EP0309804B1 - Office chair - Google Patents

Abstract

The office chair locking mechanism comprises a transverse arm (32) fixed to a support frame below the seat. Slotted housings (72,74) are fixed to the underside of the seat and close to the outer ends of the arm, with the slots in them open to the inside. A sliding and locking member (66), turning on each arm end, fits in the oblong slot in the respective housing and allows sliding of the seat when turned into a first position. It locks in the slot, when turned into a second position, in which its effective dia. between the slot walls is greater than in the first position. It can typically be of hexagonal cross-section.

Description

The present invention relates to an office chair in which the backrest can be tilted backwards from a substantially upright position. During this tilting movement, the seat is shifted at the same time, so that a synchronous seat and backrest adjustment is achieved. This is intended to create a higher level of comfort, since the seat occupies a predetermined position for each tilted position of the backrest.

Such a chair is known for example from DE-A1-33 15 237 . In this known chair, the backrest can be pivoted about a horizontal axis, the lower region of the backrest being connected in an articulated manner to the rear region of the seat. This hinge axis is movable below the horizontal axis and can be pivoted from a position behind the axis to a position in front of the axis.

Another solution of a similar chair is known from DE-A1-33 13 677 , in which likewise the backrest is pivotally mounted about a horizontal axis. An articulation point between the backrest and the seat surface can be moved forward from a position behind the horizontal axis, so that this articulation point is essentially below the horizontal axis when the backrest is tilted all the way back. This movement of the articulation point ensures that the seat area is lowered in its rear area when the backrest is tilted backwards.

In both cited references, the seat areas are connected in their front area to the support system, so that this point of suspension of the seat area is horizontally far from the center of gravity of the chair and in particular the occupied chair. Accordingly, strong locking forces are required to fix the seat in a predetermined inclined position.

This leads to technically complex and aesthetically unsatisfactory solutions, since bulky housings are required for the locking mechanism below the seat, as can be seen in particular from DE-A1-33 15 237.

The invention has for its object to provide a chair with synchronous adjustment of the seat and backrest, which has an effective and simple locking system with a short moment lever between the center of gravity of the occupied chair and the main support element, which has a very light locking pressure for fixing the seat - or back surface required.

This object is achieved by the design of a chair according to claim 1.

Fig. 1

eine perspektivische Ansicht des Bürostuhls darstellt,

Fig. 2

eine schematische Darstellung des Stuhles, welche die Verhältnisse zwischen Rückenlehne, Sitzfläche und Stützsystem sowohl in der aufrechten als auch in der geneigten Stellung darstellt,

Fig. 3

eine teilweise geschnittene Seitenansicht ist, welche den Stuhl in der aufrechten Stellung darstellt,

Fig. 4

eine seitlich teilweise geschnittene Ansicht ist, ähnlich zu Fig. 3, sie zeigt jedoch den Stuhl in seiner geneigten Stellung,

Fig. 5

eine Draufsicht auf den Stuhl ist, wobei die Sitzfläche in strichpunktierten Linien dargestellt und der Stützrahmen geschnitten dargestellt ist,

Fig. 6

eine perspektivische Ansicht eines Abschnittes der Unterseite der Sitzfläche darstellt und das Gleit/Riegelelement in der Gleitstellung bezogen auf das benachbarte Gehäuse erläutert,

Fig. 7

eine perspektivische Ansicht ähnlich Fig. 6 ist, sie zeigt jedoch das Gleit/Riegelelement in die Riegelstellung verdreht,

Fig. 8

ist eine auseinandergezogene Ansicht, teilweise geschnitten, des Stütz-Kreuzarms,

Fig. 9

ist eine perspektivische Ansicht, wobei einige Teile weggebrochen sind und stellt die Unterseite der Sitzfläche dar, wobei die vertikale Stützsäule auseinandergezogen im Abstand dargestellt ist,

Fig. 9a

ist eine perspektivische Ansicht der Montageaufnahme für die vertikale Stützsäule, entfernt von den Stützrahmen,

Fig. 10a

stellt die Unterseite einer Fußkappe dar, die vom Stuhl entfernt ist und

Fig. 10b

ist eine perspektivische Ansicht, teilweise weggebrochen, die das Ende eines jeden Beines der Stuhlbasis darstellt.

