Tightening tool and tightening tool management system

1. A tightening tool for tightening a fastener, comprising:

a motor;

a main shaft which engages with the fastener;

a clutch disposed between the motor and the main shaft, wherein the clutch rotates the main shaft by transmitting torque from the motor to the main shaft when a load acting on the main shaft is less than a predetermined value, and shuts off torque transmission from the motor to the main shaft when the load acting on the main shaft reaches or exceeds the predetermined value;

means for detecting a current flowing to the motor; and

means for determining, when torque transmission from the motor to the main shaft has been shut off, whether or not a tightening torque of the fastener is normal based upon a motor current value detected by the current detecting means at the time of torque transmission shut-off.

2. A tightening tool according to claim 1 , wherein the determining means determines that the tightening torque of the fastener is normal when the detected motor current value is within a preset range, and determines that the tightening torque of the fastener is abnormal when the detected current value is outside of the preset range.

3. A tightening tool according to claim 2 , further comprising a memory which stores a preset range for each type of tightening operation, wherein the determining means reads a preset range from the memory in accordance with the type of tightening operation, and determines whether or not the tightening torque is normal on the basis of the read preset range.

4. A tightening tool according to claim 3 , further comprising means for detecting a rotation angle of the main shaft or the motor, wherein the determining means further determines whether or not a tightening torque of the fastener is normal based upon the rotation angle of the main shaft or the motor detected by the rotation angle detecting means during a period extending from (1) a time at which a current value detected by the current detecting means exceeds a preset value to (2) a time at which torque transmission from the motor to the main shaft is shut off.

5. A tightening tool according to claim 4 , wherein the determining means (a) measures the rotation angle of the main shaft or the motor from the time at which the current flowing to the motor exceeds the preset value to a time at which the clutch is activated, and (b) determines that the tightening torque of the fastener is normal when the measured rotation angle is within a preset angle range and determines that the tightening torque of the fastener is abnormal when the measured rotation angle is outside of the preset angle range.

6. A tightening tool according to claim 5 , further comprising a memory which stores a preset angle range for each type of tightening operation, wherein the determining means reads a preset angle range from the memory in accordance with the type of tightening operation, and determines whether or not the tightening torque is normal on the basis of the read preset angle range.

7. A tightening tool according to claim 6 , wherein the determining means determines the operation type from temporal variation in the motor current value and/or temporal variation in the rotation angle of the main shaft after the current flowing to the motor has exceeded the preset value, and reads a preset angle range corresponding to the determined operation type.

8. A tightening tool according to claim 7 , further comprising means for warning an operator when the tightening torque of the fastener is determined to be abnormal by the determining means.

9. A tightening tool for tightening a fastener, comprising:

a motor;

a main shaft which engages with the fastener;

a clutch disposed between the motor and the main shaft, wherein the clutch rotates the main shaft by transmitting torque from the motor to the main shaft when a load acting on the main shaft is less than a predetermined value, and shuts off torque transmission from the motor to the main shaft when the load acting on the main shaft reaches or exceeds the predetermined value;

an ammeter detecting a current flowing to the motor; and

a microprocessor in communication with the ammeter, wherein the microprocessor determines that the tightening torque of the fastener is normal when a motor current value detected by the ammeter at a time of torque transmission shut-off is within a preset range, and determines that the tightening torque of the fastener is abnormal when a motor current value detected by the ammeter at a time of torque transmission shut-off is outside of the preset range.

10. A tightening tool according to claim 9 , wherein the clutch comprises a pair of clutch plates, and compression spring biasing one of the pair of clutch plates toward the other.

11. A tightening tool according to claim 10 , further comprising a sensor adapted to detect that the clutch has been activated, wherein the microprocessor determines whether or not the clutch has shut off torque transmission based upon an output from the sensor.

12. A tightening tool according to claim 11 , wherein the microprocessor obtains the motor current value from an output of the ammeter when the microprocessor determines that the clutch has been activated.

13. A tightening tool according to claim 9 , further comprising a memory which stores a preset range for each type of tightening operation, wherein the microprocessor reads a preset range from the memory in accordance with the type of tightening operation, and determines whether or not the tightening torque is normal on the basis of the read preset range.

14. A tightening tool according to claim 9 , further comprising rotation angle sensor adapted to detect a rotation angle of the main shaft or the motor, wherein the microprocessor further determines whether or not a tightening torque of the fastener is normal based upon the rotation angle of the main shaft or the motor detected by the rotation angle sensor during a period extending from (1) a time at which a current value detected by the ammeter exceeds a preset value to (2) a time at which torque transmission from the motor to the main shaft is shut off.

15. A tightening tool according to claim 14 , wherein the microprocessor (a) measures the rotation angle of the main shaft or the motor from the time at which the current flowing to the motor exceeds the preset value to a time at which the clutch is activated, and (b) determines that the tightening torque of the fastener is normal when the measured rotation angle is within a preset angle range and determines that the tightening torque of the fastener is abnormal when the measured rotation angle is outside of the preset angle range.

16. A tightening tool according to claim 15 , further comprising a memory which stores a preset angle range for each type of tightening operation, wherein the microprocessor reads a preset angle range from the memory in accordance with the type of tightening operation, and determines whether or not the tightening torque is normal on the basis of the read preset angle range.

17. A tightening tool according to claim 16 , wherein the microprocessor determines the operation type from temporal variation in the motor current value and/or temporal variation in the rotation angle of the main shaft after the current flowing to the motor has exceeded the preset value, and reads a preset angle range corresponding to the determined operation type.

18. A tightening tool according to claim 17 , further comprising a display warning an operator when the tightening torque of the fastener is determined to be abnormal by the microprocessor.