Abstract:
An arm assembly ( 24 ) is provided for a chair ( 12 ). The chair ( 12 ) has a forward direction and a rearward direction. The arm assembly ( 24 ) includes an armrest ( 26 ) and a support structure ( 28 ). A mounting device movably mounts the armrest to the support structure. The mounting device constrains the movement of the armrest ( 26 ) relative to the support structure to movement in a substantially horizontal plane having two degrees of freedom defined by first and second transversely arranged axes lying within the substantially horizontal plane. The armrest ( 26 ) thus move in a substantially planar fashion without changing the orientation of the armrest ( 26 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from U.S. provisional application serial No. 60/236,925, filed Sep. 28, 2000, and entitled AMR ASSEMBLY FOR A CHAIR, which provisional application is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to an arm assembly. In particular, although not exclusively, the invention relates to an arm assembly for an office chair where the armrest is adjustable for the comfort of the occupant. The invention also relates to a detachable arm assembly enabling the arm of a chair to be removed or substituted for another type e.g. a writing tablet. While the invention is described herein in terms of an office chair, it will be understood that the invention can be implemented in any type of chair including a wheelchair. Moreover, the invention might also have application beyond chairs. Arm assemblies of the invention may be attachable to a table or workstation, for example to be used as typists&#39; aids. 
     2. The Relevant Technology 
     Human beings come in all different shapes and sizes. For this reason, office chairs generally allow for adjustment e.g. seat height, seat depth. It is also known to provide for adjustment of armrests. A known adjustment includes a twisting action about a vertical axis. Another known type permits a twisting action as well as sideways movement. Another known type permits motion of the armrest in a predetermined oval path within a horizontal plane. Seat depth adjustment is one of the major adjustments required by an occupant of a chair and is to be commonly found on commercial office chairs. As the occupant adjusts their seat depth, the positioning of the armrests relative to the seat will vary even to the extent that the positioning of the armrests may be totally inappropriate for the occupant. Known office chairs do not satisfactorily meet the requirement of being easily adjustable to accommodate the seat depth position. 
     Another shortcoming of known armrest adjustment mechanisms is that they are not easy to adjust. In some known types, the user must reach under the armrest to effect the adjustment or alternatively use the opposite hand to make the adjustment. Adjustment in this fashion is awkward and cumbersome. 
     Yet another shortcoming of some of the known adjustable armrests is that they rely on force for adjustment. Therefore, while the occupant is using the chair in the normal fashion, the armrest will function as intended in the position selected by the user. However, if the user unintentionally bears considerable force against the armrests they can move. This often occurs when the occupant uses the armrests to lift himself out of the chair. This can be destabilising to the occupant and moreover, requires the occupant to re-adjust the armrests when he resumes occupancy of the chair. 
     It is therefore an object of the present invention to provide an armrest assembly which overcomes or at least addresses some of the foregoing disadvantages. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention there is provided an arm assembly for a chair wherein the chair has a forward direction and a rearward direction, said arm assembly comprising: an armrest; a support structure; a mounting device movably mounting the armrest to the support structure, wherein the mounting device constrains the movement of the armrest relative to the support structure to movement in a substantially horizontal plane having two degrees of freedom defined by first and second transversely arranged axes lying within the substantially horizontal plane to move in a substantially planar fashion without changing the orientation of the armrest. 
     It will be understood that the invention need not be limited to travel along the two pre-determined axes. It will be understood that the invention includes within its scope that the armrest is moveable along paths parallel to the first axis and additionally along paths parallel to the second axis. Thus the armrest may be permitted to move in a grid pattern. Additionally, the armrest is moveable in any direction along the substantially horizontal plane incorporating both the first and second axes. Suitably, the movement along the plane is within predetermined limits. 
     It is also preferred that the mounting device enables the armrest to adopt any one of a plurality of adoptable positions defined by the intersection of coordinate locations along the first and second axes. The term adoptable positions is thereby defined. 
     In a most preferred form of the invention, the first axis is generally aligned with the forward direction of the chair. Furthermore, the armrest might also be elongate in form with its longitudinal axis aligned with the forward direction of the chair. If the first axis is aligned with the forward direction of the chair then movement of the armrest along the second axis (or paths parallel thereto) will be in the sideways direction of the chair. 
