Abstract:
A pipe coupling with an anchoring arrangement for locking the pipes together axially. The anchoring arrangement comprises a tubular casing, a frusto-conical gripping ring with inwardly projecting gripping teeth and tensioning bolts for tightening the casing about the frusto-conical gripping ring. The frusto-conical gripping ring is formed of a plurality of arcuate segments which overlap to form a complete ring. By forming the gripping ring of segments, the gripping ring is able to adapt to pipes that are not truly circular in cross section and a better grip on the pipe may be achieved than with a single-piece gripping ring.

Description:
FIELD OF THE INVENTION 
     The present invention relates to pipe couplings and is concerned with an arrangement for anchoring pipes to lock them axially together. It is particularly concerned with pipe couplings for connecting together two plain-ended pipes. 
     BACKGROUND OF THE INVENTION 
     In our patent specification EP-A-0542779 we describe a pipe coupling for connecting together two plain-ended pipes in a fluid-tight manner, of the type consisting of a tubular casing formed with a longitudinal gap, a sealing gasket of resilient flexible material typically of rubber or synthetic rubber, arranged within the casing, and tensioning means for reducing the width of the longitudinal gap so as to tighten the casing around the gasket. In use, the sealing gasket is placed around the adjacent pipe ends and the tensioning means are tightened to clamp the gasket against the outer surfaces of the pipe ends to form a fluid-tight seal. 
     Fluid pressure in a pipeline tends to force the pipe ends apart. The coupling of the above-mentioned patent specification has an arrangement for securing the pipe against axial movement relative to the coupling under such loads. The sealing gasket has circumferential slots in its outer surface at each end of the gasket. A frusto-conical gripping ring with inwardly projecting gripping teeth is located in each slot. In use, when the tensioning means are tightened around the casing the gripping teeth penetrate the sealing gasket at the bottom of the slot and engage the outer surface of the pipe to hold the pipe ends against axial displacement. 
     The known anchoring arrangement is satisfactory for many applications. We have found however that there are circumstances where an anchoring arrangement is required that will provide a better grip, for example, for pipes made of hard material, such as some grades of stainless steel, for example 316Ti, pipes that are not truly circular in cross section due to variations in manufacturing tolerances, and pipes that have to carry fluid at very high pressure. In some marine applications, for example, pipes are required to operate at 16 bar pressure. Safety regulations require couplings to perform at four times the operating pressure, that is 64 bar. The axial load on a coupling at 64 bar may be sufficient to cause the teeth to slip on the surface of the pipe in some situations. 
     With the known coupling, under normal operation, as the axial load on the pipe increases, the frusto-conical gripping ring tends to flatten, causing the teeth to bite deeper into the pipe surface, improving the grip. However, when the pipe is of a particularly hard material or misshapen, under large axial loads such as would be produced at 64 bar, slippage may occur due to a tendency for the teeth to ‘jump’ on the surface of the pipe. We have found that this ‘jumping’ occurs because the gripping ring does not engage the surface of the pipe uniformly around its circumference. This variation in grip arises due to the inability of the gripping ring to conform to variations in the shape of the pipe and variations in the load applied at different points around the circumference by the casing due to the presence of the longitudinal gap and the increased stiffness in the region where the tensioning means are attached to the casing which may result in the casing tending to flatten in some areas. 
     The gripping ring of the known coupling is formed of a single strip of steel pressed into conical form and wound into a helix so that when in place in the coupling the strip subtends an angle of more than 360° at its centre of curvature and the ends of the strip overlap to form a complete ring. 
     SUMMARY OF THE INVENTION 
     We have found that by forming the frusto-conical gripping ring of a plurality of arcuate segments which overlap to form a complete ring, the gripping ring achieves a better grip on the pipe surface and a coupling can be produced that performs better under high axial loads than the known coupling. 
     According to the present invention an anchoring arrangement for a pipe coupling for connecting together two pipes, the anchoring arrangement comprises a casing, a frusto-conical gripping ring with inwardly projecting gripping teeth located in the casing, and tensioning means for tightening the casing about the frusto-conical gripping ring, the frusto-conical gripping ring being formed of a plurality of arcuate segments which overlap to form a complete ring. 
     In one embodiment of the invention, the gripping ring is formed in two segments. The segments comprise a major segment which subtends an arc of greater than 180° at the axis of the gripping ring and a minor segment which subtends an arc of less than 180° at the axis of the gripping ring. The casing is formed with a longitudinal gap and the tensioning means is arranged to reduce the longitudinal gap when tightened, the minor segment is located adjacent the longitudinal gap and the major segment is located to the side of the casing remote from the longitudinal gap, the major and minor segments overlapping one another on either side of the gap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, of which:— 
         FIG. 1  shows a perspective cut-away view of a coupling in accordance with the invention; 
         FIG. 2  shows an end view of the coupling of  FIG. 1 ; 
         FIG. 3  shows a longitudinal section through the coupling of  FIG. 1 ; 
         FIG. 4  shows an exploded perspective view of the gripping ring of the coupling of  FIGS. 1 to 3 ; and 
         FIG. 5  shows an end view of the coupling of  FIG. 1  with the sealing gasket removed to reveal the gripping ring of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     A pipe coupling  1  comprises tubular casing  10 , a sealing gasket  30  and two anchoring arrangements  40 . The casing  10  is formed of a rolled steel strip, formed into a tube with a longitudinal gap  13  between the free ends of the strip. The free ends of the strip that forms the casing are folded back on themselves and welded at  14  to form loops  15  along opposite edges of the longitudinal gap  13 . Pins  16  are inserted in the loops. Tensioning bolts  17  pass through transverse holes in one of the pins  16  into tapped transverse holes in the other of the pins  16 , so as to interconnect the two free ends of the outer casing. Slots  18  are cut in the loops  15  so as to provide clearance for the bolts. The axial end margins of the casing  10  are bent inwardly at right angles to the tubular web portion  20  of the casing to form flanges  19  projecting inwardly towards the centre axis of the casing. 
