With joints of this kind, it is continually necessary to find an optimum ratio between the size of the seal, which depends, inter alia, on the application area of the joint, and the level of force which is required in order to insert the pipe end into the spigot end. The various application areas may involve a high internal pressure or, on the other hand, a subatmospheric internal pressure, gas or liquid, the nature of the pipe material, etc. The abovementioned force is caused by the necessary deformation and/or compression of the sealing material. It should preferably be as low as possible in order not to make installation of the joint unnecessarily difficult, especially at less readily accessible locations.

Known solutions to this problem are, inter alia, the use of lubricant on the seal, of lubricant in the sealing material which can sweat out, and of seals with ribs for a form of labyrinth, in which case the local surface pressure at the site of the ribs still allows the insertion forces to be limited. These solutions are not satisfactory in all cases, so that it is desirable to provide further improvement.

This improvement has now been achieved by means of an annular seal according to the preamble of claim 1, which is characterized in that the axial section of the sealing ring changes in shape or flexibility in the circumferential direction.

When inserting the spigot end, which is generally completely cylindrical, into the socket, if a seal according to the invention is used, not all of the seal is deformed and/or compressed at the same time around the

circumference, but rather the deformation and/or compression initially takes place in certain areas.

Preferably, the change is in the form of waves, in which case the ratio between the circumference and the wavelength is obviously an integer (1, 2, 3 etc.).

The changeable axial section proposed according to the invention for the sealing ring can be realized in various ways. Specifically, it may relate simply to the shape, or to the thickness, or even to the use of means for affecting the stiffness in such a way that the desired change, for example a change in the form of waves, is produced.

If the seal is of the type composed of two sections which adjoin one another in the axial direction, of which one section is the sealing ring and the other section consists of material which is less elastic than the sealing ring, the inventive idea can be realized not only by simply changing the shape of the sealing ring, but also by making the axial section of the said other section also changeable in the circumferential direction. As a result, not only is the actual shape of the sealing ring changed, but also the stiffness of the entire assembly is given a wavy path.

Other properties of the said other section may also be made changeable, in particular the axial dimension and the thickness. In both cases, this influences the section which forms the sealing ring, which is situated axially further on, and where the desired effect of distributing the deformation force will then'occur.

In another embodiment, the seal may be provided with local reinforcements. These may then be formed by thickened portions or insert pieces.

The embodiments described hitherto are preferably used for tube diameters of from 100 to 160 mm.

Based on the same inventive idea of distributing the force, which is required for the deformation, in the circumferential direction over an axial displacement path of the tube part to be inserted, another possibility is a sleeve which is intended for joining two ends of smooth, cylindrical tube parts, which sleeve is provided with

internal chambers in the region of the two ends for accommodating rings made of elastic material, such as O- rings. This sleeve then, according to the invention, is characterized in that the axial position of the chamber in the circumferential direction changes location. In this case too, it is advantageous if the change runs in the form of waves and if the waves are composed of substantially straight rising and falling edges.

This type is eminently suitable for smaller tube diameters.

The invention will be explained below with reference to the appended drawing showing a few exemplary embodiments.

Fig. 1 is an axial section through part of a socket in which the seal according to the invention can be used; Fig. 2A shows an axial section through the annular seal according to the invention in a first embodiment at the location of the smallest sect ion; Fig. 2B shows a corresponding axial section through the ring at the location of the largest section; Fig. 3A shows the sectional profile of the ring on an enlarged scale at the location which corresponds to Fig.

2A; Fig. 3B shows the section of the ring at the location which corresponds to Fig. 2B; Fig. 4 is a graph which shows the curve of the force plotted against the displacement distance during insertion, as theoretically calculated, on the one hand for a known sealing profile and on the other hand for profiles according to the invention; Fig. 5A shows the results actually measured for a known profile, and Fig. 5B shows the results actually measured for a test specimen of the profile according to the invention; Fig. 6 shows, in the same way as Fig. 2B, an axial section through the ring in a second embodiment, at the location of a wave crest in the retention ring; Fig. 7A shows the section of the ring in the same way as Fig. 4, on an enlarged scale;

Fig. 7B shows a similar section to that shown in Fig. 5A, but at the location of a wave trough in the shape of the retention ring; Fig. 8 is an axial section through a sleeve for a pipe jbint, on the left designed in the traditional way and on the right provided with a third embodiment of the present invention.

