GB2273240A - Mechanism for an adjustable chair - Google Patents

Abstract

A mechanical interconnection between the seat and backrest of a chair (eg. an office chair) increases or decreases the vertical height of the backrest above the upper surface of the seat when the seat is respectively raised or lowered, so as to maintain a prearranged relationship between the seat and the backrest. The ratio of change in backrest height: change in seat height is preferably approximately 2:1; a suitable mechanism comprises a belt and pulley arrangement

Description

MECHANISM FOR AN ADJUSTABLE CHAIR The present invention relates to a mechanism for an adjustable chair and to chairs incorporating said mechanism.

The contribution of correct seating to the avoidance of strain and to the comfort of typist and keyboard operators is being increasingly appreciated, as demonstrated by E.E.C. Directive 90/270/EEC which indicates that work chairs should have "seats that are adjustable in height and back rests that shall be adjustable in both height and tilt". The present invention seeks to provide office chairs of a type which comply with this directive and additionally provide improved adjustability.

In the normal type of office chair a suitable arrangement, commonly a gas lift, is used to adjust the seat height, with the backrest being maintained at a fixed height relative to the seat during seat height adjustment. The backrest, which is commonly fitted on a sliding bracket on a bar or tube attached to the seat base, would normally be manually adjusted, after adjustment of the seat height, by releasing and tightening a ratchet or by tightening a screw, using a lever or thumb wheel, to clamp the backrest at the desired height. However, the lever or thumb wheel is commonly located behind the seat and is awkward to adjust whilst the user remains seated, as it is necessary to twist round and reach back to make an adjustment. As a result, the chair may be left with the backrest in a position which only approximates to the correct height.This problem is exacerbated when the same chair is used by people of different stature. In this case, the tedium of the need for constant readjustment of backrest height can result in seat height alone being readjusted each time with the backrest being left in an inappropriate position for at least one of the users.

The present invention seeks to provide an arrangement which, for the normal range of users, provides the correct relationship between the height of lumbar support and seat height and avoids the need for separate backrest adjustment.

Accordingly the present invention provides a mechanism for an adjustable chair of the type which comprises a base, a seat which is adjustable for height and a backrest adjustable in both height and tilt, wherein said mechanism provides a mechanical interconnection between the seat and backrest which operates automatically on operation of a seat height control to increase or decrease the vertical height of the backrest above the upper surface of the seat as the seat is respectively raised or lowered, so as to maintain a pre-arranged relationship between the relative heights of the seat and the backrest.

In a preferred embodiment of the present invention the mechanism operates such that the ratio of the change in backrest height to the change in vertical height of the upper surface of the seat is approximately 2:1. This ratio has been found to be the optimum for the normal range of stature of users.

In a particularly preferred embodiment of the present invention, the mechanism comprises: a roller which has larger and smaller diameter regions and is adapted to be mounted for upward and downward movement with the seat; a first belt attached at its one end to the smaller diameter region of the roller and adapted for attachment at its other end to a support for the backrest via a pulley arrangement; and a second belt attached at its one end to the larger diameter region of the roller and adapted for attachment at its other end to a fixed position on the chair base; the arrangement being such that upward or downward movement of the seat respectively tensions the second or first belt to rotate the roller and respectively wind on or unwind the first or second belts onto or off the roller, thereby to raise or lower the support for the backrest in synchronisation with the seat movement. Preferably, the diameters of the larger and smaller diameter regions of the roller are in an approximate ratio of 2:1. Preferably a guide is provided above the fixed position on the chair base and below the seat, to deflect the direction of movement of the first belt through approximately a right angle.

In this particularly preferred embodiment comprising the belt arrangement, the roller is the key mechanical component that enables the automatic mechanical interconnection between the seat and backrest to take place. The roller incorporates an arrangement for attaching the first and second belts, preferably enables the seat and back to move in a ratio of 2:1. Operation of the height gas lift effectively fixes the relative positions of the seat and back in any one position, when the height of the seat from the ground is fixed and thereby provides the means of locking the back and seat in any position.

In a further particularly preferred embodiment of the present invention, the mechanism comprises: an upwardly extending guide for the backrest, said guide being integral with or adapted to be secured to a support for the seat; a mounting for the backrest, adapted for upward and downward movement on the guide; a first roller located on said mounting; an attachment point located on the guide above position of the first roller, when at its greatest height; a second roller located on the guide adjacent to the attachment point; and a belt adapted to be secured at its one end to said attachment point and at its other end for attachment to a fixed position on the chair base, said belt passing downwards from said attachment point over said first and second rollers to the fixed position whereby upward movement of the support for the seat causes a relative increase of height of the backrest above the seat surface.

