Nut and tightening method

A bolt, comprising:

• a bolt shaft having a thread portion on at least a portion thereof;
• and
• a bolt head formed at an end of the bolt shaft,
• wherein:
• the bolt head has a bolt base opening formed in a portion close to the bolt shaft,
• a washer is inserted into the bolt base opening,
• the washer includes a first end portion, a second end portion and an extending portion between the first end portion and the second end portion,
• in a state where the bolt is not tightened, where the second end portion of the washer is in contact with a bottom surface of the bolt base opening, and where there is a first gap between the first end portion of the washer and the bottom surface of the bolt base opening, the washer is accommodated in the bolt base opening, and
• also in a state where the bolt is tightened, the first gap is present between the first end portion of the washer and the bottom surface of the bolt base opening.

A pair nut, comprising a first nut and a second nut,

• wherein:
• the first nut includes:
• a nut main body having a tapped hole formed therein,
• a top surface of the nut main body,
• a side surface defining the top surface, and
• a bottom surface of the nut main body located opposite to the top surface,
• a central axis of the tapped hole of the first nut matches a central axis of a bolt corresponding to the pair nut,
• the first nut has a bottom opening formed in the bottom surface thereof, the bottom opening having a diameter longer than a diameter of the tapped hole,
• a central axis of the bottom opening extends while inclining with respect to the central axis of the bolt corresponding to the pair nut,
• the second nut includes:
• a nut main body having a tapped hole formed therein,
• a top surface of the nut main body,
• a side surface defining the top surface, and
• a bottom surface of the nut main body located opposite to the top surface,
• the second nut includes a top protrusion on the top surface thereof, the top protrusion corresponding to the bottom opening of the first nut, and
• a central axis of the tapped hole formed in the top protrusion and the nut main body matches the central axis of the bolt corresponding to the pair nut.

A pair nut, comprising a first nut and a second nut,

• wherein:
• the first nut includes:
• a nut main body having a tapped hole formed therein,
• a top surface of the nut main body,
• a side surface defining the top surface, and
• a bottom surface of the nut main body located opposite to the top surface,
• a central axis of the tapped hole of the first nut matches a central axis of a bolt corresponding to the pair nut,
• the first nut has a bottom opening formed in the bottom surface thereof, the bottom opening having a diameter longer than a diameter of the tapped hole,
• the second nut includes:
• a nut main body having a tapped hole formed therein,
• a top surface of the nut main body,
• a side surface defining the top surface, and
• a bottom surface of the nut main body located opposite to the top surface,
• the second nut includes a top protrusion on the top surface thereof, the top protrusion corresponding to the bottom opening of the first nut,
• a central axis of the tapped hole formed in the top protrusion and the nut main body matches the central axis of the bolt corresponding to the pair nut,
• the pair nut is structured such that at a time of tightening the first nut and the second nut to the bolt, a part of a top surface of the top protrusion of the second nut contacts, with priority, the bottom opening of the first nut.