Fig. 1 shows part of a socket 1 of a pipe part. The socket 1 is provided on the inside with a separate sealing assembly 2. A spigot end 3 of another pipe part can be inserted into the socket in order to form a joint between these two pipe parts. The pipe parts are formed in particular from plastic material, such as polyvinyl chloride (PVC) or polypropylene (PP) or from another material which is suitable for pipes.

The sealing assembly 2 comprises a sealing ring 4 made of an elastomeric material and a retention ring 5 which is fixedly connected thereto. The sealing ring 4 is intended, when the spigot end 3 has been inserted into the socket 1, to ensure that there is a seal between the socket 1 and the spigot end 3. The retention ring is preferably produced from a rigid plastic material, for example polypropylene.

The sealing assembly 2 is preferably produced by co-injection in a manner forming the subject-matter of a previous patent application, the sealing ring 4 and the retention ring 5 being fixedly connected to one another during the moulding process. The sealing ring 4 therefore preferably consists of a thermoplastic elastomer which can easily be attached to the material of the retention ring 5.

In the embodiment shown, the sealing assembly is provided on the outside with a circumferential, outwardly projecting rib. This rib 6 positions itself, preferably in a tight-fitting manner, in a groove 7 formed on the inside of the socket 1. This ensures that the sealing assembly 2 is very accurately positioned in the axial direction with respect to the socket 1.

On the side remote from the sealing ring 4, the

retention ring 5 is provided with an outwardly projecting edge 8. This edge 8 bears against the end face 9 of the socket 1, or is at a short distance therefrom, when the sealing assembly 2 has been arranged in the socket 1. The edge 8 serves to mask the gap between the retention ring 5 and the socket 1. The edge 8 is not necessary for positioning the sealing assembly 2 in the socket 1.

Figs. 2A, 2B, 3A and 3B show an embodiment of the sealing assembly 2 in a first embodiment according to the present invention.

It can be seen from both Fig. 2A and Fig. 2B that the sealing ring 4 has been provided with a wavy profile in the circumferential direction, as indicated by 10. This is done by allowing the axial sectional shape of the sealing ring 4 to change in the circumferential direction while retaining the cross-sectional shape of the retention ring 5, which thus remains a body of revolution, as before. The cross-sectional profile of the sealing ring 4 does not change, insofar as the ribs 11, 12, 13, 14, 15 remain in the same position with respect to one another, as is clear in particular from the fact that the lines 16, 17, 18 of the ribs 11, 12, 13 situated on the inner sides run parallel to the end edge 10 in Figs. 2A and 2B. This correspondence of shape of the profile can be seen on an enlarged scale by comparing Figs. 3A and 3B. Even the rib 6, which is on the outside and is intended to be accommodated in groove 7 on the inside of the socket (cf.

Fig. 1), retains its shape as a shape of revolution about the axis.

The parts 19 which lies between ribs 6 and the first external rib 14 (cf. Fig. 3A) is, from the section at the location of a wave trough, i.e. the smallest cross- sectional area of sealing ring 4, extended and provided with an adapted profile on the inside and outside to the part 19' in the section at the location of the wave crest (Fig. 3B), where the cross-sectional profile of sealing ring 4 has acquired its greatest cross-sectional area.

Furthermore, it can be seen from Figs. 2A and 2B that the wavy profile is composed of straight rising edges

20 and equally straight falling edges 21, which are connected by transitions 22 for the crests and 23 for the troughs, which have a relatively small radius of curvature, so that these transitions are designed to be relatively sharp. This represents the preferred embodiment, which therefore is in particular different from a sinusoidal shape. The disadvantage of a sinusoidal shape is that the crests - and obviously also the troughs, which viewed from the other side are crests - are "wider" than the sharp crests 22, 23, so that at the start of insertion it would still be necessary to exert a relatively strong force.

