WO1990011707A1

WO1990011707A1 - Ergonomic chair

Info

Publication number: WO1990011707A1
Application number: PCT/US1990/001845
Authority: WO
Inventors: Marty K. Mathews; Jeffrey A. Weber
Original assignee: Krueger International, Inc.
Priority date: 1989-04-03
Filing date: 1990-04-02
Publication date: 1990-10-18
Also published as: KR920700571A; CA1312815C; EP0466777A1; US5035466A; EP0466777A4

Abstract

An ergonomic chair (10) of simplified design includes a seat support member (12), a backrest support member (14), and a base member (16) to which the seat support member and the backrest support member are both attached. The base member (16) engages the seat support member (12) in a manner that permits movement of the seat support member between a forward seated position and a rearward seated position. The base member also engages the backrest support member (14) in a manner that permits movement of the backrest support member independent of the movement of the seat support member in a first direction toward the base member and in a second direction away from the base member. The base member also serves to tilt the base member forwardly and rearwardly about a horizontal axis.

Description

ERGONOMIC CHAIR Field of the Invention

The invention generally relates to chairs suitable for home or office use. More particularly, the invention relates to chairs that are adjustable to meet differing physical and physiological demands of users. Background of the Invention

Chairs that adjust to the differing physical and physiological requirements of people are known. Heretofore, the desired ergonomic features were often obtained at the price of complexity, requiring rela¬ tively bulky and complicated mechanisms that are dif¬ ficult for a company to manufacture and for the user to assemble on site.

There thus remains a demand for user adjust¬ able chairs that contain a minimum of component parts and that are lightweight and easy to assemble on site, but at the same time provide the user with all the desired ergonomic functions, such as back-to-seat depth adjustment, forward and rearward tilt, back height adjustment, and passive lumbar support. Summary of the Invention

The invention provides a user adjustable chair that provides principal ergonomic features in a simplified and straightforward manner to accommodate in comfort people of differing physical and physiolog¬ ical requirements. The straightforward design of the chair simplifies manufacturing and provides ease of assembly on site.

The chair that embodies the features of the invention comprises a seat support member, a backrest support member, and a base member to which the seat support member and the backrest support member are attached. The base member engages the seat support member in a manner permitting movement of the seat support member between a forward seated position and a rearward seated position. The base member also engages the backrest support member in a manner per- mitting movement of the backrest support member, inde¬ pendent of the movement of the seat support member, toward and away from the base member. The invention thus provides, with a single base member, the very desirable ergonomic feature of back-to-seat depth adjustment from two standpoints; that is, by movement of the seat relative to the back, and vice versa.

In a preferred embodiment, the invention accommodates seat movement relative to the base member with a minimum of component parts. In this arrange- ment, the base member includes a channel having a gen¬ erally dovetail-shaped .configuration and a runner having a mating dovetail-shaped configuration that is slidably engaged within the channel. The seat support member can be attached to either the channel or the runner. The mating dovetail-shaped configurations of these two parts permit the desired movement of the runner within the channel, while at the same time pre¬ venting lateral separation of the channel and the runner. In this arrangement, the base member prefer- ably limits the relative movement of the channel and the runner between two selected laterally spaced posi¬ tions.

Another aspect of the invention provides pivot means for tilting the base member, and thus the attached seat support member, forwardly and rearwardly about a horizontal axis. In a preferred embodiment, an associated control mechanism selectively permits tilting of the base member in either a forward or rearward direction, while preventing pivotal movement of the base member in the other direction. The inven¬ tion thereby selectively provides the desirable ergo¬ nomic feature of forward and rearward tilt.

The invention also provides a chair having a functional member that is selectively adjustable by the user. In accordance with this aspect of the inven¬ tion, the chair includes locking means for the adjustable member that comprises a low profile lever arm pivotally movable on the chair and camming means that is brought into and out of locking contact with the adjustable member in response to movement of the lever arm. Operation of the lever arm serves to selectively lock and unlock the adjustable member in a desired relationship with the seat support member. In a preferred embodiment, the locking means biases the camming means toward locking contact with the adjustable member in response to a force that is normally applied by the user when seated in the chair. The invention thereby provides an automatic self-lock¬ ing feature to assure that the adjustable member remains in the location set by the user.

