Office chairs are known comprising a spoke-type base with feet or wheels, a column mounted on said base and provided with a gas piston for adjusting the height of the sitting plane from the floor, and a support plate supporte by said column, and on which the seating portion rests.

The back-rest support bracket and the arm-rest support rackets are generaliy connected to the seating portion support plate.

IT-B-1.288.164, filed in the name of the present applicant, describes a device for adjusting the seat back portion comprising a guide element rigid with the support plate of the seating portion, a back-rest support slide engagable with said guide salement and slidable along it, and elastic means selectively engagable with a toothed portion to ensure the stability of the position of the slide relative to the guide.

This device has however proved to be susceptible to improvement both with regard to the ease finding the correct position of the arm-rest andlor seat back portion relative to the seating portion, and with regard to avoiding undesirable slippage of the arm-rest or seat back portion relative to the seating portion if the chair is lifted via these parts.

This improvement is attained according to the invention by a height adjustment device, in particular for seat back portions and arm-rests in office chairs, as claimed in claim 1.

The invention is described in detail hereinafter with reference to the accompanying drawings, on which:

Figure 1 is a schematic side view of a seat provided with the device of the invention both for the seat back portion and for the arm-rest; Figure 2 is a sectional and transparent view of the adjustment device of the invention; Figure 3 is a longitudinal section on the line 111-111 of Figure 2; Figure 4 is a cross-section on the line IV-IV of Figure 2; Figure 5-9 show the device during adjustment ; Figure 10 is an enlarge detailed view of a tooth and counter-tooth, and Figure 11 shows a different embodiment of the device.

As can be seen from the figures the device of the invention is applied to a traditional office chair 2 comprising a spoked base 4 with feet or wheels 6, a column 8 mounted on said base and, for the seating portion 12, a support plate 10 to which are connecte the rackets 14,16 supporting the back-rest 18 and the arm-rest 20 respectively.

In the ensuing description reference is made to those bracket components supporting the seat back portion, but the same is applicable to the arm-rest components.

To the seat back portion there is applied a support 22 formed of substantially rigid plastic and provided with holes 23 for the passage of screws which engage the back-rest 18. Said support has an internal seat 24 for its guiding along the bracket 14 and comprising a cavity 26 with projections shaped to define two mutually facing toothed portions 28, the cavity 30 between adjacent teeth 31 facing downwards when the chair is assemble, and a further portion 32 interpose between the two toothed portions 28 and provided with upwardly facing counter-teeth 34 which partially occupy each

tooth cavity 30. In this manner said teeth 30 and counter-teeth 34 form a mixed-line path the purpose of which is described hereinafter.

The outer surfaces of the two toothed portions 28 form two parallel channels 36 with the lateral edges of the seat 26, and have their minor surfaces 37 diverging upwards.

A U-shaped spring 38 is fixed to the bracket 14 boy a rivet 39 and is provided at its free ends with two appendices 40 bent at 90° and housed in the seat 26.

In the absence of externat stress the distance between the appendices 40 of the spring 38 is less than the distance between the two longitudinal channels 36 but greater than the base of facing cavities 30.

The back-rest adjustment device operates in the following manner.

When in the configuration in which the back-rest is a minimum height (see Figure 5) the appendices 40 lie substantially at the upper edge of the seat 26. To ensure this position the support 22 lies with its lower end on two stops 43.

To adjust the position of the back-rest the user pulls the supports 22 upwards with the result that initially the appendices 40 interact with the upper incline surfaces 45 of the toothed portions 28 and approach each other until they pass over the first tooth, after which following the lifting action they interact with the cavity 33 of the counter-teeth 34, hence preventing any further pulling. The user then slightly pushes the support 22 downwards to enable the appendices 40 to disengage from the cavity 33 of the counter-teeth 34 and, as a result of the elastic action of the spring, position itself stably and accurately within that cavity 30 immediately downstream of the tooth which

has just been passed over. This operation is repeated until the user has positioned the back-rest at the desired height.

If this height proves excessive, to return the back-rest to a lower position the user continues to raise the support until the appendices 40 disengage from the lower teeth (see Figure 8). The elastic rection of the spring 38 then causes the appendices 40 to withdraw from each other and interfere with the incline iower surface 37 of the toothed portion 28 to be guided into the longitudinal channels 36 where they can slide freely and enable the support 22 to slide downwards on the bracket 14 (see Figure 9), to return to the position shown in Figure 5.

From the aforestated it is apparent that the adjustment device of the invention presents numerus advantages, and in particular : -it enables the back-rest and arm-rest to be positioned in a simple and convenaient manner by a stepping operation, -if the chair is lifted by the arm-rest or the seat back portion, it prevents the movable part from undesirably sliding along its racket.

In the embodiment shown in Figure 11, there is fixed to the seat back portion a substantially U-shaped first section bar to which a spring 44 is applied with its appendices bent at 90° to the section bar surface and engaging in a profile piece 46 similar to the preceding and provided on a C- section 48 having its flanges bent inwards.