     In accordance with a second aspect of the present invention there is provided an arm assembly for a chair having a forwards direction and a transverse direction, said arm assembly having a longitudinal axis such that the arm assembly is adapted to be assembled with the chair with the longitudinal axis of the arm assembly corresponding substantially to the forward direction of the chair, the arm assembly further having a transverse axis arranged such that with the arm assembly assembled with the chair, the transverse axis corresponds to the transverse direction of the chair; said arm assembly further comprising: an armrest; a support structure attachable to the chair, the armrest being mounted to the support structure by way of a mounting device such that the armrest is selectively moveable relative to the support structure forwards and backwards to adopt any one of a plurality of coordinate locations along the longitudinal axis and side to side to adopt any one of a plurality of coordinate locations along the transverse axis whereby the armrest is selectively positionable in any one of a plurality of adoptable positions defined by its respective coordinate locations along the longitudinal and transverse axes, the mounting device incorporating a locking device to releasably lock the armrest in any one of the adoptable positions. 
     The following discussion of optional features may be applicable to either of the first or second aspects of the invention set out above. 
     The invention may be further provided with guides to guide the movement along the first/longitudinal and second/transverse axes. The first guide may be provided on part of the mounting device, to guide movement of the armrest relative thereto with a corresponding first guided portion on the armrest. A second guide may be provided in fixed relationship to the support structure with a corresponding second guided portion provided on said part of the mounting device. In a preferred form of the invention, the first guide is provided on a carrier having the second guided portion for engagement with the second guide secured relative to the support structure to allow movement of the carrier relative to the support structure. 
     Preferably, the first guide is in the form of spaced tracks. Accordingly, there may be two corresponding first guided portions for engagement with the parallel spaced tracks. These may take the form of slots or channels provided on the armrest. Alternatively, the arrangement could be reversed with the first guide in the form of slots or channels with the first guided portions being in the form of one or more projections engageable in the slots or channels. 
     Similarly, the second guide may be in the form of a pair of parallel spaced opposed edges forming a part fixed relative to the support structure. The second guided portion may simply comprise the carrier being shaped for sliding engagement with the opposed edges. 
     Suitably, the length of the guides may determine the range of travel along the first and second axes. Stops may be provided to limit the range of travel along the first and second axes. As determined by the guides or by stops, the range of movement along the first axis may be larger than the range of movement along the second axis. For example, the range of movement in the forward/reverse direction may be greater than the range of movement permitted in the side to side direction. 
     The mounting device may enable the armrest to adopt any one of a finite number of adoptable positions. However, within the scope of the invention is the provision that there may be in infinite number of adoptable positions. 
     Where the mounting device enables the armrest to adopt any one of a plurality of adoptable positions defined by the intersection of coordinate locations of the first and second axes then it is preferred that there is a finite number of coordinate locations along the first and second axes with a resultant finite number of adoptable positions. This leads to a grid pattern of adoptable positions. With this in mind, there may be provided a fixed portion mounted in fixed disposition relative to at least the upper part of the support structure. This fixed portion could be part of the mounting device or alternatively an integral part of the support structure. The fixed portion may include a plurality of first engagement portions, there being a second engagement portion carried with the armrest for engagement with at least some of the first engagement portions. Alternatively, the plurality of first engagement portions may be carried with the armrest with the second engagement portion fixed relative to the support structure for engagement with at least some of the first engagement portions. In either case set out above, the arrangement of said some of the first engagement portions is such that each defines an adoptable position of the armrest when the second engagement portion is engaged therewith. In a preferred form of the invention, the arrangement of the first engagement portions may be in a grid pattern with the second engagement portion able to engage with at least some of the first engagement portions of the grid pattern. 
     The first and second engagement portions are shaped to enable engagement therebetween. Accordingly, in a preferred form of the invention, the second engagement portion may take the form of a discrete engagement member or an engagement projection. The first engagement members may be correspondingly shaped recesses/slots/holes to engagingly receive the engagement member or the engagement projection. In a most preferred form of the invention, the second engagement member is in the form of a ball and the first engagement members are in the form of dimples, shaped to receive the ball. In a most preferred form of the invention, the ball is carried by the armrest and the dimples are provided on the fixed portion fixed relative to the support structure. Accordingly, the fixed portion may include a dimple pad. 
     The recesses/slots/holes may have bevelled edges to enable the engagement members/engagement projections to be guided into position. Therefore, the ball and dimples arrangement is particularly preferred for the reason that the ball is easily locatable within any one of the dimples. 
