Electrically conductive chair support with sliding bushing

A chair for conducting the electrostatic charge of an occupant to the floor on which the chair rests, with the chair seat carried on a support unit which permits rotation of the seat and adjustment of the height of the seat above the floor. The support unit includes an outer sleeve and an inner sleeve moving vertically relative to the outer sleeve, and an electrical resistor connected between the inner and outer sleeves and in sliding contact with at least one of the sleeves, typically by means of a spring finger strip mounted in a groove of a bushing or collar.

BACKGROUND OF THE INVENTION 
This invention relates to chairs which provide for discharging the 
electrostatic charge of an occupant of the chair, which chairs are 
sometimes referred to as antistatic chairs or static protective chairs. 
The electrically conductive chairs are widely used in offices and in 
factories where it is desirable to avoid build-up of electrostatic charge 
on the person who is occupying the chair. Many chairs used in such 
situations provide for rotation of the seat relative to the support base 
and for raising and lowering of the seat relative to the support base. It 
is desirable to provide an electrically conductive path from the seat 
and/or back of the chair to the floor on which the chair rests while also 
permitting rotation and raising and lowering of the seat relative to the 
base. A number of designs have been proposed for such a product, and three 
are shown in U.S. Pat. Nos. 4,513,347; 4,625,257; and 4,747,011. In the 
electrical conductive chair, it is usually desirable to incorporate an 
electrical resistance in the electrical path between the seat and the 
floor and this is accomplished by inserting a resistor, typically a one 
megohm resistor, in series in the electrical path. 
Previous arrangements have been utilized for installing the resistor. By 
way of example, in U.S. Pat. No. 4,513,347 the resistor is in installed in 
a plastic sleeve and carried in a plastic shell attached to the chair. In 
U.S. Pat. No. 4,625,257 the resistor is mounted in an insulating sleeve 
projecting downward from the seatpost of the chair, with a conducting 
chain hanging from the resistor or from a zener breakdown device attached 
to the resistor. In U.S. Pat. No. 4,747,011 two resistors are mounted in a 
spacer between a base flange and a thrust bearing, with the leads of the 
resistors on the upper and lower surfaces of the spacer for contacting the 
thrust bearing and base flange. 
There have been problems with stability and reliability and operating life 
of these prior arrangements, and it is an object of the present invention 
to provide an electrically conductive chair with a new and improved 
arrangement for installing an electrical resistance in the electrical 
conductive path between the chair seat and floor. One problem has been 
with breaking of the leads of the resistors used in providing the desired 
electrical resistance. 
Other objects, advantages, features and results will more fully appear in 
the course of the following description. 
SUMMARY OF THE INVENTION 
A chair having a support unit for adjusting the height of the chair seat 
above the floor, with the support unit including an outer sleeve and an 
inner sleeve for providing vertical motion between the sleeves and 
rotational motion between the sleeves. An electrical resistance device is 
carried at the support unit in electrical contact with the inner and outer 
sleeves providing an electrical resistance path between the sleeves. In 
one embodiment, a spring is positioned in an annular groove in a bushing 
between the inner and outer sleeves, with the spring providing a sliding 
electrical contact with the inner sleeve and with a resistor connected 
between the spring and the outer sleeve. In another embodiment, springs 
are provided in each of two annular grooves in a collar with a resistor 
connected between the springs and with one spring providing electrical 
contact with one sleeve and with the other spring providing electrical 
contact with the other sleeve. 
The invention also includes a bushing for use with sleeves while providing 
an electrical path between the sleeves during axial and rotational motion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a chair is shown with a seat 11, a back 12, a vertical 
support post 13, a frame 14, and a plurality of rollers 15. The vertical 
support post 13 incorporates a hydraulic cylinder unit 16 which is shown 
in greater detail in FIG. 2, and which provides for raising and lowering 
of the seat with respect to the frame, and in some instances, for rotation 
of the seat about the vertical axis relative to the frame. 
