Nut runner using induction motor

Claim 1

In a nut runner for tightening screwed objects comprising: a driving motor, a driving shaft to be coupled to said screwed objects for driving the same, a reduction gear mechanism provided for transferring the revolution of said driving motor to said driving shaft, a torque detector for detecting a torque applied to said driving shaft, and a control circuit for controlling said driving motor by the use of the detected output of said torque detector, said driving motor being a two-phase induction motor having a rotor and field coils, and said control circuit comprising means to control the magnitudes, repetition frequency and phase relations of intermittent currents supplied to field coils of said two-phase induction motor so that the torque of the two-phase induction motor is controlled by control of the magnitude of the intermittent currents, the revolution speed of the two-phase induction motor is controlled by control of the repetition frequency of the intermittent currents and the forward and backward driving directions are switched by the switching of phase relations of the intermittent currents, said control circuit comprising means operating said motor at high speed during a first stage of tightening said screwed object, decelerating and quickly stopping said motor when the torque detected by said torque detector reaches a first pre-set reference value, immediately restarting said motor and operating it in the same direction at a lower speed during a second stage of tightening said object, and braking said motor to a sudden stop when the torque detected by said torque detector reaches a second pre-set reference value higher than said first reference value.