Patent Publication Number: US-6209958-B1

Title: Universal tilt mechanism for a chair

Description:
FIELD OF THE INVENTION 
     This invention relates to an office chair and, in particular, to an adjustable universal tilt mechanism which pivotally connects a seat assembly to a base. 
     BACKGROUND OF THE INVENTION 
     Conventional office chairs frequently include a seat-back arrangement which is connected to a base by a tilt mechanism. The tilt mechanism defines one or more pivot axes about which a seat or back assembly may pivot or tilt relative to the base. Office chairs typically tilt rearwardly about fixed horizontal pivot axes wherein the seat and back assemblies are rearwardly tiltable either together or independently. To resist such tilting and bias the seat and back assemblies to normal upright positions, numerous tilt mechanisms have been provided which include springs such as coil, leaf and torsion springs which oppose the tilting movement. 
     As an alternative to conventional spring arrangements, prior tilt control mechanisms have also used elastomeric pads or rings between relatively moving surfaces. The pads or rings are resilient so as to be compressed between the moving surfaces to resist the tilting movement. Some of these tilt mechanisms permit the seat to pivot in multiple directions. 
     Examples of chairs using elastomeric pads or rings which permit tilting in multiple directions are disclosed in U.S. Pat. Nos. 139,948, 3,309,137, 4,027,843, and 5,573,304. The 3,309,137 patent permits adjustment of tilting resistance by varying the compression of an elastomeric ring. The chairs disclosed in the remaining patents do not permit adjustment of the tilting resistance. 
     In another chair as disclosed in U.S. Pat. No. 4,890,886, the tilt control mechanism defines a fixed pivot axis between the seat assembly and the chair base. The tilt control mechanism further includes a plate secured to the seat assembly so as to move with the seat assembly relative to the base, and a second plate which is spaced apart from the first plate and remains stationary relative to the base. These opposing plates move relative to each other during tilting of the seat assembly, and elastomeric pads are provided between these relatively movable plates to resist tilting and bias the seat assembly to a neutral position. To adjust resistance to tilting, the elastomeric pads are movable relative to the pivot axis to thereby adjust the distance defined therebetween. In one embodiment, the pads are vertically movable. 
     However, users, such as office workers, who sit in such chairs typically move in all directions, such as sidewardly, forwardly and rearwardly when working. Conventional tilt control mechanisms having fixed axes, however, restrict such movement due to the fixed axes, and hence do not readily accommodate the usual movements of a user such as movement to the side. 
     To more readily accommodate the various movements of a user, the chair of the present invention accommodates movement of a user both forwardly and sidewardly and in fact permits the chair seat to swivel about a connection point so as to react to the user. In particular, to overcome the disadvantages of conventional chair designs which use fixed pivot axes, the chair of the present invention includes a tilt control mechanism which permits universal tilting or swiveling of the seat assembly relative to the base in substantially all horizontal directions. The seat assembly is not restrained by fixed pivot axes but instead effectively pivots about a pivot or connection point. Thus, the seat assembly can pivot forwardly and rearwardly, sidewardly and in any other horizontal direction extending radially away from the pivot point, and can also be swivelled about the connection point. Thus, as a user shifts and moves, the chair reacts to the user&#39;s movements while still providing sufficient resistance to the universal tilting movement to provide stability and control for the user. 
     The tilt control mechanism of the invention, in an embodiment thereof, includes a support member which extends upwardly from the pedestal of the base. The support member has a bearing at the top thereof which pivotally supports a pivot bracket mounted on the seat assembly. The cooperating bearing and pivot bracket effectively define a pivot point, rather than a fixed horizontal pivot axis, about which the seat assembly pivots or swivels such that the seat assembly pivots in substantially all radial directions extending away from the pivot point. 
     To provide resistance to such tilting while providing stability for the user, the tilt control mechanism in a preferred embodiment includes a cylindrical housing which is disposed in concentric and surrounding relationship to the support column wherein the housing is spaced radially outwardly from the support column to define an annular clearance space therebetween. An elastomeric doughnut-shaped ring is disposed within this annular clearance space whereby the ring is disposed concentric with the column and housing and extends radially therebetween. As the housing moves with the seat assembly relative to the column, the elastomeric material of the resilient ring is compressed and limits the tilting, and restores the seat assembly to the initial neutral position. 
     The tilt control arrangement also permits adjustment of the tilting resistance to accommodate various size users or working conditions. The tilt control mechanism includes an adjustment mechanism connected to the resilient ring which allows a user to selectively move the resilient ring toward and away from the pivot point, whereby the effective resistance to tilting is increased or decreased. 
