Pipe joint

Each of seal projections 7, 8 is positioned radially outward of an inner periphery 1a, 2a of abutting end face of each of joint members 7, 8 and has in section a contour which comprises a circular-arc portion 7b, 8b extending radially outward from the abutting end face, and a straight portion 7a, 8a extending from the circular-arc portion 7b, 8b. The circular-arc portion 7b, 8b has a center positioned radially inward of the point of intersection of the circular-arc portion 7b, 8b and the straight portion 7a, 8a. The straight portion 7a, 8a is inclined at 30 to 60 degrees with respect to the direction of axis of the joint member.

BACKGROUND OF THE INVENTION
 The present invention relates to pipe joints, and more particularly to pipe
 joints suitable, for example, for use in manufacturing semiconductors of
 which extremely high cleanliness is required.
 The present inventor has already proposed a pipe joint suited, for example,
 to use in fabricating semiconductors of which extremely high cleanliness
 is required. With reference to FIG. 6, the proposed pipe joint comprises a
 pair of tubular joint members 1, 2, an annular gasket 3 interposed between
 abutting end faces of the joint members 1, 2 and threaded means for
 joining the joint members 1, 2, the abutting end faces of the joint
 members 1, 2 being formed with annular seal projections 37, 38
 respectively, so that when the threaded means is completely tightened up,
 recesses corresponding to the respective seal projections 37, 38 are
 created in end faces of the gasket 3, with respective inner peripheries
 1a, 2a of the abutting end faces of the joint members 1, 2 becoming
 approximately flush with an inner periphery 3a of the gasket 3. The pipe
 joint is characterized in that each of the seal projections 37, 38 has a
 sectional contour which comprises a circular-arc portion 37b (38b)
 extending radially outward from the abutting end face, and a straight
 portion 37a (38a) extending from the circular-arc portion 37b (38b) (see
 JP-A-32984/1997). Referring to the same drawing, the abutting end faces
 have respective inner flat portions 15, 16 formed inwardly of the seal
 projections 37, 38, and respective outer flat portions 17, 18 positioned
 outwardly of the projections 37, 38. Indicated at 9 and 10 are
 overtightening preventing annular ridges formed at the outer peripheries
 of the respective abutting end faces.
 The pipe joint described has no liquid trapping cavity, is unlikely to
 develop in the inner periphery of the gasket creases which would permit
 deposition of dirt and further enables the worker, while tightening the
 threaded member, to perceive an altered reaction which indicates the
 completion of tightening. Thus, the pipe joint has features suitable for
 use in manufacturing semiconductors. However, the joint has the following
 problem. While such pipe joints as assembled once are frequently
 disassembled and assembled again, the joint shown in FIG. 6 has the
 drawback that the gasket 3 is difficult to remove from the joint members
 1, 2 for disassembly because when the gasket 3 is to be moved axially
 relative to the first and second joint members 1, 2, the straight portions
 37a, 38a of the seal projections 37, 38 biting in the gasket 3 give
 increased friction against the movement of the gasket.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide a pipe joint which is easy
 to disassemble and assemble again and which nevertheless has no liquid
 trapping cavity, is unlikely to develop any dirt accumulating creases in
 the inner periphery of its gasket and is adapted to notify completion of
 tightening by an altered reaction, the pipe joint thus having features
 suitable for use in fabricating semiconductors.
 The present invention provides a pipe joint comprising a pair of tubular
 joint members, an annular gasket interposed between abutting end faces of
 the joint members and threaded means for joining the joint members, the
 abutting end face of each the joint members being formed with an annular
 seal projection, so that when the threaded means is completely tightened
 up, a recess corresponding to the seal projection is created in the end
 face of the gasket opposed thereto, with the inner periphery of the
 abutting end face of each joint member becoming approximately flush with
 an inner periphery the gasket, the pipe joint being characterized in that
 the seal projection is positioned radially outward of the inner periphery
 of the abutting end face of the joint member and has in section a contour
 which comprises a circular-arc portion extending radially outward from the
 abutting end face, and a straight portion extending from the circular-arc
 portion, the circular-arc portion having a center positioned radially
 inward of the point of intersection of the circular-arc portion and the
 straight portion, the straight portion being inclined at 30 to 60 degrees
 with respect to the direction of axis of the joint.
