GB2398612A

GB2398612A - Pipe coupling

Info

Publication number: GB2398612A
Application number: GB0303958A
Authority: GB
Inventors: Andrew Welfare
Original assignee: Glynwed Pipe Systems Ltd
Current assignee: Glynwed Pipe Systems Ltd
Priority date: 2003-02-21
Filing date: 2003-02-21
Publication date: 2004-08-25
Also published as: GB0303958D0

Abstract

A pipe coupling comprises a tubular body (1) one end of which defines a socket (2) for receiving a pipe end. A pipe stiffener (16) is provided for insertion into a pipe end to be coupled,and an annular gripper ring (9) surrounds the pipe end. The gripper ring (9) has a tapered outer surface and is adapted to collapse inwardly to grip the pipe supported by the tubular stiffener in response to a radially inward force on the gripper ring (9). A locking ring with (8) with a tapered inner surface surrounds the gripper ring and is axially movable to empart a radial inward force on the gripper ring (9). Means are provided to prevent the locking ring (8) from being withdrawn axially from the socket (6). Annular seal members (10) are provided within the socket (6) to receive the end of the pipe and then constrained to move with the pipe insert stiffener (16) when the pipe and stiffener are moved in a direction towards the socket opening.

Description

PIPE COUPLING

The present invention relates to a pipe coupling and in particular relates to a compression-end type coupling for connection to the end of a plastic pipe or similar tubing. Such compression-end couplings are often referred to as "push- fit" or "stab- type" couplings.

A conventional push-fit coupling typically comprises a tubular body with a central passage therethrough which is widened towards one end to define a socket for receiving a pipe end. The inner end of the socket is defined by an annular shoulder.

A tubular pipe insert stiffener (made for instance from steel) is located within the socket and has an outer diameter corresponding to an inner diameter of the pipe and somewhat less than the inner diameter of the socket so as to define an annular gap within which the pipe end can be received. The inner end of the pipe insert stiffener has a radial flange which bears against the annular shoulder.

A pipe gripping assembly comprising a radially deformable annular gripper ring and a cooperating locking ring are located around the insert stiffener within the socket. The gripper ring has pipe gripping teeth or similar formations defined on its radially inner surface, and its outer surface is frusto-conical tapering in the direction of the entrance to the socket. The locking ring on the other hand has a generally cylindrical outer surface (which bears against the inner surface of the socket) and a tapering radial inner surface which cooperates with the tapering outer surface of the gripper ring. Means, such as a cir-clip or the like, are provided to retain the locking ring (and thus the gripper ring) within the socket. The inner radial dimension of the pipe gripper ring and locking ring are such that the pipe end can be inserted over the pipe insert stiffener and through the gripping ring and locking ring when inserted into the socket.

A seal assembly, for instance comprising one or more O-rings is typically provided at the inner end of the socket, for instance located between the pipe gripper ring and the flange of the insert pipe stiffener.

In use, a plastic pipe end is simply inserted into the socket, into the annular gap defined between the outer surface of the pipe insert stiffener and the radial inner surfaces of the gripping ring, locking ring and seal assembly, until the end of the pipe bears against the pipe insert stiffener flange. The seal assembly is dimensioned so that it will initially exert a light gripping force on the pipe, so that a subsequent attempt to withdraw the pipe will tend to pull seal assembly and thus the gripping ring with it. As the gripping ring moves with the pipe its outer tapered surface cooperates with the inner tapering surface of the locking ring (which is prevented from moving within the socket) as a result of which the gripping ring and locking ring are wedged together, forcing the gripping ring to grip the pipe more tightly (the pipe insert stiffener supports the pipe against this gripping action). The pipe is thus firmly gripped within the coupling, and indeed the grip can be so firm that any failure in the pipe under load will occur outside of the coupling.

Some variations of the above basic structure include drive means, such as a threaded nut or the like, to drive the locking ring and gripping ring together to form a tight grip on the pipe, rather than relying purely on axial force applied to the pipe to wedge the locking ring and gripping ring together. Such a coupling is for instance disclosed in US patent 4, 445,714. A similar coupling is disclosed in UK Patent No. 2198496. This describes the provision of a retaining ring inward of an O- ring seal arrangement which initially grips the pipe to ensure that the pipe cannot be withdrawn from the coupling without wedging the locking ring and gripping rings together. In other words, the retaining ring provides an initial grip on the pipe to supplement that which would be provided by the O-ring seals.

