US3134987A

US3134987A - Compact cot

Info

Publication number: US3134987A
Application number: US300577A
Authority: US
Inventors: Wallace H Bertram
Original assignee: Individual
Current assignee: Individual
Priority date: 1963-08-07
Filing date: 1963-08-07
Publication date: 1964-06-02
Anticipated expiration: 1981-06-02

Description

June 2, 1964 w. H. BERTRAM COMPACT COT Filed Aug. 7, 1965 2 Sheets-Sheet l llll l m INVENTOR.

, /f//Mf H 55477/2444 BY FJH M June 2, 1964 Y w. HfBERATRAM l 3,134,987

COMPACT 00T Filed Aug. 7, 1963 2 Sheets-Sheet 22V INVEN TOR. W /rf 55;?7914/14 United States Patent O 3,134,987 CGMPAC'I CGT Wallace H. llertrarn, South End New York Ave., La Crescente, Calif. Filed Aug. 7, 1963, Ser. No. 399,577 I4- Claims. (Cl. 5-111) This invention relates to folding or collapsible cots and, in particular, to a simple lightweight cot which can be easily folded or dismantled into a relatively small compact unit for transportation or storage. rIhis application is a continuation-impart of my prior copending application Serial No. 87,884, filed February 8, 1961.

Portable beds or cots of one type or another have been in use for many years. However, these prior art cots all possess one or more common drawbacks which hamper their widespread use in conjunction with such outdoor activities as mountain climbing, fishing, boating, hunting, camping, flying, trailering, and the like. Furthermore, there is a need, not satisfied by existing cots, for an inexpensive collapsible cot to be used as an extra bed for home use, and as an emergency bed for disaster use, particularly in conjunction with civil defense bomb shelters. Common defects in prior art cots include a lack of compactness, expensiveness, heaviness, complexity in construction, and difficulty in assemblying and disassembling.

It is accordingly an object of this invention to provide an improved collapsible cot.

Another object of this invention is to provide a folding cot of sturdy construction having a minimum of easily assembled parts which can be quickly assembled with little effort to form a neat appearing bed.

Another object of this invention is to provide a lightweight cot which when folded occupies a relatively small storage space whereby the folded cot can be easily stowed and transported.

A furthe-r object of this invention is to provide a cot which is sturdy, lightweight, foldable, easily dismantled, durable, and which is simple and inexpensive to construct.

Another object of this invention is to provide an improved body-supporting cot with supports which can be quickly attached or detached and which utilizes self-leveling side rails with a body-supporting strip or sheet of iiexible material stretched between them.

A still further object of this invention is to provide improved connections for furniture members.

Other and related objects will be apparent from the detailed description of the invention and various advantages not specifically referred to herein will be apparent to those skilled in the art on employment of the invention in practice.

I have now found that the foregoing objects and their attendant advantages can be particularly realized in a folding cot incorporating a pair of substantially parallel sectional side rails with a strong body supporting sheet stretched between them, the sections of the side rails being hingedly connected so that they can be folded parallel with each other to reduce the over-all cot length to about one-third of its extended length. The central cot supports readily attach to or detach from the side rails by means of unique furniture member connections and provide vertical support for the body-supporting portion of the cot. The cot end supports are spreader bars beneath the body-supporting sheet which connect the side rails near the cot ends and provide both vertical support and transverse bias of the body-supporting sheet stretched taut between the side rails. These end supports, interposed between the side rails, can be easily slipped oit the connectors or lugs on the side rails and are held in place primarily by the biasing action of the taut body-supporting sheet.

