Portable wheelchairs are becoming an increasingly popular alternative over the standard rigid models for wheelchair riders. The portable wheelchairs generally have a frame which folds or collapses for easy transportation, enabling a user to conveniently travel between various locations, such as from home to work, school, restaurants, the theater or any other site of interest. Typically, the portable wheelchairs are light in weight for improved maneuverability and handling. The frame is often formed from a tubular material, such as a lightweight, high-strength aircraft-grade aluminum tubing, to reduce the overall chair weight while providing the necessary strength. In efforts to further reduce the weight of the chair, the number of components comprising the chair frame has also been reduced.
Traditionally, the folding wheelchair assemblies available in the art are "side-folding" wheelchairs which typically include two opposed side frame assemblies having upper and lower horizontally extending bars and a pair of cross braces pivoted for movement about the lower frame bars. The opposite ends of each of the cross braces are pivotally mounted to a horizontally extending seat frame rod. A flexible seat is suspended between the laterally spaced seat frame rods. When the wheelchair is deployed, the seat frame is supported on brackets carried by the upper bars of the opposed side frames. The seat frame is held by the brackets in a superimposed position above the side frame assemblies. The wheelchair frame is folded or collapsed for transport by pivoting the cross braces about the respective side frame bars, raising the seat frame and drawing the opposed side frame assemblies towards one another. Typical of patented prior art side-folding wheelchair apparatus are the wheelchairs disclosed in U.S. Pat. Nos.: 4,025,088; 4,101,143; 4,273,350; 4,371,183; 4,768,797; 4,840,390; 5,154,438; and 5,328,183.
While these scissor-like folding frames are capable of folding to a reduced dimension, they are still quite large, even in the folded state. Often the wheels and wheel hubs extend outwardly of the side frames which adds to the overall width. This is particularly true if the wheels are to remain mounted to the frame when collapsed. Further, since the seat must collapse upwardly or downwardly, the backrest frame and/or the foot rest frame are prevented from folding inwardly toward the seat without requiring a complex assembly of interengaging linkages.
Another problem associated with these side folding wheelchairs is that frames generally lack torsional rigidity since the torsional loads are focused through the hinge joints rather than through other rigid or locked members. Hence, stability is affected which limits its use. Moreover, this design makes the use of a rigid seat member and rigid backrest member more difficult since they must either be hinged or removed from the frame to enable collapsing of the frame.
These problems have partially been overcome through the design of vertical folding wheelchair frames whereby the backrest frame generally folds atop and parallel to the seat assembly, while the footrest assembly folds underneath and parallel to the seat assembly. These wheelchair designs provide increased torsional rigidity and stability similar to a non-folding wheelchair, while further optionally allowing the use of a more rigid backrest member and rigid seat member. Typical of these patented vertical folding frame wheelchairs are disclosed in U.S. Pat. Nos. 4,679,816; 4,736,960; and 4,887,826.
One significant problem associated with these designs, however, is that the linkage assemblies allow the frames to be moved inadvertently between the fully collapsed or retracted position (i.e., for storage or transportation), and the fully deployed or extended position (i.e., for use). Typically, these assemblies include some type of locking mechanism, such as locking sleeves, locking pins, or the like, to lock the linkages in the deployed position to prevent collapse. This problems may be quite serious if the unfolded frames are unintentionally not locked together when the wheelchair is in use. In this situation, the weight of the wheelchair occupant, and/or the forces exerted on the frame and linkages during normal use may cause the unfolded linkages to collapse to the folded position, potentially injuring the occupant.