The invention is explained below with reference to the drawings, in which

Fig. 1

is a perspective view of the office chair,

Fig. 2

1 shows a schematic representation of the chair, which shows the relationships between the backrest, seat surface and support system both in the upright and in the inclined position,

Fig. 3

a partially sectioned side view showing the chair in the upright position Represents position

Fig. 4

3 is a partially sectioned view, similar to FIG. 3, but shows the chair in its inclined position,

Fig. 5

FIG. 2 is a top view of the chair, the seat being shown in dash-dotted lines and the support frame shown in section,

Fig. 6

3 shows a perspective view of a section of the underside of the seat surface and explains the sliding / locking element in the sliding position in relation to the adjacent housing,

Fig. 7

6 is a perspective view similar to FIG. 6, but shows the sliding / locking element rotated into the locking position,

Fig. 8

is an exploded view, partially in section, of the support cross arm,

Fig. 9

Figure 3 is a perspective view, with some parts broken away, showing the underside of the seat, with the vertical support column shown at a distance,

Fig. 9a

is a perspective view of the mounting bracket for the vertical support column, removed from the support frame,

Fig. 10a

represents the bottom of a foot cap that is removed from the chair and

Fig. 10b

Fig. 3 is a perspective view, partially broken away, illustrating the end of each leg of the chair base.

In Fig. 1 an office chair is shown, which essentially has a seat assembly 10, which has a curved backrest 12 and a seat recess 14, which are hinged together and are adjacent on the lower edge of the backrest 12 and the rear edge of the seat recess 14.

A base assembly is formed by a plurality of legs 16 that extend radially outward from a central portion and end in support rollers 18. An adjustable vertical column 17 extends upwards from the base and has a support frame 20 arranged thereon, which is located below the seat 14. A pair of side arms 22 are attached to the support frame 20 and pivotally support the seat assembly 10.

The functional mode of operation of the chair is explained with reference to FIG. 2. As shown there, the backrest 12 and the seat pan 14 are supported on the base assembly by a support frame or system 20. The backrest 12 comprises a lower lumbar region, and the backrest 12 and the seat recess 14 are connected to one another in an articulated manner in the hinge point 13.

Furthermore, the backrest 12 and the seat pan 14 are suspended above the support frame 20 on the side arms 22, which are attached at their lower ends to a transverse support arm 30 which forms the rear section of the support frame 20. The arms 22 pivotally support the backrest 12 at point 28.

In this way, the backrest seat component could normally swing or rotate about point 28, but this is prevented by a front connecting element 24 which slidably connects the front portion of the support frame 20 to the lower front portion of the seat pan 14 and locks them together.

When the connector 24 is released, the seat is freely tiltable depending on the pressure applied against the upper portion of the backrest 12. When the connecting element 24 is locked, such a tilting movement is prevented.

A vertical support column 17 extends upward from the base and the support frame 20 is attached at its upper end. The support column 17 preferably includes an adjustable gas cylinder.

As can be seen from FIG. 2, the bending back of the arms 22 creates a horizontal support axis 26 which is horizontally not far from the center of gravity of the occupied chair and which is slightly in front of the horizontal axis which passes through the hinge point 28. The arrangement of the horizontal support axis on the back of the support frame 20 substantially below the hinge axis 28 produces various desired results.

First, the torque is reduced at the point where the side arms 22 are connected to the support frame 20. Second, there is a relatively small locking pressure or force (around 200 Newtons) is sufficient to hold the backrest seat assembly in the locked position since the pressures that attempt to remove the seat from its locked position are relatively low. Third, all controls (lock, vertical adjustment, etc.) can be accommodated in the transverse support arm 30, which is very convenient for the user.