The invention will be more readily understood by ice ' URE 1;

FIGURE 3 is a side view of one of the central supports; FIGURE 4 is a side view of one of the tensioning'end supports;

FIGURE 5 is a plan view of a cot side rail in the folded position;

FIGURE 6 is an enlarged side view of the leg of one of the central supports connected to a side rail;

FIGURE 7 is an enlarged plan view of the leg of one of the central supports connected to a side rail as shown in FIGURE 6;

FIGURE 8 is a side view of the plastic connector of FIGURE 7;

FIGURE 9 is a side view of the tube cap shown in FIGURES 1, 2, 3, 5, 6 and 7;

FIGURE 10 is an enlarged plan View of the leg of an end tensioning support connected to an end rail section;

FIGURE 11 is a view in cross section taken on line A-A of FIGURE 10;

FIGURE 12, an enlarged plan View of the leg of one of the central supports connected to a side rail, is a modification of the joint embodiment illustrated in FIG- URES 6 and 7;

FIGURE 13 is a side view of the leg of one of the central supports connected to a side rail shown in FIG- URE l2;

FIGURE 14 is a plan view of a modification of the joint embodiment illustrated in FIGURES 12 and 13;

FIGURE 15 is a side View of the joint embodiment shown in FIGURE 14;

FIGURE 16 is a side View of the modified end of a central support leg shown in FIGURES 12, 13, 14 and 15; and

FIGURE 17 is a plan view of the end of the central support leg shown in FIGURE 16.

Referring now more particularly to FIGURES 1, 2, 3, 4 and 5, the cot there shown has a pair of tubing side rails 18 comprising center rail section 20 and two substantially identical end rail sections 22. End rail sections Z2 and center rail section 2li arepreferably equal in length as here shown, but other arrangements are satisfactory as long as the side rail sections are symmetrical, that is, like sections of both side rails I8 are substantially identical. Each end rail section 22 is pivotally connected at its inner end to an end of center rail section 20 by sturdy parallel substantially

vertical hinge members

24 and 26 which hold these side rail sections in alignment with each other and with the interposed leg 38 of central support 36. Pivotally connected hinge members or

side llanges

24 and 26 also allow the side rail sections to be folded together easily in order to store the cot. Stretched between the substantially parallel side rails 18 is a flexible body-supporting sheet Z8 which provides the resting or reclining surface of the cot. Sheet 28 is secured to side rails 18 as shown in FIGURE l by wrapping the longitudinal margins of sheet 28 around side rails 18 and firmly stitching the margin to the main body of sheet 28 with suitable strong longitudinal stitching 30, thus forming loops for side rails I8 to slide through. The loop openings, formed in sheet 28 at the cot ends, are partially closed by stitching 32 thereby forming a pocket into which the extremity of end rail section 22 lits thus preventing substantially rectangular sheet 28 from slipping longitudinally on the side rails While the cot is in use. Thus,

each side rail 18 is connected to the ends of a longitudinal margin of sheet 28 in such a manner that sheet 28 is biased or stretched longitudinally. Reinforced

notches

33 and 35 in sheet 2S straddle the connection points of side rails i8 and supports 34 and 36, thus preventing fabric wear or strain at these points. Sheet 28 can be made of any suitable flexible cloth, web, or fabric such as canvas, sailcloth, synthetic materials such as nylon cloth and woven plastic fabric, and the like.

Body-supporting sheet 2S is transversely stretched and the pair of side rails 13 are supported and held in spaced apart relationship by detachable tensioning end supports 34 and central supports 36. Central supports 36, which are spaced apart U-shaped tubing members each comprising substantially parallel legs 33 and base 49 (bight of U-shapcd support), support the central portion of the cot in this embodiment by means of support rivet 42 located intermediate their length. The tubing ends of legs 38 of central supports 36 are closed by tightly fitting rounded plastic caps 44, shown in FIGURE 9, as are the extremities of end rail section 22 of side rails 18. Caps 4-4 prevent dirt from entering the tubing ends and further protect users of the cot from injury in assembling and using the cot. A number of different materials can be used in manufacturing caps 44 such as plastic, rubber, wood, metal and the like. However, cap 44 can be eliminated if the tubing end is formed so as to close or partially close the end and thus provide a non-injurious, smooth or rounded surface as shown in FIGURES 16 and 17.

