the chair may select between a forward tilt or non-forward tilting configuration. 
     It will be appreciated that the selectively installable actuator or adjuster used with the forward tilt control of the preferred embodiment may be used with any chair control feature. For example, the actuator may be implemented or used in conjunction with height controls for seats, backrests, armrest, or any component of a chair, tilt adjusters, seat and backrest angle controls, seat and backrest sliding controls, or any other chair control as the application requires. 
     FIG. 18 illustrates the chair with the forward tilt mechanism configured to restrict forward tilt. The top plate flange  88  engages first cam surface  95  so that the top plate  50  cannot forward tilt in direction F about housing pivot pin  60 . The cam is further restricted from rotation due to engagement of cam positioning protrusions  102  engaging rib detent  87 . 
     With reference to FIG. 19, to reconfigure the cam so that the chair may attain a forward tilt position, a user must grasp an installed forward tilt lever  90  and rotate it in direction T as indicated in FIG.  19 . Due to the coupling of lever  90  to the forward tilt shaft  80  and consequently the cam  94 , the cam also rotates in direction T. Once the cam has rotated sufficiently forward, the top plate  50  may rotate in direction F about main pivot pin  60 . The top plate flange  88  engages second cam surface  96  once the top plate  50  attains a forward tilt position. Cam positioning protrusions  102  also engage rib detent  87  to prevent rotational translation of cam  94  in this forward tilt mode. To reconfigure the chair to the restricted forward tilt configuration, the above steps are reversed, as will be appreciated by those skilled in the art. 
     Dual-Ratio Synchro-Tilt 
     With reference to FIGS. 8-12, there will now be described the dual-ratio synchro-tilt feature of the present invention. Generally, the dual-ratio synchro-tilt feature of the present invention is the product of the geometric configuration of the chair&#39;s components. In effect, the chair reclines and forward tilts in a four-bar relationship. “Four-bar” relationship means that each of four pivot points is connected to two other of the four pivot points by a member. In the preferred embodiment, generally depicted in FIG. 8, the main pivot point  60  is connected to (a) the main bracket pin  34  by top plate  50  and (b) the synchro-tilt pivot pin  31  by housing  30 ; and the seat back rivet  29  is connected to (c) the main bracket pin  34  by backrest bracket  13  and (d) synchro-tilt pivot pin  31  by rack assembly  36 . 
     Due to the four-bar relationship of the preferred embodiment, the angle between the backrest  12  and the top plate  50 : (1) increases when the top plate is reclined from a neutral or horizontal position; and (2) increases when the top plate is forward tilted from a neutral or horizontal position. Because this angle increases during forward tilt from neutral or horizontal, a user seated in the chair is not pinched between the backrest  12  and top plate  50 . 
     Dual-Ratio Synchro-Tilt Operation 
     The operation of the dual-ratio synchro-tilt feature will now be described. The dual-ratio synchro-tilt feature controls the configuration of the chair as it is reclined or forward tilted from a neutral position. FIG. 8 depicts the chair in its neutral position. In the preferred embodiment, the neutral position refers to that position where the top plate  50  is reclined from horizontal up to 20 degrees, preferably about 3 degrees. The backrest may be at any angle from the top plate in this neutral position as desired. 
     When the chair is reclined from the neutral position, the top plate  50  tilts about housing pivot pin  60 . The geometric four-bar relationship between the main pivot pin  60 , main bracket pin  34 , seatback rivet  29 , and synchro-tilt pivot pin  31  causes the angle formed between the top plate  50  and the backrest  12  to increase from angle A 2  in the neutral position of FIG. 8 to angle A 4  in the reclined position of FIG.  10 . 
     Similarly, when the chair is forward tilted from the neutral position, the top plate  50  tilts about housing pivot pin  60 . The geometric four-bar relationship between the main pivot pin  60 , main bracket pin  34 , seatback rivet  29 , and synchro-tilt pivot pin  31  causes the angle formed between the top plate  50  and the backrest to increase from angle A 2  in the neutral position of FIG. 8 to angle A 3  in the forward tilt position of FIG.  9 . 
     Tables I-III below present the angular data obtained from a chair having the dual-ratio synchro-tilt feature of the present invention. FIG. 23 graphically presents the data of Tables I-III. The tables and graphs all demonstrate the increase in the angle between the top plate of the chair and the backrest when the chair is reclined or forward tilted from the neutral position. Variance of the data for angles between the backrest and top plate in Tables I-III is the result of the angle between the backrest  12  and the top plate  50  being adjusted between nominal, minimum, and maximum angles with the adjustable synchro-tilt feature discussed above with reference to FIGS. 11 and 12. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Nominal Adjustable 
               
