Source: {"pile_set_name": "USPTO Backgrounds"}

Pretension spiral window balances have advantages over non-pretension balances. Pretension balances have a spiral rod that is pre-wound and set with a certain amount of tension. This saves the window manufacturer having to manually turn or wind the lift force onto the spiral rod of the balance prior to attaching the end of the spiral rod to a window sash. Pretension balances also eliminate the possibility of the window manufacturer applying an incorrect number of windings (lift force) to the balance, which could result in the improper operation or outright failure of the window balance.
When a non-pretension balance is installed into the jamb channel of a window frame, the rod is free to drop below the window sash and the rod end can be accessed for winding the lift force into the balance and then attaching it to the window sash. However, this requires specific tooling and which may be prone to human error. In a pretension balance, the lifting force is machine wound. Then the pre-wound rod is locked in position by the balance manufacturer, by means of the balance spring torque, inside a special pretension balance anchor.
After a pretension balance is mounted within the jamb channel of a window frame, the rod end may not be accessible from below the window sash because it is not free to drop below the bottom end of the sash. This makes it difficult for the window manufacturer to install the balance in the jamb channel and attach the end of the spiral rod to a ledge on the window sash.
There is a need, therefore, for a device to extend the end of the spiral rod so that the spiral rod can be pulled below the sash and secure a clip which is located at the end of the spiral rod onto a ledge on the sash. A conventional window industry balance installation tool can now be attached to the extension device and used to attach the spiral rod end to a ledge on the window sash.