     Where the second engagement portion is provided in the form of a discrete member e.g. ball, roller, the ball/roller may be normally biased into a position for engagement with one of the first engagement members. 
     Conversely, the first engagement portions may comprise discrete engagement members or engagement projections such as nodules. Additionally, the second engagement portion may be formed as a recess/slot/hole to receive one of the first engagement members or projections. 
     In another specific form of the invention, the first engagement members may be in the form of rollers with the second engagement portion being shaped as a trough-shaped recess to receive any one of at least some of the rollers. 
     In a preferred form of the invention, the plurality of first engagement portions are provided on the fixed portion. The fixed portion may extend forwardly or rearwardly of the carrier. Alternatively, the fixed portion may extend out partly forwardly and partly rearwardly of the carrier. This is the preferred form because in this way, the two sides of the fixed portion define opposing guides for the carrier. Some of the first engagement portions e.g. dimples, may also be provided on the carrier. This is particularly preferred where the carrier is centrally located. Therefore, there may be more than one second engagement portions. Preferably, there are two engagement portions e.g. two balls, with the arrangement of corresponding first engagement portions e.g. dimples, on the carrier being such that both second engagement portions are not engageable with the carrier at the same time. Where it is desirable to lock the armrest into position, this arrangement will preclude the armrest from being locked to the moveable carrier which is essentially a non-locking configuration of the armrest. 
     In a most preferred form of the invention, there are two balls carried by the armrest and two portions of the dimple pad are arranged forwardly and rearwardly respectively of a centrally disposed carrier with further dimples arranged on the carrier with three dimples extending across the width of the carrier wherein the spacing between the outermost dimples on the carrier is less than the spacing of the two balls such that with one of the balls engaging the carrier, the other ball engages with one or other of the portions of the dimple pad. The two balls could be discrete balls but alternatively could be joined. 
     In an alternative less preferred form of the invention, there may be one or more balls disposed on the fixed portion with the armrest carrying the dimple pad such that the dimple pad can engage with the balls and adopt any one of the plurality of adoptable positions. 
     It will be also appreciated that the invention is not limited to balls and a dimple pad. For example, the fixed portion may comprise one or more projections e.g. nodules. The arm portion may carry one or more dimple portions shaped for engagement with the nodules with the arrangement of nodules and dimples enabling the arm portion to adopt any one of the plurality of adoptable positions. 
     The armrest may be lockable in any one of the adoptable positions. In this regard, the armrest may be selectively lockable. However, it is preferred that the armrest is normally locked in each of the adoptable positions with a release device being provided to unlock the armrest from its adopted position. As such, the release device may release the engagement between the first and second engagement portions. The release device may be in the form of a separator to separate the second engagement portion out of engagement with the first engagement portion. Preferably, the separator is in the form of a moveable slide having an engagement position whereby the first and second engagement portions are in engagement and a release position allowing disengagement of the first and second engagement portions. Where the first engagement portions are in the form of a recesses and the second engagement portion comprises discrete engagement members then the moveable slide may include a shaped recess of graduated or varying depth with a shallow part and a deep part, with the engagement position of the slide corresponding to the discrete engagement member being received in the shallow part and the release position of the slide corresponding to the discrete engagement member being received in the deep part. The slide may be biased towards the engagement position. 
     In an alternative form of the invention where the first engagement portion comprises a plurality of projections e.g. nodules and the second engagement portion comprises a correspondingly shaped recess provided on the slide member, the slide may include an inclined surface opposite the recess which is in engagement with a ramped surface, whereby the slide is slidable along the ramped surface to allow the recess of the slide to lift out of engagement with the engaged nodule. 
     The release device may be directly operable by the user. Alternatively, the release device may incorporate an actuator. Preferably, the actuator is disposed for easy reach of the hand of a user whose corresponding arm is supported by the armrest. 
     The armrest may be elongate in shape to support a forearm of the chair occupant. A flat upper surface may be provided although a trough-shaped armrest is also possible. The preferred shape is convex both in X and Y planes. Preferably, the upper surface of the armrest is padded for comfort. 
