WO1994028768A1

WO1994028768A1 - A chair

Info

Publication number: WO1994028768A1
Application number: PCT/AU1994/000317
Authority: WO
Inventors: Philip Louis Peach; David St. Barbe MORE; Alexander Stewart Richardson; Ronald Charles Hibberd
Original assignee: Philip Louis Peach; Moore David St Barbe; Alexander Stewart Richardson; Ronald Charles Hibberd
Priority date: 1993-06-11
Filing date: 1994-06-10
Publication date: 1994-12-22

Abstract

A chair is provided with a linkage mechanism for allowing pivotal movement of the seat (2) and backrest (3), a seat height adjustment mechanism for adjusting the height of the seat (2), a backrest height adjustment mechanism for adjusting the height of the backrest (3) relative to the seat (2), and a single lever (7) for controlling the linkage mechanism, the seat height adjustment mechanism and the backrest height adjustment mechanism.

Description

A Chai r

The present invention relates to chairs and, in particular, to chair adjustment mechanisms.

In a number of situations it is important to have a chair which is adjustable so as to cater for both persons of different sizes, and the different tasks which an individual is called upon to carry out whilst sitting upon the chair. The most important such situation is office or work chairs, however, adjustable chairs are not restricted to this particular situation.

It is desirable that the chair adjustment mechanism be able to cater for a wide range of individual sizes, the extremes of which are represented by short ladies of Asian descent and tall males whose hobbies include basketball. For this reason, it is necessary that the seat height above the floor be adjustable within wide limits. Similarly it is necessary that the height of the backrest above the seat be able to be adjusted within reasonably wide limits.

In addition, it is also desirable that the relative angle between the seat and backrest be able to be adjusted. It has long been recognised that it is not desirable that the seat backrest be independently adjustable since for ergono ic reasons there are combinations of seat and backrest angle which should be excluded. For this reason it is desirable to have the seat and backrest interlinked so that as the angle of one changes so the angle of the other changes in a corresponding desired relationship.

It is the object of the present invention to provide chair adjustment mechanisms which enable the abovementioned desired features to be achieved to a substantial extent.

In accordance with a first aspect of the invention there is disclosed a chair adjustment mechanism for a chair having a seat, seat height adjustment means for adjusting the height of the seat, a backrest adjustably mounted on a backrest support, backrest height adjustment means for adjusting the height of the backrest, and a pivotable linkage interconnecting the seat and backrest support which allows the seat and backrest to be moved between a forward tilted position and a rearward tilted position, said mechanism comprising biassing means for biassing the seat and backrest towards said forward tilted position, adjustable securing means for securing the seat and

SUBSTTΓUTE SHEET (Rule 26) backrest in a selected position between said foward and rearward tilted positions, a hollow lever having a knob rotatably mounted at one end thereof, a cable passing through said lever and interconnecting said knob and the backrest height adjustment means, whereby the backrest is either raised or lowered relative to said backrest support by rotation of said knob in a corresponding direction, said lever being pivotally mounted below the seat and being movable from its rest position in a first direction to actuate said seat height adjustment means to thereby allow the height of the seat to be set, and said lever being movable from said rest position in a second direction to actuate said adjustable securing means to thereby allow the angle of said seat and backrest to be set relative to the horizontal .

The adjustable securing means can, for example, comprise a lockable gas spring or hydraulic damper.

The biassing means can, for example, comprise a gas spring mechanical spring or any kind of torsion bar.

In accordance with a second aspect of the present invention there is disclosed a backrest height adjustment mechanism for adjusting the height of a chair backrest, comprising a hollow backrest support, a cable having a first threaded member at one end thereof, a bush mounted in said support and having the first threaded member rotatably mounted therein, a second threaded member slidably mounted within said bush, threadably engaged with said first threaded member, and carrying said backrest whereby rotation of said cable translates said second threaded member within said bush to thereby adjust the height of said backrest.

Conveniently, the first threaded member is formed with an external thread, and the second threaded member is formed with a corresponding internal thread.

Preferably, the backrest is mounted onto a hollow stalk which slides into the backrest support and snap engages with the second threaded member, to allow the backrest height adjustment mechanism to be transported in a knock down configuration and to be assembled without tools.