In the assembled position of the cot, the ends of legs 38 of central support 36 extend through the opening between the hingedly connected end rail section 22 and center rail section 20 as shown in FIGURES 6 and 7. Cap 44 facilitates the insertion of leg 38 into this snug opening between center rail section 29, end rail section 22, hinged 24, and linge 26. Support rivet 42 which passes through a diameter of leg 38, has ends projecting on each side of leg 3S and has the mid-sections of

hinges

24 and 26 resting upon these projecting ends. Thus, support rivet 42 comprises support means for supporting side flanges or

hinges

24 and 26 on leg 38 of central support 36.

Hinges

24 and 26 are pivotally connected to each other and to end rail section 22 by means of plastic connector 50. Connector Sti can be made of any suitably strong structural material, but a hard resilient plastic material such as polyethylene, polyvinyl chloride, and the like are preferred. Connector Si), shown in detail in FIGURE 8, has cylindrical portion 52 at one end, shaped to t tightly into the end of

tubing rail sections

20 and 22, and a mating surface 54 at the tip end which is shaped to mate with the outside surface of leg 3S. The cylindrical portion 52 of connector Sti extends far enough into the end of

rails

28 and 22 so that rivets 46 and 4S pass through the opening 55 in connectors 50, thus holding connectors Si) securely in place. Connector Sii is preferably made of plastic as here, but a modification entails connector 50 being manufactured from metal, wood, hard rubber and the like. In some cases the connecting ends of

side rail sections

20 and 22 can be shaped so as to integrally include the mating surface 54 of connector Sii.

An important feature of this invention is the abovedescribed means for pivotally supporting side rails 18 on central supports 36, thus providing a self-adjusting furniture member connection. Instead of stressing the individual connecting members as is conventional, the unequal loads typical of body supporting cots are carried to side rails 13 by sheet 28 and this loading is then transmitted by

hinges

24 and 26 directly as a vertical force component to legs 3S of central support 36 via support rivet 42 in this embodiment. In other words,

side rail sections

20 and 22 are maintained substantially parallel while in use, although occasionally vertically displaced slightly from each other on legs 38. Pivot hinges 24 and 25 of this invention thus hold side rails 1S substantially horizontal at all times even with an unbalanced loading. The effect of this pivot hinge connection is to substantially eliminate the rocking which is characteristic of most conventional cot structures.

Tensioning end supports 34 are generally U-shaped and each comprise diverging legs 66 depending from each end of base 68 (bight of support 34). On the assembled cot, each leg of end support 34 is inclined inwardly and downwardly from side rails 18 toward the longitudinal center line of body-supporting sheet 23. Thus, legs 66 project upwardly from base 68 at an angle of approximately 45. This angle of divergence is not critical, but some angle less than 90 is required in order to provide sufficient transverse tension to sheet 2S. When legs 66 of end support 34 are removably connected, as hereinafter described, to end rail sections 22 of parallel side rails 1S, support is provide for the end portions of the cot and sheet 28 is put into the required transverse tension needed to hold sheet 28 taut when supporting a body.

Tensioning end supports 34 provide the required transverse tension for the cot by connecting to the angularly disposed connector 58 as shown in FIGURES 10 and Il, the free cylindrical end of which fits snugly into the tube end of leg 66. Connector 58, usually made of a hard resilient plastic such as the polyethylene used herein, is held i'ixediy to end rail section 2t) near the end of the cot by means of rivet 69. Connector 58 is identical in structure to the previously described connector 50 except there is no opening 56 in connector 5S. rI'he end of leg 66 of end support 36, fitting snugly onto connectors 5S attached to end rail sections 22, puts sheet 2S into lateral tension, stretching it taut between side rails i8. Longitudinal tension for sheet 2S is provided by putting the capped ends of one end rail section 22 of a side rail 18 into the pocket at one end of sheet 28 formed by stitching 32 and stretching sheet 23 longitudinally until the end of the other end rail section 22 can be put into the pocket at the other end of sheet 28. Although connectors 5S depend from end rail section 22 at an angle of approximately 45 downwardly from the plane of sheet 23, the angle of 45 is not necessarily critical to operability as discussed above in relation to leg 66 of end support 34.