Other features and advantages of the inven¬ tion will become apparent upon considering the accom¬ panying drawings, description, and claims. Brief Description of the Drawings

Fig. 1 is a front elevation view of a chair that embodies the features of the invention;

Fig. 2 is a side elevation view of the chair shown in Fig. 1;

Fig. 3 is an exploded perspective view of base member and associated seat support and backrest support members of the chair shown in Fig. 1;

Fig. 4 is an enlarged assembled front view of the base member and associated seat support member of the chair shown in Fig 1;

Figs. 5A and 5B are side sectional views, taken generally along line 5-5 in Fig. 4, showing the movement of the seat support member between a rearward position (Fig. 5A) and a forward position (Fig. 5B) ;

Figs. 6A and 6B are top views, taken gener¬ ally along line 6-6 in Fig. 4, showing the movement of the seat support member between a rearward position (Fig. 6A) and a forward position (Fig. 6B) ; Fig. 7 is a side sectional view of the cam actuated locking mechanism for the backrest support member that embodies the features of the invention, shown in an unlocked position;

Fig. 8 is a side sectional view of the cam actuated locking mechanism shown in Fig. 7, but in a locked position;

Fig. 9 is a side sectional view of the cam actuated locking mechanism for the backrest that embodies the features of the invention; Fig. 10 is an enlarged exploded perspective view of the tilting mechanism for the seat support member that embodies the features of the invention;

Fig. 11 is a partial end view, in section, of the tilting mechanism shown in Fig. 10, with the associated control rod in a first, or unlocked posi- tion ;

Fig. 12 is a view of the tilting mechanism taken generally along line 12-12 in Fig. 11;

Fig. 13 is a partial end view, in section, of the tilting mechanism shown in Fig. 10, with the associated control rod in a second, or locked posi¬ tion;

Fig. 14 is a view of the tilting mechanism taken generally along line 14-14 in Fig. 13; Fig. 15 is a side section view of the tilt¬ ing mechanism shown in Fig. 10, with the seat support member tilted rearwardly;

Fig. 16 is a side section view of the tilt¬ ing mechanism shown in Fig. 10, with the seat support member tilted forwardly; and

Fig. 17 is a side elevation view of the chair shown in Fig. 1 indicating the flexing movement of the associated backrest support member. Description of the Preferred Embodiments A chair 10 is shown in Figs. 1 to 3 that embodies the features of the invention. The chair 10 comprises a seat support member 12, a backrest support member 14, and a base member 16 to which the seat support member 12 and the backrest support member 14 are attached.

The base member 16 is connected to a pedes¬ tal 18 that supports the seat member at a desired sit¬ ting height above the ground. In the illustrated and preferred embodiment, the pedestal 18 includes four wheels or casters 20 permitting easy movement of the chair 10 on the ground.

In accordance with the invention, the seat support member 12 and the backrest support member 14 are each individually movable relative to the base member 16 to accommodate persons of differing height, weight, build and physical preference.

The seat support member 12 of the chair 10 can be variously constructed. In the illustrated embodiment (as best shown in Fig. 3) , the seat support member 12 comprises a bottom plate 22 and a seat cover 24. The bottom plate 22 is preferably made of a sturdy plastic or metal material. The seat cover 24 is preferably cloth or plastic and includes a cushion material such as foam rubber. An intermediate insert 26 may also be provided between the bottom plate 22 and seat cover 24 for added support and comfort.

The base member 16 includes first means 28 that engages the seat support member 12 and permits the user to move the seat support member 12 relative to the base member 16 between a forward seated posi¬ tion (shown in phantom lines in Fig. 2) and a rearward seated position (shown in solid lines in Fig.2). The first means 28 thereby provides the desirable ergo¬ nomic feature of back-to-seat depth adjustment to meet the needs and comfort of the individual user.

The first means 28 may be variously con¬ structed. In the illustrated and preferred embodiment (as best shown in Figs. 3 and 4) , the first means 28 comprises a channel member 30 and a runner 32 which is slidably engaged within the channel member 30. Relative axial, in this case horizontal, movement between the channel member 30 and the runner 32 is thereby permitted. This relative movement is shown in Figs. 6A and 6B from the standpoint of looking down from the top of the chair 10. The relative movement is also shown in Figs. 5A and 5B from the standpoint of looking from the side of the chair 10.

The bottom plate 22 of the seat support mem¬ ber 12 is attached by suitable fasteners 34 to the runner 32 (see Fig. 4) . In this arrangement, the channel member 30 is welded or otherwise suitably attached to the pedestal 18. However, it should be appreciated that the association of seat support mem¬ ber 12 and base on the runner 32 and channel member 30 could be reversed, with the seat support member 12 attached to the channel member 30 and the runner 32 attached to the base.

