Patent ID: 8672418

Claim:
A sealing structure for a piston which is reciprocatable in an axial direction in a cylinder extending in the axial direction with an annular groove formed on one of the cylinder and an outer circumferential surface of the piston and bounded by two oppositely disposed and spaced apart interior walls, the structure comprising: a seal member made of a synthetic resin material to an annular shape and fitted in the annular groove so that a seal surface formed on an inner or outer circumferential surface thereof contacts the other of the cylinder and the outer circumferential surface of the piston for fluid-tightly sealing a fitting portion between the piston and the cylinder; a pressing member arranged on an outer or inner circumferential side of the seal member in the annular groove for urging a center portion in the axial direction of the seal member toward the cylinder or the outer circumferential surface of the piston; and an engaging portion formed on at least one end portion in the axial direction of the seal member and radially protruding for preventing the seal member from separating from the pressing member in assembling the pressing member and the seal member; wherein: the engaging portion is configured not to receive a load from the pressing member in the state that the pressing member and the seal member are fitted in the annular groove; a surface pressure which the seal member applies to the cylinder or the outer circumferential surface of the piston at an axial center portion thereof is higher than surface pressures which the seal member applies to the cylinder or the outer circumferential surface of the piston at axial end portions thereof; and the seal member possesses two oppositely disposed axial end surfaces, each axial end surface of the seal member facing toward, and being spaced apart from, a respective one of the two interior walls bounding the annular groove; wherein the seal member is provided with a slant surface which connects the seal surface with the axial end surface thereof; and wherein a connection point between the seal surface and the slant surface is located on an end portion side in the axial direction beyond a portion where the pressing member presses the engaging portion when brought into contact with the engaging portion.