US Patent 7143547 Spring assisted swing door operator

A motorized universal swing door operator includes a motor driving a gear reduction drive mechanism having an output shaft adapted to be connected to a swing door operating arm. The output shaft supports a sprocket around which is trained a chain having opposite ends connected to a link supported on a shaft which is connected to a return spring. The return spring stores and returns energy regardless of the direction of rotation of the output shaft so as to provide the universal operating capability of the operator.