In Fig. 3, the backrest 12 is shown as a top-down member, with the most forward portion of the bend located at hinge point 28 and adjacent to the user's lumbar region. The backrest 12, as shown in Fig. 5, is also curved from one side to the other. This leads to a maximum offset between the hinge 13, which connects the backrest 12 and the seat recess 14 in an articulated manner, and the bearing point 28, where the backrest is connected to the two side arms 22. The maximum offset of the hinge 13 causes a deeper vertical offset of the seat recess 14 when the seat is tilted backwards (FIG. 2). This relative movement between the seat recess 14 and the backrest 12 brings about the desired comfort.

The base includes a plurality of legs 16 that extend radially from a center. A common roller 18 supports and is attached to the free end of each leg 16. The base is a comparatively conventional base for an office chair with the exception of replaceable foot caps 19 which will be described in more detail later. The base also carries a vertical support column 17, which the vertical height adjustment of the chair enables. A commercially available adjustable gas cylinder may be provided for this purpose.

In order to connect the seat assembly 10 to the base, a support frame 20 is provided, which is best seen in FIGS. 3 to 5. The support frame 20 is arranged on the upper end of the vertical support column 17 and is essentially formed by a transverse support arm 30, a shorter cross arm 32 and a main housing 34 which connects the transverse support arm 30 and the cross arm 32.

The transverse support arm 30 is arranged at the rear end of the support frame 20, directly above the vertical support column 17, while the shorter cross arm 32 extends transversely below the front section of the seat recess 14. Both the support arm 30 and the cross arm 32 are hollow to accommodate various control components. The transverse support arm 30 forms the only support for the seat assembly 10, which is connected to it by the arms 22. To this end, the transverse support arm 30, when fully assembled, is of greater length than the width of the seat pan 14 so that it projects on either side thereof to receive the side arms 22.

Support arm 30 is hollow and includes a generally tubular portion 36 that extends in both directions from the central longitudinal axis of the frame. The hollow inner section of the tubular Section 36 is preferably hexagonal in cross-section and is tapered, as seen in FIG. 5, to accommodate an extension / connecting member 38 for a support arm extension in each end of the tubular section 36.

The extension / connector members 38 are also hollow and include a central portion 40 and a tapered hollow plug that extends axially along the axis of the support arm 30 in either direction. An inner plug portion 42 is shaped to fit within, be received by, and cooperate with the hollow conical interior of the tubular portion 36. The outwardly facing plug portion 44 receives a cylindrical end 46 of the side arm 22.

The central portion 40 may be of different lengths so as to elongate the transverse support arm 30 when used in chairs that use seat shells 14 of different widths. Therefore, if the central sections 40 are formed two or four centimeters further, the transverse support arm 30 can accommodate a seat that is four or eight centimeters wider.

A cover ring 47 is arranged on the plug section 44 between the central section 40 and the cylindrical end 46 of the side arm 22. A first lever handle 48 extends through the cover ring 47 and a corresponding opening 45 in the plug 44 and takes one end a tubular actuating rod 50. A central bore in the central section 40 of the extension / connecting element 38 supports the actuating rod 50 centrally in the support arm 30. The tubular actuating rod 50 connects the handle 48 to an actuating connector 52 which is arranged in the vertical support 17 directly above the piston of the gas spring .

In the normal retracted position, the actuator port 52 is out of engagement with the gas spring piston; however, when the handle 48 actuating the socket is pivoted into an actuating position by the user of the chair, the socket 52 is pressed down into an effective actuation of the piston of the gas cylinder so that the height of the chair can be adjusted. The actuating rod 50 is also hollow to receive a connecting rod 55 which extends completely through the transverse support arm 30 in order to hold all of the aforementioned assembled components in position with respect to one another.

A second lever handle 54 is disposed through a similar cover ring 53 with a second tubular actuation rod 56 on the opposite side of the support arm 30 to actuate the locking mechanism as described later. The lever handle 54 is also pivotally attached to the transverse support arm 30. The tubular rod 56 connects the lever handle 54 to the rear crank 58 of a connection that passes through the main housing 34. A link arm 60 connects the rear crank 58 to a front crank 62. The front crank 62 is operative connected to an actuating rod 64 of a sliding / locking element 66, which will be described below.