In the illustrated and preferred embodiment (as best shown in Figs. 3 and 4) , the channel member 30 includes a bottom wall 36 and two inwardly sloped, or beveled, sidewalls 38. This arrangement gives the channel member 30 a generally dovetail-shaped config¬ uration. The runner 32 likewise includes a top wall 40, to which the seat support member 12 is attached, and two outwardly sloped, or beveled, sidewalls 42. This arrangement forms a dovetail-shaped configuration for the runner 32 that slidably mates with the dove¬ tail-shaped configuration of the channel member 30 (as shown in Fig. 4) . This mating arrangement permits, in a straightforward manner, the desired relative axial movement between the channel member 30 and runner 32, while preventing lateral separation of the two parts. In the illustrated and preferred embodiment, limit means 44 is provided for restricting the rela- tive movement of the runner 32 within the channel mem¬ ber 30 between the two selected laterally spaced posi¬ tions corresponding to the forward and rearward seated position.

The limit means 44 may be variously con- structed. In the illustrated embodiment (as best shown in Figs. 5A/B and 6A/B) , the runner 32 is longer in axial length than the channel member 30. In this arrangement, the limit means 44 comprises downwardly depending first and second flanges 46 and 48 each attached by screws 50 to opposite end walls 52 and 54 of the runner 32. When the runner 32 reaches the position corresponding to the forward seated position (shown in Figs. 5B and 6B) , the first flange 46 abuts against the rear end wall 52 of the channel member 30, thereby preventing further forward axial movement. Likewise, when the runner 32 reaches the position corresponding to the rearward seated position (shown in Figs. 5A and 6A) , the second flange 48 abuts against the front end wall 54 of the channel member 30, thereby preventing further rearward axial move¬ ment.

The first means 28 also includes means 56 for biasing the seat support member 12 toward the rearward position. The biasing means 56 may be vari- ously constructed. In the illustrated embodiment the bias means 56 comprises one or more springs 58 attached to the channel member 30 and runner 32. In the illustrated embodiment, two springs are provided.

The springs 58 are attached at one end 58A by screws 60 or the like to the runner 32 adjacent to its forwardmost end wall 54. The opposite ends 58B of the springs 58 are attached by screws 60 or the like to the channel member 30 in a stretched, or tensioned, position, so that the normal contraction force of the springs 58 will urge the runner 32 toward the position where the second flange 48 abuts against the front end wall 54 of the channel member 30 (shown in Fig. 6A) . The seat support member 12 is thereby biased by the springs 58 toward the rearward seated position (also shown in Fig. 5A) .

When seated upon the chair 10, a user can apply an external horizontal force to move the seat support member 12 forwardly against the biasing force of the springs 58 until the forward seated position is reached (shown in Figs. 5B and 6B) . As shown in Fig. 6B, the movement will further stretch the springs 58. The user can selectively apply forces while seated on the chair 10 to adjust and retain the position of the seat support member 12 within a range of positions between the rearward and forward seated positions. Upon standing, the user removes the external forces, and the contraction force of the springs 58 will urge the runner 32 back toward the original rearward seated position shown (Figs. 5A and 6A) . Attention is now directed to the backrest support member 14 of the chair 10, which can likewise be variously constructed. In the illustrated and pre¬ ferred embodiment (see, in particular, Figs. 2 and 3) , the backrest support member 14 is of one-piece con- struction, being made from molded or extruded plastic or metal. In this arrangement, the backrest support member 14 is preformed into a generally J-shape con¬ figuration. This J-shape configuration includes a generally vertically oriented upper end portion 62, a generally horizontally oriented lower end portion 64, and an intermediate arcuate section 66.

A backrest 68 is attached to the upper end portion of the backrest support member 14. As shown in Fig. 3, the backrest 68 (like the seat support mem- ber 12) preferably includes a bottom plate 70 made of a sturdy plastic or metal material and a cloth or plastic seat cover 72 that includes a cushion material such as foam rubber. In addition, an intermediate insert 74 may also be provided between the bottom plate 70 and seat cover 72 for added support and com¬ fort.

