Abstract: An automatic screw fastening machine embodied by the invention transmits the rotating force of a motor to a driver bit via a differential planetary mechanism consisting of an input disc, a plurality of planetary cones, a cam disc, and a variable-speed ring. The automatic screw fastening machine causes a resilient supporting unit to restrict the variable-speed ring from shifting itself in the axial direction. Furthermore, the machine is provided with a torque detection unit capable of detecting the torque of reaction force applied to the variable-speed ring by effect of the load applied to the driver bit. As soon as the detect-signal value correctly matches a specific value corresponding to the predetermined fastening torque, supply of power to the motor is discontinued. In consequence, when fully completing a screw fastening process, rotation of the driver bit is substantially reduced to zero so that no rotating force can be transmitted to the driver bit.

Abstract: An automatic screw fastening machine embodied by the invention transmits the rotating force of a motor to a driver bit via a differential planetary mechanism consisting of an input disc, a plurality of planetary cones, a cam disc, and a variable-speed ring. The automatic screw fastening machine causes a resilient supporting unit to restrict the variable-speed ring from shifting itself in the axial direction. Furthermore, the machine is provided with a torque detection unit capable of detecting the torque of reaction force applied to the variable-speed ring by effect of the load applied to the driver bit. As soon as the detect-signal value correctly matches a specific value corresponding to the predetermined fastening torque, supply of power to the motor is discontinued. In consequence, when fully completing a screw fastening process, rotation of the driver bit is substantially reduced to zero so that no rotating force can be transmitted to the driver bit.