     The adjustment mechanism in this embodiment includes a drive ring or cylinder having an inclined groove or track, and an intermediate connector which is slidably connected to the inclined groove. As the intermediate connector slides along the inclined groove during rotation of the drive ring, the intermediate connector moves vertically. The intermediate connector is connected to the resilient ring to move vertically therewith. The connection of the intermediate connector to the inclined groove translates rotational movement of the drive ring into vertical movement of the resilient ring to thereby adjust the position of the resilient ring relative to the pivot point. Since the resilient ring effectively applies a biasing force to the housing as the resilient ring is compressed, this vertical movement of the resilient ring adjusts the location at which the force of the resilient ring acts relative to the pivot axis. 
     This pivot or tilt control arrangement thereby permits tilting of the seat assembly in any direction which extends radially away from the pivot point. Further, the tilting resistance can be conveniently adjusted by a person seated in the chair. 
     Other objects and purposes of the invention, and variations thereof, will be apparent upon reading the following specification and inspecting the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of a chair of the invention. 
     FIG. 2 is a partial perspective view in cross-section of a tilt control mechanism of the chair. 
     FIG. 3 is a plan view of the tilt control mechanism of FIG. 2 but with the housing top wall removed for purposes of illustration. 
     FIG. 4 is a side view of the tilt control mechanism in cross-section as taken along line  4 — 4  in FIG.  3 . 
     FIG. 5 is a diagrammatic side view of the tilt control mechanism in cross-section illustrating a resilient ring therein which is vertically movable. 
     FIG. 6 illustrates the tilt control mechanism in a tilted position. 
     FIG. 7 is a front elevational view illustrating the resilient ring and a ring-like drive member for moving the resilient ring vertically. 
     FIG. 8 is a side elevational view of a housing of the tilt control mechanism. 
     FIG. 9 is a front elevational view of a second embodiment of the tilt control mechanism. 
     FIG. 10 is a front cross sectional view of the embodiment of FIG.  9 . 
     FIG. 11 is a front elevational view in partial cross section of a third embodiment of the invention. 
     FIG. 12 is a side elevational view of a drive cylinder for an adjustment mechanism for the embodiment of FIG.  11 . 
     FIG. 13 is a plan view of the third embodiment. 
     FIG. 14 is a partial perspective view in cross-section of a further embodiment of the tilt control mechanism. 
    
    
     Certain terminology will be used in the following description for convenience and reference only, and will not be limiting. For example, the words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the arrangement and designated parts thereof. Said terminology will include the words specifically mentioned, derivatives thereof, and words of similar import. 
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the chair  10  of the invention includes a base  12 , a seat-back arrangement  14  and a tilt control mechanism  16  which connects the seat-back arrangement  14  to the base  12 . The inventive tilt control mechanism  16  not only permits vertical tilting of the seat-back arrangement  14  relative to the base  12  in a forward-rearward direction but effectively in any horizontal direction (i.e. universally) as discussed herein. 
     The base  12  may be of a conventional construction and, in the illustrated embodiment, includes a plurality of radially extending legs  18  which are supported on a support surface by casters  19 . The base  18  further includes a vertically elongate and cylindrical spindle or column  20  which projects upwardly from the legs  18  and supports the tilt control mechanism  16  on the upper end thereof. 
     The tilt control mechanism  16  also supports the seat-back arrangement  14 . The seat-back arrangement  14  may be of any construction and in the illustrated embodiment includes a seat assembly  22  having a rigid housing  23  and a horizontally enlarged cushion  24  connected thereto. 
     The seat-back arrangement  14  also includes a back assembly  26  which is connected to the seat assembly  22  by a generally L-shaped rigid upright  27 . The upright  27  has an upper end which supports a vertically enlarged back rest  28  thereon and a lower end which is connected to the seat housing  23 . 
     The back assembly  26  and seat assembly  22  can be connected together in various conventional arrangements. For example, the lower end of the upright  27  may be rigidly fixed to the seat housing  23  such that the seat assembly  22  and back assembly  26  move together in unison. Alternatively, the lower end of the upright  27  may be pivotally connected to the seat housing  23  such that the back assembly  26  is vertically tiltable relative to the seat assembly  22  while the entire seat-back arrangement  14  is vertically tiltable relative to the base  12 . 
     With respect to the tilt control mechanism  16 , this mechanism connects the seat-back arrangement  14  to the base  12  to permit universal tilting or swiveling therebetween. While many conventional tilt control mechanisms define fixed pivot axes about which the seat or back are tiltable, the tilt control mechanism  16  of this invention not only permits tilting of the seat-back arrangement  14  forwardly and rearwardly, but also in any direction relative to a central upright axis defined by the base  12 . 
     In particular, while the seat-back arrangement  14  is generally biased to the neutral position illustrated in FIG. 1, the tilt control mechanism  16  of the invention permits the seat assembly  22  to pivot and swivel about a pivot point so as to permit universal tilting of the seat assembly  22 . Thus, the seat-back arrangement  14  reacts to movements of a user forwardly and rearwardly and also sidewardly and any direction therebetween. 