     In accordance with yet another aspect of the present invention there is provided an arm assembly for a chair having a forwards direction and a transverse direction, said arm assembly having a longitudinal axis such that the arm assembly is adapted to be assembled with the chair with the longitudinal axis of the arm assembly corresponding substantially to the forward direction of the chair, the arm assembly further having a transverse axis arranged such that with the arm assembly assembled with the chair, the transverse axis corresponds to the transverse direction of the chair; said arm assembly further comprising: an armrest; a support structure attachable to the chair, the armrest being selectively moveable relative to the chair forwards and backwards to adopt any one of a plurality of coordinate locations along the longitudinal axis and side to side to adopt any one of a plurality of coordinate locations along the transverse axis whereby the armrest is selectively positionable in any one or a plurality of adoptable positions defined by its respective coordinate locations along the longitudinal and transverse axes; a locking device to releasably lock the armrest in any one of the adoptable positions wherein the locking device is operable by a user&#39;s hand positioned on the armrest. 
     Any of the features discussed in connection with any of the foregoing aspects of the invention may be incorporated into the abovementioned aspect. 
     The armrest assembly according to any of the foregoing aspects of the invention may be incorporated into a chair. Suitably two armrests will be incorporated into each chair. Preferably, the chair is an office chair. The armrest may also be incorporated into a wheelchair or used as a typist&#39;s aid at a workstation. 
     The chair and arm assembly may be designed such that the support structure is detachable from the chair. The support structure may comprise a column-like portion supporting the mounting device at its upper end. The column-like portion may include a height adjustment mechanism to enable the height of the armrest to be adjusted to suit the chair occupant. The lower end of the column-like portion may be connected to an attachment portion for detachable connection of the arm assembly to the chair. 
     In accordance with yet another aspect of the present invention there is provided a chair including: a main assembly having a seat portion, a back portion, and a support frame supporting the seat portion and the back portion; an arm assembly detachable from the main assembly, the arm assembly comprising: an armrest; a support structure having an upper end and a lower end, the support structure supporting the armrest at its upper end; and an elongate attachment portion disposed at or adjacent the lower end of the support structure, the elongate attachment portion having an external surface and including male engagement portions in defined areas of the external surface, the engagement portions including an outer male engagement surface and an inner male engagement surface spaced from each other; the main assembly further including an arm attachment portion in the form of a bore to receive the elongate attachment portion, the surface of the bore including female engagement portions in defined areas of the bore, the female engagement portions including an outer female engagement surface and an inner female engagement surface, the inner and outer female engagement surfaces being shaped and disposed such that upon insertion of the elongate attachment portion into the arm attachment portion, the outer male engagement surface and the inner male engagement surface are caused to releasably engage with the inner female engagement surface and the outer female engagement surface respectively wherein the male engagement surfaces are formed on portions of the elongate attachment portion which are non-tapering and of small length in comparison to the length of the elongate attachment portion. 
     In a preferred form of the invention, the elongate attachment portion is downwardly inclined relative to the support structure. In this way, any downward force applied to the armrest will be resolved in the elongate attachment portion in a manner tending to push the elongate attachment portion into the arm attachment portion of the main assembly. A force in the upright direction applied to the armrest will also resolve in the elongate attachment portion in the same manner due a cantilever effect. 
     The arm attachment portion which includes the female engagement surfaces may be disposed on any part of the chair such as the seat portion or a lower region of the back portion. In a most preferred form of the invention the arm attachment portion is disposed on the support frame. The support frame may include a main transom supported by a pneumatic spring. It is desirable that the arm attachment portion is defined at one end of the main transom. 
     Preferably, the length of the mutual engagement between the outer male engagement surface and the inner female engagement surface is less than 5 mm. Likewise, it is preferred that the length of the mutual engagement of the inner male engagement surface and the outer female engagement surface is less than 5 mm. 
     Preferably, the portions of the elongate attachment portion including the male engagement surfaces are of uniform cross-section. The cross-sections of the mutually engaging engagement surfaces should naturally correspond. The outer male engagement surface may be provided at the end of the elongate attachment portion but not necessarily so. An intermediate portion is defined between the outer male engagement surface and the inner male engagement surface. This may be of any shape including tapering, provided that it does not interfere with the insertion of the elongate attachment portion into the arm attachment portion. 
     The arm attachment portion could be a hollow member. However in a more preferred form of the invention, the arm attachment portion comprises first and second bands incorporated into the transom. 