Preferably two or more further rollers are provided below the second roller in positions to guide the belt from the second roller initially in a substantially vertical direction, then beneath the support for the seat, and finally in a substantially vertical direction to the fixed position on the chair base. Because the belt doubles back over the first roller behind the backrest position, the increase in height of the seat relative to the increase in height of the backrest above the seat surface is in a ratio of 2:1.

This second embodiment is, in fact, more particularly preferred as it is likely to prove cheaper, simpler and more reliable than the arrangement in which two separate belts are connected to a geared roller.

The present invention results from the appreciation of the need to link the chair seat and back together mechanically, in order that they adjust in a synchronised fashion to increase or decrease the vertical height of the seat back relative to the upper seat surface according to the height of the sitter. The present invention thus allows the dimensional relationship between the seat height and the back lumbar support of the chair to adjust automatically.

The mechanism of the invention allows the seat and back to adjust automatically by the operation of a single control, preferably a lever, and thus increases the distance between the seat and the lumbar support according to the height of the seat from the ground. The effect is that the chair "grows" according to the height of the sitter.

In a typical chair incorporating the mechanism of the present invention, as the distance between the floor and the seat increases to a maximum of 120 mm, the back-rest automatically rises by an additional maximum distance of 60 mm. This action therefore provides a ratio of adjustment of 2:1 (2 units of seat movement to 1 unit of back-rest movement).

Preferred embodiments of the present are now described, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a side elevation of a chair which incorporates the mechanism of a first embodiment of the present invention and has its seat in the lowest position.

Fig. 2 is a side elevation of the chair of Fig. 1 showing the seat in the highest raised position.

Fig. 3 is a schematic perspective view of the mechanism of the chair of Figs. 1 and 2.

Fig. 4 is a side elevation of a further chair which incorporates the mechanism of a second embodiment of the present invention and has its seat in the lowest position.

Fig. 5 is a side elevation of the chair of Fig. 4 showing the seat in the highest position.

Fig. 6 is a schematic perspective view of the mechanism of the chair of Figs. 4 and 5.

Referring to Figs. 1 and 2, an adjustable office chair, generally indicated by reference numeral 1, to which the mechanism of the present invention is applied comprises a base 2 on which are mounted seat 3 and back rest 5.

The base 2 is a conventional seat base, preferably a five point base, and includes castors 4. The base comprises a vertical column or shroud 6 which houses conventional height-adjusting means, such as gas spring 7 which is attached to a seat support 18 at point 9 by conventional means such as a bolt (not shown). Fig. 1 shows the chair at its lowest seat height, whereas Fig. 2 shows the chair with the gas spring fully extended to raise the seat to its maximum height. A conventional control such as a lever (not shown), which is normally positioned under the seat, is used to release the gas spring either to allow the seat to rise, or to be lowered when subjected to downward pressure applied by the user.In an alternative embodiment of the chair, which is not illustrated and which includes arms, it is possible to use a bowden cable to extend the control for release of the gas springs up to and along a seat arm and to provide a release lever at the outer end of the arm.

The underside of the front edge of seat 3 rests on but is not secured to bracket 11 which is welded or bolted to front support member 18. Support member 18 has a generally U-shaped cross-section comprising a horizontal portion linking trapezoid-shaped vertical plate members 15 which taper towards the front of the chair, only the nearest one of plate members 15 being visible in Figs. 1 and 2. The underside of the rear of the seat 3 is firmly attached to a bracket 13 by conventional fixings such as screws. Bracket 13 is welded or bolted to a rear support member 25, which has a generally similar shape to front support member 18 but comprises trapezoid-shaped vertical plate members 17 which taper instead towards the rear of the chair.Member 25 is hinged to front support member 18 by pivot 19 which takes the form of a free running roller arranged horizontally between vertical plate members 15 which fit within the vertical plate members 17 of the rear support member 25, in the region of the pivot 19.

Arm 30 which provides a connection between backrest 5 and rear seat support member 25 and is connected thereto towards the rear of the seat. As may be seen in Figs. 1 and 2, arm 30 curves upwards in the region of the rear of the seat to extend in a generally perpendicular direction, but with a slight backwards inclination to the vertical so as to clear the rear of backrest 5.

A sleeve 33 fits over the upright part of arm 30, with one or both of sleeve 33 and arm 30 being made of, or coated with, low-friction material to allow easy sliding movement of sleeve 33 over arm 30. A roller 57 on arm 30 provides the guide for a pulley arrangement which will be described in more detail later. At its upper and forward end, sleeve 33 carries a vertical bracket 34 which fits over and is firmly fixed to sleeve 33 and abuts the upper end of sleeve 33 in the region closest to chair back 5 at the point indicated by reference numeral 40. The backrest 5 is firmly attached to vertical bracket 34 via an intermediate bracket 36. Thus backrest 5, brackets 34 and 36, and sleeve 33 move upwards and downwards as a unit.