     The tilt control mechanism  16  (FIGS. 2 and 4) includes a pivot or support fitting  30  which is rigidly supported on the upper end of the spindle  20 . To pivotally connect the seat assembly  22  to the spindle  20 , a retainer bracket  31  is supported on the upper end of the support fitting  30  by a pivot connection defined therebetween. The retainer bracket  31  rigidly supports the seat assembly  22  thereon such that the seat assembly  22  is vertically pivotable relative to the base  12 . As described herein, the pivot connection between the support fitting  30  and retainer bracket  31  effectively defines a pivot point  70  rather than a fixed pivot axis such that the seat assembly  22  is pivotable in any horizontal direction extending radially away from the pivot point. 
     The tilt control mechanism  16  also includes an elastomeric resilient ring  34  which resists tilting of the seat assembly  22 . The resilient ring  34  is vertically movable to adjust the resistance to tilting, and the tilt control mechanism  16  further includes an adjustment mechanism  35  to adjust the position of the resilient ring  34  relative to the pivot point and thereby adjust the tilting resistance. The specific construction and function of these component parts is described in more detail hereinafter. 
     Referring to FIGS. 2 and 4, the support fitting or member  30  is a vertically-elongate cylindrical tube which is rigidly connected to the upper end of the spindle  20  (FIG. 1) in coaxial relation therewith such that the support fitting  30  defines a vertical extension of the spindle  20 . The lower end  37  of the support fitting  30  preferably defines an interior chamber  38  which opens downwardly to receive the upper end of a pneumatic cylinder  39  (FIG. 2) therein. 
     The pneumatic cylinder  39  is provided in the spindle  20  when the base  12  is height adjustable. The pneumatic cylinder  39  thereby adjusts the vertical length of the spindle  20  to adjust the height of the seat assembly  22 , which arrangement is conventional. 
     To provide access to the pneumatic cylinder  39 , the interior chamber  38  of the support fitting  30  is defined by an outer wall  42  which thickens significantly at an upper end thereof to define a bore  43  that extends vertically from the chamber  38  to the top end of the support fitting  30  and receives an actuator rod  44  vertically therethrough. The actuator rod  44  has a lower end connected to a valve on the pneumatic cylinder  39  and an upper end which projects vertically from the top of the support fitting  30 . 
     To move the actuator rod  44  vertically, the tilt mechanism  16  is adapted to support a height adjustment handle  46  (FIGS. 1 and 2) which acts on the actuator rod  44  to operate the pneumatic cylinder  39 . The height adjustment handle  46  includes a shaft  47  which extends horizontally into the tilt mechanism  16  and has a paddle-like flange  48  on the innermost end thereof. The flange  48  is disposed directly above the actuator rod  44 , and the shaft  47  is rotatable about its longitudinal axis to move the actuator rod  44  vertically by movement of the flange  48  which thereby operates the pneumatic cylinder  39  to adjust the overall height of the base  12 . 
     To pivotally support the seat assembly  22 , the support fitting  30  further includes a ball  52  on the upper end thereof. The ball  52  is formed integral with the tubular wall  42  of a rigid wear-resistant material such as steel and has a generally spherical shape. As a result, the ball  52  has an outer surface  53  which preferably defines a convex partially spherical bearing surface that primarily faces upwardly but also extends downwardly and inwardly to form an annular groove  54  and an annular upward-facing shoulder  55  near the outer wall  42 . The shoulder  55  tapers slightly downwardly. 
     The retainer bracket  31  seats on the ball  52  to define a pivot connection therewith. Since the retainer bracket  31  is also rigidly connected to the seat assembly  22 , tilting of the seat assembly  22  causes the retainer bracket  31  to pivot (i.e. swivel) relative to the support fitting  30 . 
     More particularly, the retainer bracket  31  has a ring-like mounting flange  57  which extends generally horizontally and is rigidly connected to the housing  23  of the seat assembly  22 . The flange  57  has a circular shape when viewed from above although other shapes may be provided so long as the flange  57  can be connected to or otherwise support the seat housing  23 . 
     An inside diameter of the flange  57  is formed integral with a cylindrical collar  59  which extends downwardly. The cylindrical collar  59  includes an outer wall  60  which extends vertically, and a divider wall  61 , which extends horizontally from the outer wall  60 , generally midway between the top and bottom edges of the outer wall  59  as shown in FIG.  4 . The collar  59  thereby defines a bushing seat  62  which is defined below the divider wall  61 , and a shaft chamber  63  which is defined above the divider wall  61 . 
     In the illustrated embodiment, the retainer bracket  31  is formed of steel plate or other rigid material which is formed into the desired shape. During forming, the plate material is folded downwardly, upwardly and inwardly to define the collar  59  and divider wall  61  such that the collar  59  has multiple layers of plate material while the divider wall  61  extends radially inwardly from the outer wall  60 . 
