Torque wrench having improved tightening torque measurement value

1. A torque wrench comprising:

a shaft-like strain body having a leading end coupled with a replaceable tightening portion;

pairs of first and second distortion sensors for measuring an amount of distortion of the strain body, one of the first distortion sensors and one of the second distortion sensors being arranged on one of opposite sides in a tightening force application direction of the strain body, at spaced points in an axial direction of the strain body, the other first distortion sensor and the other second distortion sensor being arranged on the other side in the tightening force application direction of the strain body, at spaced points in the axial direction of the strain body;

a housing comprising:

a front side cover part in a tubular shape adapted to receive the leading end of the strain body, a leading endface of the front side cover part being provided with a hole for receiving a proximal end of the tightening portion; and

a back side grip part in a tubular shape adapted to receive a proximal end of the strain body, the back side grip part being provided therein with a shaft extending in a direction orthogonal to the tightening force application direction and through the strain body;

a torque computation section for performing a computation of a tightening torque T while correcting an error due to a change in a point on the housing where a tightening force is applied by a user of the torque wrench, at least based on measurement results of the first and second distortion sensors; and

an output portion for outputting at least a result of the computation performed in the torque computation section as a tightening torque measurement value, wherein

a rear end of the strain body is fixed to the back side grip part to prevent movement of the rear end of the strain body in the tightening force application direction.

2. The torque wrench according to claim 1 , further comprising:

a first amplification circuit to amplify differential signals outputted from the pair of first distortion sensors;

a second amplification circuit to amplify differential signals outputted from the pair of second distortion sensors;

a first analog-to-digital converter connected downstream of the first amplification circuit; and

a second analog-to-digital converter connected downstream of the second amplification circuit, wherein

the housing further comprises a grip cap provided on a rear end side of the back side grip part,

a positioning fixing hole for receiving the rear end of the strain body is formed centrally in an inner face of the grip cap, and

the torque computation section performs the computation of the tightening torque using a following equation:

T=k 1 [( AD amax −AD amin )·( AD bmax −AD bmin )] −1 ·[ k 2 ( k 3 ( AD a −AD amin )+ k 4 ( AD amax −AD amin ))· k 5 ( AD bmax −AD bmin ))+ k 6 ( k 7 ( AD b −AD bmin )+ k 8 ( AD bmax −AD bmin ))· k 9 ( AD amax −AD amin )]

where k 1 to k 9 are constants; AD a , AD amax , and AD amin are an output value, a maximum output value and a minimum output value, respectively, of the first analog-to-digital converter; and AD b , AD bmax , and AD bmin are an output value, a maximum output value and a minimum output value, respectively, of the second analog-to-digital converter.

3. The torque wrench according to claim 1 , further comprising:

a setting portion for setting a tightening torque set value; and

a torque determination section for making a determination whether or not the torque measurement value indicated by the result of the computation by the torque computation section is close to or has attained the tightening torque set value that is set via the setting portion, the torque determination section being adapted to order the output portion to output a result of the determination.

4. The torque wrench according to claim 1 ,

further comprising a pair of sensor units attached on the opposite sides in the tightening force application direction of the strain body, the sensor units each including:

a flexible circuit board; and

one of the first distortion sensors and one of the second distortion sensors formed on the flexible circuit board.

5. The torque wrench according to claim 4 , wherein

the strain body is formed with recesses each having a length dimension corresponding to each of the sensor units, and the sensor units are affixed to the recesses.

6. The torque wrench according to claim 1 , wherein the back side grip part includes means for fixing the rear end of the strain body thereto and preventing the movement of the rear end of the strain body in the tightening force application direction.