Seating unit with adjustable lumbar device

A chair includes a lower back frame, a back construction, including a back support attached to the lower back frame, a cushion assembly attached to the back support, and a vertically adjustable lumbar frame positioned between the back support and the cushion assembly. The lumbar frame includes a transition shell and a vertically sliding lumbar device having a plurality of horizontal flexible wires. A lower edge of the transition shell is positioned on top of an uppermost horizontal flexible wire of the lumbar device. The lumbar device is operably slidably mounted between the outer shell and cushion assembly and the transition shell upper edge slides between the outer shell and the upholstery cushion assembly. Also disclosed is a chair including a Y-shaped lower back frame having two uprights and a cross-piece attached to the back support. The uprights and cross-piece of the lower back frame include an overlap flange and the back support includes a channel-shaped flange having detents. The overlap flange and channel-shaped flange mate to form an overlap joint, including a box beam for added structural support.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to co-assigned, co-pending application Ser. No. 11/757,187, filed on even date herewith, entitled HEIGHT ADJUSTABLE ARMREST, and also related to co-assigned, co-pending application Ser. No. 11/757,169, filed one even data herewith, entitled CHAIR BACK ATTACHMENT AND METHOD OF ASSEMBLY, the entire contents of both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a chair incorporating an adjustable lumbar assembly and device and an overlap joint connection. More particularly, the present invention relates to a chair having a back support, an upholstery cushion assembly and a vertically adjustable, flexible live back lumbar assembly and device positioned therebetween and a chair back support connected to a lower frame having an overlap joint connection.

Chair users and seating manufacturers have recognized the value and health benefit of providing good adjustable lumbar support. However, new lumbar devices are desired which provide optimal comfort, but are simple to manufacture and assemble, are easily adjustable, operate smoothly, and are durable and robust. A lumbar device is desired that slides more fluidly and smoothly between adjusted positions, yet is secure in its selected position and effective in its function.

Accordingly, an adjustable lumbar assembly and device are desired that solves the aforementioned problems and that has the aforementioned advantages.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a chair back construction includes an outer shell, a lumbar assembly positioned over the outer shell, and a cushion assembly positioned over the lumbar assembly and attached to the outer shell. The lumbar assembly includes a lumbar frame including a transition shell having an upper edge and a lower edge, and a vertically sliding live lumbar device having a plurality of horizontal flexible wires, where the transition shell lower edge is positioned on top of an uppermost horizontal flexible wire of the lumbar device. The lumbar device is operably slidably mounted between the outer shell and cushion assembly and the transition shell upper edge slides between the outer shell and the upholstery cushion assembly.

In another aspect of the present invention, a lumbar device is provided which includes a pair of wire retainers and a plurality of flexible wires. Each retainer includes a convex front surface which has a plurality of wells, an outside surface including oppositional tabs for slidably engaging a wave ridge of a back support to hold the lumbar device in a selected position, and a rear surface which is shaped to slidably engage a front surface of the back support. The plurality of flexible wires are positioned in the wire retainer wells and held in a horizontal position and form a curved surface which pushes in the direction of the cushion assembly to provide live lumbar support.

In yet another aspect of the present invention, a chair includes a lower back frame having two uprights and a cross-piece where the uprights and cross-piece each include an overlap flange. The chair also includes a back construction having a back support attached to the lower back frame. The back support includes two sides and a bottom where the sides and bottom each include a channel-shaped flange. The overlap flanges of the uprights and cross-piece and the channel-shaped flanges of the back support mate to form a U-shaped overlapped joint.

In another aspect of the present, invention, a chair includes a lower back frame having two uprights and a cross-piece, where the uprights each include an overlap flange. The chair also includes a back, support attached to the lower back frame, where the back support has two sides each including a channel-shaped flange having detents. The overlap flange of the uprights and the channel-shaped flanges mate to form a box beam on each side.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

For purposes of description herein, the terms “upper,” “lower,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the invention as oriented inFIG. 1. The terms “right” and “left” shall relate to the invention as oriented relative to a person in a seated position. However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise.

