1. A pipe joint in which a spigot of a first pipe is inserted into a socket of a second pipe, 5 a lock-ring storage groove is formed on an inner circumference of the socket, a lock ring stored in the lock-ring storage groove is attached to an outer circumference of the spigot, a spigot protrusion is formed on the outer 10 circumference of the spigot, and the spigot protrusion is engaged with the lock ring from a deeper portion of the socket in a separation direction of the spigot so as to prevent the spigot from separating from the socket, wherein 15 the lock ring has a first surface for preventing separation facing an opening end of the socket and a second surface for preventing separation facing the deeper portion of the socket, and in a state in which the spigot protrusion is engaged 20 with the lock ring and the first surface for preventing separation of the lock ring is in contact with an opening edge of the lock-ring storage groove around a pipe axial direction, a surface normal of a tangential plane at a contact point between the opening edge of the lock-ring 25 storage groove and the first surface for preventing separation of the lock ring passes through the second surface for preventing separation of the lock ring. 60

2. The pipe joint as claimed in claim 1, wherein an inner edge of the lock ring is included in the second surface for preventing separation, and the surface normal of the tangential plane passes 5 through the inner edge of the lock ring while passing through the second surface for preventing separation.

3. The pipe joint as claimed in claim 1, wherein an inner edge of the lock ring is included in the second surface for 10 preventing separation, and the surface normal of the tangential plane passes through the second surface for preventing separation between the inner edge of the lock ring and a position where an outer edge of the spigot protrusion is in contact 15 with the second surface for preventing separation.

4. The pipe joint as claimed in one of claims 1 to 3, wherein the first surface for preventing separation of the lock ring is shaped like a circular conical surface. 20

5. The pipe joint as claimed in one of claims 1 to 3, wherein the first surface for preventing separation of the lock ring is curved along an arc having a center on the surface normal of the tangential plane. 25

6. The pipe joint as claimed in claim 5, wherein the center of the arc of the first surface for preventing 61 separation is located at an intersection point of the surface normal of the tangential plane and the second surface for preventing separation. 5 7. The pipe joint as claimed in claim 1, wherein the lock ring is a ring having a single-cut structure with a dividing portion at a point along a circumferential direction, end faces at the dividing portion of the lock ring in the circumferential direction have engaging portions that 10 allow engagement of a spacing keeping tool, the spacing keeping tool keeping a spacing between the end faces of the lock ring increased in diameter, at least one of the end faces of the lock ring has a release portion for releasing the spacing keeping tool in 15 a separation direction that separates the spacing keeping tool inward from the end face of the lock ring in a radial direction of the lock ring, if the lock ring is stored in a normal orientation in the lock ring storage groove, the engaging portion is placed 20 near the opening end of the socket and the release portion is placed near a deep end of the socket, and the engaging portions and the release portion are exposed inward from the lock-ring storage groove in the radial direction in a state in which the lock ring is 25 increased in diameter. 62

8. The pipe joint as claimed in claim 7, wherein the release portion is an inclined face extending from the end face of the lock ring to an inner circumference face of the lock ring, and the inclined face tilts in a direction that 5 increases the spacing between the end faces of the lock ring toward inside in the radial direction.

9. The pipe joint as claimed in claim 7, wherein the release portion is a curved surface extending like an arc 10 from the end face of the lock ring to an inner circumference face of the lock ring, and the curved surface is curved in a direction that increases the spacing between the end faces of the lock ring toward inside in the radial direction. 15 10. A pipe joining method for assembling the pipe joint as claimed in one of claims 7 to 9, the method comprising: setting the lock ring into the lock-ring storage groove in the socket; inserting the spacing keeping tool from the opening 20 end of the socket into the dividing portion of the lock ring and engaging the spacing keeping tool with the engaging portions of the end faces while the lock ring is increased in diameter by using the expander tool; removing the expander tool from the lock ring and 25 keeping, by using the spacing keeping tool, the spacing between the end faces of the lock ring increased in diameter; and 63 removing the spacing keeping tool from the lock ring and reducing the diameter of the lock ring when the spigot protrusion is caused to pass through an inner circumference of the lock ring from the opening end to the deeper portion 5 of the socket by inserting the spigot into the socket.