Adjustment of the tilt of backrest 5 takes place by means of a gas spring 23 which is held its one end by pivot 21 positioned between vertical plate members 15 of the front support member 18, and at its other end by a pivot 20 located on arm 30. Release of the gas spring by a lever or similar arrangement (not shown), normally located on the opposite side of the chair to the lever for releasing height-adjusting gas spring 7, allows the upwardly extending portion of arm 30 to hinge backwards or forwards with the rear support member 25 and the chair seat 3 to slide over bracket 11, so that the tilt of backrest 5 may be adjusted as desired. For a chair with arms, the position of the adjustment lever could be transferred to the end of an arm using a similar arrangement to that already described for height-adjusting mechanism in such chairs.

The operation of the mechanism of the present invention will be understood in more detail by reference to Fig. 3 of the drawings. As may be seen, roller 55, mounted for rotation on bracket 30, as shown in Fig. 1, has a region of-larger diameter 60 corresponding to the width of belt 53.

Belt 53 is attached to roller 55 at point 63 and winds onto roller 53 in a clockwise direction. As shown in Fig. 1, from roller 55, belt 53 passes between plates 17 of rear support member 25 to pass over the further roller of pivot 19 and then pass in a substantially vertical direction to the attachment point 22 on shroud 6 of base 2.

Returning to roller 55 as shown in Fig. 3, it may be seen that there is also a region 61 of reduced diameter on the roller 55 corresponding to the width of belt 50 to which belt 50 is attached and around which belt 50 winds in an anticlockwise direction. From roller 55, belt 50 extends upwards to pass over roller 57 and then generally downwards to an attachment point 51 for its other end (as shown in Fig. 1), which is located on the inside of sleeve 33 and normally below roller 57. In effect, this arrangement thus provides a pulley which raises or lowers sleeve 33 and with it backrest 5.

The effect of upward movement of the chair may be seen in more detail with reference to Figs. 1 to 3. In Fig. 1, seat 3 is shown in its lowest position. Belt 53 has its maximum number of turns onto roller 55, whereas belt 50 has only the minimum number of turns onto roller 55, or may be almost completely unwound from roller 55. At this point, sleeve 53 is at its lowest position with belt 50 being fully extended over roller 57 with sleeve 33 being in its lowest position. Raising seat 3, as shown in Fig. 2, tensions belt 53 and causes it to unwind from roller 57 which rotates in anticlockwise direction Z. This winds belt 50 onto the roller 57 and raises sleeve 33 (and thereby backrest 5).

When the seat 3 is lowered the weight of backrest 5 and the low-friction contact between sleeve 33 and arm 30 is normally sufficient to maintain tension in the belt 50 thereby causing roller 57 to rotate anticlockwise and wind belt 53 back onto roller 57. If it were to prove necessary, it would be a simple matter to provide a spring for biasing roller 57 to rotate in the clockwise direction to assist the rewinding of belt 53.

As will be apparent from the foregoing description, the amount of relative movement of seat 3 and backrest 5 is a function of the relative diameters of regions 60 and 61 of roller 55. Referring to Figure 3, in a typical chair of the present invention, the larger diameter region 60 of roller 55 might have a diameter of 20 mm, whereas the smaller diameter region 61 of roller 55 might have a diameter of 10 mm, giving the optimum ratio of seat to backrest movement of 2:1. Additionally, in a typical chair of the present invention, if the seat height is increased to its full amount, 135 mm of belt 55 (indicated by reference letter W) is wound off roller 55. As a result, half this amount of belt 50 (67.5 mm), as indicated by reference letter V, is wound onto roller 55 again giving a ratio of 2:1 for the relative movement of seat 3 and backrest 5. Referring to Figs. 1 and 2, reference letter X indicates the height of the upper seat surface above the ground and Y indicates the height of the lumbar support centre above that surface. In a typical chair, the lowest seat height X might be 450 mm with the height Y of lumbar support centre being 150 mm (Fig. 1). At the highest seat height shown in Fig. 2, the corresponding figures for X and Y might be 585 mm and 217.5 mm, respectively, once again giving the height ratio of seat and backrest movement as 2:1.

Turning now to Figures 4 and 5 of the drawings, an alternative embodiment which achieves the same synchronised adjustment of seat and backrest is shown. The chair la which incorporates the mechanism of this embodiment of the invention is generally similar to the chair of the previously described embodiment, with identical reference numerals being used for similar parts of chair 1. The gas lift mechanism and backrest tilting arrangement are identical and need not be described further. The only point of difference in the base area is the presence of a shroud 2a which surrounds base 2 and hides the gas lift arrangement and belt attachment point 22 from view. In this instance, the lower end of belt 73 is permanently connected at attachment point 22 to the lower column of base 2 which is able to swivel with the chair but not alter in height relative to the ground.