               
                 Synchro Angles 
               
             
          
           
               
                   
                   
                 Nominal Angle 
               
               
                   
                 Top Plate Angle 
                 Between Backrest 
               
               
                   
                 from Horizontal 
                 and Top Plate 
               
               
                   
                 (Degrees) 
                 (Degrees) 
               
               
                   
                   
               
             
          
           
               
                   
                 12 
                 104.00 
               
               
                   
                 11 
                 101.32 
               
               
                   
                 10 
                 99.10 
               
               
                   
                 9 
                 97.34 
               
               
                   
                 8 
                 96.02 
               
               
                   
                 7 
                 95.11 
               
               
                   
                 6 
                 94.58 
               
               
                   
                 5 
                 94.41 
               
               
                   
                 4 
                 94.56 
               
               
                   
                 3 
                 95.00 
               
               
                   
                 2 
                 95.71 
               
               
                   
                 1 
                 96.68 
               
               
                   
                 0 
                 97.87 
               
               
                   
                 −1 
                 99.29 
               
               
                   
                 −2 
                 100.92 
               
               
                   
                 −3 
                 102.75 
               
               
                   
                   
               
             
          
         
       
     
     The examplary angular data of Table I above was collected when the angle between the top plate  50  the backrest  12  was held in position by the adjustable synchro-tilt feature described above at about 95 degrees and the top plate  50  was in its neutral position, or about 3 degrees reclined from horizontal as depicted in FIG.  8 . When manipulating the chair in this configuration, the angle between the backrest and top plate is referred to as the “nominal adjustable synchro angle.” As can be seen in Table I, when the top plate is reclined from its neutral position of about 3 degrees to about 12 degrees, the nominal adjustable synchro angle between the backrest and the top plate increases from about 95 degrees to about 104 degrees. This tabular data is conceptually represented in comparing the configuration of FIG. 8 to the configuration of FIG.  10 . As generally depicted, the angle A 2  of FIG. 8 increases to angle A 4  of FIG.  10 . 
     As further represented in Table I, when the top plate is forward tilted from a neutral position of about 3 degrees to about 3 degrees inclined to the horizontal (−3 in Table I), the nominal adjustable synchro angle between the top plate increases from about 95 degrees to about 103 degrees. This tabular data is conceptually represented in comparing the configuration of FIG. 8 to the configuration of FIG  9 . As generally depicted, the angle A 2  of FIG. 8 increases to angle A 3  of FIG,  9 . 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Maximum Adjustable 
               
               
                 Synchro Angles 
               
             
          
           
               
                   
                   
                 Maximum Angle 
               
               
                   
                 Top Plate Angle 
                 Between Backrest 
               
               
                   
                 from Horizontal 
                 and Top Plate 
               
               
                   
                 (Degrees) 
                 (Degrees) 
               
               
                   
                   
               
             
          
           
               
                   
                 9 
                 108.93 
               
               
                   
                 8 
                 107.33 
               
               
                   
                 7 
                 106.13 
               
               
                   
                 6 
                 105.31 
               
               
                   
                 5 
                 104.86 
               
               
                   
                 4 
                 104.74 
               
               
                   