As illustrated inFIGS. 1-4, one embodiment of a chair10of the present invention includes a base12, a lower back frame14, a seat16, and a back construction18. The base12includes a leg assembly19and a control housing20attached to the leg assembly19. The control housing20includes a fixed support structure230which extends laterally and upwardly on either side of the control housing (FIG. 45). An armrest22is attached to each side of the support structure. The seat16is attached to the control housing20. The lower back frame14is attached to the control housing20and extends outwardly and upwardly. As shown inFIGS. 5-7, the back construction18is attached to the lower back frame14and includes a back support (also referred to as an outer shell)24and a cushion assembly26, including an inner shell27, attached to the back support24. The back construction18also includes a lumbar assembly28which is disposed between the back support24and the cushion assembly26. The cushion assembly26is attached to the back support shell with a quick attach hooking top and side connections described further below, and a “ZIP-LOCK” type bottom connection. The back construction18also includes an upholstery attachment clip30which is wedgingly engaged between the back support24and the cushion assembly26. The upholstery attachment clip30makes assembly and disassembly of the back construction easier and less time-consuming.

The lower back frame14is Y-shaped and includes two uprights32and a cross-piece34, which connects the two uprights32(FIGS. 8,9). The lower ends of the two uprights meet at a vertex36. A lower back frame structure38extends laterally from the vertex36and attaches to the control housing20. The lower back frame14includes an inside face40, which engages the back support24and an outside face42. The inside face40of the uprights32and the cross-piece34attach to the back support24and include an overlap flange44and apertured bosses46. The outside face42of the cross-piece34includes a horizontal recess47on the cross-piece's bottom edge.

The back support or outer shell24comprises a molded polypropylene material or similar engineering-type structural material, and includes relatively stiff thoracic and pelvic sections. Referring to FIGS.5and8-11, the back support24includes an inside face48and an outside face50. The back support24also includes an upper (or thoracic) section52and a lower (or lumbar/pelvic) region54. The upper section52of the inside face48of the back support24includes a lip56along its top and side edges. A series of hooks58are evenly spaced laterally below the top lip56. The hooks58project forwardly and then upwardly. (FIG. 37). AlthoughFIGS. 5 and 10illustrate one embodiment of the present invention including a series of five hooks laterally spaced below the top edge, it is contemplated that the number of hooks can be more or less than five, preferably three to seven hooks, and more preferably four to six hooks. Also, the shape, length, style, and angle relative to the back support24of the hooks58may vary depending on the amount of upholstery on the cushion assembly26and the shape of the back support24. In one embodiment, it is contemplated that the center hook would be longer than the remaining hooks. In an alternative embodiment, the center hook is the longest, with the hooks adjacent to the left and right of the center hook being shorter than the center hook, and the remaining hooks being the shortest. In a preferred embodiment, the hooks58are all the same length and are angled about 15 to about 20 degrees relative to the back support24. A hook60on the right and left sides of the upper section52adjacent to the side lip56of the upper section52projects upwardly and then inwardly toward the middle of the upper section. In an alternative embodiment, the hooks60may project upwardly and outwardly. Adjacent to hooks60are ribs62. The ribs62are generally parallel to the side lip56. When force is applied to the sides of the cushion assembly26, the ribs prevent the cushion assembly26from disengaging from the back support.

The inside face48of the back support24also includes a pair of alignment stops64and a pair of wave ridges66. The wave ridges66are generally in the shape of a cosine or sine wave and allow for smooth and fluid adjustment of the lumbar assembly28. The alignment stops64and wave ridges66are in the lower section54of the back support24and extend into a lower portion of the upper section52. The wave ridges66terminate at a horizontal stop68. The area between the alignment stop64and the wave ridges66defines a first vertical band70. The first vertical band70is in the lower section54of the back support24and extends into a lower portion of the upper section52and includes a ramp72and two vertical lips74. The two vertical lips74provide support to the back support24. The height of the ramp72is greatest at the bottom and gradually reduces to the height of the two vertical lips74at the top of the first vertical band70. The area between the wave ridges66and the sides76of the lower section of the back support define a second vertical band78. The second vertical band78is in the lower section54of the back support24and extends into a lower portion of the upper section52. Within the second vertical band78are apertures80, which are used to fasten the back support24to the lower back frame14. Above the apertures80are a first recess82and a second recess84. The first recess82is in the lower section54of the back support24and the second recess84is in the upper section52of the back support24. The first recess82is defined as an indentation within the second vertical band78. As illustrated inFIG. 10, the first recess82is rectangular, however it is contemplated that the first recess can be square or circular in shape. The first recess82includes an aperture86for attaching the clip30, discussed further below, to the lower section54of the back support24. Alternatively, the clip30may be integral with the back support24or may be attached to the inner shell27, rendering the aperture86optional. When attached to the back support, a flange88of the clip30is disposed within the first recess82and the remaining portion of the clip30is disposed within the second recess84. (FIG. 26).