A disadvantage of such conventional push-type couplings is that they tend to be physically bulky in order to accommodate the gripping ring/locking ring and seal arrangements etc. This physical size limits the range of applications to which conventional push-type fittings can be put.

Some prior art push-type couplings, such as those disclosed in US Patent No. 4,445,714 and UK Patent No. 2198496 have the disadvantage of requiring additional components, such as the drive nut and retaining ring arrangements, to ensure a pipe cannot be withdrawn from the coupling without engagement of the gripping ring.

According to a first aspect of the present invention there is provided a pipe coupling comprising: a tubular body defining a flow passage communicating with a pipe receiving socket, the socket being defined between a pipe receiving opening at one axial end of the body and an internal abutment; a pipe stiffener having a tubular body sized to be a friction fit within a pipe end to be coupled and having an outwardly extending flange at one end thereof with a maximum diameter greater than an internal diameter defined by said internal abutment and greater than the internal diameter of said pipe end; an annular gripper ring having a toothed inner surface and a tapered outer surface arranged to surround the received pipe end and adapted to collapse inwardly to grip the pipe supported by said tubular stiffener in response to a radially inward force imposed on the gripper ring; a locking ring having a tapered inner surface adapted to surround the tapered outer surface of the gripper ring and effective when moved axially over said gripper ring in a ramping in relation to impose a radial inward force thereon; means to prevent said locking ring from being withdrawn axially from the socket; and one or more annular seal members having an internal diameter dimensioned to receive said pipe end disposed within said socket between said gripper ring and said pipe stiffener flange and such that said annular seal members are constrained to move with said insert stiffener when said pipe end and stiffener move in a direction towards the socket opening.

According to a second aspect of the present invention there is provided a pipe coupling comprising: a tubular body defining a flow passage communicating with a pipe receiving socket, the socket being defined between a pipe receiving opening at one axial end of the body and a first internal annular shoulder; the socket having an internal annular step immediately adjacent said first annular shoulder defining a second annular shoulder and a reduced diameter socket portion extending between the first and second annular shoulders; a pipe stiffener having a tubular body sized to receive a pipe end and an outwardly extending flange at one end thereof sized to fit within the reduced diameter socket portion but having an outer diameter greater than the inner diameter of the first annular shoulder; an annular gripper ring having a toothed inner periphery and a tapered outer surface arranged to surround the received pipe end and adapted to collapse inwardly to grip the pipe supported by said tubular stiffener in response to a radially inward force imposed on the gripper ring; a locking ring having a tapered inner surface adapted to surround the tapered outer surface of the gripper ring and effective when moved axially over said gripper ring in a ramping relation to impose a radial inward force thereon; means to prevent said locking ring from being withdrawn axially from the socket; one or more annular seal members disposed within said socket between said gripper ring and said second annular shoulder; wherein the seal means and/or gripper ring are dimensioned to apply a relatively light gripping force to a pipe end inserted into the socket which increases as axial force is applied in a direction tending to withdraw the pipe from the socket through cooperation of the tapering surfaces of the locking ring and gripper ring.

According to a third aspect of the present invention there is provided a pipe coupling comprising: a tubular body defining a flow passage communicating with a pipe receiving socket, the socket having a pipe receiving opening defined at one axial end of the body; a deformable annular gripper ring arranged to surround the received pipe end and adapted to collapse inwardly to grip the pipe in response to a radially inward force imposed on the gripper ring, the gripper ring having a tapered outer surface and an annular inner surface provided with axially spaced circumferential teeth each of which has an inner peripheral gripping edge having a diameter no less than the outer diameter of said pipe end; a locking ring having a tapered inner surface adapted to surround the tapered outer surface of the gripper ring and effective when moved axially over said gripper ring in a ramping relation to impose a radial inward force thereon; means to prevent said locking ring from being withdrawn axially from the socket; wherein a plurality of contact portions are formed on the inner surface of the gripper ring, each contact portion being separated from a diametrically opposed portion of the gripper ring inner surface by a distance less than the external diameter of said pipe end.