     A clip may also be provided to prevent inadvertent detachment of the arm assembly from the main assembly. Preferably, the clip is normally locked with an actuator provided to disengage the clip and release the elongate attachment portion from the main assembly attachment portion. 
     In accordance with a further aspect of the invention there is provided an armrest for a chair, wherein the armrest incorporates a guide and slide member slidably mounted thereto, the slide member accommodating an optical disc which provides operating instructions for the chair, the slide member being moveable along the guide between a retracted position and an extended position in which an occupant of the chair can access the optical disc. 
     Preferably, the slide member is manually operable by the user. Preferably, the slide is moveable from the retracted position fully received within the armrest, to an extended position forward of the armrest, the slide member extending through a forward opening in the armrest. 
     This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth. 
     The invention consists in the foregoing and also envisages constructions of which the following gives examples. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be more fully understood, one embodiment will now be described by way of example with reference to the drawings in which: 
     FIG. 1 is an exploded perspective view illustrating a chair with two detachable arm assemblies according to a preferred embodiment of the present invention; 
     FIG. 2 is a detailed view of a portion of the chair shown in FIG. 1 illustrating attachment of one of the arm assemblies; 
     FIG. 3 is a perspective view of the arm assembly with the upper padding removed; 
     FIG. 4 is a horizontal sectional view through the lower part of the arm assembly of FIG. 3 looking from below; 
     FIG. 5 a  is a perspective view of the lower part of the arm assembly of FIG. 3, partially sectioned vertically; 
     FIG. 5 b  is a perspective view from below of the parts illustrated in FIG. 5 a;    
     FIG. 5 c  is a vertical cross-section, like FIG. 5 a , except illustrating a clip to secure attachment of the arm assembly; 
     FIG. 6 is a perspective view of an upper portion of the arm assembly with parts removed for clarity; 
     FIG. 7 is a perspective view as per FIG. 6 with additional parts shown; 
     FIG. 8 a  is a detailed view of the upper part of the arm assembly with the armrest pad removed for clarity; 
     FIG. 8 b  is a longitudinal sectional view through the upper part of the armrest assembly illustrated in FIG. 8 a;    
     FIG. 8 c  is a transverse sectional view through the upper part of the armrest assembly illustrated in FIG. 8 a;    
     FIG. 8 d  is a perspective view of an armrest pad of the armrest illustrated in FIG. 1; 
     FIG. 9 is a perspective view of the armrest with a dependent upper column portion; 
     FIG. 10 is a perspective view of the armrest showing the internal detail of the upper column portion; 
     FIG. 11 is a detailed view of a lower part of the upper column portion  50  shown in FIG. 9, from the rear; 
     FIG. 12 illustrates the detail of the lower part of the upper column portion as shown in FIG. 11, received within a half portion of a stem liner; 
     FIG. 13 is a detailed view of the other half portion of the stem liner; 
     FIG. 14 is a perspective view of a modified form of an armrest in accordance with the present invention; 
     FIG. 15 is a view as per FIG. 14 with the armrest pad removed; 
     FIG. 16 is a view as per FIG. 15 with a further part removed; 
     FIG. 17 is an underside perspective view of a modified transom together with a lower part of a modified form of the arm assembly mounted to the transom; and 
     FIG. 18 is another underside perspective view as per FIG.  17 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates an office chair including a main assembly  12  having a seat portion  14  and a back portion  16 . The seat portion  14  and the back portion  16  are supported above the ground by a support frame including a wheeled base  18  and a central support column  20 . The support frame also includes a mechanism enabling the back portion  16  to recline with a synchronous tilting action of the seat portion  14  as the back portion  16  reclines. The details of the mechanism are not important to the present invention and the reader may refer to our co-pending patent application Ser. No. 60/236,925, the details of which are incorporated by reference herein. The central support column  20  houses a pneumatic spring (not shown) for height adjustment of the seat portion  14  in conventional fashion. The pneumatic spring is connected to the main transom  22  of the chair which is not illustrated in FIG. 1 but shown in FIG.  2 . The main transom  22  extends transversely across the chair and is connected to the pneumatic spring by way of central spring connection ring  23 . 
     FIG. 1 also illustrates two detachable arm assemblies  24 . The arm assemblies  24  include an upper armrest  26  which is padded for user comfort. Each arm assembly also includes an upright support structure  28 . The armrest  26  is mounted to the upper end of the upright support structure  28 . The lower end of the upright support structure has an elongate attachment portion  30  extending inwardly therefrom in a downwardly inclined angle as shown in FIG.  3 . 