In accordance with a third aspect of the invention there is disclosed a rotatable actuator knob for a chair adjustment mechanism, said knob comprising an outer tube having a recess at one end and being rotatably mounted on a hollow lever, said recess being shaped to form a first part of a two part clutch, a second part of the two part clutch housed in said recess, a cable passing through said lever into said recess and being engaged with the second part of the two part clutch, a recess cover engaged with said tube and containing biassing means between the cover and said second part of the clutch to urge said second part against said first part whereby rotation of said outer tube rotates said cable via said clutch which is free to slip in the event of said cable being restrained against further rotation.

The cable can be installed into the hollow lever either before, or after, the hollow knob and clutch are mounted on the lever. Preferably, the actuator knob is mounted on the lever by means of a snap engagement between fingers on the actuator knob and a groove in the exterior of the lever.

In accordance with a fourth aspect of the invention there is disclosed a mechanism to simultaneously adjust the angle of a seat and a backrest of a chair, said mechanism comprising a base plate mountable in a generally horizontal position atop a seat support, a seat plate pivotally mounted on said base plate, a backrest support pivotally mounted on said seat plate, a length adjustable strut pivotally mounted at each end and interconnecting said backrest support and said base plate, and a pair of pivoted levers interconnecting said base plate and backrest support.

The length adjustable strut can take the form of a gas strut, a hydraulic and/or mechanical strut, or a mechanical strut all of which are both length adjustable and self locking to the last adjusted length.

One embodiment of the present invention will now be described with reference to the drawings in which:

Preferably, the mechanism further comprises a tapered recess adapted to receive an upper portion of a chair base support, to allow the mechanism to be fitted to the chain base support without the use of tools.

Fig. 1 is a perspective view of an office chair incorporating the seat adjustment mechanism of the preferred embodiment;

Fig. 2 is an exploded perspective view of the mechanism of the chai of Fig. 1 ; Fig. 3 is a front-to-rear sectional view through the chair of Fig. 1;

Fig. 4 is a longitudinal cross-sectional view through the knob at the end of the adjustment lever;

Fig. 5 is a plan view from above of the mechanism of Fig. 2 with the seat removed;

Fig. 6 is an inverted plan view of the mechanism of Fig. 2 with the footrest and seat support removed;

Fig. 7 is a front elevation of the chair;

Figs. 8 and 9 are front-to-rear cross-sectional views showing the operation of the seat height adjustment mechanism;

Figs. 10 and 11 are front-to-rear cross-sectional views showing the operation of the seat and backrest adjustment mechanism; and

Fig. 12 is a side elevational view showing the position of the seat and backrest in two extremes of their movement, one extreme being illustrated by dotted lines.

Figs. 13, 15, 17, 19 and 20 correspond to the inverted plan view of Fig. 6, and show alternative arrangements for biassing the seat and backrest of the chair;

Figs. 14, 16 and 18 correspond to the front-to-rear sectional view of Fig. 3, and illustrate the embodiments shown in Figs. 13, 15 and 17 respectively; and

Figs. 21 and 22 are cross-sectional views through hydraulic dampers suitable for use in the embodiments shown in Figs. 13 to 19.

As seen in Fig. 1, the chair 1 of the preferred embodiment has a seat 2, a backrest 3 which is supported above the seat 2 by a backrest support 4 and a seat support 5 which is itself carried on a five armed footrest 6. Located immediately underneath the seat 2 and extending out towards the right hand edge of the seat is an adjustment lever 7 having a knob 8 at its free end.

As indicated in Fig. 1, the lever 7 can be raised in the direction of arrow A so as to vertically raise or lower the seat 2 as indicated by arrow B in order to adjust and set the seat height above the floor. In addition, the lever 7 can be lowered by being moved in the direction of arrow C so as to simultaneously tilt the backrest support 4 in the direction indicated by arrows D and tilt the seat 2 in the direction indicated by arrows E. Furthermore, the knob 8 is able to be rotated in either direction as indicated by arrow F in order to adjust and set the height of the backrest 3 above the seat 2 as indicated by arrows G in Fig. 1.

These various adjustments represent all the desired adjustment for catering not only between different sizes of individuals, but different tasks to be undertaken by a given individual whilst seated on the chair. Accordingly, it is of substantial importance that all these adjustments be able to be undertaken by means of manipulation of a single lever.

