US Patent 8220765 Spring counterbalance for rotating load

A counterbalance mechanism counterbalances an eccentric mass on a rotating shaft supported by a frame. A first pulley is coupled to and concentric with the shaft. An arm is coupled to the shaft to rotatably support a second at a first distance from the first pulley. A third pulley is fixedly coupled to the frame at a second distance from the first pulley. A fourth pulley is rotatably coupled to and concentric with the third pulley. A spring is fixed at a first end to the frame and has a second end that is coupled to the fourth pulley such that the spring provides a restoring force as the fourth pulley is rotated. A first cable has a first end that is coupled to the fourth pulley and a second end that is coupled to the frame through the third pulley after passing over the first and second pulleys.