     The elongate attachment portion  30  engages within one end of the main transom  22  in the manner illustrated in FIG.  2 . The manner of attachment is illustrated in more detail in FIGS. 4 and 5 a - 5   c  which are various views of the end of the transom  22  with the elongate attachment portion  30  engaged therein. As best shown in FIG. 5 a , elongate attachment portion  30  includes an outer male engagement surface  32  and an inner male engagement surface  34 . The outer male engagement surface  32  is spaced from the inner male engagement surface  34  thereby defining an intermediate portion  36  of tapering configuration. As will be appreciated in connection with the drawings, outer and inner define the positioning of the engagement surfaces relative to the arm assembly. 
     Each end of the main transom  22  includes an arm attachment portion in the form of a void to accommodate the elongate attachment portion  30 . The void is not a fully sided hole but rather is defined only by an inner female engagement surface  38  and an outer female engagement surface  40  at the lower side thereof. It will be appreciated that the elongate attachment portion  30  will be supported on the inner and outer engagement surfaces  38 ,  40  of the main transom  22 . Other surfaces of the elongate attachment portion  30  and the transom  22  are in contact. These are disposed at a clearance of 0.2 mm. The additional surfaces prevent side-to-side movement and up and down movement. It will be appreciated that in this context inner and outer are used from the perspective of the main chair assembly  12 . In order to assemble the arm assembly  24  with the main assembly  12 , the elongate attachment portion  30  is inserted into the main transom until the outer male engagement surface  32  engages against the inner female engagement surface  38  and the inner male engagement surface  34  engages against the outer female engagement surface  40 . The male engagement surfaces are formed on parts of the elongate attachment portion  30  which are non-tapering and of small length compared to the length of the elongate attachment portion  30 . 
     It can be seen that the elongate attachment portion  30  is downwardly inclined relative to the upright support structure  28 . In this way, any downward force applied by the chair occupant to the armrest  26  will be resolved in the elongate attachment portion in a direction tending to push the elongate attachment portion into the arm attachment portion. A force in the upright direction applied to the armrest will also be resolved in the elongate attachment portion in the same manner due to a cantilever effect. 
     Additionally, a clip is provided to secure the attachment of the arm assembly. As shown in FIG. 5 c , the clip includes a latch member  42  having a hook portion  44  engageable against a latch surface  45  (see FIG. 5 a ) formed as part of the main transom  22 . The latch member  42  has a first fulcrum  43  resting on pivot  48  and a second fulcrum  47  bearing against button  46 . The latch member  42  is formed to include an integral spring with the second fulcrum  47  biasing the button  46  outwardly. As the button  46  is pushed inwardly, the latch member  42  pivots about fulcrum  43  to release the hook portion  44  from engagement with the latch surface  45  in order that the arm assembly  24  may be detached from the main chair assembly  12 . 
     As best shown in FIG. 2, the support structure  28  includes an upper column member  50  telescopically received in a hollow lower column member  52 . The upper column member  50  and the lower column member  52  are adjustable relative to one another to effect height adjustment in a manner which will be explained in more detail in connection with FIGS. 9 through 13. Extending at a downwardly inclined angle from a lower part of the lower member  52  is leg portion  54 . The elongate attachment portion  30  is connected to the lower end of the leg portion  54 . The upper column portion  50  supports a fixed portion  56  in normally fixed disposition thereto (given that the fixed portion  56  is height adjustable as will subsequently be explained). 
     Referring now to FIG. 6, the fixed portion can be seen to include two dimple pads  57  extending on either side of a transversely slidable carrier  58 . The dimple pads  57  are two separate injection molded components self locating into the fixed portion  56 . The dimples in the dimple pads provide a plurality of first engagement portions. As shown in FIG. 8 b , the fixed portion  56  defines transverse guides for the slidable carrier  58 . The carrier  58  is thereby slidable back and forth in the sideways direction of the chair. In FIG.  c , it can be seen that the sides of the fixed portion  56  defines stops for side-ways movement of the carrier  58 . 