As shown by arrows in Fig. 2, the backrest

68 is itself preferably adjustable up and down along the upper end portion 62 of the backrest support mem- ber 14 in a manner that will be described in greater detail later. This provides the desirable ergonomic feature of backrest adjustment.

In accordance with another aspect of the invention, the base member 16 includes second means 76 engaging the backrest support member 14 and being operative for permitting movement of the backrest sup¬ port member 14 relative to the base member 16 inde¬ pendent of the movement of the seat support member 12, which is also attached to the base member 16. More particularly, the second means 76 is operative for allowing movement of the backrest support member 14 in a first direction toward the base member 16 (shown in solid lines in Fig.2), thereby allowing the user to bring the backrest 68 closer to the seat support mem- ber 12. The second means 76 is also operative for allowing movement of the backrest support member 14 in a second direction away from the base member 16 (shown in phantom lines in Fig.2) , thereby allowing the user to move the backrest 68 away from the seat support member 12. In the illustrated and preferred embodi¬ ment, the second means 76 includes means 78 for lock¬ ing the backrest support member 14 in the desired position relative to the base member 16.

The second means 76 may also be variously constructed. In the illustrated embodiment (as best shown in Figs. 7 and 8) , the second means 76 takes the form of a second channel 80 formed along the underside of the channel member 30. The second channel 80 is sized so that there is normally a clearance between the lower end portion 64 of the backrest support mem¬ ber 14 and the interior wall 82 of the channel 80. This clearance is identified by the letter C in Fig. 7. Due to the clearance C, the lower end portion 64 of the backrest support member 14 is slidably within the channel 80 (as shown by arrows in Fig. 7) . This arrangement permits adjustment of the backrest support member 14 either toward or away from the base member 16, as before described and shown in Fig. 2.

The locking means 78 may also be variously constructed. It can, for example, take the form of a conventional screw lock assembly. However, in accor¬ dance with another aspect of the invention, the lock¬ ing means 78 comprises a low profile, cam actuated locking mechanism that is best shown in Figs. 7 and 8. More particularly, the locking means 78 includes a lever arm 84 pivotally attached to the channel member 30. The lever arm 84 includes a cam¬ ming surface 86 that extends from a first surface portion 88 positioned a first radial distance from the pivot axle 92 of the lever arm 84 (Rl in Fig. 7) to a second surface portion 90 positioned a second radial distance from the pivot axle 92 (R2 in Fig. 7) . R2 is a greater radial distance than Rl.

When the lever arm 84 is located in a first position (shown in Fig. 7) , which in the illustrated embodiment is generally upright and perpendicular to the plane of the channel 80, the lower end portion 64 of the backrest support member rests against the first camming surface portion 88 out of contact with the interior wall 82 of the channel 80. In this position, the heretofore described clearance C between the lower end portion 64 of the backrest support member 14 and the interior wall 82 of the channel 80 exists. The lower end portion 64 is thereby slidably moveable within the second channel 80.

When the lever arm 84 is moved toward a sec¬ ond position (shown in Fig. 8) , which in the illus¬ trated embodiment is generally horizontal and parallel to the plane of the channel 80, the second camming surface portion 90 presses against the lower end por- tion 64 of the backrest support member 14 against the interior wall 82 of the channel 80. The above- described clearance C is eliminated. The surface con¬ tact applied by the second camming surface portion 90 and the interior channel wall 82 creates frictional forces that securely lock the lower end portion 64 of the backrest support member 14 in place within the channel 80. When the lever arm 84 is positioned in its second position, the backrest support member 14 is thereby locked into the desired position within the channel 80.

In accordance with another aspect of the invention, the locking means 78 is operative for bias¬ ing the lever arm 84 toward its second, or locked, position in response to the forces normally applied by a user when seated on the chair 10. More particu¬ larly, as shown in Fig. 8, when seated on the chair 10, the user will normally apply a resting force (designated by the letter R in Fig. 8) on the backrest 68. The resting force R, if not resisted, would tend to move the backrest support member 14 away from the seat support member 12, as shown in phantom lines in Fig. 8. The application of the resting force R upon the second camming surface portion 90 pivots the lever arm 84 in a direction toward its second, or locked position, as shown by an arrow and in phantom lines in Fig. 8 (which, in Fig 8, is a clockwise direction).

