Patent ID: 7966888

Claim:
A screw fastener comprising: a first screwdriver including a first bit configured to fasten a first screw by engaging with a recess of the first screw, and a first motor configured to rotate the first bit; a plurality of second screwdrivers each including a second bit configured to fasten a second screw by engaging with a recess of the second screw, and a second motor configured to rotate the second bit; an elevator unit configured to simultaneously ascend and descend both the first screwdriver and the second screwdriver; and a controller configured to control the first motor and the second motor so that seating of the first screw can start after torquing-up of the second screw starts, wherein the controller torques up the plurality of second screws by synchronizing driving of the second motors of the plurality of second screwdrivers, and wherein each second screw driver includes a state detector configured to detect a state of a corresponding one of the second motors and connected to the controller by a wired OR line; and a counter configured to increment a counter value by one whenever a synchronization waiting state is set, wherein each state detector switches an output to a first wired OR line from a first state to a second state in a synchronization waiting state of an odd counter value, and switches an output to a second wired OR line from the second state to the first state in a synchronization waiting state of an even counter value, and wherein the controller determines an odd-number-th synchronization when a state of the first wired OR line switches from the first state to the second state after all the second screwdrivers become in odd-number-th synchronization waiting states, and determines an even-number-th synchronization when a state of the second wired OR line switches from the first state to the second state after all the second screwdrivers become in even-number-th synchronization waiting states.