Turning now to Fig. 2, the basic components of the various mechanisms incorporated in the chair are illustrated in an exploded perspective view. The seat support 5 takes the form of a gas cylinder or gas strut 11 on top of which is mounted a base plate 12. The backrest support 4 which is of hollow tubular construction is pivotally mounted at 56 to the base plate 12 by means of a pair of support arms 13. Located above, and pivotally mounted at 10 to, the base plate 12 is a seat plate 14 which in turn carries a carrier 15 for the seat 2. The seat plate 14 is also pivotally mounted at 50 to the support arms 13.

As best seen in Fig. 3, the backrest support 4 has a bowden cable 16 which passes through the hollow lever 7 to the knob 8. The end of the bowden cable 16 terminating within the backrest support 4 is provided with a triple start coarse thread 17, and is rotatably mounted within a guide bush 18, which may be a split bush to facilitate manufacture and assembly. The bush 18 is fixed in the upper end of the backrest support 4 but provides a slidable bush mounting for a tubular backrest stalk 19, which is square or some other shape to prevent rotation in the bush 18. Fixed within the stalk 19 are two half nuts, a split nut or a nut 20 having an internally threaded bore which mates with the coarse thread 17. As a consequence, rotation of the bowden cable 16 quickly moves the nut 20 along the coarse thread 17 and therefore quickly raises or lowers the stalk 19, and hence the backrest 3, in the direction of arrows G as illustrated in Figs. 1 and 3.

As best seen in Fig. 2, the nut 20 is moulded from plastics material so as to have two pliant fingers 21 each of which carries a protrusion on its free end. The stalk 19 is formed with two (but preferably four) apertures 22 adjacent its lower end so that the stalk 19 can be pushed onto the nut 21 with the protrusions of the free ends of the fingers 21 engaging with the apertures 22 and thereby preventing the stalk 19 from being withdrawn. Four apertures makes assembly easier since orientation is less critical. This enables the chair to be sold with the backrest support 4 fitted in position, but the backrest 3 with its stalk 19 not mounted in its final position. Because of the snap engagement between the fingers 21 and apertures 22, the final assembly of the backrest 3 onto the chair can be accomplished by the eventual end user without the use of tools. This provides a substantial volume saving in relation to the transport of the chair between its initial assembly in the factory and transport to the end user.

It will similarly be appreciated from Fig. 2 that the bush 18 includes a projection 24 which snap engages with an opening 25 in the backrest support 4, thereby securing the bush 18 within the backrest support 4.

Turning now to Fig. 4, the other end of the bowden cable 16 is provided with a squared drive 27 which is received in a square aperture 28 in one part 29 of a two part clutch 30. The other part of the clutch 30 is formed from the interior surface 31 of a recess 32 in the tubular knob 8.

The recess 32 is closed by a hollow cap 33 which includes a lip

34 which snap engages with the recess 32 so as to compress a helical spring 35 between the cap 33 and the clutch part 29.

The knob 8 is retained on the lever 7, and rotatable thereabout, by means of an annular groove 36 formed in the lever 7 and into which the free ends of four arms 37 engage, only two of the arms 37 being illustrated in Fig. 4.

It will be apparent to those skilled in the art, that the spring

35 drives the two parts 29,31 of the clutch 30 together. These parts are provided with a rippled or castellated surface (not illustrated) which therefore enables rotary motion applied to the knob 8 to be transferred to the part 29 and thence to the squared drive 27 without the clutch 30 slipping. However, if the bowden cable 16 is rotated so as to place the nut 20 at either end of its range on the coarse thread 17, then the nut 20 prevents further rotation of the bowden cable 16. As a consequence, the squared drive 27 is unable to be rotated any further thereby causing the part 29 to remain stationery and the clutch 30 to slip if the knob 8 is continued to be rotated. Rotation of the knob 8 in the opposite direction enables the clutch 30 to again transmit rotary drive to the bowden cable 16 which is in turn rotated in the opposite direction.

It will be apparent that this mechanism prevents the bowden cable being over rotated and thereby damaging the mechanism which raises and lowers the backrest 3.

The mechanism which raises and lowers the seat 2 will now be described with reference to Figs. 5-9 inclusive. As best seen in Figs. 5 and 8, a Z-shaped lever 40 floats in a slot 41 formed in the base plate 12. The radially inner end 42 of the lever 40 is located directly above the actuator button 43 of the gas cylinder 11. The outer end 44 of the lever 40 abuts an ear 45 of a pivoted plate 46 which supports the lever 7 and which is pivoted on the base plate 12 at 47.

