Foldable ladder
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a foldable ladder with collapsible crosspieces having a 90-degree pivot in the middle, the crosspieces being mounted to the sidepieces likewise with 90-degree pivots. 
     2. Description of the Related Art 
     German Disclosure Publications 29 23 397 and 33 20 519 represent art related to the present invention. 
     German Disclosure Publication 29 23 397 relates to a folding ladder with two sidepieces, to each of which are hinged two crosspieces. The crosspieces consist of two parts directly hinged together. The individual swivel axes lie parallel to each other and perpendicular to the plane of the ladder, with a rigid, preferably tubular, bar connecting the hinge points of the crosspieces to each other and with a support frame placed below the lowermost crosspiece. The support legs of the support frame are longer than the crosspieces and are hinged to the sidepieces as well as to each other. The tip of the support frame is placed over the ends of the support legs and provides a bearing surface for the lower end of the rigid bar when the ladder is fully expanded. The swivel axes of the legs of the support frame at the sidepieces are placed at immovable points with respect to the sidepieces, and the swivel axis in the region of the hinging of the two support legs is movable with respect to the rigid bar. The abutment face formed by the tip of the support frame is still movable with respect to the rigid bar. 
     Thus, with this folding ladder of known construction, a central expanding bar has been provided which is not a push rod and is not pushed inside of the sidepiece. 
     German Disclosure Publication 33 20 519 discloses a folding ladder in which the crosspieces are folded in the middle and are hinged to the sidepieces. It is equipped with centrally located locking rods, which do not constitute a push rod and are not placed or pushed inside a timepiece. 
     It has been shown that prior art folding ladders cannot always satisfy practical needs. This applies in particular to the execution of a completely uniform double-up movement. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a foldable ladder which enables the crosspieces to double up while at the same time presenting no impediment to a person ascending the ladder. 
     The object of the present invention is achieved with foldable ladders of the type mentioned in the introduction by fitting to the right and left of the crosspiece stays a cross-piece connecting push rail into each sidepiece. 
     As load points for the crosspieces, a distance roller may incorporated into the middle hinge as well as into end hinges, supporting the extended crosspiece. In the center of the crosspiece, the step stay on both sides may be formed such that the two stays abut firmly against each other, supporting the pressure point at the crosspiece ends. 
     Preferably, the ascending rails are U-shaped and the material thickness, in conjunction with the likewise U-shaped crosspiece stays, is selected so that the total stability conforms to the norms adopted for ladders, with appropriate fitted parts ensuring the precision and safety. 
     One of the main objects of a foldable ladder embodying the invention, in addition to being easily accommodated in the limited space of a motor home, boat or in the house, is that it must also be easily transportable, since the foldable ladder in its folded state displays, roughly, the form of a &#34;bar&#34; with a rectangular cross section. 
     An aluminum alloy is the preferred material for the components of the foldable ladder, and the rivets preferably consist of chromium nickel steel. Aluminum provides the desired corrosion resistance and weight savings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The details of the instant invention will be discussed in greater detail with reference to the accompanying FIGS. 1 through 10, which represent preferred embodiments, in which: 
     FIG. 1 is a front view of a portion of a foldable ladder according to a first preferred embodiment of the present invention in a working position; 
     FIG. 2 is a front view of the foldable ladder of the FIG. 1 embodiment in a partially folded-together position; 
     FIG. 3 is a front view of a completely folded-together ladder of the FIG. 1 embodiment; 
     FIG. 4 shows a connecting push rail for connecting the crosspieces used in the foldable ladder of the FIG. 1 embodiment; 
     FIG. 5 is a front view of a ladder-base section of the foldable ladder of the FIG. 1 embodiment with bracketing rods; 
     FIG. 6 is a top view of a lower extended crosspiece of the FIG. 1 embodiment; 
     FIG. 7 shows the ladder completely folded together as a bar, viewed from above in cross section; 
     FIG. 8 is a top view of another embodiment of the present invention; 
     FIG. 9 is a front view of the embodiment of FIG. 8; and 
     FIG. 10 is a top view of the embodiment of FIG. 8 in the completely folded position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIGS. 1 through 4, a foldable ladder (1) includes a plurality of collapsible rungs or crosspieces (3), each of which has a central hinge (4) and two opposite hinges (9) having pivot axes (6) which are mounted inside two parallel rails or sidepieces (10). 