                 3 
                 104.90 
               
               
                   
                 2 
                 105.43 
               
               
                   
                 1 
                 106.21 
               
               
                   
                 0 
                 107.25 
               
               
                   
                 −1 
                 108.55 
               
               
                   
                   
               
             
          
         
       
     
     The examplary angular date of Table II was collected when the angle between the top plate  50  and the backrest  12  was held in position by the adjustable synchro tilt feature described above at about 105 degrees when the top plate  50  is in the neutral position, or about 3 degrees reclined from horizontal as generally depicted in FIG.  11 . When manipulating the chair in this configuration, the angle between the backrest and the top plate is referred to as the “maximum adjustable synchro angle.” In the configuration generally depicted in FIG. 11, the adjustable synchro-tilt feature more particularly, the rack assembly  36 , is shortened to length D. 
     As can be seen in Table II, when the top plate is reclined from its neutral position of about 3 degrees to about 9 degrees, the maximum adjustable synchro angle between the backrest and the top plate increases from about 105 degrees to about 109 degrees. Notably, with reference to FIG. 11, because the maximum adjustable synchro angle A 5  is so obtuse when the top plate is in the neutral position, the geometric configuration of the chair of the preferred embodiment does not allow the top plate to recline from horizontal past about 9 degrees, as indicated in Table II. It will be appreciated that alteration of the chair configuration may allow the top plate to recline to at least 30 degrees from horizontal. 
     The data of Table II is conceptually represented in comparing the configuration of FIG. 11 to the configuration of FIG.  10 . As generally depicted, the maximum adjustable synchro angle A 5  of FIG. 11 increases to angle A 4  of FIG.  10 . Notably, the angle between the top plate  50  and horizontal would be about 9 degrees instead of 12 degrees as depicted. Further, angle A 4  of FIG. 10 would be more obtuse than as depicted. Nevertheless, the concept of maximum adjustable synchro angle A 5  of FIG. 11 increasing to angle A 4  of FIG. 12 is generally illustrated. 
     As further represented in Table II, when the top plate is forward tilted from a neutral position of about 3 degrees to about 1 degree incline from the horizontal (−1 in Table I), the maximum adjustable synchro angle between the top plate increases from about 105 degrees to about 109 degrees. Because of the maximized configuration of the adjustable synchro-tilt feature, the top plate does not incline from the horizontal greater than 1 degree; however, as will be appreciated by those skilled in the art, modification may be made to the configuration to allow the top plate angle from horizontal to increase to about 20 degrees. 
     the data of Table II is conceptually represented in comparing the configuration of FIG. 11 to the configuration of FIG.  9 . As generally depicted, the maximum adjustable synchro angle A 5  of FIG  11  increases to angle A 4  of FIG.  9 . Notably, the angle between the top plate and horizontal would be about 1 degree instead of 3 degrees as depicted. Further, angle A 4  of FIG. 9 would be more pbtuse than as depicted. Nevertheless, the concept of the maximum adjustable synchro angle A 5  of FIG. 11 increasing to angle A 4  of FIG. 12 is generally illustrated. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 Minimum Adjustable 
               
               
                 Synchro Angles 
               
             
          
           
               
                   
                   
                 Minimum Angle 
               
               
                   
                 Top Plate Angle 
                 Between Backrest 
               
               
                   
                 from Horizontal 
                 and Top Plate 
               
               
                   
                 (Degrees) 
                 (Degrees) 
               
               
                   
                   
               
             
          
           
               
                   
                 12 
                 99.60 
               
               
                   
                 11 
                 97.00 
               
               
                   
                 10 
                 94.87 
               
               
                   
                 9 
                 93.22 
               
               
                   
                 8 
                 92.01 
               
               
                   
                 7 
                 91.22 
               
               
                   
                 6 
                 90.81 
               
               
                   
                 5 
                 90.75 
               
               
                   
                 4 
                 91.01 
               
               
                   
                 3 
                 91.55 
               
               
                   
