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A constant force, or “clock” spring, is a roll of prestressed strip which exerts a nearly constant restraining force to resist uncoiling. The force is constant because the change in the radius of curvature is constant. This is true if the change in coil diameter, due to buildup, is disregarded. Long extension capabilities, constant torque and virtual absence of intercoil friction have led many designers to specify constant force springs in such applications as brush springs for motors, counterbalance springs for window sashes and carriage return springs for typewriters. Constant force motor springs are used to drive mechanisms for timers, movie cameras and cable retractors.