Axial-force measurement method, tightening determination method, axial-force measurement apparatus, and tightening determination apparatus for bolt

A tightening determination apparatus for a bolt fastened to a component to be fastened, comprising:

• a fastening unit configured to fasten the bolt to the component to be fastened;
• a measurement unit configured to measure an axial force of the bolt;
• and
• a determination unit configured to determine whether or not the bolt is properly fastened to the component to be fastened, wherein
• the measurement unit is further configured to:
• measure a first axial force of the bolt temporarily fastened to the component to be fastened with a temporarily-tightening torque, the temporarily-tightening torque being a torque which is determined in advance and by which a depression is formed on a head of the bolt, by acquiring a first image representing a height distribution on the head of the temporarily-fastened bolt, measuring a first depression amount on the head of the temporarily-fastened bolt by using the first image, and using the first depression amount and a predetermined correlation between axial forces of the bolt and depression amounts on the head of the bolt;
• measure a second axial force of the bolt regularly-fastened to the component to be fastened with a regularly-tightening torque, the regularly-tightening torque being torque which is determined in advance and larger than the temporarily-tightening torque, by acquiring a second image representing a height distribution on the head of the regularly-fastened bolt, measuring a second depression amount on the head of the regularly-fastened bolt by using the second image, and using the second depression amount and a predetermined correlation between axial forces of the bolt and depression amounts on the head of the bolt;
• and
• measure an estimated axial force of the regularly-fastened bolt by using the torque with which the bolt is temporarily fastened, the torque with which the bolt is regularly fastened, and a difference between the first and second axial forces as shown in Expression 1:
• F=f2×t2/(t2−t1) (Expression 1)
• where F denotes an estimated axial force of the regularly-fastened bolt, f2 denotes a difference between the first and second axial forces, t1 denotes a torque with which the bolt is temporarily fastened, and t2 denotes a torque with which the bolt is regularly fastened, and
• the determination unit is further configured to determine whether or not an estimated axial force of the regularly-fastened bolt is equal to or larger than a predetermined target axial force, and determine that the regularly-fastened bolt is properly fastened to the component to be fastened when the estimated axial force of the bolt is equal to or larger than the predetermined target axial force.

A method for determining tightening of a bolt fastened to a component to be fastened, comprising:

• temporarily fastening the bolt to the component to be fastened with a temporarily-tightening torque, the temporarily-tightening torque being a torque which is determined in advance and by which a depression is formed on a head of the bolt;
• measuring a first axial force of the temporarily-fastened bolt;
• regularly fastening the bolt to the component to be fastened with a regularly-tightening torque, the regularly-tightening torque being torque which is determined in advance and larger than the temporarily-tightening torque;
• measuring a second axial force of the regularly-fastened bolt;
• measuring an estimated axial force of the regularly-fastened bolt by using the torque with which the bolt is temporarily fastened, the torque with which the bolt is regularly fastened, and a difference between the first and second axial forces;
• and
• determining that the regularly-fastened bolt is properly fastened to the component to be fastened when the estimated axial force of the regularly-fastened bolt is equal to or larger than a predetermined target axial force, wherein
• the measuring of the first axial force comprises:
• acquiring a first image representing a height distribution on the head of the temporarily-fastened bolt;
• measuring a first depression amount on the head of the temporarily-fastened bolt by using the first image;
• and
• measuring the first axial force by using the first depression amount and a predetermined correlation between axial forces of the bolt and depression amounts on the head of the bolt,
• the measuring of the second axial force comprises:
• acquiring a second image representing a height distribution on the head of the regularly-fastened bolt;
• measuring a second depression amount on the head of the regularly-fastened bolt by using the second image;
• and
• measuring the second axial force by using the second depression amount and the predetermined correlation between axial forces of the bolt and depression amounts on the head of the bolt, and
• the estimated axial force of the regularly-fastened bolt is derived from Expression (1):
• F=f2×t2/(t2−t1) (Expression 1)
• where F denotes an estimated axial force of the regularly-fastened bolt, f2 denotes a difference between the first and second axial forces, t1 denotes a torque with which the bolt is temporarily fastened, and t2 denotes a torque with which the bolt is regularly fastened.

An axial-force measurement apparatus for a bolt fastened to a component to be fastened, comprising:

• a fastening unit configured to fasten the bolt to the component to be fastened;
• and
• a measurement unit configured to measure an axial force of the bolt, wherein
• the measurement unit is further configured to:
• measure a first axial force of the bolt temporarily fastened to the component to be fastened with a temporarily-tightening torque, the temporarily-tightening torque being a torque which is determined in advance and by which a depression is formed on a head of the bolt, by acquiring a first image representing a height distribution on the head of the temporarily-fastened bolt, measuring a first depression amount on the head of the temporarily-fastened bolt by using the first image, and using the first depression amount and a predetermined correlation between axial forces of the bolt and depression amounts on the head of the bolt;
• measure a second axial force of the bolt regularly-fastened to the component to be fastened with a regularly-tightening torque, the regularly-tightening torque being torque which is determined in advance and larger than the temporarily-tightening torque, by acquiring a second image representing a height distribution on the head of the regularly-fastened bolt, measuring a second depression amount on the head of the regularly-fastened bolt by using the second image, and using the second depression amount and a predetermined correlation between axial forces of the bolt and depression amounts on the head of the bolt;
• and
• measure an estimated axial force of the regularly-fastened bolt by using the torque with which the bolt is temporarily fastened, the torque with which the bolt is regularly fastened, and a difference between the first and second axial forces as shown in Expression 1:
• F=f2×t2/(t2−t1) (Expression 1)
• where F denotes an estimated axial force of the regularly-fastened bolt, f2 denotes a difference between the first and second axial forces, t1 denotes a torque with which the bolt is temporarily fastened, and t2 denotes a torque with which the bolt is regularly fastened.