Bolt Fastening Structure and Turbo-Machine Using Same

A bolt fastening structure comprising:

• an upper flange and a lower flange of a casing to which an inner pressure is applied, the upper flange and the lower flange defining an abutting interface therebetween where the upper flange and the lower flange are abutted together, the abutting interface having an inner edge and an outer edge;
• a bolt hole formed through the abutting interface and having a bolt hole inner side edge and a bolt hole outer side edge, the bolt hole inner side edge being closest to the inner edge of the abutting interface, the bolt hole outer side edge closest to the outer edge of the abutting interface;
• and
• a counterbored portion formed asymmetric about a center of the bolt hole,
• wherein the abutting interface comprises:
• a first region extending from the bolt hole inner side edge to the inner edge of the abutting interface and being a region where the upper flange and the lower flange are in surface-to-surface contact with each other;
• and
• a partial contact region extending from the bolt hole outer side edge to the outer edge of the abutting interface and being a region where the upper flange and the lower flange are partially in surface-to-surface contact with each other,
• wherein, in the partial contact region, the upper flange and the lower flange are in surface-to-surface contact with each other in a second region, and
• wherein the first region has a length W1 along a line extending in an in-out direction of the abutting interface and passing through the center of the bolt hole, the second region has a length W2 along the line, and the length W1 is larger than the length W2.