As best seen in Fig. 9, by raising the lever 7 the ear 45 of plate 46 raises the outer end 44 of the lever 40. As a consequence, the inner end 42 of the lever 40 depresses the actuator button 43 of the gas cylinder 11 thereby enabling the seat 2 to be pushed downwardly against the action of the cylinder 11 until the desired seat height is reached. Fig. 9 illustrates the position of the mechanism with the seat in the lowered position whereas Fig. 8 illustrates the mechanism with the seat in a raised position.

Importantly, the Z-shaped lever 40 is not connected with the lever 7 and plate 46. As a result, when the lever 7 is depressed (to be described hereafter) the lever 40 is not moved and thus the inner end 42 of the lever 40 does not rise above the actuator button 43.

It will be apparent to those skilled in the art that the above described arrangement ensures that the operating mechanism is located below, or substantially flush with, the upper end of the cylinder 11. Consequently, the minimum seat height able to be achieved is determined essentially by the length of the cylinder 11, the height of the footrest 6 and the depth of the upholstery on the seat 2. As a consequence, particularly low seat heights are able to be obtained and this is particularly important for ladies of small stature since the seat 2 can be lowered to a level which still enables their short legs to reach the floor whilst they sit on the seat 2.

Furthermore, since the outer vertical gas spring support tube, or stand pipe, is able to travel up close to the base plate 12, the gas cylinder 11 can be selected to have a long stroke. Thus high seat settings are able to be obtained with the same arrangement. Conventionally a seat height adjustment of approximately 140mm is achievable, however, with the above described arrangement an adjustment of 157mm is achievable. This covers simultaneously both the 95% percentile of the male population and the 5% percentile of the female population.

This is to be contrasted with conventional mechanisms which can add upto 80mm to the height of the seat support 5 when the adjustment mechanism is placed on top of the seat support 5. However, the above described arrangement adds just 3mm over and above the seat support height. Thus the top of the seat plate 14 is only 3mm lower than the actuator button 43 with the seat in the fully reclined position.

The simultaneous adjustment of the seat 2 and the backrest 3 will now be described with reference to Figs. 2 and 10-12 in particular. In Fig. 12 the rearwardly inclined position of the seat 2 is illustrated in solid lines. In this position of the seat 2, the backrest 3 is also rearwardly inclined. The forwardly tilted position of the seat 2 is illustrated by broken lines in Fig. 12 and it will be seen in this position that the backrest 3 is also substantially forwardly inclined.

As best seen in Figs. 10 and 11, the support arms 13 for the backrest support 4 are pivoted to the seat plate 14 at a first pivot 50. The free ends of the support arms 13 are each pivoted at a second pivot point 51 to a short lever 52 which is in turn pivoted at a third pivot 53 to the base plate 12.

In addition, a gas strut 55 extends between the backrest support 4 and base plate 12. The gas strut 55 is pivoted at one end at a fourth pivot 56 on the support arms 13 and is pivoted at the other end about the pivot 47 for the plate 46 which carries lever 7. The gas strut 55 has an actuator button 57 which passes through the pivot 47 and bears against a curved surface 58 formed at the free end 59 of a plastics hinge 60. The other end 61 of the hinge 60 is secured to the base plate 12. The hinging action of the hinge 60 is provided by a region of reduced cross-sectional thickness between the two ends 59,61.

Alternatively, the one piece hinge 60 of Figs. 10 and 11 can be replaced by a two part arrangement 60 as illustrated in Fig. 2. Essentially the lower part 59 of the hinge is replaced by a part formed with two stub axles which are cradled in a pair of U-shaped stirrups formed on the other part which replaces end 61. The stub axles are shaped to limit the degree of movement of the free end.

As seen in Figs. 10 and 11, the free end 59 of the hinge 60 is positioned between the actuator button 57 and the undersurface of the ear 45 movable with the plate 46. As a consequence, when the lever 7 is moved downwardly in the direction of arrow C as seen In Fig. 11, the free end 59 is also moved downwardly thereby depressing the actuator button 57. Consequently, the length of the gas strut 55 can be adjusted incrementally and locked into the adjusted position.

As seen in Fig. 11, with the gas strut 55 at its maximum extent, the seat plate 14 is tilted forwardly relative to the base plate 12 and the backrest support 4 is also tilted forwardly. However, with the gas strut 55 at its minimum length as illustrated in Fig. 10, the seat plate 14 abuts the base plate 12 thereby placing the seat 2 in a rearwardly inclined position whilst the backrest support 14 has pivoted to a rearwardly inclined position.

