This invention relates to a spigot and socket joint which is particularly, but not exclusively, suitable for use with pressure piping capable of being used for example for mains water supply and distribuuon.

Traditionally, cast iron pipes were used for pressure piping, but these were difficult to handle, heavy and brittle. More recently, plastics pressure piping has been used, particularly in smaller diameters. Possible materials include polyethylene, polypropylene and PVC-U.

Conventional PVC-U materials have the positive attributes of high strength, high fracture toughness, high stiffness and low weight However, despite and because of these attributes, there are certain product properties that are not ideal. They are:-

i. Susceptibility to failure from point loads due to imperfect bedding practices - particularly in larger sizes.

ii. Susceptibility to impact damage during handling particularly at lower temperatures (≤0°C).

iii. Tendency to catastrophic failure (i.e. the complete pipe fractures when there is a failure).

Existing PVC-U pressure pipe systems are connected through push-fit joints which in general are not capable of supporting substantial end load forces. Within the pipe industry in general, there are many push fit joints which are end load capable. A common feature is a grab πng with teeth which are hard compared to the pipe mateπal PVC-U is considered too bππJe lor such a grab πng mechanism to be applied The

indentauons and loads caused by a grab ring are liable to act as stress raisers and inmate failure of the pipe mateπal.

The present applicant's co-pending British patent applicauon number 9408767.3 relates to piping, suitable for use as pressure piping, comprising at least two layers of plasucs mateπal, of different composition and/or properties, such that a first layer provides the main pressure-bearing ability, and a second layer provides resistance to fracture.

The piping may comprise only two layers in which case the preferred arrangement is that the inner layer provides pressure strength and the outer layer provides crack resistance, mechanical protection, and the ability to absorb a tensile end load, for example from a grab ring.

There is preferably a distinct discontinuity between the layers. Such a discontinuity inhibits the propagation of cracks from one layer to another, this reduces the risk of catastrophic failure even if a crack is initiated in one of the layers.

The pressure-resistant layer should have high strength and ductility.

In a preferred construction the piping has three (or more) layers of which the or an lntermediaie layer is a pressure-resistant layer, between inner and outer layers which provide protecuon (in particular mechanical protection) for the pressure -resistant layer, and resist the generation and propagauon of cracks.

In a preferred construction, the pipe consists of three layers with relauve thickness as follows: -

a An inner layer typically between 0% to 30% of the total thickness

b. A central layer typically between 60% to 90% of the total thickness.

c. An outer layer typically between 0% to 30% of the total thickness.

These thicknesses are for illustration only and the actual values will depend upon the particular balance of properties required in any pipe design.

The inner and outer layers may be of the same material or can be of different materials.

The three layers, in combination, provide improved performance related to the areas listed previously. The pipe functions as follows: -

a. The central layer provides the main pressure bearing capability; the inner and outer layers make a limited contribution to this, reflecting the thickness and material properties.

b. The inner layer has three functions as follows:-

(i) improves the resistance to slow crack propagation - as slow crack propagation usually initiates on or close to the inside bore of the pipe.

(ii) improves the impact strength-impact failures usually initiate on the bore of the pipe, particularly at lower temperatures (0°C).

(iii) contributes to fast fracture resistance in the ultimate case of the propagation of a crack.

c. The outer laver also has three functions as follows:-

(i) to provide a tough outer layer through which a grab ring or other end load capable joint can function.

(ii) as with the inner layer, contributes to fast fracture resistance in the case of catastrophic crack propagation.

(iii) resists external surface damage during the distribution, handling and installation of the pipes.

The physical attributes of the three layers differ substantially, reflecting the particular functions the layers perform. A typical application for a water pressure pipe for the UK market is as follows: -

a. The central layer is composed of a PVC blend incorporating 6phr of CPE modifier. Typically such a material would have a long term (SO yrs) stress of >23MPa, a notched Charpy level of 5-10 KJ/m ² with a brittle/ductile temperature transition of 15°C. The material will show sufficiently good resistance to slow crack propagation that a design safety factor of 1.3 can be applied.

b. The inner and outer layers are highly modified PVC, incorporating 50 phr of CPE modifier. Typically, such a material would have a short term failure stress of >15MPc, with a long term failure stress (50 yrs) of 8-10 MPA. The toughness measured in a notched Charpy mode is 70KJ/m2 with a brittle/ductile transition temperature of 2-3°C. The outer layer must also show a sufficiently high strength and thickness to allow a grab ring mechanism to function.

