Thread joint for tube

摘要

In a thread joint of a surface-sealing type used for a tube, in which a pin portion (10) including a male thread (11) formed in a tapering configuration with respect to an axis of a tube, a sealing portion (12) formed in a tapering configuration at a tip of the male thread (11), and a shoulder portion (13) formed at a tip of the sealing portion (12), and a box portion (20) including a female thread (21) formed in a tapering configuration with respect to an axis of a tube, a sealing portion (22) formed in a tapering configuration in a back part of the female thread (21), and a shoulder portion (23) formed in a back part of the sealing portion (22), are screwed in into each other and bound so that sealing portions (12,22) come into contact with each other and shoulder portions (13,23) are abut each other, wherein a precedent-stage unthread portion (14) is disposed between the male thread (11) and the sealing portion (12) of the pin portion (10), and inclination of which is smaller than that of the sealing portion (12) of the pin portion (10), and the precedent-stage unthread portion (14) and the sealing portion (12) of the pin portion (10) are connected to each other by a curve (15) tangent to the sealing portion (12), when the curve (15) includes a plurality of arcs, a distance La from a contact point X between the sealing portion (12) and the curve (15) to an end point Z of the arc tangent to the sealing portion is 1.45mm or larger, and when the curve (15) is a single arc, a distance Lb from the contact point X to a boundary Y between the precedent-stage unthread portion (14) and the sealing portion (12) is 1.45mm or larger. <IMAGE>