According to a fourth aspect of the present invention there is provided a push- fit coupling, comprising a metallic body defining a socket at one end housing push-fit pipe gripping components and having an elongate tubular portion extending from said socket suitable for welding to a metal pipe fitting or pipe.

According to a fifth aspect of the present invention there is provided a method of coupling a plastics pipe end to a metal pipe fitting or pipe, the method comprising securing a push-fit coupling to the end of the plastics pipe and welding said push-fit coupling to the metallic pipe fitting or pipe.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cross-section through a first embodiment of a pipe end coupling according to the present invention; Figure 2 is a crosssection through a second embodiment of a pipe end coupling according to the present invention; Figure 3 is a third embodiment of a pipe coupling according to the present invention; Figure 4 shows a modified gripping ring which may be included in the couplings of Figures I to 3; and Figure 5 is a cross-section through a further embodiment of the present invention.

Referring to Figure 1, the illustrated coupling is a meter box adapter designed for coupling the end of a plastic pipe (such as a gas supply pipe) to a meter box (not shown). Accordingly, the coupling comprises a body I one end of which defines a socket 2 for receiving a pipe end (not shown), and the other end of which is formed as a reduced diameter screwthreaded tubular spigot 3 for connection to the metering equipment. The central passage through the coupling thus has a relatively narrow diameter portion 4 extending through the spigot portion 3 of the body 1, which widens into the socket 2 in the region of a first annular shoulder S. The socket 2 is formed with an annular step 6 defining a second annular shoulder 7, the socket 2 having a reduced diameter at its inner region defined by the annular step 6 between the first and second annular shoulders 5 and 7.

Moving inward from the mouth of the socket 2, the body 1 houses a locking ring 8, a gripping ring 9 and a pair of O-ring seals 10. The coupling is pre-assembled by first inserting the O-rings 10 in to the socket 6 so that they abut the second annular shoulder 7 (the O-rings having an outer diameter greater than the reduced diameter portion of the socket defined by annular step 6). The pipe gripping ring 9 is then pushed into the socket. The gripping ring 9 has a conventional structure comprising an array of inner circumferential teeth 11, an outer surface which has a tapered portion 12, and a structure which allows the gripping ring to be inwardly compressed.

Typically the gripper ring will comprise a circumferential array of interlocking slots 9a extending axially into the gripping ring from its edges (this type of structure is for instance disclosed in US 4,445,714 mentioned in the introduction to this specification). Next, the locking ring 8 is inserted into the socket 2. The locking ring has a cylindrical outer diameter corresponding in diameter to the inner surface of the socket 2, and a tapered inner surface 13 which cooperates with the tapered outer portion 12 of the locking ring 9. The locking ring 8 is provided with a resilient annular projection 14 which is a snap-fit in a groove 15 defined in the inner surface of the socket 2.

The final component of the coupling is a pipe insert stiffener 16, which is a tubular member (for instance of hard plastics or other relatively hard material such as steel) having an outer diameter selected to correspond to the inner diameter of a pipe for which the coupling is intended to be used, and a radial flange 17 at its inner end which is dimensioned to fit within the reduced diameter portion of the socket 2 defined by the step 6 and about the first annular shoulder 5. In the illustrated embodiment the relative dimensions of the pipe insert stiffener 16, and flange 17, are such that the stiffener can either be inserted into the socket 2 before assembly of the seals 10, gripping ring 9, and locking ring 8, or can be inserted into the socket 2 after assembly of the other coupling components. To aid such insertion the leading edge of the flange 17 is slightly bevelled. The opposite end of the pipe insert stiffener 16 is also bevelled to aid its insertion into a pipe end.

The radial inner dimensions of the seals 10, gripping ring 9, and locking ring 8, are pre-selected such that an annular gap will be left around the pipe insert stiffener 16 to receive a pipe end, but such that the seal rings 10 will initially exert a relatively light gripping force on the pipe.