The function of the sliding / locking element 66 will be explained with reference to FIGS. 6 and 7, but it is already indicated here that a rotation of the lever handle 54 also rotates the connection from 58, 60 and 62 to the actuating rod 64 and the slide - / locking element 66 to rotate a prescribed arc.

A spring 70 is also received in the main housing 34 of the support frame 20, but the connecting arm 60 is arranged so close to the wall of the housing 34 that there can be no mutual interference between the connecting arm 60 and the spring 70. The spring 70 connects the fixed support frame 20 to the slidable seat 14 to compensate for the force that normally tends to hold the backrest 12 in the upright position. This corresponds to the prior art, and therefore the function of the spring 70 is not explained in detail.

6 and 7, the connection is shown, which slidably connects the cross arm 32 of the support frame 20 to the front portion of the seat pan and locks them against each other if necessary. A pair of spaced apart slotted housings 72 and 74 are attached to the underside of the seat 14. Each housing 72, 74 includes an inwardly elongated slot or track 76 which is adjacent to the outermost, and thus the slide / latch member 66, the cross arm 32 is opposite. By way of example only, the sliding / locking element 66 is shown hexagonally shaped.

The diameter between the surfaces of the hexagonally shaped element is substantially equal to or slightly less than the width of the slot or track 76. Therefore, when the hexagonally shaped sliding / locking element 66 is aligned in a first sliding position in which its surfaces are parallel to run along the walls of the track 76, the sliding / locking element allows a relative movement between the housing 72, 74 and the cross arm 32.

However, when the slide / latch member 66 is rotated 30 ° in response to the actuation of the lever handle 54, the effective diameter of the slide / latch member 66 increases and is larger than the space between the walls forming the track 76. In this position, the larger diameter then effectively and frictionally acts on the side walls of the slot 76 to lock the slide / latch member 66 in place, thereby preventing the seat 14 from sliding and the backrest 12 from tilting in either direction.

Because of the suspension system described above, the frictional force required to prevent movement of the chair in both directions is relatively small (on the order of 200 Newtons compared to 800 Newtons for other types of chairs). Although the slide / latch element 66 is shown as a hexagonal element, any non-circular design will accomplish the desired one Effect as long as the element has a larger effective diameter when rotated to a first position than corresponds to the second effective diameter in a second position.

The sliding / locking element 66 could therefore theoretically be formed octagonally, at right angles or elliptically in cross section. The hexagonal shape, however, appears to be a good compromise, since it causes a larger effective diameter difference than would be the case with a larger number of corners, and it only requires a 30 ° movement of the lever handle 54 to effect such a locking movement .

As described above, a mounting receptacle 80 is disposed in a recess 82 of the support frame 20 at the intersection between the transverse support arm 30 and the main housing 34. The recess 82 is essentially a cup-like seat on the underside of the frame 20. The mounting receptacle 80 includes three circumferentially spaced openings or passages 84 which receive fasteners 86 extending therethrough around the mounting receptacle 80 within the seat 82 to secure.

9a shows the upper section of the mounting receptacle 80, which is normally received in the recess 82. The mounting receptacle 80 includes a relatively large central opening 88 through which the upper end of the gas cylinder C extends and in which it is received. The Piston p of the gas cylinder extends through and over the surface of the mounting receptacle 80, so that it is easily accessible and operable through the actuating nozzle 52. The mounting receptacle 80 further includes a recess 90 in the upper portion of its side wall, which extends along the axis of the main housing.

The purpose of this recess 90 is to allow an end finger of the spring 70 to extend through this recess 90 and to be secured behind a downwardly extending flange 35 in the main housing 34. The side of the mounting receptacle 80 which is adjacent to the connection of 58 and 60 is cut free to form a seat 92 which provides clearance for the operation of the rear crank 58. Since the tubular actuating rods 50, 56 act on the outer surface of the mounting receptacle 80, an inward movement is impossible.