                 2 
                 32.36 
               
               
                   
                 1 
                 93.40 
               
               
                   
                 0 
                 94.67 
               
               
                   
                 −1 
                 96.15 
               
               
                   
                 −2 
                 97.82 
               
               
                   
                 −3 
                 99.69 
               
               
                   
                   
               
             
          
         
       
     
     The exemplary angular data of Table III was collected when the angle between a top plate  50  and the backrest  12  was held in position by the adjustable synchro-tilt feature described above at about 90 degrees when the top plate  50  is in the neutral position, or about 3 degrees reclined from horizontal as depicted in FIG.  12 . When manipulating the chair in this configuration, the angle between the backrest and top plate is referred to as the “minimum adjustable synchro angle.” In this configuration, generally depicted in FIG. 12, the adjustable synchro-tilt features, more particularly, the rack assembly  36  is lengthened to length D. As can be seen in Table III when the top plate is reclined from its neutral position of about 3 degrees to about 12 degrees, the minimum adjustable synchro angle between the backrest and the top plate increases from about 92 degrees to about 100 degrees. This tabular data is conceptually represented in comparing the configuration of FIG. 12 to the configuration of FIG.  10 . As generally depicted, the minimum adjustable synchro angle A 6  of FIG. 12 increases to angle A 4  of FIG.  10 . Notably, the angle A 4  of FIG. 10 would be less obtuse than as depicted. 
     As further represented in Table III when the top plate is forward tilted from a neutral position of about 3 degrees to about 3 degrees inclined from the horizontal (−3 in Table III), the minimum adjustable synchro angle between a top plate increases from about 92 degrees to about 100 degrees. This tabular data is conceptually represented in comparing the configuration of FIG. 12 to the configuration of FIG.  9 . As depicted, the minimum adjustable synchro angle A 6  of FIG. 12 increases to angle A 4  of FIG.  10 . 
     The data of Tables I-III is graphically represented in FIG.  23 . The Y-axis represents the angle between the backrest and the top plate, that is, the adjustable synchro angles. The X-axis represents the angle of the top plate from the horizontal. The nominal adjustable synchro angles of Table I, maximum adjustable synchro angles of Table II, and minimum adjustable synchro angles of Table III are all plotted in relation to the angle of the top plate from the horizontal in the graph. 
     As can be seen in the graph of FIG. 23, the angle between the backrest and the top plate increases when the angle of the top plate from horizontal is reclined or forward tilted from a neutral position of about 3 to about 5 degrees. Accordingly, the chair “opens up” when the user reclines or forward tilts in the chair. Thus, the user is never pinched between the seat and the backrest. As will be appreciated by those skilled in the art, the configuration of the chair may be altered so that the nominal, maximum, and minimum adjustable synchro angles are of any desired angle. 
     Chair Height Adjustment 
     With reference to FIG. 24, the chair of the present invention includes a height adjustment mechanism to alter the distance between the housing from the floor. A conventional pneumatic height adjust is included in the base  20  of the chair. A plunger  132  is disposed at the top of the pneumatic base  20 . Height adjust bracket  122  is pivotally mounted to the housing  30  by front pin  134 . Height adjust spring  126  biases the height adjust bracket  122  so that the bracket is disengaged from the plunger  132 . Height adjust shaft  128  is fixedly attached to height adjust cam  124 . A height adjust lever  100  is also attached to the height adjust shaft  128  so that rotation of the height adjust lever  138  will consequently result in rotation of the height adjust cam  124 . 