Because of the pivotal or hinged connection between center rail section 29 and end rail section 22, the sections of side rails 18 are free to fold over each other into a compact structure. This cot folding, with end supports 34 and central support 36 detached, can be done either with sheet 2S in place on side rails 18 or with sheet 28 removed as shown in the view of a folded side rail 18 in FIGURE 5.

Referring now more particularly to FIGURES 12 and 13, the apparatus there shown comprises another embodiment of my pivot hinge connection as shown in detail in FIGURES 6 and 7. The apparatus of FIGURES 12 and 13 is identical in structure and operation to that of FIGURES 6 and 7 with the following exceptions. Support rivet 42 has been omitted and another support means, yoke 74, substituted therefor. Yoke 74 is a substantially ilat horizontal bar integrally connected to the top edges of hinge 24 and hinge 26. Another exception in this alternative embodiment is that the end of leg 38, FIG- URES 16 and 17, is formed to have a rounded surface, thus substituting for cap 44 of FIGURES 6 and 7. Rounded tip 70 of leg 38 has a generally round opening as shown in FIGURE 17. Of course, either cap 44 or round tip 70 can be used satisfactorily for the end of leg 38 in this embodiment.

The support means shown in FIGURES 12 and 13 thus operates in a similar fashion to that previously described in relation to FIGURES 6 and 7. Legs 38 in this embodiment pick up the vertical loading from the pressure of yoke 74 bearing on tip 70. Thus, as with the previously described embodiment of FIGURES 6 and 7, unequal loading typically found with the use of body supporting cots is carried to side rails 18 by sheet 28 and this loading is then transmitted by yoke 74, connecting hinges 24 and 26 to each other, to tip 70 of leg 38 as a vertical force component. Here again,

side rail sections

20 and 22 are maintained substantially parallel while in use, although occasionally they are vertically displaced slightly from each other on legs 33. Yoke 74, of course, can rock back and forth on tip 70 to compensate for any movement vertically of the ends of

side rail sections

20 and 22 on legs 3S.

A further embodiment of the apparatus of FIGURES 12 and 13 is shown in FIGURES 14 and 15 wherein the structure is identical to that of FIGURES 12 and 13 except substantially at horizontal yoke 74 is replaced by yoke 76 which has a convex lower surface which tits partially into opening 72 in tip 70 of leg 38. Thus, yoke '76 can pivot on tip 70 of leg 38 in much the same manner as yoke '74 does except there is a partial mating with yoke 76 and opening 72 which provides more bearing surface than the point type contact of yoke 74.

The supports and side rail sections of the cot of this invention can be made of any suitable material which has the necessary strength, but, preferably are constructed of tubing made from a lightweight material such as aluminum or its alloys which are used in aircraft manufacture, or magnesium alloys. Thin wall steel tubing can also be used in order to provide a cot which is strong yet light in weight. The typical Weight of a cot of this invention for supporting body loads up to at least 300 pounds is about three pounds. Supports 34 and 36 are preferably reinforced at the point where they are bent by inserting a piece of smaller tubing into the section to be shaped whose outside diameter conforms to the inside diameter of the support tubing. The length of the reinforcing tube should be such as to include substantially the entire curved portion of the support. Typically, the support and side rail tubing is 0.750 inch outside diameter and has a wall thickness of 0.035 inch. For central support 36, which has a 90 bend, the bend radius typically would be about 2 inches. Therefore, for reinforcing, about 3 inches of tubing, having an 0.035 inch Wall and an outside diameter of 0.715 inch, is inserted into the bend area prior to bending. The result of this reinforcement is that the circular cross section of the support tubing does not change substantially, resulting in a stronger support with little added weight or cost. A similar reinforcing is used for end supports 34.