The above structure is for example only and is dependent upon the particular balance of properties required. Other materials which may be used in a three layer structure are as follows:-

(a) Central Layer

i. PVC-U with a design strength (50 yrs) of >26MPa_

ii PVC-T; blends of PVC incorporating a range of toughening agents, with a long term strength (50 yrs) in the range 15-23MPa.

iii CPVC.

iv The range of rigid thermoplastics shown in Table I.

v. The range of alloys blends co-polymers shown in Table II.

(b) Outer/Inner Layers

The toughness required of the inner/outer layers may be achieved in many ways; a list of possibilities is included in Table HI.

Table I Rigid Thermoplasric/Blends

PVC-U

ABS

ASA

SAN

PMMA

CPVC

POLYCARBONATE

Table ll Polymer Alloys/Blends/Co- Polymers

PVC/ABS

PVC/ASA

PVC/SAN

PVC MBS

PVC NBR

PVC/CPE

PVC VA

PVC/EVA

PVC PV

PC ABS

PC/ASA

PVC ABS/MBS

PVC/MMA

PVC/PMMA

TαWe /// Materials Potentially Suitable for nner/Outer Layers

PVC ABS

PVC/MBS

PVC/ASA

PVC/SAN

PVC/NBR

PVC/CPE

PVC/VA

PVC/EVA

PVC/PV

POABS

PC/ASA

PVC/ABS/NBR

PVC MMA

PVC PMMA

The present invention sets out to provide spigot and socket joints particularly suitable for joining lengths of the above described piping. It is to be understood, however, that this invention is not limited to such use and could be employed in many other plasucs or composite piping systems which are mtended for a wide range of other apphcauons.

According to a first aspect of the invenuon there is provided a pipe joint comprising a spigot, a grab πng and a socket, inside which the spigot can be received to form a

connection; the spigot and socket being configured such that, when the spigot is inserted in the socket an annular cavity is defined between a radially outer surface of the spigot and a radially inner surface of the socket; the cavity being adapted to receive the grab ring; the grab ring and the socket being configured such that they mutually interact to cause the grab ring to bite into the radially outer surface of the spigot, so as to resist withdrawal of the spigot from the socket, under a condition in which a force applied to either the spigot or socket or both the spigot and the socket urges withdrawal of the spigot from the socket; wherein the grab ring is provided with at least one projection for biting into the radially outer surface of the spigot and the joint comprises a means for limiting the extent to which the projection can bite into the surface of the spigot.

Preferably, a seal is also provided between the radially outer surface of the spigot and a radially inner surface of the socket The seal may be a sealing ring provided in the said cavity or in a different cavity formed between a radially inner wall of the socket and the radially outer wall of the spigot

The grab ring may have a conical radially outer sur ace and the socket may have a correspondingly conical radially inner surface, which surfaces, when moved axially against each other, cause the grab ring to contract. The means for limiting the extent of contraction of the grab ring may be an abutment provided on an inner surface of the socket which limits the axial movement of the grab ring, in relation to the socket and thereby limits the relative movement of the conical surfaces.

The grab ring may be split or axially corrugated. Alternatively, it may be defined by a plurality of discrete circumferential segments. The projection may be a tooth or one of a plurality of teeth which may be triangular in cross-section.

The sealing ring may have a generally quesuon-mark shaped cross-secuon and be installed longitudinally inwardly of the grab ring, with its tail axially directed into the sockeL The tail may be provided with ribs. The sealing ring may be provided with an abutment surface which normally faces axially outwardly for abutting the grab ring and' urging the grab ring axially outwardly and against the conical surface of the socket when the spigot is subjected to a withdrawing force.

The annular cavity may be defined by an annular housing that is fitted to a pipe end portion, so as to form part of the socket structure.

The annular housing may be adjustably movable in the axial direction, thus enabling pre-loading of the grab ring during assembly of the joint To achieve this, the housing may be in two parts, at least one of which can be moved axially relative to the other.

The annular housing may be contractable in the axial direction. Accordingly, the housing may comprise a first annular housing part including the said annular cavity and housmg the said grab ring, a second housmg part for affixauon to a pipe end portion, and means for adjusting the axial separation between the two. In a preferred embodiment one of the two housing parts comprises a threaded bore and the other comprises means for engaging a bolt the means for adjusting the axial separauon including the bolt which upon rotation co-operates with the threaded bore to bring the first housing part towards the said second housing part thereby causing the grab ring to contract and bite nto a spigot situated within the annular cavity.

The first housing part may extend axially beyond the end of a spigot-receivmg pipe end poruon and the second housing pan may be restrained upon the spigot-receivmg pipe end poruon. This restraint may be caused by a radial pro ecuon provided on the spigot-receiving pipe end poruon which has a radially outer diameter which is greater

than the radially inner diameter of a central aperture provided in the second housing part in which the spigot-receiving pipe end portion is received.

