Spool valve

1. A spool valve comprising a spool that is accommodated in a cylindrical space movably in an axial direction of the spool to change a communication state between an input port through which fluid flows in and an output port through which fluid flows out, wherein:

the input port and the output port open into the cylindrical space;

an opening of the output port and an opening of the input port exist away from each other in the axial direction on an inner wall surface that defines the cylindrical space;

the spool includes a sliding contact section that moves in sliding contact with the inner wall surface to open or close the opening of the input port;

the movement of the sliding contact section in the axial direction changes the communication state between the input port and the output port;

the spool further includes a small diameter section that has a smaller diameter than a diameter of the sliding contact section, the small diameter section having a predetermined axial length within which the diameter of the small diameter section is constant in the axial direction;

the opening of the output port is opened into an internal space that is formed between an outer peripheral surface of the small diameter section and the inner wall surface;

the sliding contact section make the opening of the input port open to the internal space by movement of the spool in the axial direction, so that the input port and the output port communicate with each other;

the spool further includes a groove on an outer peripheral surface of the spool, the groove being formed between the sliding contact section and the small diameter section in the axial direction, and the groove having a predetermined length in the axial direction and recessed radially inward of the spool;

a groove bottom of the groove in a range on one side in the axial direction including one end of the groove in the axial direction is inclined to rise further radially outward of the spool toward the one side in the axial direction;

the groove bottom of the groove in a range on the other side in the axial direction including an other end of the groove in the axial direction is inclined to rise further radially outward of the spool toward the other side in the axial direction;

the other end of the groove in the axial direction exists at the sliding contact section;

a critical state is assumed in which a position of one end of the opening of the input port in the axial direction and a position of the other end of the groove in the axial direction accord with each other with respect to the axial direction; and

in the critical state, when a straight line is extended in the axial direction as well as radially outward of the spool without changing a direction of the straight line in a circumferential direction of the spool from one end of the groove bottom in the axial direction with the same inclination as the inclination of the groove bottom at the one end of the groove in the axial direction relative to the axial direction, the straight line passes through the opening of the output port to contact a wall surface of the output port.

2. The spool valve according to claim 1 , wherein:

the output port includes:

a recess that is recessed radially outward of the spool from the inner wall surface; and

a passage that extends from the recess radially outward of the spool; and

the straight line contacts a wall surface of the recess.

3. The spool valve according to claim 1 , wherein:

the groove extends across the sliding contact section and the small diameter section;

the one end of the groove in the axial direction exists at the small diameter section; and

the small diameter section is reduced in diameter at the one end of the groove in the axial direction, in a step-wise manner.

4. The spool valve according to claim 3 , wherein:

the spool further includes another sliding contact section on one side of the small diameter section in the axial direction;

the another sliding contact section moves in sliding contact with the inner wall surface;

the internal space spreads between the sliding contact section and the another sliding contact section in the axial direction;

a part of the small diameter section further on one axial side than the one end of the groove in the axial direction includes:

a first section having a smaller diameter than a diameter of a part of the small diameter section where the groove exists; and

a second section having a larger diameter than the diameter of the first section and a smaller diameter than a diameter of the another sliding contact section; and

the second section exists on one side of the first section in the axial direction.

5. The spool valve according to claim 1 , wherein:

the groove is one of two grooves that exist 180 degrees away from each other in the circumferential direction; and

the output port spreads by 180 degrees in the circumferential direction and penetrate in a radial direction of the spool.

6. The spool valve according to claim 1 , further comprising a rotation restriction section that restricts rotation of the spool in the circumferential direction relative to the inner wall surface.

7. The spool valve according to claim 1 , wherein the groove exists only on an outer peripheral surface of the sliding contact section.

8. The spool valve according to claim 1 , wherein:

the groove includes a predetermined range that continues from the other end of the groove in the axial direction to the one side in the axial direction; and

a width of the groove in the circumferential direction becomes larger toward the one side in the axial direction in the predetermined range.

9. The spool valve according to claim 1 , wherein:

the groove includes a predetermined range that continues from the one end of the groove in the axial direction to the other side in the axial direction; and

a width of the groove in the circumferential direction becomes larger toward the other side in the axial direction in the predetermined range.

10. The spool valve according to claim 1 , wherein:

the groove is one of a plurality of grooves that exist at positions away from each other in the circumferential direction;

the plurality of grooves include combinations of grooves; and

axial positions of the other ends of the plurality of grooves in the axial direction are different between the combinations of grooves.

11. The spool valve according to claim 1 , wherein:

the groove is one of a plurality of grooves that exist at positions away from each other in the circumferential direction;

the plurality of grooves include combinations of grooves; and

widths of the plurality of grooves in the circumferential direction are different between the combinations of grooves.

12. The spool valve according to claim 1 , wherein:

the groove is one of two grooves that exist at positions away from each other in the circumferential direction;

each of the two grooves does not have a mirror image symmetry with a plane including a center of the spool in the circumferential direction and an axis of the spool serving as a symmetry plane; and

the two grooves are positioned to have a mirror image symmetry with another plane including the axis of the spool serving as a symmetry plane.

13. The spool valve according to claim 1 , wherein the groove bottom is curved, and continues smoothly.

14. A spool valve comprising a spool that is accommodated in a cylindrical space movably in an axial direction of the spool to change a communication state between an input port through which fluid flows in and an output port through which fluid flows out, wherein:

the input port and the output port open into the cylindrical space;

an opening of the output port exists on one side of an opening of the input port in the axial direction away therefrom on an inner wall surface that defines the cylindrical space;

the spool includes a sliding contact section that moves in sliding contact with the inner wall surface to open or close the opening of the input port;

the movement of the sliding contact section in the axial direction changes the communication state between the input port and the output port;

the spool further includes:

a small diameter section that has a smaller diameter than a diameter of the sliding contact section and that continues with one side of the sliding contact section in the axial direction;

another sliding contact section that moves in sliding contact with the inner wall surface on one side of the small diameter section in the axial direction; and

another small diameter section having a smaller diameter than a diameter of the another sliding contact section on one side of the another sliding contact section in the axial direction;

the opening of the output port is opened into an internal space that is formed between an outer peripheral surface of the another small diameter section and the inner wall surface;

another internal space is formed between the sliding contact section and the another sliding contact section;

the spool makes the opening of the input port open to the another internal space by the movement of the sliding contact section to the other side in the axial direction, and makes the another internal space open to the internal space by the movement of the another sliding contact section to the other side in the axial direction, so that the input port and the output port communicate with each other;

the spool further includes a groove on an outer peripheral surface of the spool, the groove having a predetermined length in the axial direction and recessed radially inward of the spool;

a groove bottom of the groove in a range on one side in the axial direction including one end of the groove in the axial direction is inclined to rise further radially outward of the spool toward the one side in the axial direction;

the groove bottom of the groove in a range on the other side in the axial direction including an other end of the groove in the axial direction is inclined to rise further radially outward of the spool toward the other side in the axial direction;

the other end of the groove in the axial direction exists at the sliding contact section;

a critical state is assumed in which a position of one end of the opening of the input port in the axial direction and a position of the other end of the groove in the axial direction accord with each other with respect to the axial direction; and

in the critical state, when a straight line is extended to the one side in the axial direction as well as radially outward of the spool without changing a direction of the straight line in a circumferential direction of the spool from one end of the groove bottom in the axial direction with the same inclination as the inclination of the groove bottom at the one end of the groove in the axial direction relative to the axial direction, the straight line contacts a region of a wall surface that defines the another internal space, the region configured by components other than the spool.

15. The spool valve according to claim 14 , wherein the groove bottom is curved, and continues smoothly.