     To connect the retainer bracket  31  to the support fitting  30 , the bushing seat  62  receives a generally diametrically split cylindrical bushing  66  through the open bottom of the collar  59 . The bushing  66  includes an outer circumferential surface  67  which is tight-fittingly received within the wall  60 , and a generally spherical bearing surface  68  on the hollow interior thereof which faces downwardly. The bearing surface  68  has a concave shape which corresponds to the convex shape of the ball  52 , and the bushing  66  is secured in the collar  59  of the retaining bracket  31  and is also fitted onto the ball  52  such that the opposing bearing surfaces  68  and  53  are in slidable contact with each other. 
     The retainer bracket  31 , bushing  66  and ball  52  thereby define a pivot connection between the chair base  12  and the seat assembly  22 . To reduce friction, the bushing  66  preferably is formed of acetal or equivalent similar materials. 
     Since the opposing bearing surfaces  53  and  68  extend circumferentially and are generally spherically curved, a pivot point  70  is defined at the center of the ball  52 , about which the entire seat assembly  22  pivots or swivels. In particular, the seat assembly  22  is able to vertically pivot in any horizontal direction that extends radially outwardly from the pivot point  70  and can also be swiveled about the connection point. This universal tilting of the seat assembly  22  thereby allows the seat assembly  22  to tilt and, in effect, to react to movements by the chair occupant whether forwardly, rearwardly, sidewardly, or any direction therebetween. 
     To assist in securing the bushing  66  to the ball  52 , the bearing surface  68  of the bushing  66  preferably converges radially inwardly into the groove  54  formed on the ball  52 . While the resilient ring  34  resists and limits the universal tilting as described herein, the bushing  66  and outer wall  60  also may swing downwardly and contact the shoulder  55  if tilting of the seat assembly  22  is excessive. The shoulder  55  thereby defines a positive stop which in this embodiment is annular to provide a symmetrical stop that limits tilting equally in all directions. Alternatively, an asymmetric positive stop may also be provided. 
     In the preferred embodiment, the opening  71  has a sufficiently large diameter so as to avoid contact with the actuator rod  44 . To achieve this result, the opening  71  preferably has a circular shape when viewed from above (FIG. 3) and tapers upwardly outwardly when viewed from the side (FIG.  4 ). However, the opening  71  may also be permitted to contact the actuator rod  44  to limit tilting and thereby act as a positive stop. If the opening  71  is circular as illustrated, the stop arrangement would be symmetric. To provide an asymmetric stop arrangement, the opening  71  may have an asymmetric shape such as an ellipse. More specifically, the major axis would extend in a forward and rearward direction to limit forward tilting to a first angle (such as 12 degrees), while the minor axis would extend sidewardly to limit sideward tilting to a second angle (such as 8 degrees) which is smaller than the first angle. Tilting which is between forward and sideward tilting would thereby be limited to an intermediate angle which varies between the first and second angles. 
     Still further, the opening  71  could have other asymmetric shapes to vary the tilt angles. For example, the opening  71  could be egg-shaped wherein forward tilting would be limited to a greater extent than rearward tilting. 
     To adjust the chair height, the retainer bracket  31  also supports the height adjustment handle  46  thereon. In particular, the handle shaft  47  is rotatably supported by opposite sides of the outer collar wall  60  and extends radially inwardly into the shaft chamber  63 . As shown in FIGS. 3 and 5, the shaft  47  is offset from the center of the collar wall  60  such that the flange  48  is disposed above the opening  71  formed through the center of the divider wall  61 . As illustrated in FIG. 2, the actuator rod  44  extends vertically through this opening  71  such that rotation of the shaft  47  causes the flange  48  to drive the actuator rod  44  downwardly and actuate the pneumatic cylinder  39 . 
     The retainer bracket  31  also supports a cylindrical housing  75  near the outer diameter of the mounting flange  57 . The housing  75  is rigidly secured at the upper end thereof to the mounting flange  57 , and includes an outer wall  76  having an interior surface  77  which is disposed coaxial and concentric with an outer surface  78  of the support fitting  30  when the seat-back is in its normal upright or neutral position. 
     The interior surface  77  and the outer surface  78  preferably are disposed in spaced apart relation such that an annular clearance space  80  is defined radially therebetween. The clearance space  80  extends vertically between the bottom of the housing  75  and the shoulder  55  formed on the support fitting  30 . When the seat assembly  22  is in the neutral position (FIG.  1 ), the opposing, surfaces  77  and  78  preferably are parallel to each other such that the clearance space  80  has a uniform radial width along its vertical length. 
     Referring to FIG. 3, the outer housing wall  76  includes a plurality, here six, of circumferentially spaced vertical grooves  82  which extend vertically and open radially inwardly through the interior surface  77 . As described herein, these grooves  82  serve as guides when the resilient ring  34  is moved vertically. 
