A PIPE FITTING

Abstract: A pipe coupling comprising a tubular spigot (1) for insertion into the end of a pipe (8) and a sleeve (7) for positioning over the end of the pipe (8) into which the spigot (1) has been inserted. The outer surface of the spigot (1) defines a series of circumferential ribs (4) separated by circumferential grooves (5). When the coupling is assembled the sleeve (7) traps the pipe end (8) against the spigot (1) such that the ribs (4) are pressed into the inner wall of the tube and material displaced by the ribs (4) is pressed into the grooves (5). The mean cross-sectional area of the space defined between the radially outer surface of the spigot (1) and the radially inner of the sleeve (7) in the assembled coupling reduces from the end (2) of the spigot which is first inserted into the pipe. Thus, the compression of the pipe end (8) increases towards the end of the pipe, ensuring that relatively thick walled pipes are tightly gripped close to the spigot end (2) whereas relatively thin walled pipes are tightly gripped by the ribs (4) and grooves (5) remote from the spigot end (2).

Description: - / - A PIPE FITTING This invention relates to a pipe fitting. Plastics pipes are widely used in, for example, fuel gas and water distribution networks where it is important that all connections between pipes and between pipes and various appliances are very reliable. The connections must be leak proof and strong enough to prevent them from being pulled apart as a result of any end load on a pipe. Such an end load may result from thermal expansion or contraction in the pipe's length. Medium density polyethylene pipes have been widely used in Britain, particularly for gas and water distribution, and such pipes have often been interconnected using couplings of the type described in British Patent Number GB B1596112. These couplings essentially comprise a sleeve which is slid over the end of the pipe to be connected and a circumferentially grooved and ribbed tubular spigot which is pushed into the pipe end. The ribs on the spigot have an external diameter slightly greater than the internal diameter of the pipe, whereas the sleeve has an internal diameter approximately the same as the external diameter of the pipe. The coupling is assembled by pulling the sleeve on to the end of the pipe containing the spigot, thus deforming the pipe which is compressed between the sleeve and spigot. Some of the plastics material is forced into the circumferential grooves in the spigot, thereby forming a very strong and leak proof connection. The spigot may be integral with an appliance to which the pipe is to be connected or integral with a second spigot thus facilitating pipe to pipe connections. Pipe couplings of this sort have been in successful use for many years, but it is necessary to exercise strict control over the manufacturing tolerances of both the pipe to be connected and the spigot and sleeve components. If, for instance, the pipe is produced with a wall thickness which is too large, or a spigot is produced with an outside diameter too large, or a sleeve produced with the inside diameter too small, then the annular gap between the insert and the sleeve may be too small to accommodate the pipe material. Thus when the sleeve is slid over the end of the pipe containing the spigot to- assemble the coupling a relatively large amount of the pipe material is forced ahead of the sleeve and high compressive forces are included in the pipe material. This results in a very large stress being applied by the sleeve, in which circumstances it has been found that the ability of a coupling to resist end loads is reduced. It appears, from tests, that when the stress on the pipe inside the coupling is large, substantially all of the end load is taken by the material adjacent to the rib nearest the end of the spigot, there then being a tendency for the pipe to break in the region of high stress adjacent the end of the spigot. In contrast, however, if the pipe and coupling component tolerances are maintained strictly within predetermined limits, then end load tests result in the pipe failing outside the coupling. It is possible to manufacture pipes and coupling components within very tight tolerances but only at significant cost. Furthermore, the tolerances are more critical for the high density materials which are now being more widely used in preference to medium density polyethylene. Accordingly relaxing pipe and/or coupling component tolerances, if this could be achieved without risking coupling reliability, would result in substantial economic benefits. It is an object of the present invention to obviate or mitigate the above problems. According to the present invention, there is provided a pipe coupling comprising a tubular spigot for insertion into the end of a pipe, and a sleeve for positioning over the end