Patent ID: 6851720

Claim:
A hydraulic joint structure comprising: a first member having a first tubular joint portion and a hydraulic passage extending to the first tubular joint portion, the first tubular joint portion having a female thread portion at an end thereof; a second member having a second tubular joint portion and a hydraulic passage extending to the second tubular joint portion, the second tubular joint portion having a male thread portion at an end thereof for screw-engagement with the female thread portion; and an annular seal member disposed between the female portion and an end surface of the first tubular joint portion, the seal member abutting, in a liquid-tight manner, the first tubular joint portion at its outer circumference and the second tubular joint portion at its inner circumference, wherein the first member has a first pressure-contact surface constituted by the end surface of the first tubular joint portion, whereas the second member has a second pressure-contact surface, the second pressure-contact surface surrounding the second tubular joint portion and being brought into contact with the first pressure-contact surface in order to prevent loosening of the thread portions and to form a downstream space on the downstream side of the seal member, with respect to a direction of flow of any leaking working fluid; and a passage is formed on at least one of the first and second pressure-contact surfaces in such a manner that when working fluid leaks from the upstream side of the seal member to the downstream space via a clearance between the seal member and the first and second tubular joint portions, the passage permits the working fluid to flow from the downstream space to an external space in the state in which the first and second pressure-contact surfaces are in pressure contact, wherein the passage is formed in such a manner that the passage extends in a circumferential direction of the thread portions, and communicates at one end with the downstream space and at the other end with the external space, wherein the passage is constituted by a spiral groove formed on at least one of the pressure-contact surfaces.