Alternatively the second housing pan may include a flange, which is connectable to a flange provided at a pipe end.

The second housing pan may include a second annular cavity including a second grab ring, the second cavity and second grab ring being configured such that they mutually interact to cause the grab ring to bite into the radially outer surface of the spigot- receiving pipe end portion, so as to resist withdrawal of the spigot-receiving pipe end portion from the second cavity, under a condition in which a force applied to either the annular housing, the spigot-receiving pipe end portion or both urges the withdrawal of the spigot-receiving pipe end portion from the second annular cavity; wherein the radially outer grab ring is provided with at least one projection for biting into the surface of the spigot-receiving pipe end portion.

The second grab ring may have a conical radially outer surface inclined oppositely to the said first grab ring and the second annular cavity may have a correspondingly conical radially inner surface, which surfaces, when moved axially against each other, cause the grab ring to contract

The second grab ring may be split or axially corrugated. Alternatively, it may be defined by a plurality of discrete circumferential segments. The projection may be a tooth or one of a plurality of teeth, which may be triangular in cross section.

The axial separation between the first and second housing pans may be adjustable in the axial direcuon, thus enabling the conical surfaces of the first and second cavities to be brought to act upon the respecuve conical surfaces of the first and second grab

rings, causmg the grab rings to bite into the spigot and spigot-receivmg pipe end poruon respecuvely.

To enable axial adjustment the annular housmg may compnse a first housing pan, including the first annular cavity and a second housmg pan including the second annular cavity, wherein one of the housmg parts is telescop.cally received within the other. The housmg parts may compnse complimentary screw threads, enabling one pan to be screwed into the other to effect axial contraction of the annular housing. The outer housmg pan may include a plate for abutting, an axial end face of a spigot- receivmg pipe end portion after fitting of the housing pan over the spigot-receivmg pipe end portion. The plate may be threadably engaged with an inner wall of the housmg plate for effecting axial movement of the housing pan upon the spigot- receivmg pipe end portion and thereby causing the grab πng to bite into a radially outer surface of the spigot-receiving pipe end portion.

As an alternative means to enable the axial contraction, the annular housmg may comprise first and second housmg parts, respectively including the first and second annular cavuies, and a bolt which can be rotated whilst axially fixed in relation to one of the first and second housmg parts and co-operate with a threaded bore on the other housmg part, to adjust the axial separation between the two. The annular housmg may further compnse an annular plate, for locauon against an end face of a spigot-receivmg pipe end portion and comprising an aperture for receivmg and axially holding the bolt so as to enable the second housmg pan to be fitted over the spigot-receivmg pipe end poruon and to be urged axially towards the pipe end, by turning the bolt whilst held fixed by the plate, and thereby cause the grab ring to bite into a radially outer surface of the spigot-receivmg pipe end poruon.

According to a second aspect of the invention, there is provided a pipe joint comprising a spigot a socket a locking ring and a support means; the socket and spigot being so configured that the spigot can be received within the socket to form a connection therebetween and in such a manner as to define an annular cavity between a radially outer surface of the spigot and a radially inner surface of the socket; the locking ring being collapsible and the suppoπ means being adapted to maintain it in an uncoliapsed condition; the arrangement being such that the locking ring can be located in a region of the socket which serves to define the said cavity in conjunction with the spigot and held in position in an uncoliapsed condition by the suppon means in such a manner that the spigot can be inserted into the socket with at least a portion of the spigot projecting through the locking ring; the spigot comprising means for separating the suppon means from the locking ring, so as to allow the locking ring to collapse onto the spigot and resist the withdrawal of the spigot from the socket

The suppon means is preferably a suppon ring which fits coaxially within the locking ring. The spigot may comprise an annular projection which has a greater external diameter than the internal diameter of the suppon ring, such that when the spigot is inserted into the socket the annular projection pushes the suppon πng from within the locking ring and on into the socket The projection and locking ring may be configured so as to maintain the locking ring in an uncoliapsed condition until the spigot is inserted into the socket by a certain amount after which the locking ring collapses onto the spigot due to removal of the support so as to at least partially encircle the projection, thereby providing an obstacle to the withdrawal of the spigot from the socket The locking ring may be provided with a first region of a relatively small diameter, for location relatively longitudinally outwardly in the cavity and a second region of relatively large diameter, for location relatively longitudinally inwardly the cavity, such that the annular projecuon maintains the locking πng in an uncoliapsed state as it passes through the region of relatively small diameter and allows the locking πng to collapse as it passes through the region of relauvely larger diameter.

One of the locking ring and suppon ring may be provided with a projection and the other with a recess; the recess and projection being configured to interengage to maintain the position of the suppon ring within the locking ring.