On the outside face50of the back support24, the side and bottom edges of the lower section54include a channel-shaped flange92and92aand apertures94and94a, (FIGS. 8and11). As shown inFIG. 11, on the side edges of the lower section54, apertures94are located within a recess96. To attach the lower frame14to the back support24, the inside face40of the lower back frame14is placed in contact with the lower section54of the outside face50of the hack support24, such that the overlap flange44on the lower frame14and channel-shaped flange92and92aon the back support24mate to form an overlap joint. (SeeFIGS. 11-16). The overlap flanges of the uprights and the cross-piece and the channel-shaped flanges of the back support mate to form a U-shaped overlap joint. Fastening means, such as screws, are inserted through apertures94and94aand anchored in the apertured bosses46. The resulting overlap joint formed on the bottom edge of the back support is illustrated inFIG. 16. On the side edges of the outside face50of the back support24, the overlap flange44of the lower back frame14mates with the channel-shaped flange92of the back support24to form an overlap joint including a box beam98(FIGS. 13,14). The box beam98provides additional structural support to the lower section of the back support. Notably, this overlap joint configuration allows for variations in manufacturing tolerances of the lower back frame and back support, thereby facilitating assembly of the back construction18.

As illustrated inFIG. 5, in one embodiment of the present invention, a vertically adjustable lumbar assembly28is positioned between the cushion assembly26and the back support24. Referring toFIGS. 18-23, the lumbar assembly28is vertically adjustable to provide optimal comfort to a seated user and includes a lumbar frame including a transition shell102, a lumbar device support104including side handles106, and a lumbar device108including lumbar-energy wires109. The transition shell102includes slits110, which extend from near a transition shell upper edge112to a transition shell lower edge114to form vertical strips116. In another embodiment, the transition shell is not a part of the lumbar frame, but rather is separate and attached to the wires109of the lumber device108. The lumbar device support104includes a first vertical flange118which projects outwardly from the lower portion of the lumbar frame28and a second vertical flange120which is oriented approximately perpendicular to the first vertical flange118. Projecting outwardly from the second vertical flange120are handles106. The second vertical flange120also includes fastening apertures122for attaching the lumbar device108to the lumbar device support104. The lumbar device support104also includes openings124located where the first and second vertical flanges118and120intersect to form a corner.

The lumbar device108includes a pair of lumbar-energy wire retainers126and126ain which a plurality of flexible lumbar-energy wires109are held in a horizontal position. In a preferred embodiment, the length of the lumbar-energy wires109are longer than a distance, D, between the wire retainers126and126a, such that the wires slightly bend when placed in the wire retainers. (SeeFIG. 20). The slightly bent, wires form a curved surface128, which when the lumbar device is attached to the lumbar frame, pushes in the direction of the back support24of the back construction18to provide active lumbar support. Also, in this embodiment, the wires exert an outward force on the wire retainers126and126awhich facilitates contact between tabs146and the wave ridges66. In alternative embodiments, the curved surface can be formed using a center vertical strap spanning the front of the wires, a central vertical strap wrapped around the lumbar assembly, or a tensioning element placed in front or back of the lumbar assembly which includes a plurality of loops wrapped around the wires109. As the strap or tensioning element is tightened the wires109will push in toward the back support.