We claim:
1. A pipe joint in which
a spigot of a first pipe is inserted into a socket of
a second pipe,
5 a lock-ring storage groove is formed on an inner
circumference of the socket,
a lock ring stored in the lock-ring storage groove is
attached to an outer circumference of the spigot,
a spigot protrusion is formed on the outer
10 circumference of the spigot, and
the spigot protrusion is engaged with the lock ring
from a deeper portion of the socket in a separation
direction of the spigot so as to prevent the spigot from
separating from the socket, wherein
15 the lock ring has a first surface for preventing
separation facing an opening end of the socket and a second
surface for preventing separation facing the deeper portion
of the socket, and
in a state in which the spigot protrusion is engaged
20 with the lock ring and the first surface for preventing
separation of the lock ring is in contact with an opening
edge of the lock-ring storage groove around a pipe axial
direction, a surface normal of a tangential plane at a
contact point between the opening edge of the lock-ring
25 storage groove and the first surface for preventing
separation of the lock ring passes through the second
surface for preventing separation of the lock ring.
60
2. The pipe joint as claimed in claim 1, wherein an inner
edge of the lock ring is included in the second surface for
preventing separation, and
the surface normal of the tangential plane passes
5 through the inner edge of the lock ring while passing
through the second surface for preventing separation.
3. The pipe joint as claimed in claim 1, wherein an inner
edge of the lock ring is included in the second surface for
10 preventing separation, and
the surface normal of the tangential plane passes
through the second surface for preventing separation
between the inner edge of the lock ring and a position
where an outer edge of the spigot protrusion is in contact
15 with the second surface for preventing separation.
4. The pipe joint as claimed in one of claims 1 to 3,
wherein the first surface for preventing separation of the
lock ring is shaped like a circular conical surface.
20
5. The pipe joint as claimed in one of claims 1 to 3,
wherein the first surface for preventing separation of the
lock ring is curved along an arc having a center on the
surface normal of the tangential plane.
25
6. The pipe joint as claimed in claim 5, wherein the
center of the arc of the first surface for preventing
61
separation is located at an intersection point of the
surface normal of the tangential plane and the second
surface for preventing separation.
5 7. The pipe joint as claimed in claim 1, wherein the lock
ring is a ring having a single-cut structure with a dividing
portion at a point along a circumferential direction,
end faces at the dividing portion of the lock ring in
the circumferential direction have engaging portions that
10 allow engagement of a spacing keeping tool, the spacing
keeping tool keeping a spacing between the end faces of the
lock ring increased in diameter,
at least one of the end faces of the lock ring has a
release portion for releasing the spacing keeping tool in
15 a separation direction that separates the spacing keeping
tool inward from the end face of the lock ring in a radial
direction of the lock ring,
if the lock ring is stored in a normal orientation in
the lock ring storage groove, the engaging portion is placed
20 near the opening end of the socket and the release portion
is placed near a deep end of the socket, and
the engaging portions and the release portion are
exposed inward from the lock-ring storage groove in the
radial direction in a state in which the lock ring is
25 increased in diameter.
62
8. The pipe joint as claimed in claim 7, wherein the
release portion is an inclined face extending from the end
face of the lock ring to an inner circumference face of the
lock ring, and the inclined face tilts in a direction that
5 increases the spacing between the end faces of the lock
ring toward inside in the radial direction.
9. The pipe joint as claimed in claim 7, wherein the
release portion is a curved surface extending like an arc
10 from the end face of the lock ring to an inner circumference
face of the lock ring, and the curved surface is curved in
a direction that increases the spacing between the end
faces of the lock ring toward inside in the radial direction.
15 10. A pipe joining method for assembling the pipe joint
as claimed in one of claims 7 to 9, the method comprising:
setting the lock ring into the lock-ring storage
groove in the socket;
inserting the spacing keeping tool from the opening
20 end of the socket into the dividing portion of the lock
ring and engaging the spacing keeping tool with the engaging
portions of the end faces while the lock ring is increased
in diameter by using the expander tool;
removing the expander tool from the lock ring and
25 keeping, by using the spacing keeping tool, the spacing
between the end faces of the lock ring increased in
diameter; and
63
removing the spacing keeping tool from the lock ring
and reducing the diameter of the lock ring when the spigot
protrusion is caused to pass through an inner circumference
of the lock ring from the opening end to the deeper portion
5 of the socket by inserting the spigot into the socket.