By means of the invention it is, therefore, possible to provide a push-fit joint which is capable of supponmg substantial end load forces without raising stress or provoking failure of the pipe material.

Embodiments of the invention will be described below by way of example and with reference to the accompanying drawings, in which: -

Figure 1 is a view, in partial longitudinal section, of a first embodiment of a joint in accordance with a first aspect of the present invention;

Figure 2 shows the same view of the joint of Figure 1, but under the condition where the joint is subjected to end load pressure;

Figure 3 is a view, in longitudinal section, of a second embodiment of a joint in accordance with the first aspect of the present invention;

Figure 4 is a view, in longitudinal secuon, of a third embodiment of a joint in accordance with the first aspect of the present invention;

Figure 5 is a view, in longitudinal section, of a fourth embodiment of a jomt in accordance with the first aspect of the present invention;

Figure 6 is a view, in longitudinal secαon, of a fifth embodiment of a joint in accordance with the first aspect of the present invention;

Figure 7 is a view, in partial longitudinal section, of an embodiment of a joint according to a second aspect of the invention, prior to completion of assembly;

Figure 8 is a view identical to Figure 7, but showing the joint in the assembled state; and

Figure 9 is a view identical to Figures 7 and 8, but showing the joint being subjected to end load pressure.

The joint shown in Figure 1 comprises four components, namely a pipe spigot 20, a pipe socket 40, a grab ring 50 and a sealing ring 60.

The spigot 20 is circular in cross-section and cylindrical It is provided with a chamfered edge 22 in order to facilitate insertion of the spigot into a socket

The socket 40 is also circular in cross-section. However, its wall has a varying radius, which serves to define eight integral, but distinct coaxial regions in the longitudinal direction. Taking these regions sequentially from the mouth of the socket the first region 41 is cylindrical. The inner diameter of this region is slightly larger than the outer diameter of the spigot 20.

Moving longitudinally inwardly from the mouth, the second region 43 of the socket is inclined radially outwardly and is relatively short This region defines, on its radially inner surface, an abutment surface 45. Extending from the longitudinally inner end of the region secuon 43 is the third region 42. This region also inclines radially outwardly, moving in the longitudinally inward direcuon. However, this region is

much longer m the longitudinal direcuon than the second region 43 and is inclined with a gentler slope than the second region. The radially inner surface of the third region 42 of the socket defines a camming surface 47.

The fourth region 44 of the socket is cylindncal. This region is connected with the longitudinally innermost end of the third region 42. The fourth region 44 has the greatest radius of all of the socket regions.

The fifth, sixth, seventh and eighth regions of the socket (46,48,49 and 40a, respecuvely) bring the diameter of the socket back down to the diameter of the first region 40. The eighth region 40a essentially corresponds m shape and diameter to the first region 40. The regions are arranged to provide a cylindncal portion 48 (the sixth region) with a diameter intermediate that of the first/eighth region and the fourth region. This cylindrical region 48 is extends on each side mto an inclined region 46, 49 which step the diameter between the eighth region 40a, which has the relauvely narrowest diameter and the fourth region 44, which has the relauvely greatest diameter.

The second to seventh regions of the socket define an annular recess 100 in the inner surface of the socket

The fourth to seventh regions of the socket are configured to accommodate mtemally a sealing nng 60 (descπbed below). The second and third secuons of the socket are configured to accommodate mtemally and manipulate a grab πng 50 (descπbed below).

The grab nng 50 has an inclined radially outer surface 52. which is inclined at an angle similar to that of the radially inner surface 47 of the second secuon 42 of the socket 40 A radially inner surface of the grab πng 50 is provided with a senes of teeth 56. which

are triangular in cross-section. The teeth 56 are ananged so that, when the grab ring 50 is located within the socket and with its inclined surface 52 abutting the inclined surface 47 of the second section 42 of the socket 40, the teeth 56 are directed radially inwardly and longitudinally inwardly of the socket. This configuration serves to resist . movement of a spigot longitudinally outwardly in relation to the socket. Each longitudinal end of the grab ring 50 is provided with a flat abutment surface 58, 54.

The grab ring 50 is defined by a number of discrete circumferential segments, which are separated by intermediate radial gaps when the grab ring is uncoliapsed. As the grab ring collapses, the segments are urged closer together, reducing the separation between them and, therefore, the gap thicknesses. When the grab ring is fully collapsed, the segments touch each other, effectively limiting the further collapse of the ring and preventing projection of the teeth 56 any further into the spigot 20.