In use, a pipe end is simply pushed into the socket over the pipe insert stiffener 16 (if pre-assembled with the coupling - alternatively the pipe insert stiffener 16 can be first inserted into the end of the pipe and then pushed into the socket together with the pipe end) so that the flange 17 of the pipe insert stiffener bears against the first annular shoulder 5, and the end of the pipe (not shown) bears against the flange 17.

Subsequently, an initial attempt to withdraw the pipe from the coupling will draw the seals 10 and thus gripping ring 9 along with it. Since the locking ring 8 is prevented from being withdrawn from the socket 2 by projection 14 engaged in groove 15, the tapered inner surface 13 of the locking ring ramps over the tapered outer portion 12 of the locking ring, compressing the gripping ring 9 against the pipe wall (which is supported internally by the insert stiffener 16). The greater the axial force applied to attempt to withdraw the pipe from the coupling, the greater will be the gripping force on the pipe.

Supported on the outer surface of the body I is an arrangement for engaging the wall of a meter box comprising a flange member 19 and a split washer 20. These components function in the manner described in British patent 2117470 (and will not be described in detail here). However, with the fitting according to the present invention the flange member 19 defines an annular recess 21 to receive one end of a protective sleeve (mentioned further below).

The above described coupling has a number of features which contribute to reducing the physical bulk of the coupling in accordance with the present invention.

The tapering surfaces 12 and 13 of the gripping ring 9 and locking ring 8 respectively have a relatively small taper angle, of the order of 5 to 20 (preferably of the order of 7 ). This enables the internal, and thus, outer diameter of the body 1 to be minimised.

Given this relatively small angle, and the absence of a separate drive member for forcing the locking ring 8 and gripping ring 9 together (as is for instance is provided in the coupling disclosed in US patent 4,445,714 referred to above) the coupling according to the present invention is designed to allow greater than usual travel of the pipe end. This is achieved by virtue of the annular step 6 which defines the second annular shoulder 7 providing an inner location for the sealing rings l O whilst allowing the pipe and insert stiffener 17 to be inserted beyond the seals 10. This ensures that the necessary degree of axial movement (corresponding to the axial length of the step 6) can be accommodated when the pipe is initially pulled back without the pipe being withdrawn from the seals 10.

Another advantage of this embodiment of the invention, is that a protective sleeve (not shown) can be fitted around the relatively narrow diameter body 1 and over the pipe. For instance, within the UK gas industry it is conventional to provide a GRP sleeve to protect any exposed gas pipes from W and mechanical damage. With conventional fittings the sleeve is positioned around the pipe and extends a short way into the end of the coupling socket, which necessitates a relatively large diameter socket. With the present invention the plastic sleeve (not shown) is fitted around the mouth of the socket to be retained in position by, for instance, the recessed portion 21 of the flange member 19.

A Farther advantage of the described fitting is that the pipe insert stiffener 16 can be inserted before or after the other components of the coupling are assembled.

This can be particularly advantageous when the pipe to be connected to the coupling is not quite round. In this case the insert stiffener 16 can first be pushed into the end of the pipe before insertion in the coupling. The bevel on the end of the pipe insert stiffener 16 will assist this.

Figure 2 illustrates a second embodiment of the present invention. This coupling is designed as a house entry tee. The coupling arrangement is the same as that described above, again comprising an internally stepped socket 2 to receive the end of a pipe and housing a pipe insert stiffener 16, O-ring seals 10, gripping ring 9, and locking ring 8. The assembly and operation of these components is the same as that described above and will not be described again here. This embodiment simply demonstrates another possible application of the push-fitting according to the present invention and differs from the embodiment of Figure 1 in that the modified coupling body 22 has a screw threaded internal bore 23 communicating with the coupling socket 2 and a tee pipe 24. A plug 25 is screwed into the bore 23 and carries a seal member 26. The position of the plug can be adjusted to selectively open or close the path from the socket 6 to the tee pipe 24 and is conventional in design. The end of the bore 23 is sealed with a screw threaded plug member 27. This arrangement is again conventional. An annularly recessed flange member 28 is disposed around the body 22 in the region of the socket 6 to provide an annular location for an end of a protective sleeve (not shown) which may be fitted over a pipe (not shown) connected to the coupling in the manner described above in relation to the meter box adapter of Figure 1.

