Mattress foundations, sometimes referred to as "box springs", consist generally of a wooden frame having multiple cross pieces which may be constructed of steel, and a plurality of wire form springs attached to the frame and cross pieces, and at a top portion to a wire grid or "deck" which serves as a supporting surface for a mattress. The use of non-coil type springs in mattress foundations is disclosed, for example, in U.S. Pat. Nos. 4,339,834, 4,739,977, 4,921,228 and 4,932,535. Mattress foundations of this type, i.e., utilizing limited deflection non-coil springs connected to a foundation frame and a wire grid support deck, have the advantages of providing a firm but flexible support for the wire grid deck and limited deflection characteristics to avoid overstressing and permanent deformation.
U.S. Pat. No. 4,921,228 discloses a non-coil spring mattress foundation wherein a generally horizontal base portion of the spring is attached to the foundation frame by staples, and a generally horizontal deck attaching portion is secured to the wire grid of the deck without separate fasteners by underlapping and overlapping with a linear section of the grid which traverses the spring to interlockingly engage the grid. The base portions of the spring can alternatively be attached to steel cross pieces of the foundation frame by insertion of a base portion of each leg of the spring through aligned slots formed in the cross-piece, as disclosed in U.S. Pat. No. 4,470,584.
Other types of non-coil springs having generally horizontal upper portions can be assembled into foundation structures by overlap/underlap interlocking with linear sections of the grid. For example, FIG. 5 illustrates a foundation assembly described by U.S. Pat. No. 4,932,535 which has a unitary wire body spring 100 having a horizontal load bearing portion 102 which is interlockable with a wire grid support deck 104, and generally vertical deflectable spring legs 106 which are configured to maximize lateral stability and minimize skewing at full deflection.
A significant difficulty encountered in the customary hand assembly of this type of foundation is the insertion and interlocking of each spring into the wire grid deck. In particular, to interlock the horizontal load bearing portion of the spring with the linear section of the wire grid requires that the vertical legs of the spring be inserted past the grid, and opposite corners of the horizontal load bearing section manipulated under the single wire of the grid in opposite directions by rotating the spring, with the section of the top portion of the spring connecting the two corners crossing over the top of the wire. This complicated assembly task is necessarily done by hand for each of the approximately thirty to thirty-five springs in each queen size foundation. The process is made even more difficult in foundations in which the base portion of each spring is secured to the frame by insertion of the spring feet into aligned transverse slots in the frame cross-sections (as described by the '584 patent). In this case, the spring feet must be positioned on opposite sides of the cross-section for insertion into the slots as the top portion is manipulated into the interlocked position with the wire grid.
Although the basic configuration of the spring described in the '535 patent has good deflective and support characteristics when assembled in a foundation in this manner, the difficulty of the assembly process emanates from the compound intersection shown in FIG. 5 of wire grid section cross wire 108 with the downwardly angled bend 114 between torsional member 110 and downwardly angled member 112. The close binding proximity of the downwardly angled bend 114 with cross wire 108 of the grid acts to resist insertion of the spring into the fully interlocked and aligned position. Specifically, an upper surface of the wire at downwardly angled bend 114 impinges directly upon cross wire 108 as the spring is forcibly rotated into interlocking engagement with the grid. This resistance is substantial due to the rigidity of the wire section of the grid and the spring and therefore increases the difficulty of the hand assembly process significantly. This difficulty of assembly of course contributes to increased production time and assembler fatigue.