The sealing ring 60 is, generally, question-mark-shaped in cross-section. It is arranged in the socket with the tail 64 of the question-mark aligned parallel to the longitudinal axis of the socket The tail 64 is ribbed in cross-section. The remaining, enlarged portion of the question-mark shape of the sealing ring is defined by a generally cylindrical portion 62, which merges at each end into a generally radial portion 63,61, each of which is also inclined slightly in the longitudinal direction. These portions are inclined towards each other, so as to define, together with the cylindrical portion 62, an annular recess 65 on the radially inner surface of the sealing ring which is trapezoidal in cross-section. This recess accommodates a metal blocking ring to hold the sealing ring in place.

Preferably, the sealing ring is made from an elastomer.

The joint is assembled by first locating the sealing nng 60 and the grab ring 50 within the socket caviry 100. as shown in Figure 1. The spigot 20 is then pushed mto the

socket mouth 40 until the end 22 of the spigot is situated within the eighth, longitudinally innermost region 40a of the socket. A suitable index can be provided on the outer surface of the spigot 20 to enable a person assembling the joint to judge the conect degree of insertion. The assembled joint will now appear as shown in Figure 1. It can be seen that the tail section 64 of the sealing ring is sandwiched between the sixth section 48 of the socket and the spigot The enlarged section 61,62,63 of the sealing nng is sandwiched between the fourth section of the socket and the spigot 20. The longitudinally innermost radial section 63 of the sealing ring is seated against the inner wall of the inclined fifth section 46 of the socket The end of the seal tail 64 is seated against the innermost surface of the inclined seventh section 49 of the socket The grab ring 50 is wedged between the inclined third section 42 of the socket and the radially outer surface of the spigot 20. At this stage the inclined surface 52 of the grab ring abuts the radially inner surface 47 of the third section 42 of the socket The teeth 56 of the grab ring 50 abut the radially outer surface of the spigot 20. The grab ring 50 and the sealing ring 60 are spaced from each other in the longitudinal direction.

When the joint is subjected to an end load, the spigot 20 is urged longitudinally out of the socket 40. This condition is shown in Figure 2. It can be seen that as the spigot 20 is drawn out of the socket the seal 60 moves in the same direction as the spigot 20 due to friction between the seal and the spigot and, where it exists, due to internal pressure from within the joint End face 66 of the seal is brought mto abutment with end face 58 of the grab ring 50. This urges the grab ring 50 longitudinally outwardly. The inclined surface 52 of the grab nng slides against the inner surface 47 of the inclined third region 42 of the socket. This causes the grab ring 50 to clamp radially inwardly against the outer surface of the spigot 20. This camming acuon of the inclined surfaces causes the teeth 56 of the grab πng 50 to bite mto the surface of the spigot 20.

The end surface 54 of the grab nng 50 and the abutment surface 45 of the second region of the socket are configured such that radial movement of the grab ring 50 is restricted by their abutment so as to limit the radial compression of the grab ring 50 and, therefore, the biting action of the teeth 56, to a certain predetermined extent. This ensures that the teeth 56 do not over-project into the surface of the spigot and that the spigot 20 is not over-compressed by the grab ring 50. This serves to reduce the risk of damage to the spigot and is particularly important where a multiple layer pipe, such as described above, is employed. This is because it would be necessary to ensure that the relatively soft inner layer of the spigot is not penetrated by the teeth.

Figure 3 shows a second embodiment of a joint in accordance with the first aspect of the invention. This embodiment comprises generally similar components to the first embodiment that is to say a pipe spigot 220, a pipe socket 240, a grab ring 250 and a sealing ring 260.

The primary difference between this embodiment and the first embodiment is that the components which essentially define the socket structure are bolted to a flange 270 which is provided at the end of a piece of piping 272. In the illustrated example, the socket structure is bolted to the flange 270 by means of a number of nuts and bolts 274, 276 which locate through apertures 278, 280, 282 provided respectively in the two primary housing parts 242, 244 which define the socket structure 240 and the flange 270. Of course, alternative means of fixing the socket structure to the flange could be used, as appropriate.

The socket structure 240 can be seen as a pair of housmg parts 242, 244 which can be moved relative to one another in the direcuon of the longitudinal axis of the piping. The first housing pan 242 corapπses an annular recess 246 which corresponds, to the annular recess 100 provided in the first embodiment, to the extent that it comprises an

inclined radially inner surface 247 which co-operates with an inclined radially outer surface 252 of a grab ring 250.

The operation of the radially inner sur ace 247 and the grab ring 250 is exactly the same as in the first embodiment. That is to say, axial movement of the first housing pan 242 causes contraction of the grab ring 250. This contraction is caused by the inclined surface 252 of the grab ring sliding against the inner surface 247 of the first housing 242. As a result the teeth 256 of the grab ring 250 bite into the surface of the spigot 220.

