Abstract

A screw tightening apparatus comprises a motor-driven screwdriver bit supported on a base for tightening a screw with a torque which is detected as a reactive force acting on the base by a torque detector. An output signal from the torque detector is amplified and delivered to a comparator in which the signal is compared with a signal from a torque setting device which is indicative of a desired torque with which the screw is to be tightened. When the signals agree with each other in the comparator, the latter produces an output signal which is supplied to a switch which in turnde-energizes an electric or pneumatic motor of the screw tightening apparatus. An adjustable time-delay circuit is connected between the comparator and the switch to delay the signal from the comparator for a period of time which may be varied dependent on the material into which the screw is to be driven and the pitch and material of the screw in use. The screwdriver bit is coupled to an output shaft of the motor through an electromagnetic clutch which is de-energizable by the switch simultaneously with the de-energization of the motor.