A further feature of this invention is that the cot protects itself from overloading in that the typical support heights combined with the flexibility of the side rails, cooperate synergistically with the body-supporting fabric to yield suiciently to rest excessive loads on the cot-supporting surface, thus never permitting overstress of the frame or fabric. Also, the heavy or low points on a reclining adult body are spaced on my cot to fall between the supports, thus resulting in a more even distribution to the several supports. A typical cot of this invention is about 6 feet 5 inches long, 26 inches Wide and about 6 inches high. Preferably, the overall height of a 26- inch cot will not exceed 8 inches. As the Width of the cot increases the height should increase so that a SO-inch wide cot might typically have an S-inch height, and preferably would not exceed 10 inches.

The cot of this invention, because of its inherent flexibility, readily contours to the slope of the ground or other supporting surface with no rocking. This ability to conform to the supporting surface is accomplished with no loss in comfort or structural rigidity since the slidably self-adjusting side rails allow shifting of the support-side rail juncture with no loss of stability.

The symmetry of the cot of this invention contributes markedly to its economy, collapsibility, ready assembly, and to its ease of storage. The side rails and legs can be bundled together and wrapped or rolled up in sheet 28 to form a compact package. A typical disassembled cot packs into a rectangular container 27 inches long, 7 inches wide, and 31/2 inches deep. From the foregoing description, it is apparent that a cot has been provided which is simple, strong, light in weight, compact when folded, and is constructed and arranged so that it is convenient to fold and unfold, Furthermore, the cot of this invention provides a comfortable and durable piece of camp furniture which can be easily transported, set up, and knocked down, yet suiting the comfort of the most discriminating user.

Various other changes and modifications are apparent from the description of this invention and further modifications will be obvious to those skilled in the art. Such modifications and changes are intended to be included within the scope of this invention as defined by the following claims.

I claim:

1. A folding cot comprising:

a pair of side rails, each side rail having a middle section and a pair of end sections at opposite ends of said middle section, said end sections each having an inner end and an outer end;

a flexible body-supporting sheet connecting said side rails;

hinges connecting each end of said middle section to the inner end of one of said end sections, the tip of said ends of said middle section and the tip of said inner ends of said end sections being shaped so as to have a surface which mates with a center support hereinafter described, each of said hinges including a pair of substantially parallel side anges and a first pivot means extending through one end of said side franges and the end of a middle section interposed between said side flanges and a second pivot means extending through the other end of said side flanges and the inner end of an end section interposed between said side flanges;

a pair of center supports located beneath said side rails and said body-supporting sheet, each center support comprising a substantially U-shaped member having a base and legs, the end portion of each of said legs extending above said side rails and being in removable sliding relationship between said mating surfaces of said tips of said middle section and said end section, said mating surfaces being located between said side flanges so that a load on said bodysupporting sheet will force substantially the entire mating surface of said tips of said end and middle sections to tightly engage said end portion of said legs;

support means for supporting said side flanges on said end portion of each of said legs of said center supports;

and a pair of end supports having a base with upwardly diverging members at each end of said base, said end supports being interposed between said side rails to releasably hold them in spaced apart relationship and to stretch said sheet between said side rails, said diverging members of said end supports being removably connected near the outer end of said end section of said side rail, said base of said end support being located at substantially the same level beneath said body-supporting sheet as the base of said U-shaped center support.