As with the previous embodiment the first housing pan 242 comprises an abutment surface 245 which co-operates with an end surface 254 provided at the longitudinally outer axial end of the grab ring 250. The end surface 254 of the grab ring 250 and the abutment surface 245 of the first socket housing pan 242 are configured such that radial movement of the grab ring 250 is restricted by the abutment so as to limit the biting action of the teeth 256 to a certain predetermined extent.

The second socket housing pan 244 comprises an axial lip 247 which projects co- axially within the first socket housing pan 242. This lip 247 has a dual purpose. First by projecting radially into the first housing pan 242, it provides a seat for the first housing part Second, it provides a stop for the grab ring 250, thereby limiting its movement longitudinally inwardly and forcing the action between the inclined surfaces 247 and 252 to cause radial compression of the grab ring, rather than mere movement of the grab ring axially into the socket.

The lip 247 also provides a convenient seat for the seal 260, which in this case is in the form of an O-ring. Of course, the seal could be provided in any suitable position along the overall radiallv inner surface of the socket 240.

The second housmg part 244 also compnses a radially inwardly directed flange 248, which provides an end stop for the inserted spigot 220.

Each of the first housmg pan 242 and the second housmg pan 244 compnses a respective radially outwardly directed flange 243, 249. These flanges have the same diameter as the flange 270 provided on the pipe 272.

The socket housing parts are factory fitted to the pipe 272.

The two-part nature of the socket housmg provides two mam advantages. The first is that the grab nng can be easily inserted mto the first housmg part 242, which is then fitted over the second housmg part 244, to hold the grab nng 250 in place. This allows the grab nng to be relauvely stiff, because there is no need to compress it radially to install it.

The second advantage is that after insertion of the spigot 220 mto the assembled socket housmg 240, the nuts and bolts 274, 276 can be tightened, thus axially compressmg the socket and pre-loading the grab πng 250 This initially secures the joint pnor to use and also enables the grab nng to bite mto even relatively hard pipe matenals.

Figure 4 shows a third embodiment of the first aspect of the invention.

Like the second embodiment the third embodiment mcludes a socket structure 340 which has a first socket housmg pan and a second socket housmg pan 344 This arrangement is not however, mtended to define the whole of the socket assembly Instead, the socket structure 340 of the third embodiment is intended to attach to a spigot-receivmg pipe end portion 346 formed at the end of a pipe 372

To this end, the socket structure 340 compnses a first πng 344 which fits around a radially outer surface of the spigot-receiving pipe end portion 346. It is restrained from outward axial movement by the profile of the spigot-receiving pipe end portion 346, which includes a radially outwardly projecting annular portion 386. The first housing part 342 of the socket structure 340 projects axially from the end of the spigot-receiving pipe end portion 346. The first and second housing parts of the socket structure 340 are connected by means of a series of nuts 374 and bolts 376 which locate through corresponding respective apertures 378 and 380 provided in the first and second housing parts of the socket structure 340.

The first housing pan 342 provides an annular recess 348 for accommodating a grab ring 350. The grab ring and recess 350, 348 operate to cause contraction of the grab ring 350 by means of a camming process in exactly the same way as the previous two embodiments. The two-pan nature of the socket structure 340 provides the same benefits as in the second embodiment That is to say, once a spigot has been inserted in the socket the nuts 376 can be tightened on the bolts 374 to cause axial movement of the first housing pan 342 relative to the second housing part 344, which is restrained by the radial projection 380, to cause a pre-loading of the grab ring 350.

The spigot-receiving pipe end poruon 346 of the pipe 372 comprises an annular recess 390 located radially within the radial projection 386. This annular recess 390 accommodates an annular seal 360 which has a cross sectional profile generally similar to the seal shown in the first embodiment of the invention.

Reference to Figure 4 will show that the spigot-receivmg pipe end portion 346 is double- walled. This is because during manufacture it is expanded following extrusion, in order to achieve the desired profile. This expansion causes a thinning of the wall in this region and the second, outer layer is. therefore, added for purposes of strength. Whilst the outer laver would normally be formed from the same maieπal as the

remainder of the portion and, indeed, the pipe 372, this is not absolutely necessary. What is important is that the outer layer exhibits the required physical properties discussed in the foregoing introduction.

Figure 5 discloses a fourth embodiment of the first aspect of the invention. This joint differs from the joint shown in Figure 4, in the way that the second housing pan 444 is attached to the spigot-receiving pipe end portion 446 of the pipe 472.

