Molded Chair Shell

Abstract

An ell-shaped i.e. L-shaped, molded plastic shell comprising a seat portion and a back portion, both joined by an integral concavoconvex nonflanged waist portion is provided with stiffening structure medial of the waist portion.

Description

CROSS REFERENCE

U.S. Pat. No. 3,133,765, Gideon A. Kramer, issued May 19, 1964.

SUMMARY OF THE INVENTION

In ell-shaped chair shells, where the seat and back portions are integrally joined by a cupped waist portion, the common practice has been to reinforce the margins of the waist portion by providing edge flanges or thickened structural edge reinforcements. It has been observed that when such a shell is nonrigidly mounted and is subject to flexural and torsional stresses and strains during use, whereby the waist portion is more or less than normally cupped, surface fractures have occurred at the waist margins tending to shorten the life of the shell. The ordinary solution to such a problem would be to rigidly mount the shell against or onto a rigid substructure or additionally reinforce and to build up the edges of the waist to resist expected forces, particularly those tending to elongate the waist surfaces. Since the functional requirement of self-adjustability demands that the shell be permitted to flex and distort from its "at rest" form, this invention is concerned with overcoming the attendant problem. It has been discovered that by proceeding in opposition to the prior art, that is by providing relatively thin waist margins and by appreciably building up or reinforcing the medical waist area between the waist margins, substantial and unexpected resistance to otherwise damaging flexural and torsional forces can be provided, thus extending and prolonging the useful life of the chair shell. A further object has been to overcome such problems with respect to chairs in which such a shell is flexibly secured in a frame at two laterally spaced points under the seat portion and at a single central point behind the middle of the back portion that permit the desired distortion of the shell relative the rigid under structure.

DRAWINGS DESCRIBED

FIG. 1 is a side view of a chair incorporating a frame and a chair shell produced according to this invention;

FIG. 2 is a flattened diagrammatic view graphically showing various formation thicknesses of a preferred form of a molded chair shell;

FIGS. 3, 4 and 5 are cross-sectional views taken, respectively, in the planes 3-3, 4-4 and 5-5 of FIG. 2;

FIG. 6 is a cross section of the waist portion of the shell of FIG. 1 showing the concavoconvex nature of the shell at the waist portion;

FIG. 7 is a partial plan view of an alternate construction according to this invention;

FIG. 8 is a cross section on line 8-8 of FIG. 7;

FIG. 9 is a cross section on line 9-9 of FIG. 7;

FIG. 10 is a schematic sketch illustrating a functional design principle embodied in the chair shell of this invention;

FIG. 11 is a cross section of the waist portion of a flanged chair shell of the prior art;

FIG. 12 is a cross section of an alternate edge reinforced waist section of a prior art chair shell design; and

FIG. 13 is a cross section of an alternate structure stiffening and reinforcing the shell waist portion.

DETAILED DESCRIPTION

It has been found in chair constructions of the type shown in U.S. Pat. No. 3,133,765, utilizing a material with elongation characteristics too limited to permit the degree of flexing required without exceeding the recommended limits thereof, where, when using any such materials, flexure occurs in the cupped waist portion of the chair shell between the seat portion and the back portion during use under loading conditions, that severe strains have been concentrated in the waist region tending to produce surface fissures that in time may develop into cracks and fractures appreciably shortening the useful life of the chair shell. The prior practice was to provide flanges extending along at the edges of the waist portion and marginally along the adjacent edge portions of the seat and back to reinforce and bolster the shell at this point. Typical of such prior art construction is the flange 16, seen in FIG. 11, at the margin of the waist portion of shell 18. The fissures mentioned generally occurred in the region indicated 20 in FIG. 11. Their occurrence becomes particularly apparent when the shell is flexed in directions tending to open the ell-shaped whereupon the forces applied to the shell tend to close the ell-shape. In that case the surface 20 is subjected to compressive forces tending to reduce its surface dimensions. An alternate solution to the problem is illustrated in FIG. 12 in which, in lieu of the flange 16, as shown in FIG. 11, the chair shell 22 at the marginal edges of its waist portion has a thickened edge 24 materially greater than the medial thickness of the waist 22 to resist the flexural and torsional forces. In the region 26 of shell 22 it was noted, nevertheless, that the fissures and fractures mentioned likewise formed too early in the life of the shell. Extensive study and an analysis has disclosed that in the structures of FIGS. 11 and 12 under flexure of the seat and back portion, forces were applied greatly exceeding the practical safe limits of structural strength of the fiberglass-reinforced molded material of which the chair was formed. It has been determined that the practical limits of such material in elongation should not exceed the range of 3,000 to 4,500 micro inches per inch of surface which is equivalent to approximately 15 percent of the ultimate tensile property of said material used. With structures such as shown in FIGS. 11 and 12, surface elongation has been determined to be materially greater and exceeding the stated range by two to three times.