The resting force R normally applied by the user, then, in effect biases the lever arm 84 toward its locked position. The resting force R normally exerted by the user will then maintain a tightening force upon the lever arm 84 to maintain the pressing engagement between the second camming surface portion 90 and the ad acent surface of the lower end portion 64 of the backrest support member 14. The cam actuated locking arrangement pro¬ vides for a low profile locking assembly. It also is easily manipulated by the user and resists loosening. In the illustrated and preferred embodiment (see Fig. 17) , the material from which the backrest support member 14 is made permits the user, by leaning back upon the backrest, to resiliently bend or flex the upper end portion 62 relative to the arcuate mid¬ dle portion 66 when the backrest support member 14 is suitably locked in position. The J-shape configura¬ tion provides added strength to lend the necessary back support while providing the resiliency necessary to accommodate the desired flexing movement. Added comfort and lumbar support is thereby provided. Preferably, as before discussed, the back¬ rest 68 is adjustable up and down along the upper end portion 62 of the backrest support member 14. For this purpose (see Figs. 3 and 9), a third channel 94 is formed along the rear surface of the backrest 68, which slidably receives the end portion 62. In this arrangement, as also shown in Fig. 9, means 96 is also provided for locking the backrest 68 in a desired position. The means 96 includes the same cam-actuated locking arrangement just described. For this reason, the component parts of the locking means 96 are given the same reference numerals in the drawings. The desirable ergonomic feature of back height adjustment is thereby provided.

In the illustrated and preferred embodiment, the base member 16 also includes third means 98 opera¬ tive for pivoting the base member 16 about a horizon¬ tal axis with respect to the pedestal 18. This pro¬ vides still additional degree of adjustability to meet the needs and comfort of the user. While the third means 98 may be variously constructed, in the illustrated embodiment (best shown in Fig. 10) , a bracket assembly comprising two gener¬ ally U-shaped brackets 100 and 102 is provided. One U-shape bracket 100 nests within the other U-shape bracket 102, and for this reason the brackets will be referred to as the inner and outer brackets, respec¬ tively. The brackets 100 and 102 are attached by pins 104 about a common pivot axis, so that the brackets 100 and 102 will pivot relative to each other (see Figs. 15 and 16) .

In the illustrated arrangement, the inner bracket 100 is attached to the pedestal 18, and the outer bracket 102 is attached to the base member 16. The brackets 100 and 102 are thereby arranged to per- mit pivotal movement of the base member 16, and thus the entire seat support member 12 attached thereto, in a forward direction (shown in Fig. 16 to be generally counterclockwise about the pivot axis 104) and in a rearward direction (shown in Fig. 15 to be generally clockwise about the pivot axis 104) in response to external tipping forces applied by the user.

The third means 98 also includes means 106 for normally biasing the brackets 100 and 102 to retain the seat support member 12 carried by the base member 16 in a generally horizontal position (shown in Fig. 2) . While the biasing means 106 can be variously constructed, in the illustrated embodiment (see Fig. 10) , two pairs of springs 106 A/B are provided. The pairs 106 A/B are attached by pins 108 at opposite ends of the nested brackets 100 and 102 in the path of pivotal movement (see also Figs. 15 and 16) .

When the seat member atop the base member 16 is in a horizontal position, the springs are in a nor¬ mally rested, unstressed position (shown in Figs. 4 and 10) . As shown in Figs. 15 and 16, pivotal move- ent of the seat support member 12 either forwardly or rearwardly in response to an external force will com¬ press one pair of springs 106 A/B while stretching the opposite pair of springs 106 A/B. Upon removal of the external force, the pairs of springs 106 A/B independently return to their rested positions and, in doing so, cooperatively return the seat support member 12 to its normal horizontal position.

The base member 16 further includes pivot control means 110 operatively connected with the third means for selectively permitting pivotal movement of the base member 16 in one desired direction while pre¬ venting pivotal movement of the base member 16 in another direction. In the illustrated embodiment, the pivot control means 110 in use permits forward tilting motion of the seat support member 12, while preventing rearward tilting motion of the seat support member 12. The invention thus provides the desirable ergonomic feature of forward and back tilt on a selective basis. The pivot control means 110 can be variously constructed. In the illustrated and preferred embodi¬ ment, a pivot control rod 112 is provided that extends through the nested brackets 100 and 102 along an axis 114 that lies above the pivot axis 104 of the bracket assembly (see Figs. 11 and 13) . More particularly, the control rod 112 passes through a pair of inner and outer entry openings 116 and 118 located along the axis 114 on one side of the nested inner and outer brackets 100 and 102, respectively. The control rod 112 also passes through an oppositely spaced pair of inner and outer exit openings 120 and 122 located along the axis 114 on the opposite side of the nested inner and outer brackets 100 and 102, respectively.