Reference to Figure 5 will immediately show that a second housing part 444 is connected to the spigot-receiving pipe end portion 446 by means of a grab ring 480, located within an appropriately orientated annular recess 490. The grab ring 480 and recess 490 operate in the same way to those of the previous embodiments, but face in the opposite direction to the grab ring 450 and recess 448 provided in the first housing pan 442.

This arrangement greatly facilitates the manufacturing process, as the second housing pan 444 can be installed upon the spigot-receiving pipe end portion 446 of the pipe 442 and the nuts and bolts 474, 476 can be tightened, in order to pre-load the grab ring 480 and secure the position of the second housing 444 upon the spigot-receiving pipe end portion 446. This is achieved by providing an annular plate 495 between the first and second housing parts 442, 444. The annular plate 495 comprises a series of apertures 496 which correspond in position to those provided in the first and second housing pans. Each aperture 496 is internally threaded, so that rotation of the bolt 476 will cause relative axial movement between the second housing pan 444 and the plate 495. In practice, the plate 495 is located against the end face of the spigot-receiving pipe end portion 446 and each bolt 476 is turned. This urges the second housing pan 444 towards the plate 495, causing radial compression of the grab ring 480 and providing the requisite pre-loading.

As with the previous two embodiments, each nut 474 can be rotated upon a respective bolt 476 in order to pre-load the grab ring 450 located in the first housing pan after insertion of the spigot 420.

Figure 6 shows a fifth embodiment of the first aspect of the invention.

This embodiment like that of Figure 5, includes a two-pan socket structure which has two grab rings 550, 558 accommodated within respective annular recesses. In this case, the second housing pan 544 comprises an internally threaded mouth portion 545 which receives an externally threaded portion 543 of the first housing pan 542. The annular plate 595 has a threaded radially outer surface which co-operates with the radially inner threaded surface of the second housing pan 544.

This arrangement provides an extremely facile method of assembly. In practice, the second housing pan 544 is first located upon the spigot-receiving pipe end poruon 546 of the pipe 572. The annular plate 595 is screwed axially into the second portion 544 by means of notches 596, which co-operate with an appropriate tool. As the plate 595 is screwed into the second housing pan 544, it bears against the axial end face of the spigot-receiving pipe end portion 546. This has the affect of moving the second housing portion 544 in the axially outward direction of the pipe end. This causes the inclined surface of the second housing part 544 to co-operate with the inclined surface of the grab ring 580 and cause the grab nng to compress, thus pre-loading the grab ring 580 and securing the second housmg pan in position upon the spigot-receivmg pipe end portion 546.

After insertion of the spigot 520 within the socket 540, the first housing pan 542 is screwed mto the second housmg pan 544 using gπpping portions 547. This has the effect of pre-loading the grab πng 550 provided in the first housmg pan.

Although the second, third, fourth and fifth embodiments of the first aspect of the invention utilise pre-loading of the joints to provide initial bite into the pipe, the main securing force is provided by the flow/pressure of the internal water. This increases the bite as the pressure increases.

In all embodiments, an end stop provided in the socket housing prevents the or each grab ring from collapsing too far if it continues to the end of the inclined surface. The teeth of each grab ring are of a limited, predetermined height so that they are fully engaged into the associated piping at working pressure. This ensures that the cylindrical internal elements of the grab ring on either side of and between the teeth provide support to the outside of the pipe and prevent the differential radial movements which could otherwise occur in the region of the tooth causing failure in the pipe.

Any of the grab rings can be made from hard metal inserts which are co-moulded into plastic or aluminium or other metal casting or fabrications.

The annular housings are preferably made from cast aluminium, but other materials could be used.

An embodiment of a joint in accordance with a second aspect of the invention is shown in Figures 7 to 9. This joint comprises four main components, namely a pipe spigot 30, a pipe socket 70, a support ring 80 and a locking ring 90.

The spigot 30 comprises two cylindrical regions 31,33 of similar diameter and a chamfered end 38 to facilitate insertion of the spigot 30 into a socket 70. An annular portion having an enlarged external diameter is provided longitudinally inwardly of the mouth of the spigot 30. intermediate the two cylindrical portions 31.33. This is formed by a cylindrical region 34 of increased diameter which is integrally formed in the wall

of the spigot 30 and merges into the rest of the spigot by means of sloping wall regions 32 and 36. Sloping portions 32 and 36 define annular sloping surfaces on the radially outer surface of the spigot

The socket 70 comprises a first cylindrical portion 71, which defines the mouth of the socket Moving longitudinally inwardly, this blends into an inclined wall region 72 which extends radially outwardly and merges into a second cylindrical region 73 which is much shorter in length than cylindrical region 71 and which defines a region of greater diameter than region 71. Moving further longiωdinally inwardly, section 73 merges into a second inclined portion 74, which also extends radially outwardly and transforms into a third cylindrical region 75, of a roughly similar longitudinal length to cylindrical region 73. Cylindrical region 75 defines the region of greatest diameter of the socket Moving further longitudinally inwardly, cylindrical region 75 blends into a further inclined region 76 which slopes radially inwardly and merges into a fourth cylindrical region 77. Cylindrical region 77 defines a diameter roughly equivalent to that of the first cylindrical region 71. Moving further longitudinally inwardly, section 77 transforms into a sloping region 78, which is inclined radially inwardly and blends into a fifth cylindrical region 79, which defines a region of narrowest diameter.