A third embodiment of the invention is illustrated in Figure 3. This is a "weld- transition" designed to provide a transition between a plastic pipe and a metal pipe or fitting. Accordingly, the coupling is provided with an elongated steel (or other metal) body 30, defining a socket 2 at one end, the other end being suitable for welding to an existing metal pipe or fitting (not shown).

Most of the coupling components correspond to those described above (and like reference numerals are therefore used) except that a flanged stop ring 31 is provided in place of the internal socket step 6. The stop ring 31 has a tubular portion 31a which bears against the inner wall of the socket 6 and an annular flange 31b which provides a locating shoulder for the end flange 17 of the tubular insert 16. The opposite axial end of the tubular portion 3 la of the ring 31 provides a small annular shoulder for locating the O-ring seals 10. Thus, the axial end of the tubular portion 31 a is equivalent to the second annular shoulder 7 of the embodiments of Figure 1 and 2, the tubular portion of the ring defines the reduced diameter internal portion of the socket (which with the above embodiments is defined by the annular step 6) and the flange 3 lb corresponds to the first annular shoulder 5 of the embodiments described above. Otherwise, the structure and operation of the coupling is identical to that described above.

With reference to Figure 3, the present invention for the first time provides a push-fit coupling which is compact enough to be suitable for use as a weld transition.

The conventional manner for providing a weld transition is to use a fitting such as that disclosed in US patent 4,293,147 and modifications thereof. A difficulty with such fittings is that they require factory assembly to the end of the plastic pipe. The present invention has the advantage that it can be assembled on-site by an unskilled person.

With the above described embodiments of the invention an initial annular gripping force provided by the O-ring seals 10 is relied upon to initially urge the gripping ring 9 into cooperation with the locking ring 8. However, one effect of reducing the angle of taper of the gripping ring 9 and the locking ring 8 is that the axial distance through which the pipe end must travel before the gripping ring 9 bites into the pipe surface is increased. In fact, the increased travel can be such that the compressive force exerted by the annular O-ring seals is reduced as the O- ring seals move away from the annular shoulder 7/flange 17. A modified gripping ring 32 which addresses this problem is illustrated in Figure 4.

Referring to Figure 4, the illustrated gripping ring 32 is similar to the gripping ring 9 described above in that it has an outer surface with a tapered portion 33 and axially spaced circumferential teeth 34 defined on its inner surface. Also in common with the gripping ring 9, gripping ring 32 has a structure which allows it to be inwardly compressed by virtue of a plurality of axial slots 35 extending into the gripping ring 32 from one axial end. In this case, all slots 35 extend into the gripping ring 32 from the same axial end and terminate short of the other, with the exception of one slot 35a which extends all the way through the gripping ring so that the ring is split. The gripping ring 32 is thereby divided into a plurality of circumferentially spaced gripping portions or 'fingers' 36.

In order to minimise the resistance to initial insertion of a pipe end, the arcuate gripping periphery of each gripping tooth 34 has an inner diameter no less than the nominal outer diameter of the pipe. However, to provide an initial light gripping force the inner surface of the gripping ring is provided with a plurality of contact portions 37. Each contact portion 37 extends along a chord defined between two points on the gripping periphery of the first gripping tooth portion of each gripping finger 36. The distance between diametrically opposed contact portions 37 is therefore less than the diameter of the gripping periphery of the gripping tooth, and moreover is less than the nominal outer diameter of the pipe end.

Accordingly, substituting the gripper ring 9 of any of the above described couplings with the gripping ring 32 will have one important effect on operation of the coupling. That is, as the pipe end is inserted into the socket and through the gripping ring 32 the contact portions 37 will exert an initial gripping force on the pipe sufficient to ensure that the gripping ring 32 travels with the pipe end and is brought into cooperation with the locking ring 8 without the need to rely on a gripping force provided by the annular O-rings 10. This therefore ensures that the pipe end cannot be withdrawn from the coupling without the locking ring and gripping ring coming into cooperation and thus biting into the pipe end. This therefore overcomes the problems associated with increased travel of the pipe end without the need to include additional drive members as used in the prior art as for instance mentioned above.