As best seen in Fig. 6, a compression spring 63 retained between the fourth pivot 56 and the base plate 12 acts to urge the seat 2 and backrest 3 into the forwardly tilted position indicated by broken lines in Fig. 12 when the gas strut 55 is released. Thus the sitter is able to sit on the seat 2 whilst holding the lever 7 down and press backwardly against the backrest 3 until the desired degree of inclination is achieved. Then the lever 7 is released into its rest position so as to move the free end 59 away from the actuator button 57 thereby fixing the gas strut 55 in its current position.

Alternatively, if the sitter depresses the lever 7 whilst sitting on the seat 2, and then leans forwardly, the rear of the seat rises and the backrest 3 moves forwardly. Again when the desired posture is reached, the lever 7 is released to lock the seat and backrest. By arranging the spacing of the various pivots 10, 47, 50, 51, 53 and 56, and the spacing therebetween, the desired range and rate of movement of the seat 2 and backrest 3 can be achieved. In this connection, the preferred range of movement of the seat 2 is 15° between a forward position with the seat tilted 10° above the horizontal and a backwards position in which the seat is tilted 5° below the horizontal. Similarly, the preferred range of movement for the backrest is 35° being 15° forward of the vertical and 20° rearwardly from the vertical. Thus there is very little movement of the seat in the region of the knees to achieve a "knee action tilt".

Since the backrest moves through 35° and the seat moves through 15°, the backrest moves at a faster rate than the seat. However, this rate is not constant. With the backrest near its most rearward position, for a given change of the seat angle, a greater change in backrest angle results. Conversely, with the backrest near its most forward position, for a given change of the seat angle, a lesser change in backrest angle results. It is thought that this arrangement provides a better ergonomic relationship between the seat angle and the backrest angle than other arrangements.

Since because the the relationship between the rate of movement of the seat and the backrest is fixed, this cannot be adjusted by the user. Clearly the seat 2 and backrest 3 cannot be independently adjusted. So the user, who is almost certainly unskilled in the desired ergonomic relationship between the seat and backrest, is only able to select one of a number of combinations of positions all of which are ergonomically advantageous.

Figs. 13 and 14 show an alternative embodiment of the seat adjustment mechanism shown in Fig. 3 and Fig. 6, in which the gas strut 55 is replaced by a hydraulic damper 70. The hydraulic damper 70 has the same dimensions as the gas strut 55 shown in Fig. 6, and is actuated by depression of an actuator button 72, in the same way as the gas strut 55 is actuated by depression of the actuator button 57. Furthermore, the hydraulic damper 70 is pivoted at the pivots 47 and 56 in the same manner as the gas strut 55. However, unlike the gas strut 55, the hydraulic damper 70 does not serve a biassing function, but rather serves as a damper for damping the pivotal movement of the seat 2 and backrest 3, and as a locking device for locking the seat 2 and backrest 3 in a selected position.

The hydraulic damper 70 is shown in greater detail in Fig. 21, and comprises a piston 74 slidably mounted in a cylinder 76 which is partially filled with oil 78. A second piston 80 is also slidably mounted within the cylinder 76. The second piston 80 is biassed towards the first piston 74 by a compression spring 82. The portion of the cylinder 76 behind the second piston 80 contains air 84 at atmospheric pressure, and is vented to the outside of the cylinder 76 by a small vent 86.

The first piston 74 is provided with a valve 88, which can be opened by depression of the actuator button 72 in order to allow the oil 78 to pass through the first piston 74 and thereby allow the first piston 74 to move through the oil 78. As the first piston 74 is moved further into the cylinder 76, the piston rod 90 occupies an increasing volume within the cylinder 76, and the second piston 80 therefore moves backwards, against the action of the spring 82, in order to accommodate the volume within the cylinder 76 occupied by the piston rod 90.

It should be appreciated that, because the oil 78 is incompressible, the first piston 74 is locked in position as soon as the valve 88 is closed. In particular, once the valve 88 is closed, further movement of the first piston 74 towards the second piston 80 is prevented, since such movement would result in a vaccuum being formed in the portion 92 of the cylinder 76 lying behind the first piston 74. For this reason, once the first piston 74 is locked in position, the rigidity of the hydraulic damper 70 is completely independent of the strength of the compression spring 82. The compression spring 82 has a very low spring constant, which results in a tiny biassing force on the first piston 74 (due to the different surface areas of the two sides of the first piston 74) when the valve 88 is open. For all practical purposes the biassing force is negligible, and can be ignored.

