| 摘要 |
PROBLEM TO BE SOLVED: To suppress the concentration of stress in a portion in which the cross-sectional area of a frame material is reduced, in the case of gradually changing the cross-sectional area of a frame material by reducing the amounts of three wall surfaces which are adjacent to one another among a plurality of wall surfaces constituting the frame material having a polygonal cross section, by changing all the amounts of the three wall surfaces gradually so as to adjust the strength in each part of a back frame.SOLUTION: Among wall surfaces forming a first frame 11, in a cross-sectional reduction part of which the cross-sectional area is partly reduced, the amounts of three wall surfaces 11Aa, 11Ab and 11Ac which are adjacent to one another are reduced, and as the one surface 11Ab among the three wall surfaces is moved away from a cross-sectional reduction starting portion in the cross-sectional reduction part, the removal amount of the wall surface is gradually increased, and the wall surface is removed into a V shape 11Ae. As the two wall surfaces 11Aa and 11Ac facing each other across the wall surface 11Ab between them are moved away from a portion where the wall surface 11Ab is removed and disappears as a starting point, the removal amounts of the wall surfaces are gradually increased. |