Referring toFIGS. 20,21,26and27, the lumbar-energy wire retainers126and126ainclude a convex front surface130, a rear surface132, an outside side surface134, and an inside side surface136. The convex front surface130includes apertures138used in attaching the lumbar device108to the lumbar device support104, and wells140for holding the lumbar-energy wires109in the lumbar retainers126and126a. Preferably, the front surface130includes four wells140, however if is contemplated that the front surface could include three or more wells. As presently configured, each well140includes a pair of lumbar-energy wires109. The pair of horizontal wires109are formed using a single wire, bent into the shape of a rectangle with one end of the rectangle including the two ends of the wire. To facilitate assembly of the lumbar device, the two ends of the wire may be connected by means such as resistance welding. However, alternatively, the ends may be left separated. One end of the rectangle is positioned in a well140of wire retainer126with the opposite end of the rectangle positioned in an opposite well of wire retainer126a. In another embodiment of the present invention, one or more single wires may be positioned in a given well, however this is not preferred. When using individual wires, the ends of the wires are attached to the wire retainers by means of a hook, which increases the depth of the wire retainers.

The rear surface132of wire retainers126and126ais slidably engaged to the ramp72of the back support24(FIGS. 24-27). Notably, the rear surface132and the ramp72are concentric, which allows the lumbar device108to remain engaged with the ramp when the device is vertically adjusted. This concentric configuration prevents the formation of gaps between the lumbar assembly28and back: support24, thereby providing smooth adjustment of lumbar support. The rear surface132also includes tabular flanges141. The tabular flanges141are on both ends of the rear surface132and extend inward. The tabular flanges141provide structural stability to the wire retainers126and126a. When the lumbar assembly28is in contact, with the back support, the tabular flanges141abut the alignment stops64of the back support24. The rear surface132also includes apertured bosses142used in attaching the lumbar device108to the lumbar device support104. The outside surface134of the pair of wire retainers includes oppositional detents144. The detents144include tabs146which slidably engage the wave ridges66to hold the lumbar assembly28in a selected position.

As assembled, the wire retainers126and126aare positioned under the lumbar device, support104. A fastening means is inserted through apertures122and apertures138and into apertured bosses142. One end of a rectangle providing a pair of lumbar-energy wires109are positioned in a well140of the lumbar-wire retainer126. The pair of horizontal lumbar energy wires109pass through an opening124in the lumbar device support104, extend across the lumbar device support, pass through an opening124on the opposite side of the lumbar device support, allowing the second end of the rectangle providing the pair of lumbar-energy wires109to be positioned in a well140of the lumbar wire retainer126apositioned under the opposite side of the lumbar device support. The lower edge of the vertical strips116of the transition shell102is positioned on top of the uppermost lumbar-energy wires109. To vertically adjust the lumbar assembly28, a user engages at least one handle106, preferably two handles106, and moves the handle in either an upward or downward direction. As noted above, the wire retainers are slidably engaged to ramp72of the back support24. As the lumbar assembly is adjusted upward, the wire retainers also slidably engage vertical lips74. The tabs146on wire retainers126and126aslidably engage the wave ridges66on the back support to hold the lumbar assembly28in a select position. Vertical adjustment of the lumbar assembly is limited in an upward direction by the horizontal stop68on the back support and in a downward direction by the lower end of the ramp72. The transition shell upper edge112slides between the back support24and the cushion assembly26when the lumbar assembly is adjusted vertically. Notably, the transition shell102allows the horizontal wires109of the lumbar device108to slide vertically between the cushion assembly26and the back support24without objectionable friction. The transition shell102also distributes stress across the horizontal wires109.

The cushion assembly26includes a cover assembly147similar to the cover assembly disclosed in U.S. Pat. No. 6,220,661, issued Apr. 24, 2001, entitled “CHAIR BACK AND METHOD OF ASSEMBLY,” the entire contents of which are incorporated herein in its entirety by reference for its teachings, a cushion148and an inner shell27, (FIG. 5). The cover assembly147includes an upholstery front panel and a rear panel forming a sock that can be inverted and pulled upwardly onto the cushion148and inner shell27as the cover assembly is inverted. The rear panel includes a fabric section which hangs downwardly from the front panel and has a strip of stiff material149sewn along its lower edge to form a stiffened edge flange. The strip of stiff material149, such as polyethylene, and is generally in the shape of an inverted “J”. (SeeFIG. 17). The stiffened edge flange can be pressed or “zipped” into, and frictionally retained in, a horizontal recess47of the lower back frame14.

The cushion148comprises a polyethylene terephthalate (PETE) matting, preferably including recycled content, or alternatively, polyurethane foam and includes a rear surface shaped to mateably receive the inner shell27.