With the sharper crests/troughs 22/23 as preferably proposed by the invention, it is possible when inserting the spigot end 3 in the manner shown in Fig. 1 for its end edge to come into contact first with the rib 11, at the locations which in Figs. 2A and 2B are referred to as troughs in the wave form, since they are situated furthest to the left. Such a location of first contact in the line 16 is indicated in Fig. 2B by 24. When pushed on further, the location of first contact moves on either side along the crest line 16 of the first rib 11 in the direction of the two arrows shown on either side of the location 24. In an even later stage, the end edge of the spigot end 3 will also make contact with locations such as 25 in the crest line 17 of rib 12, with a gradual shift of the contact locations, again in both directions, and so on. In this process, the material of the sealing ring 4 is, of course, deformed and/or compressed in the traditional manner, but always with an extremely uniform distribution of the force required to do this, i.e. overall with a much lower force than when the spigot end 3 is inserted into a sealing ring 4 which in the traditional way is a perfect body of revolution.

The first embodiment of the invention which has been described so far comprises two complete waves in the half section of Fig. 2A or Fig. 2B, i.e. four complete wave periods over the circumference. The result of this is that there are four points of initial contact when the spigot end 3 is inserted. Experience has shown that this provides

a reduction of the insertion force which has proven effective in practice, so that four wave periods over the circumference are preferred by the inventors.

The following figures show the effect of the inventive idea. Fig. 4 shows calculated graphs representing the insertion force plotted against the displacement of the spigot end 3. Line a represents the force curve with the traditional constant shape, i.e. the body of revolution.

The curves b, c and d show the effect of the wavy profile as illustrated in Figs. 2A to 3B, specifically for respective wave amplitudes of 2.0 mm, 2.5 mm and 3.0 mm.

Strictly speaking, the embodiment shown in the abovementioned figures corresponds to curve d, with a wavy amplitude of 3.0 mm. Figs. 5A and 5B show the results of tests carried out on a sample. Fig. 5A shows the curve of the force as a function of the insertion distance for a traditional design, and Fig. 5B shows the curve for the design according to the invention, as illustrated in Figs.

2A to 3B. The reduction in the force is not as great as that which was calculated in theory, but nevertheless the force has fallen from a maximum value of 450 newtons to 325 newtons. It is to be expected that the results of tests carried out on series-production samples will be closer to the calculated result shown in Fig. 4. It can thus be seen that the maximum force has been reduced to slightly more than half the force which was previously required. The difference with three complete waves over the entire circumference is not great. If the number of waves over the circumference were to be reduced to two, there are only two crests or points of first contact. This means that there is a risk of the spigot end 3 becoming tilted, i.e. of an angular difference between the axis of the spigot end 3 and the axis of the socket 1. This will correct itself automatically, but the irregularity will entail an increasing force in a subsequent stage.

Figs. 6, 7A and 7B show a second embodiment of the inventive idea. The ribs 12, 13, 14, 15 and the rib 6 have precisely the same shape and dimensions as in Fig. 3A, and these parts of the sealing profile 2 are rotationally

symmetrical: in Figs. 7A and 7B they have the same shape and are situated at the same location. However, if Fig. 7A is compared with Fig. 3A, it will be seen that the first rib 11 is no longer present on the inside. Instead of this, as indicated in Fig. 7B at 26, the inside of the ring 2 simply runs straight on in the axial direction. The end edge 27 of the retention ring 5 in Fig. 7B has a location and shape which correspond to the location and shape of the retention ring 5 in the first embodiment, both in Figs. 3A and 3B. However, in Fig. 7A it can be seen that the ring 5 is provided at that location with an axially directed extension 28, which takes the place of that part 26 of the inner wall of the sealing ring 2 which runs axially straight on. As can be seen from Fig. 6, this axial extension 28 has a wavy profile: the end edge is denoted by the line 29. With regard to the preferred features of this wave form, the same applies as that which was stated in connection with the first embodiment: a wave with linear edges and a small radius of curvature, and four complete waves over the circumference.

