Fluid pressure cylinder

1. A fluid pressure cylinder, comprising:

a cylinder tube having a central axis and an inner peripheral surface that is cylindrical about the central axis;

a piston that is accommodated in the cylinder tube, is slidable against the inner peripheral surface of the cylinder tube, and defines a first side chamber and an opposite, second side chamber within the cylinder tube;

a sealing ring held in a sealing ring accommodation groove formed in a circumferential direction in an outer peripheral surface of the piston, the sealing ring configured to cut off the first side chamber from the opposite, second side chamber;

a first piston ring that is held in a first piston ring accommodation groove formed in the circumferential direction in the outer peripheral surface of the piston, and includes a fractured portion which enables a diameter of the first piston ring to increase and reduce;

a second piston ring that is held in a second piston ring accommodation groove formed in the circumferential direction in the outer peripheral surface of the piston, and includes a fractured portion which enables a diameter of the second piston ring to increase and reduce, wherein the first piston ring accommodation groove is located between the second piston ring accommodation groove and the sealing ring accommodation groove;

a backup ring that is attached, without being accommodated in a groove, to the outer peripheral surface of the piston between the sealing ring and the first piston ring, wherein the backup ring has a first backup ring end and a second backup ring end opposite to each other in a central axis direction, the first backup ring end is kept in constant contact with the sealing ring and, simultaneously, the second backup ring end is kept in constant contact with the first piston ring to enable the first piston ring to support a load exerted, via the backup ring, on the sealing ring in the central axis direction; and

a bearing ring that is attached, without being accommodated in a groove, to the outer peripheral surface of the piston between the first piston ring and the second piston ring, the bearing ring configured to support the piston slidably relative to the cylinder tube, wherein the bearing ring has a first bearing ring end and a second bearing ring end opposite to each other in the central axis direction, the first bearing ring end is kept in constant contact with the first piston ring and, simultaneously, the second bearing ring end is kept in constant contact with the second piston ring.

2. The fluid pressure cylinder as defined in claim 1 , wherein

the backup ring, the bearing ring, the second piston ring and the first piston ring are disposed on the piston on a first side of the sealing ring relative to the central axis direction, and

the fluid pressure cylinder further comprises, on a second side of the sealing ring which is opposite to the first side in the central axis direction:

a further first piston ring that is held in a further first piston ring accommodation groove formed in the circumferential direction in the outer peripheral surface of the piston, and includes a fractured portion which enables a diameter of the further first piston ring to increase and reduce;

a further second piston ring that is held in a further second piston ring accommodation groove formed in the circumferential direction in the outer peripheral surface of the piston, and includes a fractured portion which enables a diameter of the further second piston ring to increase and reduce, wherein the further first piston ring accommodation groove is located between the further second piston ring accommodation groove and the sealing ring accommodation groove;

a further backup ring that is attached, without being accommodated in a groove, to the outer peripheral surface of the piston between the sealing ring and the further first piston ring, wherein the further backup ring has a first further backup ring end and a second further backup ring end opposite to each other in the central axis direction, the first further backup ring end is kept in constant contact with the sealing ring and, simultaneously, the second further backup ring end is kept in constant contact with the further first piston ring; and

a further bearing ring that is attached, without being accommodated in a groove, to the outer peripheral surface of the piston between the further first piston ring and the further second piston ring, wherein the further bearing ring has a first further bearing ring end and a second further bearing ring end opposite to each other the central axis direction, the first further bearing ring end is kept in constant contact with the further first piston ring and, simultaneously, the second further bearing ring end is kept in constant contact with the further second piston ring.

3. The fluid pressure cylinder as defined in claim 2 , wherein

the piston has first and second axial ends opposite to each other in the center axis direction, the first axial end is on the first side of the sealing ring, and the second axial end is on the second side of the sealing ring,

each of the sealing ring accommodation groove, the first piston ring accommodation groove, the second piston ring accommodation groove, the further first piston ring accommodation groove, and the further second piston ring accommodation groove is formed in the outer peripheral surface of the piston between the first and second axial ends, and not at the first and second axial ends,

the outer peripheral surface of the piston has no groove between the second piston ring accommodation groove and the first axial end, and

the outer peripheral surface of the piston has no groove between the further second piston ring accommodation groove and the second axial end.

4. The fluid pressure cylinder as defined in claim 3 , wherein

the sealing ring comprises an elastically deformable material,

each of the first piston ring, the second piston ring, the further first piston ring, and the further second piston ring comprises a metal material, and

each of the bearing ring and the further bearing ring comprises synthetic resin.

5. The fluid pressure cylinder as defined in claim 1 , wherein the sealing ring comprises an elastically deformable material.

6. The fluid pressure cylinder as defined in claim 5 , wherein the first piston ring and the second piston ring comprise a metal material.

7. The fluid pressure cylinder as defined in claim 6 , wherein the bearing ring comprises synthetic resin.

8. The fluid pressure cylinder as defined in claim 1 , wherein

the piston has axial ends relative to the center axis direction, and

each of the sealing ring accommodation groove, the first piston ring accommodation groove, and the second piston ring accommodation groove is formed in the outer peripheral surface of the piston other than the axial ends.

9. The fluid pressure cylinder as defined in claim 1 , wherein

the fractured portion of the first piston ring is constituted by end portions of the first piston ring and the fractured portion of the second piston ring is constituted by end portions of the second piston ring, and

the end portions of the first piston ring and the end portions of the second piston ring respectively have L-shaped cutouts.

10. The fluid pressure cylinder as defined in claim 1 , wherein

the backup ring, the bearing ring, the second piston ring and the first piston ring are disposed on the piston on a first side of the sealing ring relative to the central axis direction,

the fluid pressure cylinder further comprises, on a second side of the sealing ring which is opposite to the first side in the central axis direction:

a further first piston ring that is held in a further first piston ring accommodation groove formed in the circumferential direction in the outer peripheral surface of the piston, and includes a fractured portion which enables a diameter of the further first piston ring to increase and reduce; and

a further backup ring that is attached, without being accommodated in a groove, to the outer peripheral surface of the piston between the sealing ring and the further first piston ring, wherein the further backup ring has a first further backup ring end and a second further backup ring end opposite to each other in the central axis direction, the first further backup ring end is kept in constant contact with the sealing ring and, simultaneously, the second further backup ring end is kept in constant contact with the further first piston ring,

the piston has first and second axial ends opposite to each other in the center axis direction, the first axial end is on the first side of the sealing ring, and the second axial end is on the second side of the sealing ring,

each of the sealing ring accommodation groove, the first piston ring accommodation groove, the second piston ring accommodation groove and the further first piston ring accommodation groove is formed in the outer peripheral surface of the piston between the first and second axial ends, and not at the first and second axial ends, and

the outer peripheral surface of the piston has no groove between the further first piston ring accommodation groove and the second axial end.

11. The fluid pressure cylinder as defined in claim 10 , wherein

the sealing ring comprises an elastically deformable material,

each of the first piston ring and the second piston ring comprises a metal material, and

the bearing ring comprises synthetic resin.

12. The fluid pressure cylinder as defined in claim 8 , wherein

the outer peripheral surface of the piston has

first and second portions to which the backup ring and the bearing ring are attached, and

a third portion between the second piston ring accommodation groove and one of the axial ends of the piston, wherein

the third portion extends continuously from the second piston ring accommodation groove all the way to said one of the axial ends of the piston, and

the first and second portions of the outer peripheral surface of the piston are at the same radius from the central axis as the third portion of the outer peripheral surface of the piston.