of a pipe into which the spigot has been inserted to thereby trap the pipe end between the spigot and sleeve, the outer surface of the spigot defining a plurality of circumferential ribs separated by circumferential grooves, wherein the mean cross-sectional area of the space defined between the radially outer surface of the spigot and the radially inner surface of the sleeve in the assembled coupling reduces from the end of the spigot which is first inserted into the pipe. As a result of the reducing cross-sectional area, the space available to accommodate the pipe end reduces progressively with distance from the free end of the spigot. Thus thick-walled pipes are securely gripped adjacent the free end of the spigot, and thin walled pipes are securely gripped away from the free end of the spigot. Preferably the mean cross-sectional area of the space radially outside the groove and rib closest to the said first inserted end is greater than the mean cross-sectional area of the space radially outside the groove and rib next closest to the said first inserted end, the mean cross-sectional area reducing for each adjacent groove and rib pair along the length of the spigot. The reduction in cross- sectional area may be achieved for example by tapering the spigot, or by "stepping" adjacent rib and groove pairs along the length of the spigot. Alternatively the ribs may be of constant width and the grooves of progressively varying width along the length of the spigot, or the grooves could be of constant width and the ribs of progressively varying width along the length of the spigot. A specific embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which; Figure 1 is a part sectional view of a spigot of a coupling according to the present invention; Figure 2 is an enlarged cross sectional view of part of the spigot of Figure 1; Figure 3 is a part sectional view of a coupling according to the present invention incorporating the spigot of Figure 1 prior to assembly; Figure 4 shows in section part of the coupling of Figure 1 fully assembled; and Figure 5 is an enlarged cross sectional view of part of an alternative spigot of a coupling in accordance with the present invention. Referring to Figures 1 and 2, the spigot consists of a tapered tubular body 1 with an additionally tapered narrow end 2 and an external annular flange 3 at its wider end. The tubular body 1 is formed with circumferential ribs 4 separated by circumferential grooves 5, on its external surface. In axial cross-section, the grooves 5 have straight sides and a straight base with obtuse angles being formed between the base and sides, and the ribs have a flattened crest. All the ribs 4 and grooves 5 are of the same height and depth respectively, the bases of the grooves 4 and the crests of the ribs 4 having the same angle of taper. This angle of taper is indicated by the two broken lines, one of which is parallel to the main spigot axis and the other of which is parallel to the crests of the ribs and the base of the grooves. The flange 3 is provided with bolt holes 6 so that the spigot may be connected to another device (not shown), e.g. another spigot to facilitate pipe to pipe connections. Alternatively, a conventional loose flange will be provided, the loose flange acting on a stub projecting from the spigot. The spigot of Figure 1 is used with a sleeve 7 shown in Figure 3. In use, the sleeve 7 is first slipped over the end of a pipe 8. The internal diameter of the sleeve 7 is substantially the same as the external diameter of the pipe 8, whereas the external diameter of the ribs 4 is greater than the internal diameter of the pipe 8. A hydraulic or mechanical press (not shown) is first used to force the spigot into the pipe end, causing the pipe end to radially expand, and is then used to force the sleeve 7 over the end of the pipe 8 surrounding the spigot body 1. The end of the pipe 8 is thus compressed between the sleeve and the spigot. As shown in Figure 4, material from the inner surface of the pipe is forced to flow over the ribs 4 and into the grooves 5. The degree to which each groove 5 is filled by the pipe material forced into it depends on the thickness of the pipe wall and the difference between the diameter of the base of the groove 5 and the internal diameter of the sleeve 7. Thus, because the spigot body is tapered, the first groove 5 near the end of the spigot is only slightly filled, each successive groove 5 then being filled to a greater degree as the diameter of the spigot body increases. This continues along the length of the spigot. Beyond this point material from the pipe is forced to flow along the length of the coupling as the sleeve is pushed home. However, before this point is reached, there will be a number of grooves 5 which are not completely filled. Thus it can be seen that by tapering the spigot body 1, there will