Owing to the presence of the projecting part 28, which changes in the form of waves, of the retention ring 5, the sealing ring 2 acquires an amended shape - as mentioned for the most general definition of the inventive idea - but also acquires a stiffness against deformation in the radially outwards direction which also changes in the form of waves. It is this stiffness which is the decisive factor for the curve of the resistance force experienced when inserting the pipe end 3, and thus the result is again a resistance force which spreads gradually in the circumferential direction from four starting points in order in this way to reach a lower total force than with the traditional structure in which both the retention ring 5 and the sealing ring 2 were bodies of revolution; the result will be comparable to that illustrated in Figs. 4 and 5B.

This change in shape, but above all the change in stiffness into a wave form in the circumferential direction, can also be achieved in still further ways. In

particular, consideration may be given to thickened portions of the sealing ring 2, in particular in the area denoted in Figs. 3A and 3B by 19 and 19', but also by providing a thickening at the location of the ribs 11 to 15 in the same figures. Yet another possibility for achieving this solution is by providing insert pieces in the sealing ring 2, which insert pieces are made from a material of greater stiffness, i.e. lower elasticity, than the material of the sealing ring 2, so that the result is again a substantially wavy total stiffness curve, without changing the shape of the retention ring 5 in the first embodiment in accordance with Figs. 3A and 3B, in which this ring is a body of revolution.

The two latter variants, i.e. changing the properties of the elastic sealing ring not only through the dimensions but also through the use of a different material of a different elasticity, can be used in particular for seals which generally comprise only an elastic sealing ring, without such additions as the retention ring 5. The retention ring 5, which is known per se, essentially represents merely a practical exemplary embodiment, without this retention ring itself being necessary to realize the essence of the inventive idea.

Obviously, the seal described can be used in a socket at a pipe end or in an accessory piece which is provided with one or more sockets.

A third embodiment of the invention is illustrated in Fig. 8. This figure relates to a sleeve 31 of the type comprising a body 32 which is substantially cylindrical on the inside and outside. In the centre, it is provided with an inwardly facing stop rib 33, against which pipe pieces such as 34 can be pressed from both sides. The left-hand side of the figure shows the traditional design, in which the body is provided with a semi-circular chamber or groove 35 which runs around the entire circumference and in which there is accommodated a simple O-ring 36 which provides the seal after the pipe end has been inserted.

In the right-hand half of the figure, it can be seen that the groove 37 according to the invention is

arranged in the inner wall of the body part 32 in a plane which is not perpendicular to the pipe axis, but rather is at an angle to this axis. An O-ring 38 is again accommodated in this groove, which O-ring in this case merely has to have a slightly greater circumferential length than ring 36 in view of the greater groove length when this groove is positioned at an angle inside the same cylinder.

When a pipe end 34 is inserted, the first contact will be formed at the location where groove 37 and ring 38 are closest to the end face 39, i.e. at the top in the figure, with the result that only a limited insertion force is required. As a result, the contact spreads gradually, further downwards in the figure, until the pipe 34 has moved right past ring 38 and butts against rib 33. As it is distributed over a greater displacement path, the force which has to be exerted at any moment is then considerably less than in the traditional design, as shown on the left- hand side of the figure, in which it is necessary when inserting the pipe end to overcome the entire resistance force to the deformation of the O-ring 36 at the same time.

It can be said that the assembly or the sealing ring forms one single wave, again composed of straight edges.

The figure also shows a variant of this same embodiment in which there are two waves. Dashed lines 37' show a groove of which half is inclined in one direction and the other half in the other direction with respect to the axis. When the ring 38 is accommodated in this groove, there will be two locations of initial contact. Although it is then necessary to overcome the resistance to deformation of ring 38 over approximately half the displacement path, so that the force will be proportionally greater at any moment, the advantage is the symmetry, since there is a risk with the variant shown in solid lines and formed by a single wave that the pipe piece 34 will become tilted during the insertion.

Other variants allowing a change in the form of waves to the properties of the ring to be obtained, as described in connection with the first and second embodiments - change in thickness, stiffness, use of a rigid dry ring and insert pieces - can in principle be used here too. The rings do not have to be O-rings of identical form in all sections; the ring section may also change. The use of ordinary O-rings instead of rings of a special profile have the advantage of a low cost price.