Since the mounting receptacle 80 is separated from the frame 20, various desired results are achieved. First of all, all frames are easier to adapt to gas cylinders. Second, the mounting bracket includes 80 different, rather complicated passages, seats and recesses. Therefore, it is much easier to manufacture a smaller and separate part such as the mounting receptacle 80 than a part, possibly a cast part, for the entire support frame 20. The mounting receptacle additionally creates that it receives and connects the frame and the gas cylinder , a hold for the rear spring tongue, and holds the actuation for the gas spring as well as the actuation for the rear crank 58 in place.

A decorative foot cap 100 is removably attached to the end of each leg 16 of the base. The foot cap 100 is made of molded flexible polymeric material having a body portion 102 and a longitudinally extending finger 104 that extends a short distance along the surface of the leg 16. The finger 104 is received by a seat 106 on the leg 16.

The underside of the toe cap 100 includes an opening 108 through which a mounting boss of the roller 18 extends to hold the toe cap 100 in place. The foot cap 100 is simply placed in place by bending it and sliding it onto the exterior of the foot 16. While roller 18 is removed, finger 104 is moved into place within seat 106. When the roller 18 is inserted in place through the opening 108, the foot cap 100 is securely secured.

The foot cap 100 can be removed by the opposite procedure. The decorative foot cap 100 creates a protective function for the end of the legs 16, which are quickly scratched or unsightly in conventional furniture designs. Since the attachment and removal of the foot cap 100 is easy, the chair can appear refurbished by simply replacing the foot caps 100. In addition, the appearance of the chair can be changed by replacing the foot caps 100 with foot caps of other colors or shapes.

As shown in FIGS. 3 and 4, the side arms 22 may have a full length in one embodiment. In this case, they extend forward and upward from their lower point and then backward to the attachment point 28 on the backrest 12. In another embodiment for secretaries or similar purposes, it may be desirable to have a shorter arm formation. In these cases, the arm extends vertically upward, then backwards for a shorter distance to the attachment point 28 on the backrest 12.

2, it is advantageous to support the seat assembly 10 from the support frame 20 with the smallest possible torque, which is initiated at the attachment point 26 by maintaining a predetermined ratio between three transverse axes, namely a first transverse or horizontal axis which extends along the rear edge of the seat element 14 and the lower edge of the backrest 12 at a point 13 where this seat element 14 and the backrest 12 are hingedly connected to one another; a second horizontal axis extending through the lumbar region at point 28 where the rear ends of the arms 22 are hinged to the backrest 12; and a third horizontal axis that coincides with the transverse support arm 30.

The second horizontal axis through the lumbar region lies behind the third axis, which coincides with the transverse support arm 30. The The first transverse or horizontal axis is movable depending on the tilting of the backrest 12 between a first upright position in which the first horizontal axis is arranged substantially behind the second horizontal axis described and a second tilted position in which the first horizontal axis is closer to or is even arranged substantially below the second horizontal axis.

It is important that the chair should be designed so that the first horizontal axis cannot be moved to a position that is more forward than below the second horizontal axis. In that case, the seat pan 14 would then be raised in response to further tilting of the backrest 12, which would interfere with one of the main objectives of the present invention.

However, as shown and described, the seat pan 14 lowers slightly when the backrest 12 is tilted backwards. While the relationships between the first and second horizontal axes are described in the above-mentioned German patent, the arrangement of the third horizontal axis is reserved for the present invention.

Due to the unique manner in which the chair is suspended from the transverse arm 30 (third horizontal axis) and the relative arrangement between the arm 30 and the first and second horizontal axes, the horizontal distance between the center of gravity of the occupied chair and the transverse one Support arm 30 reduced. An advantage of the arrangement of the first, second and third horizontal axes relative to one another is therefore also the resulting low locking pressure which is necessary to fix the chair in its locked position compared to the embodiments of the prior art.

