Inverted constant force window balance systems are depicted in, for example, U.S. Pat. Nos. 5,353,548 and 5,463,793, the disclosures of which are hereby incorporated by reference herein in their entireties. Inverted constant force window balances utilize a housing or shoe that carries a coil spring having a free end secured to a window jamb channel with a mounting bracket, screw, or other element. As the coil spring unwinds, the recoil tendency of the spring produces an upward force to counter the weight of the window sash. The shoe may be a tilt-in shoe that allows the window sash to tilt inwards for cleaning and/or installation/removal purposes. As the window sash tilts, a locking mechanism holds the shoe in place to prevent the coil spring from retracting the shoe in the absence of the weight of the sash.
Existing tilt-in inverted constant force window balances, however, suffer from several shortcomings. First, as with many types of balance shoes, the locking shoes used with inverted constant force window balances are dimensioned such that they can not easily be inserted into the window jamb channel. Second, particularly heavy window sashes may require more than a single spring on each side to provide an adequate counterbalance. While it is possible to add additional springs in regular constant force window balances (in which the coil springs are located in a fixed position at the top of the window jamb channel), adding additional springs to inverted constant force balances requires modifications of the shoes, or the addition of supplemental or companion spring carriers. Third, dust and debris from new construction or aging installations may enter the coil spring, thereby preventing proper operation thereof. What is needed then, is an inverted constant force balance that addresses these and other shortcomings.