     To support the height adjustment handle  46 , the outer housing wall  76  includes a bore  83  (FIGS. 2 and 3) which rotatably supports the handle shaft  47 . The housing wall  76  also includes a horizontally elongate rectangular slot  84  (FIGS. 2 and 8) on the side opposite the bore  83 . 
     Since the housing  75  is connected to the retainer bracket  31 , the housing  75  moves with the seat assembly  22  during tilting thereof. During tilting, the lower edge of the housing  75  on one side thereof moves toward the support fitting  30  as generally shown in FIG. 6, and relative movement occurs between the opposing surfaces  77  and  78 . 
     To control tilting, the resilient ring  34  is provided in the clearance space  80  as shown in FIG.  4 . In particular, the resilient ring  34  has an annular shape which fits into the clearance space  80  in concentric relation with the support fitting  30  and the housing  75 . The resilient ring  34  has a radial width which fits closely between the opposing surfaces  77  and  78  but permits vertical sliding within the clearance space  80 . 
     Preferably, the resilient ring  34  comprises an inner band  86  and an outer band  87  which define inner and outer diameters, respectively, of the ring  34 . The inner and outer bands  86  and  87  are formed of metal to resist wear as the resilient ring  34  moves vertically along the axial length of the clearance space  80  although other suitable materials may be used and either or both bands  86  and  87  could be eliminated. 
     The inner and outer bands  86  and  87  are joined together by an elastomeric material  88  which extends radially therebetween and permits the inner and outer bands  86  and  87  to move relative to each other. The material  88  is preferably bonded or adhesively secured to the bands  86  and  87 . Any suitable resilient and durable material may be used. In the preferred embodiment, the elastic material  88  is a natural rubber of 40-60 durometers. 
     During tilting of the chair  10 , the housing  75  moves relative to the support fitting  30  which thereby compresses the resilient material  88  on one side of the support fitting  30  and stretches the resilient material  88  on a diametrically opposite side thereof as shown in FIG.  6 . This compression and stretching serve to resist tilting of the seat assembly  22  and, in particular, generates a force acting on the housing  75  which increases as the angle of tilt increases. When the load on the seat assembly  22  is released, the resilient ring  34  biases the housing  75  and restores the seat assembly  22  to the neutral position. 
     While the housing  75  is disposed radially outwardly of the resilient ring  34 , this arrangement may be modified, for example, by positioning the resilient ring  34  about the exterior of the housing  75  and providing a further annular housing which is fixed to the base  12  and is disposed radially outwardly of the resilient ring. In this modified arrangement, the resilient ring would still be positioned between a fixed surface and a movable surface which moves in response to tilting of the seat assembly. As a result, the resilient ring resists tilting and biases the seat to the neutral upright position. 
     To vary the biasing force being applied to the housing  75 , the resilient ring  34  is vertically movable. In particular, as shown in FIG. 5, vertical movement of the resilient ring  34  varies the vertical distance between the resilient ring  34  and the pivot point  70 , for example, from distance D 1  to distance D 2  which thereby varies the tilting resistance (i.e. torque). 
     To adjust the position of the resilient ring  34 , the adjustment mechanism  35  is connected to the resilient ring  34  to permit manual adjustment of the tilting resistance. As shown in FIGS. 2,  3  and  7 , the adjustment mechanism  35  includes a rotatable drive member such as a drive ring  91  disposed within the upper end of the housing  75 . The drive ring  91  has an annular shape and fits concentrically in the radial space  92  between the collar wall  60  and the housing wall  76 . The drive ring  91  fits closely but is rotatable within this radial space  92  as described herein. 
     To effect manual rotation of the drive ring  91 , the drive ring  91  includes a horizontal bore  93  which extends radially therethrough. The bore  93  is aligned with the horizontal slot  84  in the housing  75  such that an adjustment handle  96  for tilting resistance extends radially through the slot  84  and into the bore  93  as shown in FIG.  2 . The handle  96  is confined vertically in the slot  84  but is movable horizontally therealong such that the handle  96  can be swung manually in a horizontal plane to effect rotation of the drive ring  91 . The opposite ends of the slot  84 , however, limit the range of motion for the drive ring  91 . 
     Preferably, the handle  96  is threadedly engaged with the bore  93  so that confinement of the handle  96  in the slot  84  prevents vertical movement of the drive ring  91 . During assembly, the drive ring  91  is first slid into the radial space  92  and then the handle  96  is engaged therewith such that handle  96  prevents removal of the drive ring  91  and the resilient ring  34  which is connected thereto. 
     The handle  96  also supports a cylindrical shroud  94  for enclosing the tilt mechanism  16 . The shroud  94  generally has a tapered shape and rotates in combination with the handle  96 . 