Referring now to Figure 5, this illustrates a pipe coupling similar to that disclosed above but exemplifying another aspect of the present invention.

Components corresponding to the components of the couplings described above are identified by the same reference numerals. Thus, it can be seen that the coupling comprises a body I one end of which defines a socket 2 for receiving a pipe end and the other end of which is formed as a reduced diameter screw-threaded tubular spigot 3 for connection to appropriate equipment. The central passage through the coupling has a relatively narrow diameter portion 4 extending through the spigot portion 3 of the body 1, which widens into the socket 2 in the region of a first annular shoulder 5.

Moving inward from the mouth of the socket 2, the body 1 houses a locking ring 8 a gripping ring which may correspond in structure to that illustrated in Figures I to 3 or to that illustrated in Figure 4 and will thus be identified by the reference 9/32, and a pair of O-ring seals 10.

Assembly and operation of the locking ring 8, gripping ring 10/32, and 0rings 10 are essentially as described above. The coupling of Figure 5 differs from the earlier described couplings in that it includes a modified pipe insert stiffener 38 which has a flange adapted to replace the second annular shoulder 7 of the couplings illustrated in Figures 1 to 3. That is, the modified insert pipe stiffener 38 has an end flange which has both an annular radially extending portion 39a and an annular axially extending portion 39b which defines an annular recess 40. The outer diameter of the flange 39 corresponds to the inner diameter of the socket 2 and the inner diameter of the axially extending portion 39b of the flange corresponds generally to the outer diameter of the pipe end. The axial portion 39b of the flange thus defines an annular end face 39c which effectively replaces the annular shoulder 7 of the earlier described embodiments. In other words, the O-ring seals 10 are located between the gripping ring 10/32 and the axial end face 39c of the stiffener flange.

From a user perspective, operation of the coupling is the same as that described above. That is, the pipe end to be coupled is inserted into the socket 2 around the tubular portion of the pipe insert stiffener 38. However, with this coupling the axial end of the pipe is received within the recess 40. The tubular body portion 38 is dimensioned to be a relatively snug friction fit within the pipe end so that any attempt to withdraw the pipe end from the socket 2 will pull the pipe stiffener 38 along with it. Because the O-rings 10 are located by the stiffener flange 39, their position with respect to the pipe end will remain constant as the pipe end moves. This avoids any possibility that the pipe end may be withdrawn from the O-ring seals 10 before the pipe is fully gripped by the gripping ring 10/32. This is a potential problem which can occur when the taper of the gripping ring 10/32 and locking ring 8 is small with a corresponding increase in pipe travel as mentioned above.

It will be appreciated that the annular recess 40 is not essential but is preferable to ensure that the end of the pipe clears the O-rings 10. It will also be appreciated that the axially extending portion 39b of the flange 39 could be replaced by a separate ring member located between the O-rings 10 and a radially extending flange 39. As a further modification, the external surface of the tubular portion of the stiffener 38 could be provided with grooves or other formations to help the pipe end grip the pipe stiffener so that the two move together.

It will also be appreciated that the coupling disclosed in Figure 5 could be embodied in a variety of pipe fittings, including a house entry tee as illustrated in Figure 2 or a 'weld-transition' as illustrated in Figure 3. It will be further appreciated that couplings according to the present

invention can Loran part of a variety of fittings not limited to the meter box adapter, house entry tee and weld transition fittings described above. Other applications of the present invention will be readily apparent to the appropriately skilled person.