An alternative embodiment of the hydraulic damper 70 is shown in Fig. 22, in which the compression spring 82 is replaced by a sealed gas cavity 94, which biasses the second piston 80 towards the first piston 74. Like the spring 82, the gas cavity 94 exerts only a small

SUBSTITUTE SHEET (Rule 2b; force on the second piston 80, and thus makes a negligible contribution towards biassing of the first piston 74 when the valve 88 is open.

The hydraulic damper 70 therefore has two useful features, namely: a damped movement of the piston rod 90 and first piston 74 with respect to the cylinder 76; and the ability to lock the piston rod 90 and cylinder 76 in selected relative positions.

Returning to Figs. 13 and 14, biassing of the seat 2 and backrest 3 towards the forwardly tilted position, shown in broken lines in Fig. 12, is effected by first and second elastomeric torsion bars 96 and 98. The first elastomeric torsion bar 96 pivotally connects the base plate 12 to the seat plate 14 at pivot axis 47, and the second elastomeric torsion bar 98 pivotally connects the support arms 13 to the seat plate 14 at the first pivot 50. Each elastomeric torsion bar 96, 98 comprises an inner steel tube 100 surrounded by a coaxial outer steel tube 102, with a cylindrical rubber member 104 extending along the length of the elastomeric torsion bar 96, 98 between the inner and outer steel tubes 100 and 102. The rubber member 104 is vulcanised to the inner and outer steel tubes 100 and 102, and provides a biassing force which resists relative rotation between the inner and outer steel tubes 100 and 102 about their single longitudinal axis. As shown in Fig. 13, the inner steel tubes 100 of the first and second elastomeric torsion bars 96 and 98 are welded to the seat plate 14, the outer steel tube 102 of the first elastomeric torsion bar 96 is welded to the base plate 12, and the outer steel tube 102 of the second elastomeric torsion bar 98 is welded to the support arms 13.

The embodiment shown in Figs. 15 and 16 is the same as that shown in Figs. 13 and 14, except that the first and second elastomeric torsion bars 96 and 98 are replaced by first and second torsion springs 104 and 106, which extend around pivot axles 108 and 110 respectively, and bias the base plate 12, seat plate 14 and support arms 13 in the same manner as the elastomeric torsion bars 96 and 98.

The embodiment shown in Figs. 17 and 18 is the same as that shown in Figs. 13 and 14, except that the first and second elastomeric torsion bars 96 and 98 are replaced by first and second torsion bars 112 and 114 respectively. Each torsion bar 112, 114 comprises a steel bar 116, and two extension tubes 118. The extension tubes 118 are welded to the seat plate 14, and the ends of the steel bars 116 are secured within the ends 120 of the extension tubes 118. The steel bar 116 of the second torsion bar 114 is welded at its centre to the U-shaped bracket 122, which is fixed to the support arms 13. Likewise, the steel bar 116 of the first torsion bar 112 is welded at its centre to the base plate 12. When the seat 2 is tilted backwards the steel bars 116 twist about their longitudinal axes, thus biassing the seat 2 towards the forward position shown in dotted lines in Fig. 12.

The embodiment of Fig. 19 is the same as that of Figs. 13 and 14, except that biassing of the seat 2 towards its forward position is effected by a non-lockable gas spring 124, which takes the place of the spring 62 of the embodiment of Fig. 6. The gas spring 124 simply acts as a biassing means, and is not affected by operation of the lever 7.

The embodiment of Fig. 20 is the same as that shown in Fig. 6, but with the spring 63 removed. The gas strut 55 performs the function of biassing the seat 2 towards the forward position, and of locking the seat 2 in selected positions.

It will be appreciated that other embodiments are possible. For example, in the embodiment of Fig. 6 the gas strut could be replaced by the hydraulic damper 70, so that the biassing force is provided entirely by the spring 63.

As best seen in Figs. 5 and 6, the base plate 12 is provided with a ring 65 having a Morse taper. The upper end of the gas cylinder 11 is also arranged to have a corresponding tapered surface thereby enabling the end user to simply place the seat 2 and its associated mechanism on top of the upper end of the seat support 5 when assembling the seat at its final destination. As a consequence, the seat 2 need not be packed in a carton above the footrest 6 spaced at a height determined by the seat support 5. Instead, the seat 2 can lie "vertically" alongside the seat support 5 thereby again reducing the size of the container needed to ship the chair to its end user.