The inner shell27comprises a polypropylene panel and is adhered to the cushion148as needed to maintain the stability of the cushion assembly26. As illustrated inFIG. 29, the inner shell27includes a top section152, and side perimeter bands154and a bottom perimeter band156that extend down the side edges and along the bottom of the inner shell27, which define an opening159. The inner shell27also includes evenly spaced apertures160, including an overhang161, across the top that correspond to hooks58and an aperture162, including an overhang163, located on the upper left and upper right portions of the inner shell which correspond to hooks60. (FIG. 30). The front side of the bottom perimeter band156includes ridges157and valleys158. (FIG. 29). The ridges157and valleys158provide structural support to the inner shell27. On the rear side164of the inner shell27, illustrated inFIG. 30, adjacent to apertures162are ribs166. The ribs166are parallel to the side edges of the inner shell27. A pair of horizontal ribs167connect the overhang163and the rib166. The horizontal ribs167provide structural support to the overhang163and ribs166. As illustrated inFIG. 30A, when the back construction18is assembled, ribs62on the inside face48of the back support24and ribs166on the rear side164of the inner shell27are parallel and abut longitudinally. This configuration prevents the sides of the cushion assembly from excessively pulling in towards the center of the back construction and causing gaps between the cushion assembly and back support.

The top section152of the inner shell27may also include, slits168which extend upwardly from the opening159and terminate below apertures160. The area between the slits168defines a flap170. The slits168and flap170enhance the flexibility of the inner shell. The inner shell27also includes two or more apertures172on the side perimeter bands158and an associated hook174adjacent an aperture172, extending downwardly and outwardly from each aperture172(FIGS. 30,31). Preferably, the inner shell27includes two apertures172and hooks174, however, it is contemplated that the inner shell may include more than two apertures172and hooks174. Notably, the hooks174extending from each aperture172are oriented perpendicular to the hooks58located across the top of the back support24. The hooks can be oriented in an outboard or inboard direction, preferably an outboard direction. This configuration prevents the hooks in the back construction from disconnection caused by the application of forces on the chair during use. In an alternative embodiment, the more than two apertures172and hooks174may be on the back support24.

A clip30(FIGS. 32 and 33) is attached to the back support24as illustrated inFIG. 26. In another embodiment, it is contemplated that the clip30is not connected to the back support24by means of fasteners, but rather the clip is a part of, and integral with, the back support. In yet another embodiment, the clip30may be either attached20or integral with the inner shell27. The clip30is preferably made of steel, but may also be composed of any stiff metal or plastic material. The clip30includes a main body portion176, and a side flange178. When the clip30is attached to the back support24or inner shell27, the clip also includes a fastening flange88. As illustrated, the main body portion176includes a side edge177and an angled edge180and is in approximately the same plane as the fastening flange88. The main body portion176and fastening flange88are connected by a bent portion182. The geometry of the bent portion182may vary depending on the construction of the back support24or inner shell27. The fastening flange88includes an aperture184through which a fastening means attaches the clip30to the back support24. Where the clip30is integral with the back support24or inner shell27, the bent portion182and the fastening flange88are optional. An edge of the main body portion176abuts an edge of the side flange. The side flange178is approximately perpendicular to the main body portion176, and includes at least one angled ramp186. In a preferred embodiment, the clip30includes two or more angled ramps186. The side flange176also includes at least one stop187where the width of the side flange88is greatest, and at least one indentation188located between a stop and a subsequent angled ramp, where the width of the side flange178narrows, and an end portion190. Preferably, the side flange176includes two or more stops187and indentations188. As noted above, the clip30may be attached to or integral with either the back support24or inner shell27. When the clip30attaches to the back support24, the fastening flange88of the clip30is placed into the first recess82, allowing the remainder of the clip, namely the main body portion176and side flange178, to be disposed within the second recess84, and then a fastening means is inserted through the aperture184of the fastening flange88and into the fastening aperture86of the back support24. (FIG. 34). When disposed within the second recess84, the main body side edge177abuts the back support lip56.

To assemble the back construction18, first the clip30is attached to the back support24as discussed above or alternatively, attached to the inner shell27. Where the clip is integral with the back support or inner shell, this step is not necessary. Next, for a chair10including a lumber assembly28, the lumbar assembly28is positioned over the back support24such that the wire retainers126and126aare positioned on the ramps72, the tabular flanges141abut the alignment stops64, and the tabs146on the wire retainers are slidably engaged with the wave ridges66on the back support24. (FIGS. 35 and 26). Alternatively, the lumbar assembly can be positioned over the back support first and then the clips attached to the back support. If the chair10does not include a lumbar assembly28, this step is not necessary.