Claims

1. A pipe coupling comprising: a tubular body defining a flow passage communicating with a pipe receiving socket, the socket being defined between a pipe receiving opening at one axial end of the body and an internal abutment; a pipe stiffener having a tubular body sized to be a friction fit within a pipe end to be coupled and having an outwardly extending flange at one end thereof with a maximum diameter greater than an internal diameter defined by said internal abutment and greater than the internal diameter of said pipe end; an annular gripper ring having a toothed inner surface and a tapered outer surface arranged to surround the received pipe end and adapted to collapse inwardly to grip the pipe supported by said tubular stiffener in response to a radially inward force imposed on the gripper ring; a locking ring having a tapered inner surface adapted to surround the tapered outer surface of the gripper ring and effective when moved axially over said gripper ring in a ramping in relation to impose a radial inward force thereon; means to prevent said locking ring from being withdrawn axially from the socket; and one or more annular seal members having an internal diameter dimensioned to receive said pipe end disposed within said socket between said gripper ring and said pipe stiffener flange and such that said annular seal members are constrained to move with said insert stiffener when said pipe end and stiffener move in a direction towards the socket opening.

2. A pipe coupling according to claim 1, wherein the pipe stiffener flange defines an annular recess surrounding the tubular body of the pipe stiffener for receiving a short length of said pipe end such that when inserted into the socket said pipe end extends to an axial distance beyond said annular seal member(s).

3. A pipe coupling according to claim 1, wherein a spacer ring is positioned between the stiffener flange and the annular seal member(s) defining an annular gap for receiving a short length of said pipe end such that when inserted into the socket said pipe end extends to an axial distance beyond said annular seal member(s).

4. A pipe coupling according to any preceding claim wherein said socket abutment is an annular shoulder defined by the socket body.

5. A pipe coupling comprising: a tubular body defining a flow passage communicating with a pipe receiving socket, the socket being defined between a pipe receiving opening at one axial end of the body and a first internal annular shoulder; the socket having an internal annular step immediately adjacent said first annular shoulder defining a second annular shoulder and a reduced diameter socket portion extending between the first and second annular shoulders; a pipe stiffener having a tubular body sized to receive a pipe end and an outwardly extending flange at one end thereof sized to fit within the reduced diameter socket portion but having an outer diameter greater than the inner diameter of the first annular shoulder; an annular gripper ring having a toothed inner periphery and a tapered outer surface arranged to surround the received pipe end and adapted to collapse inwardly to grip the pipe supported by said tubular stiffener in response to a radially inward force imposed on the gripper ring; a locking ring having a tapered inner surface adapted to surround the tapered outer surface of the gripper ring and effective when moved axially over said gripper ring in a ramping relation to impose a radial inward force thereon; means to prevent said locking ring from being withdrawn axially from the socket; one or more annular seal members disposed within said socket between said gripper ring and said second annular shoulder; wherein the seal means and/or gripper ring are dimensioned to apply a relatively light gripping force to a pipe end inserted into the socket which increases as axial force is applied in a direction tending to withdraw the pipe from the socket through cooperation of the tapering surfaces of the locking ring and gripper ring.

6. A pipe coupling according to claim 5, wherein the first annular shoulder and said annular step defining said second annular shoulder are formed by portions of the inner wall of the socket defined by said body.

7. A pipe coupling according to claim 5, wherein the first annular shoulder, second annular shoulder, and annular step, are provided by a ring member which is generally L-shaped in axial cross-section having a tubular portion defining said step, a radially inwardly extending annular flange at one axial end of the tubular portion defining said first annular shoulder, the other axial end of the tubular portion defining said second annular shoulder.

8. A pipe coupling according to claim 7 wherein the body is provided with an internal annular formation against which said ring member abuts.

9. A coupling according to any one of claims 5 to 8, wherein the outer diameter of the pipe stiffener flange is such that the pipe stiffener can be inserted in the end of a pipe prior to insertion of the pipe into the socket, through said gripping ring, locking ring, and annular seals.

10. A pipe coupling according to any preceding claim, wherein the said inner surface of the annular gripper ring is provided with an array of axially spaced circumferential teeth each of which has an inner peripheral gripping edge having a diameter no less than the outer diameter of said pipe end, and wherein a plurality of contact portions are formed on the inner surface of the gripper ring, each contact portion being separated from a diametrically opposed portion of the gripper ring inner surface by a distance less than the external diameter of said pipe end.