2. A folding cot comprising:

a pair of side rails, each side rail having a middle section and a pair of end sections at opposite ends' of said middlersection, said end sections each having an inner end and an outer end;

a ile'iible body-supporting sheet connecting said side rai s;

hinges connecting each end of said middle section to the inner end of one of said end sections, the tip of said ends of said middle section and the tip of said inner ends of said end sections being shaped so as to have a surface which mates with a center support hereinafter described, each of said hinges including a pair of substantially parallel side flanges and a first pivot means extending through one end of said side anges and the end of a middle section interposed between said side flanges and a second pivot means extending through the other end of said side anges and the inner end of an end section interposed between said side tianges;

a pair of center supports located beneath said side rails and said body-supporting sheet, each center support comprising a substantially U-shaped member having a base and legs, said legs having projecting means located intermediate the length of said legs, the end portion of each of said legs extending above said side rails and being in removable sliding relationship be tween said mating surfaces of said tips of said middle section and said end section, said mating surfaces being located between said side flanges, and the midsection of said side tianges resting on said projecting means located on said legs of said center supports so that a load on said body-supporting sheet will force substantially the entire mating surface of said tips of said end and middle sections to tightly engage said end portion of said legs;

3. A folding cot as dened in claim 2 wherein said end sections and said middle section of each of said side rails are substantially equal in length.

4. A folding cot as defined in claim 2 wherein said side tianges of said hinges and the legs of said center supports are substantially perpendicular with respect to said body-supporting sheet` 5. A folding cot as defined in claim 2 wherein the axis of each of said upwardly diverging members of said end supports intersects the axis of said base of said end support at an angle of about 45.

6. A folding cot as defined in claim 2 wherein said connection of said end section of said side rail and said upwardly diverging member of said end support comprises:

a lug depending inwardly and downwardly towards the longitudinal center line of said sheet, said lug having a free end shaped to tit snugly into the end of said diverging member of said end support, and the end of said diverging member fitted onto said free end of said lug.

7. A folding cot as defined in claim 2 wherein each end of each of said side rails is removably attached to each end of a longitudinal margin of said body-supporting sheet so as to longitudinally bias said body supporting sheet.

8. A folding cot comprising:

a flexible body-supporting sheet connecting said side rails;

hinges connecting each end of said middle section to the inner end of one of said end sections, the tip of said ends of said middle section and the tip of said inner ends of said end sections being shaped so as to have a surface which mates with a center support hereinafter described, each of said hinges including a pair of substantially parallel side flanges connected at their top edge to a substantially tiat yoke and a rst pivot means extending through one end of said side flanges and the end of a middle section interposed between said side tianges and a second pivot means extending through the other end of said side flanges and the inner end of an end section interposed between said side anges;

a pair of center supports located beneath said side rails and said body-supporting sheet, each center support comprising a substantially U-shaped mcrnber having a base and legs, the end portion of each of said legs extending above said side rails and being in removable sliding relationship between said mating surfaces of said tips of said middle section and said end section, said mating surfaces being located between said side tianges, and the tip of said end portion of each of said legs positioned beneath said yoke connecting said hinges so that said yoke is resting on said tip, so that a load on said body-supporting sheet will force substantially the entire mating surface of said tips of said end and middle sections to tightly engage said end portion of said legs;

9. A folding cot as defined in claim 8 wherein said end sections and said middle section of each of said side rails are substantially equal in length.

10. A folding cot as defined in claim 8 wherein said side tianges of said hinges and the legs of said center supports are substantially perpendicular with respect to said body-supporting sheet.

11. A folding cot as deiined in claim 8 wherein the axis of each of said upwardly diverging members of said end supports intersects the axis of said base of said end support at an angle of about 45.

12 A folding cot as defined in claim 8 wherein said connection of said end section of said side rail and said upwardly diverging member of said end support comprises:

a lug depending inwardly and downwardly towards the longitudinal center line of said sheet, said lug having a free end shaped to fit snugly into thc end of said diverging member of said end support, and the end of said diverging member iitted onto said free end of said lug.

13. A folding cot as defined in claim 8 wherein each end of each of said side rails is removably attached to each end of a longitudinal margin of said body-supporting sheet so as to longitudinally bias said body-supporting sheet.

14. A folding cot as defined in claim 8 wherein there is an opening in the tip of said end portion of each 0E said legs of said center supports and wherein said yoke has a convex lower surface which partially tits movably into said opening in said tip, said convex lower surface being larger than said opening in said tip.