In the embodiment illustrated, a guide 71 for the backrest forms part of rear support member 25 and extends upwardly to the rear of backrest 5. A mounting 73 for the backrest 5 is adapted for upward and downward movement on the guide 71, by sliding or otherwise but largely hidden from view. A first roller 75 is located at the lower end of mounting 73 and moves upwards and downwards with backrest 5 and its mounting 73. The upper end of the belt 73 is permanently connected to the uppermost point of the seat mechanism which rises and descends with the seat at attachment point 83 on guide 71. From the attachment point, the belt 73 passes downwards over roller 75 and the back upwards to a second roller 80 which is located at the upper part of guide 71 in a position adjacent to attachment point 83.The belt 73 then passes downwards in a generally vertical direction over further rollers 87 and then under seat 3 to pivot and roller 19 from where the belt again passes vertically to attachment point 22.

In a typical seat, roller 80 is mounted in the seat mechanism and will also rise and descend with the seat a distance of say 100mm, whereas the roller 75 is mounted on the backrest and can only move in height a maximum distance of 50 mm.

The operation of the mechanism of this embodiment may be seen in more detail in Figure 6 of the drawings which shows belt 73 secured at the upper attachment point 83 and the lower attachment point 22 and arranged for use, as shown in Figures 4 and 5. Rollers 80, 89 and 19 are fitted into the seat mechanism. Roller 75 forms part of the backrest mounting and moves independently of the seat mechanism. The vertical height of roller 75 in relation to that of roller 80 is equal to the vertical height of the backrest in relation to that of the seat.

Length A varies by means of a gas lift under the seat. It follows that length B is controlled to vary by half that of length B ( giving a ratio of 2:1 ).

The skilled reader will appreciate that alternative mechanical arrangements to those specifically described, such as the use of gears, may be adopted to achieve the mechanical interconnection between seat and backrest in accordance with the present invention, also, for example, a cable could be substituted for the belt and pulleys could replace the rollers.

It will of course be understood that the present invention has been described purely by way of example, and modifications of detail can be made within the scope of the invention.

References herein to vertical height and movement are to be taken to be relative to the chair in its normal orientation for use.

Claims (10)

Claims:

1. A mechanism for an adjustable chair of the type which comprises a base, a seat which is adjustable for height and a backrest adjustable in both height and tilt, wherein said mechanism provides a mechanical interconnection between the seat and backrest which operates automatically on operation of a seat height control to increase or decrease the vertical height of the backrest above the upper surface of the seat as the seat is respectively raised or lowered, so as to maintain a pre-arranged relationship between the relative heights of the seat and the backrest.

2. A mechanism according to claim 1 in which the mechanism operates such that the ratio of the change in backrest height to the change in vertical height of the upper surface of the seat is approximately 2:1.

3. A mechanism according to claim 1 comprising: a roller which has larger and smaller diameter regions and is adapted to be mounted for upward and downward movement with the seat; a first belt attached at its one end to the smaller diameter region of the roller and adapted for attachment at its other end to a support for the backrest via a pulley arrangement; and a second belt attached at its one end to the larger diameter region of the roller and adapted for attachment at its other end to a fixed position on the chair base; the arrangement being such that upward or downward movement of the seat respectively tensions the second or first belt to rotate the roller and respectively wind on or unwind the first or second belts onto or off the roller, thereby to raise or lower the support for the backrest in synchronisation with the seat movement.

4. A mechanism according to claim 3 in which a guide is provided above the fixed position on the chair base and below the seat, to deflect the direction of movement of the first belt through approximately a right angle.

5. A mechanism according to claim 3 or 4 in which the diameters of the larger and smaller diameter regions of the roller are in an approximate ratio of 2:1.

6. A mechanism according to claim 1 comprising: an upwardly extending guide for the backrest, said guide being integral with or adapted to be secured to a support for the seat; a mounting for the backrest, adapted for upward and downward movement on the guide; a first roller located on said mounting; an attachment point located on the guide above position of the first roller, when at its greatest height; a second roller located on the guide adjacent to the attachment point; and a belt adapted to be secured at its one end to said attachment point and at its other end for attachment to a fixed position on the chair base, said belt passing downwards from said attachment point over said first and second rollers to the fixed position whereby upward movement of the seat support causes a relative increase of height of the backrest above the seat surface.

7. A mechanism according to claim 6, comprising two or more further rollers below the second roller positioned to guide the belt from the second roller initially in a substantially vertical direction, then beneath the support for the seat, and finally in a substantially vertical direction to the fixed position on the chair base.

8. A mechanism for an adjustable chair substantially as hereinbefore described with reference to figures 1 to 3 of the accompanying drawings.

9. A mechanism for an adjustable chair substantially as hereinbefore described with reference to figures 4 to 6 of the accompanying drawings.

10. A chair incorporating a mechanism according to any of the foregoing claims.