     To accommodate the height adjustment handle  46 , the upper surface  97  of the drive ring  91  also includes a generally pie shaped notch  98  opening upwardly therefrom. The notch  98  permits the height adjustment handle  46  to extend radially therethrough, and extends circumferentially to permit the drive ring  91  to be rotated. As the drive ring  91  rotates, the handle shaft  47  slides horizontally along the notch  98  as indicated generally by arrow  99  in FIG.  3 . 
     To translate rotational movement of the drive ring  91  into vertical movement of the resilient ring  34 , the drive ring  91  also includes a plurality and preferably two inclined slots  101  in the outer circumferential surface  102  thereof. When viewed from above as shown in FIG. 3, an upper end of each slot  101  opens upwardly through the upper ring surface  97 , while an open side opens radially through the outer circumferential surface  102 . 
     The slots  101  angle downwardly and circumferentially away from the upper ring surface  97  as shown in FIG.  7 . Preferably, the slots  101  extend clockwise away from the upper surface  97 , and the drive ring  91  is formed of nylon or of other suitable plastic material. 
     To effect vertical movement of the resilient ring  34 , the adjustment mechanism  35  also includes an intermediate connector or tie bracket  105  for each one of the slots  101 . In particular, a pair of tie brackets  105  are provided to connect or tie the drive ring  91  and the resilient ring  34  together. 
     Referring to FIGS. 3 and 7, each tie bracket  105  includes a vertically elongate bar  106  and a cylindrical pin  107  at each opposite end thereof. The lowermost pin  107  fits into a corresponding bore formed radially in the outer band  87  of the resilient ring  34 . When the tie brackets  105  are connected to the resilient ring  34  as shown in FIG. 4, the resilient ring  34  is movable vertically with the tie bracket  105 . When assembled, the bar  106  of each tile bracket  105  fits into and slides vertically in the groove  82  formed in the inside of the housing  75  whereby the grooves  82  guide vertical sliding of the resilient ring  34 . 
     The uppermost pin  107  of the tie brackets  105  is slidably received in a corresponding one of the slots  101  and is slidable therealong. Thus, as the drive ring  91  is rotated, the upper pins  107  slide along the circumferential length of the slots  101  although, since the slots  101  are inclined, the rotation of the drive ring  91  effects vertical movement of the tie brackets  105  and the resilient ring  34  which is connected thereto. For example, as shown in FIG. 7, rotation of the drive ring  91  counterclockwise in the direction of arrow  109  causes the upper pin  107  to move relative to the slot  101  in the direction of arrow  110  which thereby causes the resilient ring  34  to move downwardly in the direction of arrow  111  and adjust the resistance force. 
     With the above-described arrangement, the chair  10  not only provides universal tilting but the tilting resistance is adjustable to accommodate different size users or to provide different tilting characteristics. 
     In operation, the seat assembly  22  can tilt about the pivot point  70  in any direction extending radially away from the pivot point. As the seat assembly  22  tilts, the housing  75  moves relative to the support fitting  30  which thereby compresses the resilient ring  34  on one side thereof. This compression of the resilient ring  34  generates a resistance force which is applied to the housing  75  by the resilient ring  34 . Once tilting is completed, the resiliency of the ring  34  causes the seat assembly  22  to return to its neutral position. 
     Since resistance to tilting may need to be adjusted depending upon the characteristics and requirements of an occupant, the resilient ring  34  is vertically movable to adjust the distance between the pivot point  70  and the location on the housing  75  to which the resistance force is applied. 
     The position of the resilient ring  34  is adjusted by moving the handle  96  clockwise or counterclockwise. Rotation of the handle  96  causes the drive ring  91  to rotate which thereby causes the resilient ring  34  to move vertically due to the cooperation of the tie brackets  105  and inclined slots  101 . In particular, rotation of the drive ring  91  causes relative vertical and horizontal movement between the slots  101  and the pins  107  of the tie brackets  105 . As a result, the chair  10  of the invention provides universal tilting and ready adjustment of the resistance forces to improve the comfort and health of an occupant. 
     FIGS. 9 and 10 illustrate a second embodiment for the tilt control mechanism  16 - 1 . The second embodiment incorporates a number of common components as described herein, which common components are designated by the same reference numeral in combination with “−1”. These common components have similar structures and functions to those described above, and the following disclosure is directed primarily to the differences therebetween. 
     More particularly, the tilt control mechanism  16 - 1  includes a support fitting  30 - 1  which is supported on the base  12 , and includes a cylindrical outer wall  42 - 1  to which a ball  52 - 1  is attached. The ball  52 - 1  defines a convex bearing surface  53 - 1  which faces upwardly. A central bore  43 - 1  also is provided to permit actuation of a pneumatic cylinder as provided in a height-adjustable base. 
