Abstract: A power transmission device comprising a rotatably mounted drum for housing a spring mechanism, one end of the spring mechanism being fixed to the interior of the drum, and the other end of the spring mechanism being fixed to a sprocket gear wheels provided on the drum axis. There is also provided a slip clutch adapted to engage the second sprocket gear wheel, a freewheel, connected to the slip clutch, a drive wheel coupled to the freewheel, at least one crank coupled by a drive to the drive wheel for rotating the drive wheel, and a speed control governor coupled to the drum. Cranking of the drive wheel drives the freewheel and then the slip clutch and sprocket gear wheel to wind the spring mechanism, causing the drum to rotate under spring-tension at a speed regulated by the governor so as to provide mechanical power.