     The carrier  58  additionally includes two longitudinal guides  62 . The guides  62  are each of T-shaped configuration as shown. As best shown in FIG. 8 a , the guides  62  extend through longitudinally extending slots  64  provided in a base portion  66  of the armrest, the ends of the slots defining stops for longitudinal movement. The base portion  66  of the armrest  26  is thereby slidable forward and backward along or parallel to a first longitudinal axis A (FIGS. 1 and 3) by the sliding action of the longitudinal guide  62  within the slots  64 . Since the longitudinal guide  62  are formed or the carrier  58 , as the carrier  58  slides sideways, the base portion  66  of the armrest  26  will be likewise carried in the sideways direction along or parallel to a second transverse axis B (FIGS.  1  and  3 ). 
     It will be understood that with the combined action of the longitudinal guide  62  and the transverse guide  60 , the armrest will be able to move in any direction along the horizontal plane defined by the transverse guide and the longitudinal guide  62  within predetermined limits established by the stops. In order to establish a finite number of adoptable positions which the armrest  26  can adopt within the plane, the armrest carries a pair of discrete engagement members in the form of metal balls  70  in fixed disposition relative to each other to provide second engagement portion. Together, these balls can move across the carrier  58  and engage with respective ones of the dimples  72  provided on either of the dimple portions  57 . Additionally, the carrier  58  also has dimples  74 . These are arranged at a spacing whereby only one of the balls can engage with the carrier  58  at a time, with the other ball engaging with the dimple  72  from one or other of the dimple pads  56 . 
     The balls  70  are carried by the base portion  66  of the armrest  26  by way of slide  80  as shown in FIG.  7 . On the underside of slide  80  as shown in FIG. 8 b , two recesses  82  are provided, each for receiving one of the balls  70 . Each recess is graduated to include a shallow part and deep part, the orientation of the shallow and deep parts being the same in each case. When the balls  70  are located in the shallow part of the recesses  82 , the balls will be held into engagement with the engaged dimples. In order to release the balls  70  from engagement with the dimples  72  the slide  80  is slid in its lengthwise direction so that the deep part of the recess is engaged with the balls  70 , thereby releasing the balls  70  from engagement with the dimples  72 . 
     As shown in FIG. 8 a , the slide  80  is slidably mounted within the base portion  66 . A pivotable actuator  84  is pivoted to the base portion  66  about pivot  86 . The pivotable actuator  84  is an L-shaped member with a depressor portion disposed conveniently on the armrest in the vicinity of the user&#39;s thumb. The slide  80  is biased by spring  88  towards an engagement position whereby the balls  70  reside in the shallow part of the recess in the slide. Therefore, the actuator  84  is operated against the bias of the spring  88 . The end of the slide  80  opposite to the actuator  84  also incorporates an abutment projection  90  which in the release position of the slide member, engages against stop  92 . 
     In operation, the user depresses actuator  84  with his or her thumb thereby moving the slide  80  to the release position whereby the balls  70  are released from engagement with the dimples  72 . With the user holding his thumb against actuator  84 , the base portion  66  can be slid forwards and backwards along the longitudinal guides  62 . Additionally, the base portion  66  can be moved from side to side by movement of the carrier  58  along the transverse guides. Additionally, synchronous movement of the base portion  66  on the guide  62  and the carrier  58  on the transverse guides is possible so that the base portion  66  can move in any direction along the plane within predetermined outer limits. When the user releases the actuator  84 , the balls  70  will bear against the dimple pads  57  and/or against the central portion of carrier  58 . A slight adjustment of the armrest  26  will then locate the balls  70  within the closest dimples. The armrest will then be locked into the adopted position until the user again depresses the actuator  84 . 
     FIG. 9 illustrates the upper half of the armrest  26  with the upper column portion  50  removed from the lower column portion  52  within which it is normally telescopically received. The upper column portion  50  is a substantially solid column member. FIG. 10 illustrates the internal components of the upper column portion  50  including a height adjustment actuator  101 , a roller carriage  102  and connecting rod  103 . As can be seen in FIG.  10  and more clearly in FIG. 13, the roller carriage  102  has a graduated recess  105  provided therein having a deep part and a shallow part. A roller  104  is received within the graduated recess  105 . 
     Referring to FIG. 11, the roller carriage  102  is slidable within the lower end of the upper column portion  50  as defined by a locating receptacle  106  in which the roller carriage  102  is housed. The roller carriage  102  is moveable within the receptacle  106  as the actuator  101  is operated by the user. The roller carriage is normally biased by a spring (not shown) into the lower most position illustrated in FIG.  11 . 
