| 摘要 |
PROBLEM TO BE SOLVED: To prevent damages of a screw head and a backflow preventing ring by setting a gap between an outer periphery of a conical part of the head and an inner surface of a cylinder to a specific value, flattening a rear end face of the conical part and a front surface of the ring in a shape that a part of an inner diameter or more of the ring is brought into contact with the front surface. SOLUTION: A rear end face 6a of a conical part of a screw head and a front surface 4a of a backflow preventing ring are flat surfaces in shape that a part of an inner diameter or more of the ring is brought into contact with the front surface. Thus, a contact surface of the face 6a of the part 6 of the head with the surface 4a of the ring can be increased. A pressure and wear of this surface can be reduced. At this time, an outer diameter (d) of the part 6 is increased to further increase a contact area of the face 6a with the surface 4a and a gap (t) between the outer periphery of the part 6 and an interior of the cylinder is set to 0.2 to 2 mm or preferably 0.05 to 1 mm. |