     The seat assembly  22 - 1  is pivotally connected to the support fitting  30 - 1  by a retainer bracket  31 - 1 . The retainer bracket  31 - 1  includes a relatively thick central plate  120  which has a downward opening recess  121  at the center thereof. The recess  121  defines a concave bearing surface  68 - 1  which cooperates with the bearing surface  53 - 1  to define a pivot connection therebetween. 
     The retainer bracket  31 - 1  also includes an annular mounting flange  57 - 1  and a cylindrical housing  75 - 1  which projects downwardly therefrom in concentric relation with the support fitting  30 - 1 . Similar to the embodiment of FIGS. 1-9, the housing  75 - 1  moves relative to the support fitting  30 - 1  and compresses a resilient ring-like member  34 - 1  therebetween. 
     An adjustment mechanism  35 - 1  also is provided to move the resilient ring  34 - 1  vertically. As described herein, the adjustment mechanism  35 - 1  cooperates with a plurality and preferably three vertically elongate slots  224  which are formed through the housing  75 - 1  and are angularly spaced apart. 
     The adjustment mechanism  35 - 1  operates substantially the same as the above-described adjustment mechanism  35  in that a rotational driving motion is converted into vertical movement of the resilient ring  34 - 1 . However, the drive member  19 - 1  herein is cylindrical rather than ring-shaped and is disposed radially outwardly of the resilient ring  34 - 1  rather than vertically aligned as in the first embodiment of FIGS. 1-9. 
     The drive cylinder  19 - 1  fits over the outside of the housing  75 - 1  in concentric relation therewith, and is rotatable by manual movement of an actuator handle  96 - 1  which projects radially outwardly therefrom. The drive cylinder  19 - 1  includes a plurality and preferably three inclined slots  101 - 1  which extend both circumferentially and vertically. The slots  101 - 1  extend entirely through the wall of the drive cylinder  19 - 1  and are aligned in communication with the vertical slots  224 . 
     To adjust tilting resistance, the drive cylinder  19 - 1  and the resilient ring  34 - 1  are coupled together by intermediate connectors  105 - 1 . The intermediate connectors  105 - 1  are formed as pins which have a threaded end that is inserted horizontally through the aligned inclined grooves  101 - 1  and vertical slots  224  and is threadedly engaged with the outer band  87 - 1  on the resilient ring  34 - 1 . As a result, manual rotation of the drive cylinder  19 - 1  causes the pins  105 - 1  to move vertically as they slide along the grooves  101 - 1 , whereby the pins  105 - 1  move vertically up or down the slots  224  to effect a corresponding vertical movement of the resilient ring  34 - 1 . 
     These pins  105 - 1  also permit the resilient ring  34 - 1  to be inserted into the annular clearance space  80 - 1  and then secured in place since the pins  105 - 1  are confined vertically within the slots  224 . As can be seen, the drive member thereby may be formed as a cylinder disposed radially outwardly of the housing  75 - 1  and resilient ring  34 - 1 . 
     Alternatively, referring to FIGS. 11-13, a third embodiment of the tilt control mechanism  16 - 2  is illustrated. In this alternate embodiment, a drive cylinder  19 - 2  is located radially inwardly of a housing  75 - 2  directly next to a resilient ring  34 - 2  which is in contact therewith. 
     The tilt control mechanism  16 - 2  is formed similar to the first and second embodiments in that a retainer bracket  31 - 2  is pivotally connected to the ball  52 - 2  of a support fitting  30 - 2 . The pivot connection defined thereby provides substantially the same universal tilting movement as provided by the above-described embodiments. 
     The retainer bracket  31 - 2  further includes the cylindrical housing  75 - 2  formed similar to the housing  75 - 1  in that a plurality and preferably three vertically elongate slots  228  are spaced equally about the housing  75 - 2 . The slots  228  are formed the same as the slots  124  and cooperate with the adjustment mechanism  35 - 2  in the same manner as described herein. 
     The drive cylinder  19 - 2  in this third embodiment, however, is located radially between the resilient ring  34 - 2  and the housing  75 - 2  such that the resilient ring  34 - 2  acts on and slides along the drive cylinder  19 - 2 . Nevertheless, the resilient ring  34 - 2  effectively acts on the housing  75 - 2  since the housing  75 - 2  is in slidable contact with the drive cylinder  19 - 2 , such that the resilient ring  34 - 2  defines a tilting resistance. 
     With this arrangement, intermediate connector pins  105 - 2  project radially through the vertical slots  228  and the inclined grooves  101 - 2  which are formed in the drive cylinder  19 - 2 , and then threadedly engaged with the outer band  87 - 2  of the resilient ring  34 - 2 . Thus, rotation of the drive cylinder  19 - 2  causes vertical movement of the resilient ring  34 - 2  in the same manner as described previously. 