     In FIG. 2, the pivotal movement of crosspiece segments 3a which causes each crosspiece to &#34;double up&#34;  is synchronized from crosspiece to crosspiece (3) by a means of two push rails or linkages (2) on the left and right and tied by rivet joints (19) to the crosspiece segments 3a whereby the intersection point of pivot axes (19) to the push-rail rivets (16) must be positioned with great precision. 
     FIG. 1 shows the ladder (1) positioned so that crosspiece segments 3a are firmly anchored with flat end surfaces (7) in the working position (with the crosspieces in the extended position) abutting flat inner surfaces (10a) of the sidepieces (10). Opposite flat end surfaces (8) of a pair of crosspiece segments 3a also abut at the central hinge (4) above the distance rollers (5) to reinforce and maintain the horizontal disposition of the crosspieces (3) provided by the coaction of the hinges (9) fitted inside the sidepieces (10) at the opposite ends of the crosspieces (3). 
     In FIG. 3, the ladder can be seen in its folded position as a space-saving rectangular bar, with opposite sides (17) of the sidepieces (10) juxtaposed. 
     Referring to FIGS. 5 and 6, each of the sidepieces (10) is U-shaped in cross-section and includes an end wall (10a) and two, parallel sidewalls (10b) and (10c). The linkages (2) have bottom ends (2a) which abut the block (12) when the crosspieces are in the horizontally extended position. Each crosspiece segment (3) has a top wall (3b) and two opposite sidewalls (3c). When the crosspieces are in the horizontally extended position, the linkages (2) slide in between and in contact with opposing surfaces of the sidewall (10c) of the sidepieces (10). Moreover, the pivot points (2b) of the linkages (2) are vertically aligned with the blocks (12) and spaced inwardly from the vertically aligned side pivot points (10d) of the sidepieces (10). A locking detent (20) may be used to provide a locking capability. Handles (15) facilitate carrying the folded ladder. 
     In FIG. 4, a linkage (2) is shown with rivets (16). 
     According to FIG. 5, in the working position of the foldable ladder the bracketing rods (21) are anchored by the central holding bracket (22). The rods (21) are locked into position in the ladder-foot base portion (23) for support by pins 23a, whereby a counterbrace is provided which tends to steady the ladder. The crosspieces are stabilized by the distance roller (5) which includes a locking core pin (30) which stops rotational movement when rotated about pin (28) to the position shown in FIG. 5. The locking pin (27) connects the connecting rods (21) to the crosspieces (3a) through the roller (5), thus providing means for locking the ladder in the extended position. 
     In FIG. 6, viewed from above, spacers (26) are shown on opposite sides of the crosspiece (3) for determining the distance between the U-shaped sidepieces (10). The crosspiece linkages (2) serve to raise and lower synchronously the crosspiece segments on both sides. The rails are propped in the extended state for support, as shown in FIG. 1 by ends (13) of the push rails (2) abutting blocks (12). 
     FIG. 7 shows the folded-together ladder, as viewed from above, as being almost square in cross section. The middle support roller (5) is shown at (24) providing for stabilization. The movable linkages (2) by means of the spacers (26), slide into the sidepieces (10). The round middle point is the end detent. The U-shaped sidepieces (10) are shown pushed together. 
     FIGS. 8-10 show further embodiments of the invention by way of example. 
     In FIGS. 8 and 9 are shown: at (31) the sidepieces and at (32) the collapsible crosspieces, at (33) the displaced linkages, at (34) the hinge rivets determining the precise distance from crosspieces to crosspieces for the uniform pushing of the crosspieces, at (35) the spacers fitted to the linkage on the pivot pin (39) connecting (31) and (32), and at (36) spacers for the mutual displacement of the crosspieces, at (37) a support distance roller, at (38) the pin connecting the crosspieces. Pivot pins (39) anchor the crosspieces to the sidepieces. Pivot pins (34) ensure that the linkages connect the crosspieces at all points of the 90-degree torque for synchronous up-and-down movement whereby in the end folded-together position the linkages protrude slightly into the opposite sidepieces, as shown in FIG. 10. At (41) is disposed a detent for the extended crosspiece. In this way, the stabilization of the bar-shaped ladder as a folded-together ladder is also shown.