References Cited in the file of this patent UNITED STATES PATENTS 2,854,261 Spaide Sept. 30, 1958 2,973,888 Beardsley Mar. 7, 1961 FOREIGN PATENTS 526,373 Belgium Feb. 27, 1954 1,176,418 France Apr. l0, 1959

Claims

1. A FOLDING COT COMPRISING: A PAIR OF SIDE RAILS, EACH SIDE RAIL HAVING A MIDDLE SECTION AND A PAIR OF END SECTIONS AT OPPOSITE ENDS OF SAID MIDDLE SECTION, SAID END SECTIONS EACH HAVING AN INNER END AND AN OUTER END; A FLEXIBLE BODY-SUPPORTING SHEET CONNECTING SAID SIDE RAILS; HINGES CONNECTING EACH END OF SAID MIDDLE SECTION TO THE INNER END OF ONE OF SAID END SECTIONS, THE TIP OF SAID ENDS OF SAID MIDDLE SECTION AND THE TIP OF SAID INNER ENDS OF SAID END SECTIONS BEING SHAPED SO AS TO HAVE A SURFACE WHICH MATES WITH A CENTER SUPPORT HEREINAFTER DESCRIBED, EACH OF SAID HINGES INCLUDING A PAIR OF SUBSTANTIALLY PARALLEL SIDE FLANGES AND A FIRST PIVOT MEANS EXTENDING THROUGH ONE END OF SAID SIDE FLANGES AND THE END OF A MIDDLE SECTION INTERPOSED BETWEEN SAID SIDE FLANGES AND A SECOND PIVOT MEANS EXTENDING THROUGH THE OTHER END OF SAID SIDE FLANGES AND THE INNER END OF AN END SECTION INTERPOSED BETWEEN SAID SIDE FLANGES; A PAIR OF CENTER SUPPORTS LOCATED BENEATH SAID SIDE RAILS AND SAID BODY-SUPPORTING SHEET, EACH CENTER SUPPORT COMPRISING A SUBSTANTIALLY U-SHAPED MEMBER HAVING A BASE AND LEGS, THE END PORTION OF EACH OF SAID LEGS EXTENDING ABOVE SAID SIDE RAILS AND BEING IN REMOVABLE SLIDING RELATIONSHIP BETWEEN SAID MATING SURFACES OF SAID TIPS OF SAID MIDDLE SECTION AND SAID END SECTION, SAID MATING SURFACES BEING LOCATED BETWEEN SAID SIDE FLANGES SO THAT A LOAD ON SAID BODYSUPPORTING SHEET WILL FORCE SUBSTANTIALLY THE ENTIRE MATING SURFACE OF SAID TIPS OF SAID END AND MIDDLE SECTIONS TO TIGHTLY ENGAGE SAID END PORTION OF SAID LEGS; SUPPORT MEANS FOR SUPPORTING SAID SIDE FLANGES ON SAID END PORTION OF EACH OF SAID LEGS OF SAID CENTER SUPPORTS; AND A PAIR OF END SUPPORTS HAVING A BASE WITH UPWARDLY DIVERGING MEMBERS AT EACH END OF SAID BASE, SAID END SUPPORTS BEING INTERPOSED BETWEEN SAID SIDE RAILS TO RELEASABLY HOLD THEM IN SPACED APART RELATIONSHIP AND TO STRETCH SAID SHEET BETWEEN SAID SIDE RAILS, SAID DIVERGING MEMBERS OF SAID END SUPPORTS BEING REMOVABLY CONNECTED NEAR THE OUTER END OF SAID END SECTION OF SAID SIDE RAIL, SAID BASE OF SAID END SUPPORT BEING LOCATED AT SUBSTANTIALLY THE SAME LEVEL BENEATH SAID BODY-SUPPORTING SHEET AS THE BASE OF SAID U-SHAPED CENTER SUPPORT.