     A bridging member  22  of rolled steel is located inside the casing  10  and spans the longitudinal gap  13 . The bridging member is curved about the centre axis of the casing. The end margins of the bridging member  22  are bent inwardly at right angles to the part-cylindrical web portion  23  of the bridging member to form flanges  24  projecting inwardly towards the centre axis of the casing. The casing  10  on either side of the longitudinal gap overlaps the bridging member. The axial length of the bridging member  22  is slightly less than that of the casing  10  so that, where they overlap, the flanges  24  fit inside the flanges  19 . 
     The sealing gasket  30  is of an elastomeric material, for example, rubber. The inner surface of the gasket is formed with two sets of annular sealing ribs  32  which project inwardly towards the centre axis of the seal. Near its ends the inner side of the sealing gasket is formed with raised sealing surfaces  33 . The extreme ends of the gasket are formed by axial extensions, which form end seals  34 . 
     The gasket  30  fits inside the casing formed by the casing  10  and the bridging member  22  with the side flanges  19  and  24  fitting into a recess  35  formed behind the end seals  34 . 
     A frusto-conical slot  36  is formed in the outer surface of the gasket at each end of the gasket. The outer end of each slot lies at the axial end of the outer surface, the inner end of the slot lies close to the inner surface of the raised sealing surface  33 . The slope of the slot is such that the inner end of the slot is nearer the axial middle of the gasket than the outer end. 
     An anchoring arrangement in the form of a frusto-conical ring  40  is located in each of the slots  36 . Each ring  40  is formed of two arcuate segments  40   a  and  40   b . The segment  40   a  is a minor segment and subtends at its centre of curvature an angle of less than 180°, say between 170° and 90°, typically approximately 140°. The segment  40   b  is a major segment and subtends at its centre of curvature an angle of more than 180°, say between 200° and 270°, typically approximately 250°. The segments  40   a  and  40   b  are placed together in the slot  36  so that they overlap in the regions  45  and form a complete frusto-conical ring. 
     The ring segments  40   a  and  40   b  are made of metal, preferably hard steel. The inner edges of the ring segments are cut at intervals to form teeth  42 . When the coupling is assembled the teeth sit in the bottom of their respective slot  36  and the outer edges  43  of the ring segments sit in the angle  44  formed by the flanges  19  and  24  and inner surfaces of the tubular web portions  20  and  23  of the casing and the bridging member. 
     In use, the ends of two plain ended pipes  60  are inserted into the coupling from opposite ends as shown in  FIG. 3 . A space of, say, 10 mm may be left between the pipe ends to allow for angular deflection, or to avoid abrasive wear, or to dampen vibration. With the coupling in place, the bolts  17  are tightened to clamp the coupling to the pipes. As the bolts  17  are tightened, the edges of the casing  10  on either side of the longitudinal gap  13  are drawn together, thereby causing the casing to apply a radially compressive force to the sealing gasket and the frusto-conical ring. The bridging member is arranged so that it spans the longitudinal gap  13 . In this way the bridging member supports the gasket in the region of the gap  13  where there is no support from the casing. 
     The tightening of the tensioning bolts  17  presses the annular sealing ribs  32  and the sealing surfaces  33  into sealing contact with the outer surfaces of the pipe ends. At the same time, the gripping ring  40  is pushed inwardly towards the centre axis of the coupling by the casing  10  and the bridging member  22 , the outer edge of the ring being gripped by the casing in the angle between the cylindrical web portion  20  and  25  of the casing and the flanges  19  and  24 . The compression of the gripping ring causes the edges of its teeth  42  to penetrate through the gasket  30  at the bottom of the slot  36  and bite into the surface of the pipe, thereby anchoring of the coupling to the pipe against axial movement. The overlapping parts of the ring segments  40   a  and  40   b  can slide over one another in the slot allowing the ring to shrink in diameter as it is compressed by the tightening of the casing. 
     The extensions  34  are trapped between the flanges  19  and  24  on the casing  10  and the bridging member  22 , on the one hand, and the outer surfaces of the pipe ends on the other, to form end seals to prevent liquid and dirt from entering the coupling from outside. 
     The slot  36  in which the toothed ring segments are placed is sufficiently wide with regard to the thicknesses of the ring segments that they can slide over one another when the coupling is tightened and find their own settlement. A lubricant may be applied to the ring segments to enable their overlapping portions to slide over one another more easily. The outer-edges are located in the angle between the flanges and web portions of the casing and the bridging member. The positions of the inner edges are determined by the position where they bites into the surface of the pipe. As the coupling is tightened the teeth bite deeper into the pipe. The angle of approximately 45 degrees which is established between the ring and the pipe surface when the ring first bites is maintained as the coupling is tightened. 
     We have found that a coupling with the anchoring arrangement as described above can perform satisfactorily under test pressures of 64 bar with pipes of 316Ti stainless steel. 
     Modifications of the arrangement described above are possible within the scope of the invention. For example, the gripping ring may be made of more than two segments. Where the gripping ring is formed of more than two segments, each segment overlaps the segments on either side of it circumferentially so that the overlapping segments form a complete ring around the axis of the pipe coupling.