As can be seen in Fig. 10, the entry open- ings 116 and 118 are generally circular in configura- tion, whereas the exit openings 120 and 122 are gener¬ ally elongated in configuration in a direction gener¬ ally perpendicular to the base member 16 (see also Figs. 12 and 14) . As can also be seen in Fig. 10, the outer entry and exit openings 118 and 122 are radially larger than the inner entry and exit openings 116 and 120. As can be seen in Figs. 12 and 14, when the brackets 100 and 102 are nested together, the forward side edges 124 of the elongated inner and outer exit openings 120 and 122 are generally aligned, while the rearward edges 126 are not.

The control rod 112 is movable along the axis 114 between spaced first and second positions. In the first position (shown in Fig. 11) , the control rod 112 extends through the inner and outer entry openings 116 and 118, but not through both exit open¬ ings 120 and 122. A tab 128 on the control rod 112 abuts against the interior of the inner bracket 100 when the control rod 112 is in this first position to prevent withdrawal of the rod 112 altogether from the entry openings 116 and 118.

Due to the arrangement and relative dimen¬ sions of the inner and outer entry openings when the control rod 112 is in its first position (see Figs. 11 and 12) , pivotal movement of the brackets 100 and 102 can proceed unimpeded in both the counterclockwise, or forward tilting, direction and clockwise, or rearward tilting, direction (as shown in Fig. 12) .

In its second position (shown in Figs. 13 and 14) , the control rod 112 extends through both in¬ ner and outer entry openings 116 and 118 and both inner and outer exit openings 120 and 122. A tab 130 on the control rod 112 abuts against the interior of the inner bracket 100 when the control rod 112 is in this second position to prevent over-extension of the rod from the exit openings 120 and 122.

As shown in Fig. 14, due to the partial alignment and relative dimensions of the inner and outer exit openings 120 and 122 when the control rod 112 is in its second position, pivotal movement of the brackets 100 and 102 as above describe can proceed unimpeded only in the counterclockwise, or forward tilting, direction. Interference of the control rod 112 against the aligned forward edges 124 of the exit openings 120 and 122 prevents pivotal movement in the clockwise, or rearward tilting, direction.

It should be appreciated that different results can be obtained by altering the relative dimensions and alignment of the inner and outer exit openings 120 and 122. For example, by aligning the rearward edges 126 of the exit openings 120 and 122 and providing interference of the control rod 112 against these aligned rearward edges 126, forward tilting movement can be prevented, while allowing rearward tilting movement. Also, by aligning both the forward and rearward edges 124 and 126 of the exit openings 120 and 122 and providing interference of the control rod 112 simultaneously against both aligned forward and rearward edges 124 and 126, all tilting movement of the base member 16 can be prevented.

It should also be appreciated that base mem¬ ber 16 that embodies the features of the invention can be attached to the pedestal 18 with a bracket (not shown) preventing all forward and rearward tilting movement.

Various features of the invention are set forth in the following claims.

Claims

We claim:

1. A chair comprising: a seat support member; a backrest support^'member; and base member to which said seat support member and said backrest support member are attached, said base member including: first means engaging said seat support member for permitting movement of said seat support member between a forward seated position and a rear- ward seated position relative to said base member; and second means engaging said backrest support member for permitting sliding, generally hori¬ zontal movement of said backrest support member toward and away from said base member, independent of the movement of said seat support member.

2. A chair according to claim 1 further comprising a pedestal connected to said base for sup¬ porting said seat support member at a desired sitting height above the ground; wherein said base member includes third means operative for tilting said base member about a horizontal axis relative to said pedestal.

3. A chair according to claim 2 wherein said base member includes means operatively connected with said third means for selec¬ tively permitting tilting movement of said base member in one direction while preventing tilting movement of said base member in another direction.

4. A chair according to claim 2 wherein said third means includes means for biasing said base member toward a generally horizontal position while allowing tilting of said base member about said horizontal axis in response to external force.

5. A chair according to claim 1 wherein said first means includes means for biasing said seat support member toward said rearward seated position.

6. A chair according to claim 1 wherein said first means includes a channel, a runner slidably engaging said channel to permit rel¬ ative movement between said channel and said runner, and limit means for restricting the relative movement of said channel and said runner between two selected laterally spaced positions corresponding to said for¬ ward and rearward seated position; and wherein said seat support member is attached to one of said channel and said runner.