always be at least some grooves 5 only partially filled. If an end load was applied to the pipe, that load would be distributed between the ribs 4 rather than being concentrated adjacent the rib nearest the spigot end. If a coupling is used with a pipe having a relatively thin wall, the grooves and ribs remote from the spigot end will provide the required engagement with the pipe. If used with a relatively thick walled pipe, the grooves and ribs adjacent the spigot end will provide the necessary engagement. Thus pipe couplings of this type do not have to be manufactured to the very tight tolerances required of existing couplings, and also have a greater tolerance to accommodate variations in pipe dimensions, which is outside the control of coupling manufacturers . Figure 5 shows part of an alternative spigot in which the tapering of the spigot body 1 is achieved by "stepping" successive rib and groove pairs. Figure 5 also shows an alternative configuration for the end of the spigot body 1, which is stepped as opposed to tapered. As a further alternative, the axial dimensions of successive ribs and/or grooves may vary. For example, given constant rib and groove diameters the ribs may be of constant width and the grooves of progressively reducing width, or the grooves could be of constant width and the ribs of progressively increasing width. It will be understood that any of the aforementioned embodiments of the spigot could be combined in any combination, for example a tapered insert with successively wider grooves, taper angles which vary along the length of the spigot, or different step heights between adjacent grooves and ribs. As a further alternative, a combination of a parallel spigot with a tapered sleeve bore could achieve the desired loud distinction. The taper of the spigot and/or the varying dimensions of the grooves, can be chosen to suit the characteristics of any particular plastics material and the required compression along the length of the coupling. The present invention provides a range of possible advantages, for example: 1. Reduction of maximum pipe compression at the groove nearest the spigot end, thus removing the problem of premature yielding adjacent that groove. 2. Distribution of end load along the length of the coupling. 3. Ability to vary the compression along the length of the coupling to suit the characteristics of different types of plastics material. 4. Ability to increase the manufacturing tolerances of the spigot and sleeve, therefore allowing more economic methods of manufacture to be used. 5. Ability to reduce the number of grooves required to give desired levels of mechanical performance, and therefore to decrease the length of the coupling and its cost. 6. Ability to reduce the required maximum coupling assembly loads, and thus the size of assembly equipment, therefore reducing equipment costs and making jointing easier. 7. Ability to accommodate larger tolerances on plastics pipe diameters and wall thickness. SUBSTITUTE SHEET

Claims: CLAIMS: 1. A pipe coupling comprising a tubular spigot for insertion into the end of a pipe, and a sleeve for positioning over the end of a pipe into which the spigot has been inserted to thereby trap the pipe end between the spigot and sleeve, the outer surface of the spigot defining a plurality of circumferential ribs separated by circumferential grooves, wherein the mean cross-sectional area of the space defined between the radially outer surface of the spigot and the radially inner surface of the sleeve in the assembled coupling reduces from the end of the spigot which is first inserted into the pipe. 2. A pipe coupling according to claim 1, wherein the mean cross- sectional area of the space radially outside the groove and rib closest to the said first inserted end is greater than the mean cross-sectional area of the space radially outside the groove and rib next closest to the said first inserted end, the mean cross-sectional area reducing for each adjacent groove and rib pair along the length of the spigot. 3. A pipe coupling according to claim 1 or 2, wherein the spigot is tapered. 4. A pipe coupling according to claim 1 or 2, wherein the sleeve is tapered. 5. A pipe coupling to claim 1 or 2, wherein adjacent rib end groove pairs along the length of the spigot are of different outside diameter. 6. A pipe coupling according to claim 1 or 2, wherein the ribs are of constant width and the grooves are progressively varying widths along the length of the spigot. 7. A pipe coupling according to claim 1 or 2, wherein the grooves are of constant width and the ribs are of progressively varying widths along the length of the spigot. SUB

Inventors: SMITH TREVOR REGINALD, WELFARE ANDREW, SMITH, TREVOR, REGINALD, WELFARE, ANDREW
Applicants: WASK RMF LTD, WASK-RMF LIMITED
Application year: 1992
Main classification: F16L33
IPC codes: F16L33