     To adjust the distance between the housing  30  and the floor, the height adjustment mechanism of the present invention illustrated in FIG. 24 is activated. A user may rotate the lever  138  in either a counterclockwise or clockwise manner. Consequently cam  124  is rotated in either direction as well. Height adjust cam  124  engages the height adjust bracket  122  so that the height adjust bracket  122  rotates counterclockwise about the front pin  34 . The user must exert enough rotational force to overcome the height adjust spring  126 . Consequently, the height adjust bracket engages the plunger  132  of pneumatic base  20  to expel air from or take air into the pneumatic base  20 . As is known in the art, the user may adjust his or her weight to lower or raise the housing with respect to the floor. 
     Backrest 
     With reference to FIG. 25, the chair of the present invention includes a backrest height adjustment mechanism. Backrest bracket  13  is pivotally coupled to the top plate  50  by main backrest bracket pin  34 . The backrest bracket  13  is be pivotally connected to a rack assembly  36  at seatback rivet  29 . A backrest sleeve  176  couples to backrest bracket  13 . Backrest plate  178  is slidably received in sleeve  176 . Backrest height adjust shaft  172  is rotatably disposed through the backrest bracket  13 . The backrest height adjust lever  170  is operably connected to the backrest height adjust cam. Ball  174  is disposed within ball sleeve  175 . Cam lobe  173  is engagable against the ball  174  so that the ball will engage the backrest plate  178  and prevent the backrest plate from adjusting vertically. Backrest spring  177  engages top plate  50  and backrest height adjust shaft  172  so that the backrest bracket  13  tends to rotate about main bracket pin  34  in a clockwise manner. 
     With reference to FIG. 25, vertical adjustment of the backrest  12  will now be described. In an engaged position, the cam lobe  173  is forcibly engaged against ball  174  which consequently is forced against backrest plate  178 . This force is sufficient to prevent backrest plate  178  from sliding within sleeve  176  and consequently adjusting the height of the backrest  12  with respect to the backrest bracket  13 . 
     To adjust the height of the backrest with relation to the backrest bracket, a user must rotate the backrest lever  170  in a counterclockwise or clockwise manner. As a consequence of this rotation, the cam lobe  173  rotates and disengages from the ball  174 . Consequently, the ball  174  disengages from the backrest plate  178 . Accordingly, the backrest plate  178  may slide vertically within a sleeve  176 . The user may adjust the backrest  12  upward by manually lifting the backrest  12  upward. Downward displacement of the backrest  12  is motivated by gravity. 
     Once the user obtains a desired backrest height, the user rotates the lever in a direction opposite from which she originally rotated the lever to re-engage the cam lobe  173  against the ball and consequently the ball against the backrest plate. This will effectively lock the backrest plate into engagement with the ball and the sleeve  176  so that the backrest  12  remains in this vertical position. 
     The backrest spring  177  of FIG. 25 also aids in returning the backrest to a predetermined angle with respect to the top plate  50 . For example, when a user disengages the rack assembly  36  so that the rack  38  is free to extend or retract from the body  40  as described above, the backrest will tend to tilt toward the top plate  50 , that is, forward, due to biasing forces exerted on the cam shaft  172 . The spring naturally rotates the backrest bracket  13  clockwise about main backrest bracket pin  34 . Accordingly, the angle between the backrest  12  and the top plate  50  will be reduced when the rack assembly  36  is disengaged and the user does not exert any rearward force against the backrest  12 . However, the backrest spring  177  may be overcome by force exerted on the backrest by a user so that the angle between the backrest  12  and the top plate  50  is increased when the rack assembly  36  is disengaged. Once the user attains a desired angle between the backrest  12  and the top plate  50  he or she may re-engage the rack assembly  36  into a locked mode. 
     Tilt adjust Mechanism 
     With reference to FIGS. 26 and 27 the tilt adjust mechanism located in housing  30  will now be described. The tilt adjust mechanism generally includes height adjustment tray  166 , spring  168 , and tilt adjust rack assembly  136 . The height adjustment tray  166  pivotally mounted to the housing  30  via front pin  134 . Tension spring  168  is coupled on one end to the spring adjustment tray  166  and on the other end to top plate  50  in a conventional manner. Elastomer  169  is placed within or around the spring  168  to provide dampening. It will be appreciated that any other biasing element may be substituted for the spring  168 . Further, any conventional elastomer or dampening material may be used in place of or in combination with elastomer  169 . 