The seat 11 and/or the back 12 include electrical conductive material which 
provides an electrical path from an occupant on the chair to the upper end 
of the hydraulic cylinder unit 16. Similarly, the frame 14 and rollers 15 
include material providing an electrically conductive path from the 
hydraulic cylinder unit to the floor upon which the chair rests. All of 
the above construction may be conventional in nature, such as that shown 
in prior patents, and is not set out in detail herein. The new feature of 
the present invention is the arrangement for providing an electrically 
conductive path through the hydraulic cylinder unit, with an electrical 
resistance in this path. 
As shown in FIGS. 2-4, the hydraulic cylinder unit 16 includes an inner 
sleeve 21 and an outer sleeve 22, each made of an electrically conducting 
material, typically steel. The upper end of the inner sleeve is joined to 
the seat 11, and the lower end of the outer sleeve is joined to the frame 
14. 
In the embodiment illustrated, a bushing 23 of electrically insulating 
material is positioned between the inner and outer sleeves, and is fixed 
to the outer sleeve by one or more screws 24. The inner sleeve 21 slides 
vertically in the bushing 23 which provides for maintaining alignment of 
the inner and outer sleeves. 
A piston rod 26 of the hydraulic cylinder unit slides in the inner sleeve 
21 and is connected at its lower end to a flange 27 at the lower end of 
the outer sleeve 22. A thrust bearing 28 is positioned at a shoulder 29 on 
the lower end of the piston rod 26 and rests on an electrically insulating 
washer 30. A spring clip 31 is installed at the lower end of the piston 
rod to retain the rod, and a upper washer 32 is positioned about the rod 
above the thrust bearing for limiting downward movement of the inner 
sleeve 21 along the piston rod 26. 
An inner annular groove 33 is provided in the bushing 23, and an 
electrically conductive spring 34 is positioned in the groove 33. The 
spring may be of conventional construction, and typically is a strip 35 
with a plurality of spaced parallel fingers 36. An appropriate length of 
the spring is cut and is placed in the groove 33, with the strip 35 
against the inner wall of the groove and with the individual fingers 36 
facing inward to provide a resilient electrically conductive engagement 
with the outer wall of the inner sleeve 21. An electrical resistor 38 has 
one lead 39 connected to the spring through an opening 40 in the bushing 
23. The other lead 41 of the resistor is positioned along a shoulder 42 of 
the bushing so that the lead is pressed against the inner wall of the 
outer sleeve 22 when the bushing is installed in the sleeve. With this 
arrangement, the resistor provides an electrical path between the inner 
sleeve and the outer sleeve. 
An alternative and presently preferred embodiment of the invention is shown 
in FIGS. 5-7, where components corresponding to those of FIGS. 2-4 are 
identified by the same reference numerals. 
A collar 45 of electrically insulating material is positioned at the upper 
end of the outer sleeve 22, and preferably is held in place by screws 46 
which pass through aligned openings in a cover or cap 47 and in the collar 
45 into threaded openings 48 of the outer sleeve. The collar 45 is shown 
as a single piece in the drawings, but can be made as one piece or as two 
or more pieces, as desired. The cover 47 may be utilized to provide 
mechanical protection, but is not an essential element of the invention. 
Two internal annular grooves 51, 52 are provided in the collar 45, and 
springs 53, 54 are positioned in the grooves 51, 52, respectively. The 
springs 53 and 54 may be the same in construction as the spring 34. The 
resistor 34 is connected to each of the springs, with lead 39 in an 
opening 57 and with lead 41 in an opening 58. 
In each of the embodiments, the hydraulic cylinder unit 16 is operated in 
the conventional manner for raising and lowering the seat and for rotating 
the seat relative to the base. The resistor provides an electrical 
resistance path between the inner and outer sleeves, with the desired 
electrical resistance in the path during the rotational and vertical 
motions of the chair. A conventional hydraulic cylinder unit can be 
utilized, merely requiring for the 
embodiment of FIGS. 2-4, the groove 33 and opening 40, and for the 
embodiment of FIGS. 5-7, the openings 48 for the mounting screws 46. 
With the construction of the present invention as shown in the embodiment 
of FIGS. 5-7, the electrical grounding path is outside the two sleeves of 
the cylinder unit and the connections of the electrical resistor are not 
in a lood bearing condition and hence there is no wear of the ends of the 
resistor leads. Also the externally portioned resistor with its cover is 
protected from contamination, such as by the grease associated with the 
sliding components.