How To Fix a Window that Won't Stay Open
How To Replace a Sash Cord
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Friction alone ordinarily will not hold the upper sash in place. Likewise the lower sash could not stay in the open position either, without some help. This help comes in the form a of sash cord, pulley and counter-weight.
The sash cord fastens to the left and right edges of each sash, then feeds through an opening at the top of each channel, over a pulley and then down to a counter-weight. As you lift up or pull down on a sah, the counterweights are balanced against the sash weight to allow the window to glide easily and to stay in the position they are put into.
Older windows use ropes and lead weights. In some cases a special type of flat link sash chain was used instead of rope. In some newer windows the sash cord is replaced by a high strength material that resembles twine. Another newer style sash cord uses a system similar to an automatic retracting tape measure. A thin metal strip connects the sash to a spring loaded mechanism that coils or uncoils the strip as the window is opened or closed.
In the newest models of some windows, no sash cord is used at all. Instead they rely on proprietary "sash balancers", mechanisms that rely on an internal system of pulleys and springs. In the case of sash balancers, typically the entire mechanism is replaced. The parts can be ordered from the window manufacturer.