     The rearmost portion of the spring adjust tray  166  defines cross pin slots  162 . Within the cross-pin slots is a tension cross pin  160 . The cross pin  160  is threaded onto tension cross pin bolt  164 . Tension cross pin bolt  164  is itself attached to housing  30  in any manner appreciated by those skilled in the art. Tension cross pin bolt  164  also has secured thereto a conventional tension driven gear  156 . Tension drive gear  154  drives tension driven gear  156 . Tension drive gear  154  is attached to tilt adjust shaft  152  so that the tension drive gear does not rotate relative to the shaft  152 . Tilt adjustment shaft  152  has tilt adjustment knob  150  disposed at an end thereof. 
     Synchro angle pull sleeve  70  is concentrically disposed about the tilt adjust shaft  152 . The synchro angle pull sleeve  70  is slidable in relation to the tilt adjust shaft  152 . The synchro angle pull sleeve  70  has cable  46  coupled thereto in a conventional manner at synchro angle pull sleeve tab  72 . The cable  46  is operably attached to rack assembly  36  described above. The cable  46  may be guided within the housing with cable guides (not shown) as desired. The cable is disposed through the port  74  and under the bottom of the housing  30  to the cable inlet  47  of the rack assembly  36 . 
     The tilt adjust mechanism illustrated in FIGS. 26 and 27 also includes a tilt adjust rack assembly  236  which is identical to and operates in the same manner as the rack assembly  36 . The tilt adjust rack assembly  236  is connected to the front pin  134  and the top plate pin  135  to provide adjustment in the angle of the top plate from horizontal. When in an engaged mode, the tilt adjust rack assembly  236  prevents the top plate from tilting. 
     The rack assembly  236  is actuatable by tilt adjuster pull sleeve  110  which is circumferentially disposed about the height adjust shaft  128 . Cable  246  is connected in a conventional manner at one end to the top plate rack assembly  236  and at the other end to the top plate adjustable pull sleeve tab  272 . The cable  246  may be guided by cable guides (not shown) to fit within the housing as desired. As will be appreciated by those skilled in the art, the remote pull sleeve actuators  70  and  110  for the rack assemblies  36  and  236  may be replaced with any conventional remote cable actuating mechanism. 
     The operation of the tilt adjust mechanism illustrated in FIGS. 26 and 27 will now be described. The force required to tilt the top plate  50  in relation to the housing  30  about housing pivot pin  60 , that is, the tilt tension, may be selected by adjusting the compression of the spring  168  and elastomer  169 . For example, if the user desires that more force be required to tilt back in her chair, the spring  168  must be preloaded with compressive force. Alternatively, if the user desires that top plate  50  recline quickly from the horizontal, the spring  168  and elastomer must be unloaded. 
     With reference to FIGS. 26 and 27, the user may adjust the pre-load, or compression, of the spring  168  and elastomer  169  by rotating the tilt adjust knob  150 . To increase the compression in the spring and elastomer  168  and  169  and the force required to recline in the chair, the user rotates the knob in a clockwise manner. This rotates tilt adjustment shaft  152  and the attached drive gear  154 . Drive gear  154  drives driven gear  156  which rotates cross pin screw  164 . Rotation of cross pin screw  164  urges rotation of the cross pin  160 . However, because the cross pin  160  is retained within cross pin slots  162  of the spring adjust tray  166 , the cross pin  160  does not rotate; rather it effectively threads onto the cross pin screw  164 . Consequently, the cross pin  160  is urged toward the driven gear  156 . 
     Because of its coupling to the cross pin  160  at the cross pin slots  162 , the spring adjust tray  166  is urged to rotate counterclockwise about front pin  134 . In turn, the front portion of  165  of the spring adjust tray  166  moves upward. The spring  168  and elastomer  169  are compressed between the front portion  165  of the spring adjust tray  166  and the top plate  50 . Accordingly, the compression of the spring  168  and elastomer  169  is increased. Moreover, when the top plate rack assembly  236  is disengaged as discussed below by pulling the tension pull sleeve  110 , more force is required to rotate the top plate  50  about the housing pivot pin  60  and recline or tilt in the chair. 