     To effect rotation of the drive cylinder  19 - 2 , a pair of ears  131  project upwardly therefrom as seen in FIG.  13 . In particular, the ears  131  are diametrically opposed and project upwardly through a pair of arcuate slot-like tracks  133  formed in a central plate  134  of the retainer bracket  31 - 2 . The ears  131  are curved when viewed from above as seen in FIG. 13, and therefore are able to slide horizontally along the tracks  133 . 
     The retainer bracket  31 - 2  also includes a mounting flange  57 - 2  which extends radially outwardly of the tracks  133  and ears  131  and includes mounting holes  135  (FIG. 13) which receive fasteners (not illustrated) therethrough to connect the retainer bracket  31 - 2  to the seat assembly. To secure the drive cylinder  19 - 2  to the retainer bracket  31 - 2 , a mounting ring  136  is connected to the ears  131  by fasteners  137  (FIGS.  11  and  13 ). The fasteners  137  extend through corresponding holes  132  (FIG. 12) formed in the ears  131 . When connected together, the mounting ring  136  is slidably supported on top of the flange  57 - 2 . 
     To rotate the drive cylinder  19 - 2 , an actuator handle  96 - 2  projects through diametrically opposed sections of the mounting ring  136  as seen in FIG.  13 . Thus, horizontal movement of the handle  96 - 2  causes rotation of the mounting ring  136  and a corresponding rotational movement of the drive cylinder  19 - 2 . This movement thereby adjusts the vertical position of the resilient ring  34 - 2  due to the above-described cooperation of the slots  101 - 2 , grooves  228  and pins  105 - 2 . 
     The actuator handle  96 - 2  also is rotatable about its horizontal longitudinal axis to serve as an actuator for a pneumatic cylinder. In particular, the handle  96 - 2  includes a cam projection  139  which overlies an opening  71 - 2  formed in the central plate  134 , which opening  71 - 2  provides access to the valve of the pneumatic cylinder. An actuator rod, like actuator rod  44 , preferably is connected between the pneumatic cylinder and the cam projection  139  such that rotation of the handle  96 - 2  causes vertical actuation of the pneumatic cylinder. Thus, the handle  96 - 2  serves two functions. 
     In the above-described embodiments, the resilient ring  34  ( 34 - 1  and  34 - 2 ) is annular so as to act circumferentially around the support fitting  30 . This annular shape is preferred since the resilient ring  34  provides a uniform resistance to universal tilting of the seat assembly  22 . 
     In particular, the continuous ring provides for better transmission and generation of forces since the stretching and compressing of the material can be more readily transferred circularly around the entire ring, and this also leads to better durability. Also, the circular ring reacts the same irrespective of the plane of vertical tilt and thus provides good and uniform tilt resistance whether tilt is to front, back, side, or any angle therebetween. 
     While the elastic material  88  preferably is annular so as to extend around the entire resilient ring  34 , the elastic material  88  may also be formed as separate elastomeric blocks or connectors which are connected radially between the inner and outer bands  86  and  87  but are circumferentially spaced apart. Further, the elastic material  88  could be a spring steel, formed, for example, as coil springs wherein the springs are circumferentially spaced apart like the elastomeric blocks. 
     Referring to FIG. 14, a further embodiment of the invention is illustrated. In particular, the tilt mechanism  16 - 3  as illustrated in FIG. 14 is substantially identical to the tilt control mechanism illustrated in FIGS. 1-8 and the prior disclosure relative to this first embodiment is equally applicable to FIG.  14 . Generally, the tilt mechanism  16 - 3  includes a support fitting  30 , resilient ring  34 , retainer bracket  31  and an adjustment mechanism  35  having a drive ring  91 . In the tilt control mechanism  16 - 3 , however, a locking arrangement is provided which permits rotation of the drive ring  91  by the handle  46  but provides a locking effect to resist unwanted rotation of the drive ring  91 . 
     In the illustrated embodiment of FIG. 14, the upper surface of the drive ring  91  includes a ball detent unit  134  which projects upwardly therefrom and is a commercially available component. The movable ball  135  of the ball detent unit  134  is removably engaged with one of a series of concave recesses  136  which receive the ball therein. The ball detent unit  134  is spring-loaded such that the ball  135  slides out of the corresponding recess  136  during rotation of the drive ring  91  but engages a further one of the recesses  136  once the drive ring is adjusted. 
     Other locking arrangements may also be provided. For example, a ratchet-like bracket may be provided on the exterior of the seat assembly where the tilt handle  46  engages one of a series of notches in the bracket. The handle  46 , however, is slidable along the bracket to another one of the notches which again resists movement of the handle after adjustment. It will be appreciated that other suitable locking arrangements may also be provided to resist rotation of the drive ring  91 . 
     Although particular embodiments of the invention have been disclosed in detail for illustrative purposes, it will be recognized that variations or modifications of the disclosed apparatus, including the rearrangement of parts, lie within the scope of the present invention.