Claims (15)

1. An office chair comprising

   a) a seat (14) and a back (12) with a lumbar area, the seat (14) and the back (12) being connected in the manner of a hinge and articulated together about a first horizontal axis (at 13), which extends along the rear area of the seat (14) and the lower edge of the back (12), the back (12) being of curved construction from top to bottom,

   b) a support system, which comprises a vertical column (17) extending upwards from a base and a transverse supporting arm (30), the transverse supporting arm (30) running underneath the seat (14) along a third horizontal axis,

   c) a pair of side arms (22), the side arms (22) being fastened to opposite ends of the transverse supporting arm (30) and extending upwards and then rearwards herefrom, the upper ends of the side arms (22) articulatedly accommodating opposing side edges of the back (12),

   d) connecting means, which slidably connect and optionally lock the support system with the front section of the seat (14),
   wherein

   e) the first horizontal axis (at 13) is movable in dependence on tilting of the back (12) between a first, upright position and a second, tilted position, whereby, if the back (12) is tilted backwards, the seat (14) undergoes slight lowering,

   f) the back (12) is of curved construction about a second horizontal axis (at 28), which runs through the lumbar area,

   g) the support system comprises a supporting frame (20), which comprises the transverse supporting arm (30),

   h) the transverse supporting arm (30) is arranged at the upper end of the vertical column (17) and runs at right angles thereto,

   i) the supporting frame (20) further comprises a front element (cross-arm 32) and an elongate element (main housing 34),

   k) the hinge point at the upper end of each of the two side arms (22) lies at a point (28) which is substantially aligned with the second horizontal axis,

   l) the second horizontal axis (at 28), which extends through the lumbar area, lies behind the transverse supporting arm (30),

   m) in the first, upright position the first horizontal axis (at 13) lies substantially behind the second horizontal axis (at 28), and in the second, tilted position the first horizontal axis (at 13) runs substantially beneath the second horizontal axis (at 28),

   n) the horizontal distance between the centre of gravity of the occupied chair and the transverse supporting arm (30) is reduced if the back is tilted backwards,

   o) the connecting means connect and optionally lock the front element (cross-arm 32) of the supporting arm (20) with the front section of the seat (14), the connecting means comprising:

   o1) a cross-arm (32), which forms the front end of the supporting frame (20) and runs perpendicularly thereto in both directions therefrom;

   o2) a slit housing (72, 74), which is fastened to the bottom of the seat (14) and adjoins each end of the cross-arm (32), each of the housings (72, 74) having an inwardly directed elongate slit (76) ;