Next, the cushion assembly26including the inner shell27is positioned above the back support24and optionally, the lumbar assembly28. (FIG. 36). By moving the cushion assembly26downward over the back support24, the hooks174slide through the clips30. (FIG. 31). In particular, a lower hook174of the inner shell27passes through the angled edge180and the uppermost angled ramp186of the clip30until the lower hook reaches a stop187. Notably, the angled edge180and the angled ramp186allow each hook174to slide easily through the clip30. The stop187suspends forward movement of the hook174, to allow positioning of the cushion assembly26, back support24, and optionally, the lumbar assembly28. The stop187makes it difficult, but not impossible, for the hook to move backward to allow disassembly of the back construction, if desired. Partial assembly positioning of the hooks on the back support through the apertures on the inner shell27is accomplished when an associated hook174reaches an indentation188. Where the inner shell27includes two apertures172and hooks174, assembly of the back construction includes three stages. Once the cushion assembly, back support, and optionally, the lumbar assembly are positioned as desired (first stage), application of additional downward force will cause the hook174to pass through the stop187to reach the indentation188. As each hook174slides through the upholstery attachment clip30, the assembler is able to first place hooks60located on the upper left and upper light portions of the back support24through apertures166located on the upper left and upper right portions of the inner shell27(second stage) and then place hooks58across the top of the back support through apertures160across the top of the inner shell (third stage). (FIGS. 37,38). Forward movement of the hooks174is finally stopped when the end portion190of the clip30is reached. As the number of hooks174increases, the possible number of assembly stages also increases. Where the clip30is attached to, or integral with, the inner shell27and the back support includes the more than two apertures172and hooks174, assembly of the back construction18is similar to that discussed above.

Referring toFIGS. 39-43, the base12includes a center hub193, a leg assembly19having a plurality of radially extending legs196, and a leg cover198adapted to cover the legs196inside the center hub193is a cylinder192, which is attached to a control housing20. The legs196include a vertical tube section200at the leg outer end202for receiving a pintle204of a caster206. The leg outer end202has a concave shape to mateably engage tire vertical tube section200, thereby partially encircling the vertical tube section200. In a preferred embodiment, the vertical tube section200is welded to the leg outer end202. The legs196also each have a top surface208divided by a weld along its centerline210and have an aperture212offset to one side of the centerline210near the leg outer end202of the respective legs.

The apertures212(FIG. 42) have an oblong shape, and each side includes two tines213that extend about one-third to one-half of the way into the aperture212. The tines213on each side are spaced apart so that they define a space for receiving a respective boss217, but so that the fines213frictionally engage the sides of the boss217to prevent its removal. By locating the apertures212offset to one side of the tubular section of legs196, the complete apertures212can be preformed in the sheet metal before forming the tube, and therefore the apertures212can be more accurately formed. Also, the weld line in the tube does not have to skip or avoid the apertures212. Also, the weld line can be located in a symmetrical location on the tubular section, so that the forming and welding processes for forming the tubular legs196are more consistent and controllable. The leg cover198includes reinforcement ribs214that stiffen side flanges216of the leg cover198.

As shown inFIGS. 44 and 45, the control housing20includes a primary energy mechanism218and a top plate220. The primary energy mechanism218is similar to that disclosed in co-assigned U.S. Pat. No. 6,991,291, filed Feb. 1, 2005, entitled “BACK CONSTRUCTION FOR SEATING UNIT HAVING SPRING BIAS,” the contents of which are incorporated herein by reference for its teachings. The mechanism218includes an elongated horizontal tab222across the front of the mechanism218and a pin224on each side of the mechanism. The top plate220includes a slit226across the front of the plate220. On each side of the top plate220is an aperture228and a fixed side support230to which the armrest22is attached. To attach the top plate220to the primary energy mechanism218, the horizontal tab222is inserted into slit226. The top plate220is then pivoted such that the aperture228on the top plate is aligned with the pin224on the mechanism. The pin is then inserted through the aperture.