7. A chair according to claim 6 wherein said channel includes a generally dovetail-shaped configuration, and said runner includes a mating dovetail-shaped configuration that slidably engages said channel configuration to permit relative axial movement while preventing lateral sep¬ aration of said channel and said runner.

8. A chair according to claim 7 wherein one of said channel and runner is longer in axial length than the other one of said channel and runner, and wherein said limit means co - prises a lip extension on the longer one of said chan¬ nel and said runner that engages a side edge of the other one of said channel and said runner to prevent further axial movement therebetween.

9. A chair according to claim 1 wherein said second means includes means for locking said backrest support member in a desired position relative to said base member.

10. A chair according to claim 9 wherein said locking means comprises a lever pivotally movable on said base member and cam means operatively connected with said lever for selectively engaging and disengaging said backrest support member in response to pivotal movement of said lever.

11. A chair according to claim 9 wherein said second means includes means for biasing said locking means toward a position that locks said backrest support member in response to the forces normally applied by a user when seated on the chair.

12. A chair comprising a seat support member; a base member including first means engaging said seat support member and being operative for per- mitting movement of said seat support member between a forward seated position and a rearward seated posi¬ tion relative to said base member, said first means including a channel having a generally dovetail-shaped configuration, a runner having a mating dovetail- shaped configuration that slidably engages said channel configuration to permit relative axial move¬ ment while preventing lateral separation of said channel and said runner, and limit means for restric¬ ting the relative movement of said channel and said runner between two selected laterally spaced positions corresponding to said forward and rearward seated position, said seat support member being attached to one of said channel and said runner.

13. A chair according to claim 12 wherein one of said channel and runner is longer in axial length than the other one of said channel and runner, and wherein said limit means com- prises a lip extension on the longer one of said chan¬ nel and said runner that engages a side edge of the other one of said channel and said runner to prevent further axial movement therebetween.

14. A chair according to claim 12 wherein said first means includes means for biasing said seat support member toward said rearward seated position.

15. A chair comprising: a seat support member; a base member to which said seat support member is attached; and a pedestal connected to said base for sup¬ porting said seat support member at a desired sitting height above the ground; said base member including pivot means oper¬ ative for tilting said base member forwardly and rear- wardly about a horizontal axis relative to said pedes¬ tal, and control means operatively connected with said pivot means for selectively permitting tilting of said base member in one forward or rearward direction while preventing pivotal movement of said base member in the other forward or rearward direction.

16. A chair according to claim 15 wherein said pivot means includes means for biasing said base member toward a generally horizontal position while allowing tilting of said base member about said horizontal axis in response to external force.

17. A chair according to claim 15 wherein said pivot means comprises an outer bracket, an inner bracket nested within said outer bracket, and pin means coupling said inner and outer brackets for tilting said inner and outer brackets relative to each other about a common pivot axis, and and further including spring means opera¬ tively attached to said inner and outer brackets for biasing said brackets toward a generally non-tilted position.

18. A chair according to claim 15 wherein said pivot means comprises an outer bracket, an inner bracket nested within said outer bracket, and pin means coupling said inner and outer brackets for tilt- ing said inner and outer brackets relative to each other about a common pivot axis, and spring means op- eratively attached to said inner and outer brackets for biasing said brackets toward a generally non-tilt¬ ed position, and wherein said control means includes a pivot control rod extending through said inner and outer brackets along a second axis that is spaced from the pivot axis of said brackets, said control rod being passable through a pair of inner and outer entry open- ings located along said second axis on one side of said inner and outer brackets, said inner and outer entry openings being of different sizes, said control rod being further passable through an oppositely spaced pair of inner and outer exit openings located along said second axis on the opposite side of said inner and outer brackets, said inner and outer exit openings being of different sizes and being aligned along at least one peripheral edge and non-aligned along at least an opposite peripheral edge when said brackets are in said untilted position, said control rod being movable along said second axis between a first position, in which said control rod extends through both of said inner and outer entry openings but not both of said inner and outer exit openings, to allow tilting of said inner and outer brackets, and a second position, in which said control rod extends through both inner and outer entry openings and both inner and outer exit openings adjacent said aligned peripheral edge thereof to selectively prevent tilting of said inner and outer brackets in said one direct ion.