   o3) an actuating rod (64), which extends through the cross-arm (32) and ends in a slide/bolt element (66) on both sides thereof, the slide/bolt element (66) projecting beyond the cross-arm into a working connection with the elongate slit (76) in the slit housings (72, 74), the slide/bolt element being constructed rotatably between a first sliding position and a second bolting position, the working diameter of the slide/bolt element (66) between the walls which form the slit (76) being greater in the second bolting position than in the first sliding position.
2. An office chair according to claim 1, characterized in that in the second position the working diameter of the slide/bolt element (66) exerts a frictional bolting force of substantially 200 Newtons.
3. An office chair according to claim 1, characterized by connecting means (rear crank 58, connecting arm 60, front crank 62), which connect the transverse supporting arm (30) and the actuating rod (64), which latter extends through the cross-arm (32) to actuate the slide/bolt element (66) between the first and second positions.
4. An office chair according to claim 3, characterized by a lever handle (54), which is arranged swivellably on the transverse supporting arm (30), the connecting means (rear crank 58, connecting arm 60, front crank 62) being actuated in dependence on a movement of this lever handle (54).
5. An office chair according to claim 4, characterized in that the connecting means (rear crank 58, connecting arm 60, front crank 62) extend through the transverse supporting arm (30) and the cross-arm (32) and actively connect the lever handle (54) and the slide/bolt element (66).
6. An office chair according to claim 1, characterized in that the cross section of the slide/bolt element (66) has a hexagonal form.
7. An office chair according to claim 1, characterized in that the back (12) is curved from side to side in addition to its curvature from top to bottom, whereby the horizontal distance between the first horizontal axis (at 13) and the second horizontal axis (at 28) is increased to obtain a greater downwards directed displacement of the seat (14) in dependence on the tilting of the back (12).
8. An office chair according to claim 1, characterized in that the transverse supporting arm (30) comprises a connecting element (38), which is fixed at each of its ends, the connecting element (38) comprising a first conical and hollow plug portion (42), which extends into the supporting arm (30), and a second hollow and conical plug portion (44), which extends in the opposite direction, each side arm (22) being fastened to one of the second hollow conical plug portions (44), whereby the length of the transverse supporting arm (30) can be extended, various connecting elements (38) of differing lengths being used, so that wider or narrower seat arrangements can be obtained.
9. An office chair according to claim 1, characterized in that the vertical column (17) comprises a gas cylinder, which comprises an actuating connecting piece (52) at its upper end which extends into the supporting frame (20) approximately to the point of intersection of the transverse supporting arm (30) with the main housing (34), an actuating bar (50) being carried inside the transverse supporting arm (30) and being movable between a first withdrawn position and a second position in which it is in active loading with the gas cylinder piston and a lever handle (48) being attached to the transverse supporting arm (30) which is connected with the actuating connecting piece (52) by the first actuating bar (50), to move the actuating connecting piece (52) between the withdrawn and the actuating positions.
10. An office chair according to claim 9, characterized by connecting means (rear crank 58, connecting arm 60, front crank 62), which connect the transverse supporting arm (30) and the actuating rod (64), which extends through the cross-arm (32), to rotate the slide/bolt element (66) and by a lever handle (54), which is arranged swivellably on the transverse supporting arm (30) and is connected with the connecting means inside the transverse supporting arm (30) by a second actuating bar (56) and finally by an assembly receptacle (80) which is inserted releasably into a recess (82) in the supporting frame (20) at the point where the supporting frame (20) is fixed on the upper end of the vertical column (17), the assembly receptacle (80) comprising a through-opening (88) to receive and fix the supporting frame (20) at the top end of the gas cylinder, and the assembly receptacle (80) further comprising bearing surfaces to hold the first actuating bar (50) and the second actuating bar (56) in their positions and bear upon them.
11. An office chair according to claim 1, characterized by a base for supporting the vertical column (17), the supporting base comprising a plurality of radially extending legs (16), each ending freely and comprising a roller (18), which is attached releasably to the underside of the leg, moreover by a decorative foot cap (100) which is fastened releasably to the free end of each leg (16) and comprises an opening (108) in its underside through which the roller (18) extends, to hold the foot cap (100) in position.
12. An office chair according to any one of claims 1 to 11, characterized by:

    a) a cross-arm (32), which is fastened to the lower supporting frame and extends perpendicularly to the axis running from front to back underneath the seat element;

    b) a slit housing (72, 74), which is fixed to the bottom of the seat (14) and adjoins each outer end of the cross-arm (32), each of the housings (72, 74) comprising an inwardly directed elongate slit (76);

    c) a slide/bolt element (66), which is arranged rotatably at each end of the cross-arm (32) and extends in a working connection into the elongate slit (76) in the adjacent slit housing (72, 74), the slide/bolt element being constructed to be rotatable between a first, sliding position and a second, bolting position, the working diameter of the slide/bolt element (66) between the walls which form the slit (76) being greater in the second position than in the first, sliding position.
13. An office chair according to claim 12, characterized in that in the second position the working diameter of the slide/bolt element exerts a bolting frictional force of substantially 200 Newtons.
14. An office chair according to claim 12, characterized by a connecting arm (60), which is connected with the actuating rod (64) which extends through the cross-arm (32) to rotate the slide/bolt element between the first and second positions.
15. An office chair according to claim 12, characterized in that the cross-sectional arrangement of the slide/bolt element (66) is of hexagonal construction.

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