 As the threaded means is tightened, the seal projections of the joint
 members come into intimate contact with the respective end faces of the
 gasket, opposed thereto, and the inner peripheral portion of the abutting
 end face of each joint member then comes into intimate contact with the
 inner peripheral portion of the gasket end face opposed thereto, producing
 a sealing effect and eliminating any liquid trapping cavity. Although the
 gasket deforms most greatly at the position where each seal projection is
 located, this position is not the inner periphery of the gasket end face
 but is outward of the inner periphery, so that the gasket portion
 deforming most greatly is outward of the inner peripheral portion.
 Consequently no crease is formed in the inner periphery of the gasket,
 obviating the likelihood of dirt collecting in creases. If the straight
 portion included in the contour of the projection extends axially of the
 joint, there arises the problem that the gasket is difficult to remove
 when the pipe joint is to be disassembled, whereas the straight portion,
 which is inclined, obviates this problem. Moreover, the projection permits
 the worker to perceive an altered reaction more easily than when it is
 wholly in the form of a circular arc. Thus, the projection has an
 advantage of a projection which is entirely in the form of a circular arc
 and that of a projection which has an axial straight portion.
 If the inclination is smaller than 30 degrees, the projection will have the
 drawback of rendering the gasket difficult to remove, i.e., the problem
 attributable to the presence of the axial straight portion, while if the
 inclination is greater than 60 degrees, there arises a problem
 attributable to the projection which is arcuate in its entirety, i.e., the
 drawback that an altered reaction is difficult to perceive. Accordingly,
 the inclination is preferably 30 to 60 degrees, more preferably about 45
 degrees.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 An embodiment of the invention will be described below with reference to
 the drawings. In the following description, the terms "left" and "right"
 are used based on FIG. 1.
 Referring to FIGS. 1 and 2, the pipe joint shown comprises a first tubular
 joint member 1, a second tubular joint member 2, an annular gasket 3
 interposed between the right end face of the first joint member 1 and the
 left end face of the second joint member 2, and a retainer 5 holding the
 gasket 3 and held by the first joint member 1. The second joint member 2
 is fastened to the first joint member 1 by a nut 4 provided on the member
 and screwed onto the first joint member 1. The abutting end faces of the
 respective joint members 1, 2 are formed, each on its approximate radial
 midportion, with annular seal projections 7, 8, respectively, and have
 annular overtightening preventing ridges 9, 10, respectively, at the outer
 peripheral portion of each member.
 The gasket 3 has opposite flat end faces at right angles with the axis of
 the joint, and is provided on its outer periphery with a slipping-off
 preventing portion 3b in the form of an outer flange. This portion 3b
 makes it easy for the worker to cause the retainer 5 to hold the gasket 3
 thereto and cause the joint members 1, 2 to hold the retainer 5 thereto.
 The joint members 1, 2 and the gasket 3 are made of SUS 316L. The joint
 members 1, 2 have an inside diameter equal to the inside diameter of the
 gasket 3. A suitable stainless steel other than SUS 316L or other metal is
 usable for making the joint members 1, 2 and the gasket 3.