The foregoing describes only one embodiment of the present invention and modifications, obvious to those skilled to the art, can be made thereto without departing from the scope of the present invention.

Claims

Cl ai ms

1. A chair adjustment mechanism for a chair having a seat, seat height adjustment means for adjusting the height of the seat, a backrest adjustably mounted on a backrest support, backrest height adjustment means for adjusting the height of the backrest, and a pivotable linkage interconnecting the seat and backrest support which allows the seat and backrest to be moved between a forward tilted position and a rearward tilted position, said mechanism comprising biassing means for biassing the seat and backrest towards said forward tilted position, adjustable securing means for securing the seat and backrest in a selected position between said foward and rearward tilted positions, a hollow lever having a knob rotatably mounted at one end thereof, a cable passing through said lever and interconnecting said knob and the backrest height adjustment means, whereby the backrest is either raised or lowered relative to said backrest support by rotation of said knob in a corresponding direction, said lever being pivotally mounted below the seat and being movable from its rest position in a first direction to actuate said seat height adjustment means to thereby allow the height of the seat to be set, and said lever being movable from said rest position in a second direction to actuate said adjustable securing means to thereby allow the angle of said seat and backrest to be set relative to the horizontal .

2. A chair adjustment mechanism as claimed in claim 1, wherein the adjustable securing means comprises a lockable gas spring.

3. A chair adjustment mechanism as claimed in claim 1, wherein the adjustable securing means comprises a hydraulic damper.

4. A chair adjustment mechanism as claimed in claim 1 , 2 or 3, wherein the biassing means comprises a gas spring.

5. A chair adjustment mechanism as claimed in claim 1 , 2 or 3, wherein the biassing means comprises a mechanical spring.

6. A chair adjustment mechanism as claimed in claim 1 , 2 or 3, wherein the biassing means comprises any kind of torsion bar.

7. A backrest height adjustment mechanism for adjusting the height of a chair backrest, comprising a hollow backrest support, a cable having a first threaded member at one end thereof, a bush mounted in said support and having the first threaded member rotatably mounted therein, a second threaded member slidably mounted within said bush, threadably engaged with said first threaded member, and carrying said backrest whereby rotation of said cable translates said second threaded member within said bush to thereby adjust the height of said backrest.

8. A backrest height adjustment mechanism as claimed in claim 7, wherein the first threaded member is formed with an external thread, and the second threaded member is formed with a corresponding internal thread.

9. A backrest height adjustment mechanism as claimed in claim 7 or 8, wherein the backrest is mounted onto a hollow stalk which slides into the backrest support and snap engages with the second threaded member, to allow the backrest height adjustment mechanism to be transported in a knock down configuration and to be assembled without tools.

10. A rotatable actuator knob for a chair adjustment mechanism, said knob comprising an outer tube having a recess at one end and being rotatably mounted on a hollow lever, said recess being shaped to form a first part of a two part clutch, a second part of the two part clutch housed in said recess, a cable passing through said lever into said recess and being engaged with the second part of the two part clutch, a recess cover engaged with said tube and containing biassing means between the cover and said second part of the clutch to urge said second part against said first part whereby rotation of said outer tube rotates said cable via said clutch which is free to slip in the event of said cable being restrained against further rotation.

11. A rotatable actuator knob as claimed in claim 10, wherein the cable can be installed into the hollow lever either before, or after, the hollow knob and clutch are mounted on the lever.

12. A rotatable actuator knob as claimed in claim 10 or 11, wherein the actuator knob is mounted on the lever by means of a snap engagement between fingers on the actuator knob and a groove in the exterior of the lever.

13. A mechanism to simultaneously adjust the angle of a seat and a backrest of a chair, said mechanism comprising a base plate mountable in a generally horizontal position atop a seat support, a seat plate pivotally mounted on said base plate, a backrest support pivotally mounted on said seat plate, a length adjustable strut pivotally mounted at each end and interconnecting said backrest support and said base plate, and a pair of pivoted levers interconnecting said base plate and backrest support.

14. A mechanism as claimed in claim 13, wherein the length adjustable strut is a gas strut, or a hydraulic strut, which is both length adjustable and self locking to the last adjusted length.