     To reduce the amount the spring  168  and elastomer  169  are compressed so that the chair may be reclined from horizontal with ease, the user may rotate the tilt adjustment knob counterclockwise. Upon rotation, the drive gear  154 , driven gear  156 , and cross pin screw  164  rotate. Consequently, the cross pin  160  is urged to rotate; however, because it is restricted from rotation by cross pin slots  162 , the cross pin  160  threads off of the pin screw  164 . Accordingly, the cross pin  160  engages the cross pin slots  162  and rotates the spring adjustment tray  166  clockwise about front pin  134 . In this manner, the front portion  165  of the spring adjustment tray  166  rotates clockwise. This increases the distance between the top plate  50  and the spring adjustment tray  166 . Accordingly, the compression in the spring  168  and elastomer  169  is reduced. 
     With the compression reduced, the top plate may rotate about housing pivot pin  60  with minimal effort. Moreover, when the tilt rack assembly  236  is disengaged, as discussed below, the chair may be reclined from horizontal with minimal effort. 
     As will be appreciated by those skilled in the art, the directions of rotation of any of the tilt adjustment mechanisms including all gears, trays, etc. may be reversed. Other mechanisms may also be substituted for those of the preferred embodiment to increase or decrease the force required to recline or tilt the chair. 
     The operation of the tilt rack assembly will now be described. In its engaged mode, the rack assembly  236  prevents top plate  50  from rotating about housing pivot pin  60 . Accordingly, the top plate  50  cannot recline or forward tilt. The tilt rack assembly  236  may be disengaged to allow the top plate  50  to rotate in relation to the housing  30  about housing pivot pin  60 . To disengage the tilt rack assembly  236 , a user must pull sleeve  110  outward from the housing  30  as indicated in broken lines. The functioning of the rack assembly  236  is identical to that of the rack assembly  36  described above; the user pulls the pull sleeve  104  until the rack is disengaged and rack  238  is free to extend or retract from the rack body  240 . Accordingly, the weight of the user seated in the seat coupled to the top plate  50  compresses the spring  168  and elastomer  169 . Simultaneously, the top plate  50  may rotate in relation to the housing  30  about housing pivot pin  60 . 
     With the tilt rack assembly in a disengaged mode, the user may lean back in the chair so that the top plate  50  reclines a predetermined angle. Alternatively, the user may remove her weight from the top plate  50 , that is, the seat (not shown), so that the spring  168  and elastomer  169  pushes the top plate  50  in a clockwise manner about housing pivot pin  60  to a forward tilted position. 
     The user may select the angle at which she prefers the top plate  50  to be in relation to the horizontal and lock the top plate into that position by re-engaging the tilt tension rack assembly  236 . Re-engagement is actuated by pulling a second time on the pull sleeve  110 . As in the above description of the operation of rack assembly  36 , the top plate rack assembly will re-engage so that the rack  238  is no longer free to extend and retract from the rack body  240 . Thus, the top plate  50  is in fixed relation to the housing  30  and may no longer rotate about housing pivot pin  60 . 
     Alternatively, the user may leave the tilt adjust rack assembly  236  in a disengaged mode, so that the top plate  50  may recline and tilt freely. The user may also use the tilt adjust rack assembly to adjust the pre-load or compression of the spring and elastomer as discussed above. It will be appreciated that other actuators may be substituted for the tilt adjust rack assembly to control the reclination or tilt of the top plate. 
     The above descriptions are those of the preferred embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. Any references to claim elements in the singular, for example, using the articles “a,” “an,” “the,” or “said,” is not to be construed as limiting the element to the singular.