 The retainer 5, which is integrally made from a stainless steel plate,
 comprises a ring portion 25, a gasket holding portion 26 having three
 claws 28 inwardly projecting from the right end of the ring portion 25 and
 adapted to hold the outer periphery of the gasket 3, and a joint member
 holding portion 29 engageable with the right end of the first joint member
 1. The three claws 28 have slight resiliency. Each of the claws 28 has a
 rightwardly bent outer end to provide a bent lug 28a having slight
 resiliency. The gasket 3 is fitted to the inside of the claws 28 with the
 bent lug 28a in intimate contact with the gasket 3, whereby the gasket 3
 inside the retainer 5 is prevented from moving radially and axially
 thereof. At the position where each of the three claws 28 is formed, the
 ring portion 25 has a pair of axial cutouts to provide a holding pawl 30.
 Three pawls 30 thus formed provide the joint member holding portion 29.
 The three holding pawls 30 resiliently grip the right end outer surface of
 the first joint member 1, whereby the retainer 5 is held to the first
 joint member 1. The claws 28 and the holding pawls 30 are not limited to
 three in number; four claws or pawls may be provided.
 The nut 4 has an inner flange 11 formed at its right end and fitting around
 the second joint member 2. The nut 4 has at its left end an internally
 threaded portion 12, which is screwed on an externally threaded right
 portion 14 of the first joint member 1. The second joint member 2 has an
 outer flange 13 on the outer periphery of its left end. A thrust ball
 bearing 6 is interposed between the outer flange 13 and the inner flange
 11 of the nut 4 for preventing the joint member 2 from rotating with the
 nut 4.
 FIGS. 3 and 4 show the feature of the pipe joint of the invention in
 greater detail. Each of the seal projections 7, 8 has in section a contour
 which comprises a circular-arc portion 7b (8b) extending radially outward
 from the abutting end face of the joint member 1 (2), and a straight
 portion 7a (8a) extending from the circular-arc portion 7b (8b). The
 circular-arc portion 7b (8b) is in the form of a circular arc centered
 about a point positioned radially inward of the point of intersection of
 the circular-arc portion 7b (8b) and the straight portion 7a (8a). The
 straight portion 7a (8a) is inclined at 45 degrees with respect to the
 direction of axis of the joint.
 Each of the abutting end faces includes an inner flat face 15 (16) and an
 outer flat face 17 (18) at the respective inner and outer sides of the
 seal projection 7 (8). The inner flat face 15 (16) is positioned axially
 closer to the gasket 3 than the outer flat face 17 (18).
 The overtightening preventing ridges 9, 10 project rightward and leftward
 beyond the projections 7, 8, respectively, and are adapted to press the
 retainer 5 on opposite sides thereof when the nut 4 is to be tightened up
 more than is properly. These ridges 9, 10 protect the seal projections 7,
 8 of the joint members 1, 2 from damage before assembling, obviating the
 serious influence to be otherwise exerted on the sealing properties.
 FIG. 3 shows the pipe joint as tightened manually by the nut 4. As the nut
 4 is tightened, the extremity of each of the seal projections 7, 8 first
 comes into contact with the end face of the gasket 3 opposed thereto as
 illustrated in the drawing. At this time, an inner clearance G1 is present
 between the inner flat face 15 (16) of the joint member 1 (2) and the left
 (right) end face of the gasket 3, and an outer clearance G2 greater than
 the clearance G1 exists between the outer flat face 17 (18) of the joint
 member 1 (2) and the left (right) end face of the gasket 3. There is a
 still greater clearance G3 between the overtightening preventing annular
 ridge 9 (10) and the retainer 5. Thus, G1&lt;G2&lt;G3. As the nut 4 is further
 tightened as by a wrench from the manually tightened state, the gasket 3
 deforms, reducing the inner clearance G1 to zero. At this time, the outer
 clearance G2 is not zero. When the joint has been tightened up properly,
 the outer clearance G2 also diminishes to zero as shown in FIG. 4, the
 inner flat face 15 (16) comes into intimate contact with the inner
 peripheral portion of the left (right) end face of the gasket 3, and the
 inner periphery 1a (2a) of the joint member 1 (2) becomes substantially
 flush with the inner periphery 3a of the gasket 3. Consequently, no liquid
 trapping cavity remains. Even at this time, the clearance G3 between each
 annular ridge 9 (10) and the retainer 5 has not reduced to zero. When the
 nut 4 is further tightened, the clearance G3 between the ridge 9 (10) and
 the retainer 5 diminishes to zero, creating greatly increased resistance
 to tightening, whereby overtightening is prevented.