Obviously the chair shell can be made so rigid and be so inflexibly mounted in a chair frame that no flexure in the waist portion between the seat and the back portion is permitted. This, however, defeats the purpose of this chair design which is purposely flexible. The seat portion and the back portion are mounted in supporting frame structure at two points laterally spaced apart and located beneath the seat portion and at a single centrally located point behind the back portion. Thus, when a person seated normally erect in the chair tends to straighten his legs with respect to his back, the forward edge of the seat is deflected downward and the upper part of the back is deflected up and rearward with a resultant tendency to increase cupping of the waist portion. Alternatively, if a seated person tends to sit in a more erect than normal position, placing the weight of the body more rearward on the seat, the front of the seat portion tends to rise under his knees and the upper back tends to move downward and to swing forward against the shoulders as the normal ell-shape of the chair shell is somewhat closed. This results in flattening or less cupping of the waist portions.

Referring to FIG. 1, the chair comprises supporting frame structure including an upright column 30 having splayed feet 32 to bear on the floor. Rising above column 30 and extending rearward is an ogee-shaped arm 34 which is medial of the frame. Forward curving support arm 36 is bifurcated to provide two arms 38 that spread from the central column 30 forward and laterally to support the seat. Central arm 40 medial of flared arms 38 carries a stop or bumper 42.

The chair shell 44 comprises a seat portion 46 and a back portion 48 integrally joined by the concavoconvex flangeless waist portion 47. A flexible mount 50 is secured to the center back 48 and is also secured to bar 34 by a fastening 52. In a similar manner each of the arms 38 is likewise secured beneath the seat portion at points laterally spaced apart with respect to the median of the shell 44 and approximately in the fore-to-aft middle of seat portion 46. Thus, it will be seen that the forward edge of seat 46 is cantilevered with respect to the remainder of the structure. Bar 40 and the bumper 42 carried thereby may be used to limit downward deflection of seat 46. When the chair is used as described above, a chair shell designed to accommodate predetermined stresses and strains may have applied to it forces that exceed the structural limitations and flexural characteristics of the material. As an example, an overweight person or, as is more likely, a normal weight person may sit on the chair in such manner that an excessive amount of loading force is applied to the forward portion of the seat ahead of the pivots thus tending to longitudinally straighten the waist portion. This may subject the skin surfaces of the shell to excessive compressive and tensile forces. It is to avoid such overstressing and overstraining of the shell that bumpers 42, as mounted on bars 40, may be provided. The spacing between the bumpers 42 and the bottom of the shell in the "at rest" position permits the designer to vary the stopping action of the bumper with respect to any particular shell design. It is, of course, possible that a shell may be designed with such heavy cross sections that the stops may not be needed to prevent excessive forces. This, however, defeats the purpose of the invention because such a heavy shell would not have the desired flexure characteristics.

FIG. 2 shows a preferred form of fiberglass-reinforced molded shell as it would appear when flattened to schematically depict its shape and structural thicknesses. The shell is formed by laying up fiberglass-reinforcing filaments and fluid plastic material prior to molding in a variety of thicknesses in the pattern shown in FIG. 2. Extending from the central portion of the back 48 through the concavoconvex waist portion 47 and into the seat portion 46 is an area 60 which desirably has a final molded thickness in the range of between about 0.175 inch to about 0.250 inch. Outward of and surrounding area 60 is area 62 which has a final molded thickness in the range of between about 0.090 inch and about 0.120 inch. For the purposes of resisting fracture and avoiding damage at the edges of the shell 44, such as might occur when the chair is overturned and said edges would strike a hard surface, edge areas 64, 66, 68 and 70 are built up to a substantial thickness comparable to area 60.

In a preferred form of chair shell found entirely practical, area 60 has a thickness of 0.109 inch, area 62 has a thickness of 0.120 inch and the marginal area 72 of the waist 47 has a thickness of 0.100 inch. At the points of attachment to the mounts 50 and 54 the shell is thickened to 0.300 inch in areas 74 and 76, in the latter of which is mounted stud 78, as seen in FIG. 5, by which mount 54 is attached to seat portion 46. By slightly downwardly curling the edge 73 of the waist margin, the points of highest strain are shifted inward of the outer edge and thus the outer edge elongation strain reduced.

It will be noted, with reference to FIG. 2, that area 60 crosses the waist portion 47 between the back portion 48 and seat 46 and that in the seat area it is bifurcated or forked as it progresses from the waist area toward the point of attachment of mounts 54. Further, in the preferred form shown in FIG. 2, area 60 in the bifurcated portions tends to progress toward the forward edge of the seat portion and then to be joined across the front of the seat portion. This is for structural purposes of reinforcing the front edge of the seat.