The locking ring 90 is split and is configured such that its radially outer surface fits comfortably against the radially inner surfaces of cylindrical regions 73 and 75 and inclined region 74 of the socket and also against the radially inner surfaces of cylindrical regions 73 and 70 and inclined region 72 of the socket It is also configured so that its radially inner surface fits comfortably against the radially outer surfaces of cylindrical regions 30 and 34 and inclined region 32 of the spigot. To achieve this, the locking ring 90 is defined by a first cylindrical portion 92 of a relatively nanow diameter, a second cylindrical poruon 96 of a relatively greater diameter with an integral intermediate inclined poruon 94. The locking nng 90 has a wall thickness which is significantly less than that of the spigot 30 and the socket 70 and which is

slightly less than the difference between the outer diameter of the cylindrical region 31 of the spigot 30 and the inner diameter of the first cylindrical region 71 of the socket 70. The locking ring is provided with an annular groove 98 on the radially innermost wall of cylindrical portion 92.

The support ring 80 is generally cylindrical and comprises an annular projecting ring 82 on its radially outer surface. One longitudinal end of the support ring 80 is provided with a radially inwardly directed bevelled edge 84 at its mouth.

Initially, the locking ring 90 is installed within the socket 70 in such a position that the radially outer surfaces of the portions 96, 94 and 92 contact the radially inner surfaces of portions 75, 74 and 73 of the socket 70 respectively. The locking ring 90 is manufactured in such a way that it would tend to collapse down from this diameter if left to its own devices. Therefore, in order to hold the locking ring 90 in this position, the support ring 80 is fitted coaxially within the locking ring 90 such that its annular projection 82 is received within the annular recess 98 in the locking ring. This arrangement holds the locking ring 90 in place and allows a spigot to the inserted into the socket.

Once the socket 70, locking ring 90 and support ring 80 are assembled in this manner, they are ready to receive the spigot 30. As the spigot 30 is inserted, the cylindrical portion 33 passes inside the support ring 80, unimpeded. When the inclined portion 36 comes into contact with the support ring 80, the radially outer surface of the inclined portion 36 abuts the bevelled edge 84 of the support ring, and disconnects the suppon ring from within the locking ring, thereafter transporting it longimdinally inwardly into the socket until it meets the radially inner surface of inclined portion 78 of the socket, which serves to arrest its further movement After the support ring is dislodged from inside the locking ring, the locking ring is held in the expanded state by the radially outer surface of cylindncal poruon 34 of the spigot 30. However, as the spigot 30 is

urged further mto the socket the cylindncal poruon 34 moves past cylindncal portion 92 of the lockmg nng, which can then collapse about the radially outer surfaces of the cylindncal portions 34, 32 and 31. During all steps, the longitudinal movement of the lockmg rmg is prevented by the radially inner surface of the inclined portion 76 of the socket

When the spigot 30 has been inserted m this manner, the jomt is in the condition shown in Figure 8 of the drawings. In this figure it can be seen that the support nng 80 has now become clearly separated from the lockmg nng 90

When the jomt is subjected to end load pressure, the jomt enters the condition shown m Figure 9. In such a circumstance, the load causes the spigot 30 to travel longitudinally outwardly m relation to the socket 70. Duπng this movement the lockmg nng is urged the same direction as the spigot by the radially outer surfaces of portions 34, 32 and 31 of the spigot This progress continues unimpeded until the radially outer surfaces of portions 96, 94 and 92 of the lockmg nng abut the radially innermost surfaces of portions 73, 72 and 70 of the socket When this occurs, the lockmg nng is forced against the radially innermost surface of portion 72 of the socket by the radially outermost surface of portion 32 of the spigot This effectively wedges the lockmg nng in posiuon and prevents further movement of the spigot m the longimdinally outward direcuon of the socket.

It is possible that either or both of the lockmg nng and die support nng could be provided m the form of a stnp, which is fed mto the socket through a hole provided m the wall of the socket. A seal may be provided between a radially outer surface of the spigot and a radially inner surface of the socket