Piston bearing structure for fluid pressure cylinder

1. A piston bearing structure for a fluid pressure cylinder in which a piston is supported in a cylinder tube, the piston bearing structure comprising:

one seal ring housed in a housing groove of the piston and held in sliding contact with the cylinder tube; and

two backup wear rings housed in the housing groove, held in sliding contact with the cylinder tube and sandwiching the seal ring,

wherein

the housing groove includes

a first groove which houses the seal ring, and

two second grooves which are shallower than the first groove and connected to opposite sides of the first groove, and

each of the backup wear rings has an L-shaped cross-sectional shape including

a wear ring portion which is housed in a corresponding one of the second-grooves and is adapted to receive a load of the piston, and

a backup ring portion which is housed in the first groove, faces the seal ring and is integrally formed with the wear ring portion,

wherein

the wear ring portion includes a pair of opening end portions to form a gap opening,

the backup ring portion includes a pair of bias cut opening end portions which are held in contact with each other in a state where the gap opening is maintained,

the backup wear ring includes a pressure relief groove which faces the housing groove and extends over the backup ring portion and the wear ring portion, and

the pressure relief groove allows opposite sides of the backup wear ring to communicate between the piston and the cylinder tube, and faces the housing groove over an entire extent of the pressure relief groove in the backup ring portion and the wear ring portion.

2. The piston bearing structure for the fluid pressure cylinder according to claim 1 , wherein

the seal ring has an X-shaped cross-sectional shape including two pairs of lips projecting to inner and outer peripheries of the seal ring,

the lips projecting to the inner periphery of the seal ring are formed with an annular chamfered portion held in contact with a bottom surface of the first groove,

a depth U of the pressure relief groove is smaller than a difference between

(i) a chamfer width T that is a distance from the bottom surface of the first groove to outer peripheral ends of the chamfered portions and

(ii) a clearance S that is an opening width of a clearance provided between an inner peripheral surface of the backup ring portion and the bottom surface of the first groove, and

an end of the pressure relief groove faces a part of the chamfered portion.

3. The piston bearing structure for the fluid pressure cylinder according to claim 1 , wherein

in a cross-section along an axial direction of each of the backup wear rings, the pressure relief groove extends continuously over an entire extent in which the backup wear ring faces the housing groove.

4. The piston bearing structure for the fluid pressure cylinder according to claim 1 , wherein, in each of the backup wear rings,

the bias cut opening end portions are inclined with respect to an axis of said backup wear ring, and

when the bias cut opening end portions are held in contact with each other, the opening end portions are spaced from each other by said gap opening in a circumferential direction of said backup wear ring.

5. The piston bearing structure for the fluid pressure cylinder according to claim 4 , wherein

each of the second grooves includes

a side surface, and

a bottom surface connected to the side surface,

the first groove includes

opposite side surfaces respectively connected to the bottom surfaces of the second grooves, and

a bottom surface between the opposite side surfaces and connected to the opposite side surfaces, and

in a cross-section along an axial direction of each of the backup wear rings, the pressure relief groove extends continuously over an entire extent in which the backup wear ring faces

the side surface of the corresponding second groove,

the bottom surface of the corresponding second groove,

a corresponding one of the opposite side surfaces of the first groove, and

the bottom surface of the first groove.