Screw fastening method for fastening object member and screw fastening device

A screw fastening device configured to use a screw fastening member to couple a fastening object member by screw tightening with a target tightening torque corresponding to a predetermined target tightening axial force, the screw fastening device comprising:

• a tightening mechanism configured to tighten or loosen the screw fastening member disposed on the fastening object member;
• a torque measurement mechanism configured to measure a torque when the screw fastening member is tightened or loosened by the tightening mechanism;
• and
• a control unit, wherein
• the control unit is configured to
• perform a temporary tightening operation of tightening the screw fastening member in a tightening direction up to a first tightening torque using the tightening mechanism, and then perform a loosening operation of loosening the screw fastening member in a loosening direction opposite to the tightening direction,
• detect a maximum value of a loosening torque during the loosening operation using the torque measurement mechanism, and then perform a re-tightening operation of tightening the screw fastening member in the tightening direction up to a second tightening torque using the tightening mechanism, and
• calculate the target tightening torque using a loosening direction torsion angle of a bolt of the screw fastening member during the loosening operation and a tightening direction torsion angle of the bolt during the re-tightening operation.

A screw fastening device comprising:

• a screw fastening member configured to couple a fastening object member by screw tightening with a target tightening torque corresponding to a predetermined target tightening axial force;
• a tightening mechanism configured to tighten or loosen the screw fastening member disposed on the fastening object member;
• a torque measurement mechanism configured to measure a torque when the screw fastening member is tightened or loosened by the tightening mechanism;
• and
• a processor configured to execute a program stored in a memory, wherein
• the processor configured is configured to, when the program is executed by the processor, cause the screw fastening device to execute:
• tightening the screw fastening member in a tightening direction up to a first tightening torque using the tightening mechanism, and then loosening the screw fastening member in a loosening direction opposite to the tightening direction,
• detecting a maximum value of a loosening torque during loosening the screw fastening member using the torque measurement mechanism, and then re-tightening the screw fastening member in the tightening direction up to a second tightening torque using the tightening mechanism, and
• calculating the target tightening torque using a loosening direction torsion angle of a bolt of the screw fastening member during loosening the screw fastening member and a tightening direction torsion angle of the bolt during re-tightening the screw fastening member.