Weight responsive chair

1 . A chair comprising:

a base including a ground engaging portion;

a seat supported on the base;

a backrest connected to the seat and the base, the backrest pivotally connected to the base at a recline axis, the backrest pivotable about the recline axis between an upright position and a reclined position;

a recline tension spring biasing the backrest in the upright position; and

a weight responsive mechanism including:

a front link pivotally connected between the base and the seat,

a rear link pivotally connected between the seat and the backrest, and

a resilient element connected between the seat, the rear link and the backrest,

wherein the force of a user occupying the seat pivots the front link and the rear link against a resistance created by the resilient element an amount proportional to the weight of the user to move the seat downwardly towards the base and forwardly away from the recline axis and increase the effective length of the moment arm between the user and the recline axis, such that the force to pivot the backrest about the recline axis varies as a function of the weight of the user.

2 . A chair comprising:

a base including a ground engaging portion;

a seat supported on the base;

a backrest connected to the seat and the base, the backrest pivotally connected to the base at a recline axis, the backrest pivotable about the recline axis between an upright position and a reclined position;

a recline tension spring biasing the backrest in the upright position; and

a weight responsive mechanism including:

a front link pivotally connected between the base and the seat,

a rear link pivotally connected between the seat and the backrest, and

a resilient element connected between the seat, the rear link and the backrest,

wherein the force of a user occupying the seat pivots the front link and the rear link against a resistance created by the resilient element an amount proportional to the weight of the user to increase the effective length of the moment arm between the user and the recline axis, such that the force to pivot the backrest about the recline axis varies as a function of the weight of the user; and

including a secondary recline spring between the base and the backrest, the secondary recline spring contributing to the biasing force of the recline tension spring when the backrest is moved to the reclined position.

3 . The chair of claim 2 wherein the secondary spring is unloaded when the backrest is in the upright position.

4 . The chair of claim 3 wherein the amount of contribution of the secondary spring acting to bias the backrest in the upright position is adjustable when the backrest is in the upright position.

5 . The chair of claim 4 wherein the secondary spring is a compression coil spring having a first end and a second end, the first end fixed in position on the base, the second end adjustably positioned on the backrest, wherein adjustment of the second end of the secondary spring changes a distance between the second end and the recline axis and changes the amount of contribution of the secondary spring to the recline resistance.

6 . The chair of claim 5 wherein the secondary spring dynamically adjusts as a function of the weight of the user.

7 . The chair of claim 6 wherein the second end of the secondary spring is caused to move with respect to the backrest in response to the weight of a user sitting on the seat.

8 . The chair of claim 7 including a drive linkage connected between the seat and the second end of the secondary spring, wherein movement of the seat upon a force provided by the weight of a user causes movement of the second end of the spring.

9 . The chair of claim 8 wherein the backrest includes an index surface, and wherein the second end of the secondary spring includes an indexer connected to the second end of the spring, the indexer retained in place by the index surface when the backrest is reclined.

10 . A chair comprising:

a base having a ground engaging portion and a mechanism portion;

a seat for supporting a user; and

a backrest connected to the seat and the base, the backrest pivoting with respect to the base between an upright position and a reclined position about a recline axis, the backrest biased in the upright position by a recline spring;

wherein the seat is supported on the base by a four-bar linkage, the four-bar linkage including the seat, the mechanism portion of the base, a front link between the seat and the mechanism portion of the base, and a rear link between the seat and the mechanism portion of the base, wherein the rear link includes an upper rear link and a lower rear link, the upper link connected to the seat, and the lower rear link connected to the base;

wherein the four-bar linkage is biased in a first position by a weight sensing spring;

wherein a force from a user sitting on the seat acts against the weight sensing spring and pivots the front link, the upper rear link and the seat downward and forward with respect to the backrest and the recline axis and increases the distance between at least a portion of the seat and the recline axis.

11 . A chair comprising:

a base having a ground engaging portion and a mechanism portion;

a seat for supporting a user; and

a backrest connected to the seat and the base, the backrest pivoting with respect to the base between an upright position and a reclined position about a recline axis, the backrest biased in the upright position by a recline spring;

wherein the seat is supported on the base by a four-bar linkage, the four-bar linkage including the seat, the mechanism portion of the base, a front link between the seat and the mechanism portion of the base, and a rear link between the seat and the mechanism portion of the base, wherein the rear link includes an upper rear link and a lower rear link, the upper link connected to the seat, and the lower rear link connected to the base;

wherein the four-bar linkage is biased in a first position by a weight sensing spring;

wherein a force from a user sitting on the seat acts against the weight sensing spring and pivots the front link, the upper rear link and the seat forward with respect to the recline axis and increases the distance between at least a portion of the seat and the recline axis; and

including a secondary recline spring between the base and the backrest, the secondary spring contributing to the biasing force of the recline tension spring when the backrest is moved to the reclined position.

12 . The chair of claim 11 wherein the secondary recline spring includes a first end connected to the mechanism portion of the base and a second end connected to the backrest, wherein the position of the second end is adjustable with respect to the position of the recline axis.

13 . The chair of claim 12 wherein the secondary recline spring is dynamically adjustable as a function of the user's weight.

14 . The chair of claim 10 wherein the weight sensing spring is a resilient material, and wherein a first portion of the weight sensing spring is connected to the upper rear link.

15 . The chair of claim 14 wherein the weight sensing spring is capable of flexing from an upright position to a forward position upon the force of the weight of a user sitting on the seat.

16 . The chair of claim 15 wherein the upper rear link includes a first end mounted to the seat, and a second end pivotally connected to the lower rear link at a rear seat pivot axis.

17 . The chair of claim 16 wherein the weight sensing spring includes a second portion that extends from the rear seat pivot axis to the backrest, the second portion flexing as the upper rear link is pivoted.

18 . The chair of claim 17 wherein the weight sensing spring is a one-piece, V-shaped resilient member having a first leg that is coextensive with the upper rear link and forms the first portion of the weight sensing spring and extends between the seat and the rear seat pivot axis and a second leg that extends from the seat pivot axis to the backrest and forms the second portion of the weight sensing spring.

19 . The chair of claim 18 wherein the lower rear link is pivotally connected to the upper rear link at the rear seat pivot axis, and pivotally connected to the mechanism portion of the base at the recline axis.

20 . A chair comprising:

a base;

a backrest supported by the base at a recline axis, the backrest pivotable about the recline axis between an upright position and a reclined position, the backrest biased in the upright position by a recline spring;

a seat supported by the base and having a surface for supporting an occupant; and

a weight responsive mechanism adjusting the force required to pivot the backrest to the recline position as a result of an occupant sitting on the seat, independent of the pivoting of the backrest, and as a function of the weight of the occupant, the weight responsive mechanism including:

a front link between the base and the seat,

an upper rear link between the seat and the backrest,

a lower rear link between the upper rear link and the base, the backrest attached to the lower rear link, and

a weight sensing spring urging the upper rear link toward a first position, wherein the force of an occupant on the surface of the seat acts against the weight sensing spring to pivot the front link and the upper rear link out of the first position and downwardly and away from the backrest.