11. A pipe coupling comprising: a tubular body defining a flow passage communicating with a pipe receiving socket, the socket having a pipe receiving opening defined at one axial end of the body; a deformable annular gripper ring arranged to surround the received pipe end and adapted to collapse inwardly to grip the pipe in response to a radially inward force imposed on the gripper ring, the gripper ring having a tapered outer surface and an annular inner surface provided with axially spaced circumferential teeth each of which has an inner peripheral gripping edge having a diameter no less than the outer diameter of said pipe end; a locking ring having a tapered inner surface adapted to surround the tapered outer surface of the gripper ring and effective when moved axially over said gripper ring in a ramping relation to impose a radial inward force thereon; means to prevent said locking ring from being withdrawn axially from the socket; wherein a plurality of contact portions are formed on the inner surface of the gripper ring, each contact portion being separated from a diametrically opposed portion of the gripper ring inner surface by a distance less than the external diameter of said pipe end.

12. A pipe coupling according to claim 10 or claim 11, wherein each contact portion extends substantially along a chord of the inner annular surface of the gripper ring.

13. A pipe coupling according to claim 12, wherein each chord extends between two points defined on the gripping periphery of a respective one of said circumferential teeth.

14. A pipe coupling according to claim 13, wherein each contact portion forms a chord between two points defined by the same circumferential tooth, said contact portions being circumferentially spaced around the annular inner surface of the gripper ring.

15. A pipe coupling according to any one of claims 10 to 14, wherein each contact portion is diametrically opposed another contact portion.

16. A pipe coupling according to claim 14, wherein said tooth is adjacent one axial end of the gripper ring.

17. A pipe coupling according to claim 16, wherein said tooth is located at the axial end of said coupling ring close to said socket opening.

18. A pipe coupling according to any preceding claim, wherein the annular gripper ring is divided into a plurality of deformable portions by a plurality of slots each extending from one axial end of the gripper ring towards, but falling short of, the other.

l9. A pipe coupling according to claim 18, wherein each slot extends form the same axial end of the gripper ring.

20. A pipe coupling according to claim 19, wherein said slots extend into the gripper ring from the axial end closest the socket opening.

21. A pipe coupling according to any one of claims 18 to 20, wherein one of said slots extends along the entire axial length of the gripper ring so that the annular gripper ring is circumferentially split.

22. A pipe coupling according to any one of claims 18 to 21, wherein said slots extend axially.

23 A pipe coupling according to any preceding claim, wherein the annular taper of the locking ring and gripping ring is between 5 and 20 .

24. A coupling according to claim 23, wherein said angle is of the order of 7 .

25. A pipe coupling according to any preceding claim, wherein radially outer and axial end edges of the pipe stiffener are bevelled.

26. A pipe coupling according to any preceding claim, wherein said one or more annular seal members comprises two adjacent O-ring seals.

27. A pipe coupling according to any preceding claim, wherein said means preventing said locking ring from being withdrawn axially from the socket comprises a resilient annular projection extending from an outer surface of the locking ring which is a snap fit within a annular groove provided in the internal wall of the socket.

28. A pipe coupling according to any preceding claim, wherein said body includes a metal tubular portion extending away from said socket and having an axial length longer than the axial length of said socket, suitable for welding to a metal pipe or pipe fitting thereby providing a weld transition from a plastic pipe end to be received within the coupling and said metal pipe or pipe fitting.

29. A coupling according to anyone of claims 1 to 27, wherein said body has a screw threaded tubular spigot portion extending away from said socket.

30. A pipe coupling according to claim 29, wherein said coupling is a meter box adapter for connecting a plastic pipe end to fluid flow metering equipment.

31. A pipe fitting according to anyone of claims 1 to 27, comprising a lateral passage communicating with said tubular body to provide a tee connection between a plastic pipe end to be received within the coupling and another pipe or fitting.

32. A push-fit coupling, comprising a metallic body defining a socket at one end housing push-fit pipe gripping components and having an elongate tubular portion extending from said socket suitable for welding to a metal pipe fitting or pipe.

33. A method of coupling a plastics pipe end to a metal pipe fitting or pipe, the method comprising securing a push-fit coupling to the end of the plastics pipe and welding said push-fit coupling to the metallic pipe fitting or pipe.