 With the pipe joint described, the gasket 3 and the seal projections 7, 8
 are so dimensioned as to eliminate the clearances G1, G2 between the
 gasket 3 and the seal projections 7, 8 when the tightening torque reaches
 the proper value. For example, the seal projections 7, 8 have a height of
 0.1 mm from the respective inner flat faces 15, 16, and the circular arc
 is 0.5 mm in radius. The outer flat faces 17, 18 are recessed from the
 respective inner flat faces 15, 16 by 0.02 mm. When the nut 4 has been
 rotated by hand, the distance between the retainer 5 and the
 overtightening preventing ridges 9, 10 is 0.17 mm. The inner flat faces
 15, 16 contact the gasket 3 when the nut 4, as manually rotated, is
 further rotated through 56.7 degrees, and the outer flat faces 17, 18
 contact the gasket when the nut 4 is rotated through 68 degrees from the
 manually rotated position. The distance between the joint members 1, 2 as
 positioned by manual tightening decreases by 0.317 mm when the nut has
 been tightened through exactly 90 degrees eventually, causing the inner
 flat faces 15, 16 and the outer flat faces 17, 18 to bite into the gasket
 3 by 0.06 mm and 0.04 mm, respectively. When the nut is further rotated
 through 5.7 degrees for tightening, the ridges 9, 10 come into contact
 with the retainer 5, whereby overtightening is precluded. The distance
 between the retainer 5 and the overtightening preventing ridges 9, 10 when
 the nut 4 is tightened by hand may be smaller than 0.17 mm, for example,
 0.15 mm.
 With the pipe joint of the invention, the frictional force against the
 axial movement of the gasket 3 relative to the first and second joint
 members 1, 2 is approximately equivalent to the corresponding force to be
 involved in the case where each seal projection has in section a
 circular-arc contour including no straight portion, so that the joint is
 free of the problem that the gasket 3 is difficult to separate from the
 joint members 1, 2 for disassembling.
 FIG. 5 shows the relationship between the angle of rotation of the nut 4
 plotted as ordinate and the tightening torque as abscissa, as established
 for the pipe joint of the invention. The drawing reveals that the slope
 alters when the angle of rotation of the nut 4 is about 70 degrees. Stated
 more specifically, the increase of the tightening torque relative to the
 angle of rotation of the nut 4 is small and the graph is consequently
 steep until the outer flat faces 17, 18 contact the gasket 3. After the
 contact of the outer flat faces 17, 18 with the gasket 3, however, the
 area of contact of each joint member 1 (2) with the gasket 3 is great,
 resulting in a great increase in the tightening torque relative to the
 angle of rotation of the nut 4 and accordingly making the slope of the
 graph approach the horizontal, hence an apparent alteration in the slope
 when the angle of rotation of the nut 4 is about 70 degrees. When the seal
 projection has in section a circular-arc contour including no straight
 portion, the slope remains unaltered at an angle of rotation of the nut of
 about 70 degrees although not illustrated.
 Table 1 shows the foregoing features of the invention as summarized. The
 table reveals that the present invention has overcome all the problems
 conventionally encountered.
 TABLE 1
 Invention Comp. Ex. 1 Comp. Ex. 2
 (see FIG. 4) (see FIG. 6) (not shown)
 Shape of projection Circular arc + Circular arc + Circular arc
 inclined straight axial straight only
 portion portion
 Liquid trapping None None None
 recess
 Creasing of gasket No No No
 inner periphery
 Perceived reaction Altered Altered Unaltered
 of tightening
 Removal of gasket Smooth Difficult Smooth
 for disassembly