To prevent the deformation of a skirt plate and use a bumper without fixing by inserting and fixing a grove from the axial direction to the bumper parallel to the axial direction of a guide groove engaged with the stretched part of the skirt plate stretched and fixed the wider plate than the thickness of a swing frame main body end to the upper/lower parts of the main body end.

A skirt plate 10 is composed of a wider plate than the thickness of a swing frame main body 1 end and a stretched part is formed by fixing so as to stretch vertically from the swing frame main body 1 end. A bumper 2 has a nearly trapezoid shape section, on whose bottom side a guide groove 20 engaged with the stretched part is formed along the axial direction. The bumper 2 is fixed by inserting from the stretched part of the rear end of the skirt plate 10 until its front end is contacted with a skirt rib 12 while moving the guide groove 20 along the axial direction. As the bumper 2 prevents the coming off of the rear end of a released shirt plate 10 from the stretched part, the deformation of the skirt plate 10 can be prevented and the bumper 2 can be used without fixing.