     As shown in FIGS. 12 and 13, the lower end of the upper column portion  50  is received within two half portions of a stem liner. One of the half portions  108  is illustrated in FIG.  12 . The other half portion  109  is illustrated in FIG.  13 . The two half portions  108 ,  109  together define a central conduit within which the lower part of the upper column portion  50  is telescopically received. The half portions of the stem liner  108 ,  109  fill part of the void within the hollow lower column portion  52 . The ribs  110  assist with filling the void and also adds strength to the half portions of the stem liners. Additionally, one half portion of the stem liner has integrally formed leaf springs (not shown) to bias the stem liner towards one side of the lower column portion  52 . Furthermore, one or both of the half portions of the stem liner have internal integrally formed leaf springs to reduce any slack between the upper column portion  50  and the stem liner. 
     The half portion  108  illustrated in FIG. 12 is provided with a series of spaced trough-like recesses  100 . The roller  104  which in part protrudes through an opening  111  (See FIG. 9) is shaped to engage with any one of the trough shaped recesses  100 . As already explained, the roller carriage  102  is normally biased by a spring into a lowermost position illustrated in FIG.  12 . In this position, the roller  104  will be received in the shallow part of the graduated recess  105  while it is in engagement with one of the trough-like recesses  100 . 
     When it is desired to adjust the height of the armrest  26 , the user operates the actuator  101  in order to move the roller carriage  102  upwardly. The roller  104  in effect is moved down the graduated recess  105  towards the deep part until it is no longer held between the base of the recess  105  and the trough shaped recess  100 . Once engagement between the roller  104  and the trough shaped recesses  100  is released, the upper column portion  50  can be moved relative to the lower column portion  52 . Once the height of the armrest has been satisfactorily adjusted, the user releases the actuator. With possibly some slight adjustment required, the roller  104  will locate within the closest trough shaped recess  100  and in doing so, the roller carriage  102  will move with the bias of a spring (not shown) into a locking configuration with the roller  104  located in the shallow part of the recess and in engagement with a recess  100 . 
     FIG. 13 illustrates the other half portion  109  of the stem liner, the inner side having two spaced tracks in the form of elongate recesses  112 . The roller carriage  102  has two spaced guides  114  which are configured to be slidingly received within the tracks  112 . At the upper end of the half portion  109 , the tracks terminate and accordingly, this termination point will define an upper limit beyond which the guides  114  cannot be upwardly moved. This prevents the upper column portion  50  being fully withdrawn from the lower column portion  52 . 
     FIG. 14 illustrates a modified form of the armrest  26   a . The upper portion of the armrest  26   a  which is padded for user comfort includes an opening  118  in which a slide  120  is received. FIG. 15 is a view as per FIG. 14 except with the padded part of the upper portion removed. The slide  120  is slidably received within a slide casing  125 , mounted on the base portion  66 . The slide  120  can clearly be seen in FIG.  16 . The slide includes a shallow rebate  127 . An optical disk such as a CD (not shown) is received in the rebate  127 . The CD is used with a computer to graphically illustrate the operating features of the chair  10 . 
     FIGS. 17 and 18 illustrate of modified form of the main transom  22   a . Several of the features of the main transom  22   a  are not described in detail here but are described in Provisional Application Serial No. 60/236,925 in connection with the description of the features and operation of the chair as a whole. The main transom  22   a  is supported from the base  18  by a pneumatic spring at central spring connection ring  23   a.    
     Only the lower part of the modified form of the arm assembly is shown in FIGS. 17 and 18. This includes a lower column member  52   a  and an attachment portion  130  extending inwardly therefrom. The upper part of the modified form of the arm assembly may be the same as shown in the preceding drawings with the lower column member  52   a  telescopingly receiving the upper column member  50 . 
     The attachment portion  130  is made up of two parts including an outer attachment section  130   a  which is integrally formed with the lower column portion  52   a . Further, the attachment portion  130  includes an inner attachment section  130   b  which is bolted to the outer attachment section  130   a  by means of bolts  131  and  132 . A further bolt  133  fastens the whole attachment portion  130  to the main transom  22   a . The inner attachment section is provided mainly for aesthetic reasons. 
     The foregoing describes only one embodiment of the present invention and modifications can be made thereto without departing from the scope of the invention.