It will be understood that the areas depicted in FIG. 2 are schematic and that adjacent areas flair one into the other. During the laying-up or forming process, the fiberglass reinforcing is laid down to progressively merge between the main areas. In other words, there are transitional tapering areas between the indicated principal areas for the purposes of producing a smooth surfaced shell in its final molded shape and for the purposes of avoiding abrupt changes in dimensional thickness to avoid concentrations of undesirable forces. It will, of course, be apparent to those skilled in the art, that this chair shell may also be formed by well-known molding techniques.

It is believed that the principle on which this chair shell design functions can best be understood by reference to FIG. 10. In FIG. 10 a hollow-walled hemisphere in the nature of a flexible hemispherical shell, for example, a half a rubber ball, is shown. When outward force is applied at two opposite points as indicated, the result is a distortion of the shell at the two intermediate opposite points in opposite directions, inward as indicated by arrows. In other words, as the extension forces tend to displace two opposite sides outward, the two intermediate sides tend to cup in and be stretched. Conversely, when force is applied in the opposite manner, there is a resultant outward movement of the intermediate sides. A similar reaction has been found to occur in the concavoconvex waist portion of fiberglass-reinforced plastic chair shells of the type disclosed. This is illustrated in FIG. 6 wherein when the back and seat portions are closed together, the waist 47 tends to assume a broader curvature indicated by dotted line 80; and when the back and seat portions are flexed tending to open the normal ell-shape, the waist portion assumes a lesser curvature indicated by line 82. These changes in curvature at the waist portion reduce the application of forces at the waist margin that would otherwise produce the objectionable fissuring or fracturing.

In FIGS. 7, 8 and 9 is shown an alternative construction in which the chair shell 90, which may be of substantially uniform thickness throughout, is provided with auxiliary flexible means that bolster and reinforce the shell between its point of attachment to the resilient mount 50 and its points of attachment to the resilient mounts 54. A flexible preformed arcuate resilient metallic member 92, having the upstanding portion 94 and the bifurcated legs 96 and 98, is provided. Member 92 is secured mechanically or adhesively in close proximity to the back and underside of the shell 90 separated from the chair frame structure by the resilient mounts 50 and 54,54.

Member 92 has a predetermined flexibility of such a nature that it, added to the flexibility of the shell 90, in the medial waist portion of said shell, is less than the flexibility of the otherwise unsupported margins of the waist portion 47 which is preferably flangeless. In this arrangement there is appreciably less resistance to flexure at the waist margins than in the stiffer medial portion of the waist thus permitting flexing without undue elongation at the waist margins.

Further modes of stiffening or strengthening a shell of substantially uniform thickness is shown in FIG. 13. The shell 100 has a main rib 102 and, as desired, supplementary ribs 104 extending through the back side of the waist portion of the shell. By proportioning ribs 102, 104, various degrees of stiffness or resilience can be obtained without interfering with waist cupping while avoiding the application of excessive elongation forces at the margins of the waist.

In the foregoing has been set forth the details of a preferred embodiment of the invention. It will, of course, be recognized by those skilled in the art that alterations and modifications without departure from the principles of the invention may be made.

Claims

What I claim is:

1. In a chair, comprising:

an ell-shaped chair shell of fiberglass-reinforced molded plastic material having a seat portion and a back portion integrally joined thereto by a flexible concavoconvex waist portion;

said shell being secured by resilient mounts to supporting frame structure at two points laterally spaced apart and located beneath the seat portion and at one centrally located point behind the back portion;

the resilient mounts as said points permitting the concavoconvex waist portion to cup as the relative positions of the seat and back portions change under loading; and

the improvement, comprising: flexible means separated from said frame by said mounts and in close proximity to said shell;

said flexible means extending between said one centrally located point and each of said two laterally spaced apart points and imparting to said shell, medial of said waist portion, a flexural rigidity appreciably greater than the flexural rigidity of the margins of said waist portion.

2. A chair according to claim 1 in which said flexible means comprises a metallic member positioned closely adjacent the back and bottom surfaces of said shell.

3. A chair according to claim 1 in which said flexible means comprises fiberglass-reinforcing structure incorporated integrally in said shell and thickening the same medial of said waist portion relative the margins thereof.

4. A chair according to claim 3 in which said shell, medial of said waist portion has a molded thickness in the range between about 0.175 inch to about 0.250 inch and the margins of said waist portion have a molded thickness in the range between about 0.090 inch and about 0.120 inch.

5. A chair according to claim 4 in which said shell, medial of said waist portion has a molded thickness of about 0.190 inch and the margins of said waist portion taper therefrom to a thickness of about 0.100 inch.

6. A chair according to claim 1 in which said frame structure includes stop means operable to limit downward deflection of the forward seat portion.