A socket of this type is generally known in the state of the an and can be constructed with two ends to form a coupling piece. This is used, for example, for joining conduits that are provided with a screw thread. With this arrangement the thread is provided on the free end of the conduits and the socket is screwed onto said thread The external diameter of the socket is greater than the external diameter of the pipe or conduit concerned which has been provided with the thread.

This means that a completely smooth whole is not obtained. In some applications a smooth join between two pipes i sessential. Onc example is pipelines in greenbouses.

These are fivquenfly used for moving so-called pipe trolleys over them, the wheels moving along the pipes. It will be nndatstood that with such an application differences in the diameter of the pipe as a consequence of coupling pieces and the like are inadmissible, It in for this reason that currently the majority of pipes are joined to one another by welding. This welding is carried out in the greenhouse. The working conditions there are generally disadvantageous and the government is imposing increasingly mom stringent requirements with regard to the working environment, with the result that this welding is becoming increasingly more difficult to carry out. Apart from the welding itself, that is to say the fumes generated by this, there is the problem of protecting the exterior of the pipes against corrosion, In general, such pipes are supplied in various way. Firstly it ia possible to provide the pipe with a conosion-resiBtant coating over its entire length. However, problems arise at the point where welding has to take place. Galvaniing is in general not permitted because the (zinc) vapours produced dunny welding am harmful to health.

Another possibility is to supply the pipe with so-called paint-free ends, In any event it is necessary after welding to tat the weld location separately with a corosion-rcaistant agent. Depending on the protection, the option frequently chosen is to provide this entire pipe with a further corrosion-resistant coating. It will be understood that conditions in a greenhouse are far from optimum and that the use of certain methods of protection is not possible or is not permissible in greenhouses.

US 3 489 445 A discloses a joint (Fig. 3) between two aolid mds. One of the m is

provided with a bore with internal thread, whilst the other rod is provided with a stepped- down end with external thread. As in the case of certain bolt constructions, the thread is not directly at the start of the bore but some distance away from this. In the intervening section there is an appreciable gap between the bom and the stepped-down end of the other rod.

WO 94111664 disclosas a socket consisting of an intenml threaded section. A conical surface is provided close to the end of the socket. However, thia suriace extends over a very small portion of the thickneaa of the socket wall so that a smooth transition would not be produced if a corresponding conical aection of a pipe wem to be screwed in contact with this.

The aim of the present invention ia tp ptovide a joint between two pipes, which joint is, of course, firstly watertight because such pipes am used for the purposes of heating by means of water. Secondly, the aim is to make the joint smooth so that it is possible for pipe tolleys to travel over it. Thinly, the aim of the invention is to implement the various features without welding, so that it is posible to provide pipee pmvided with a conoaion- resistant coaling over their entire length and for which no or hardly any after-treatments are needed, This aim ia realiaed with a socket « described above in that the length (b) of aaid accomodation at least corresponds to the length (a) of said screw thread and in that the entry opening of said aceomodatian is provided with a conically permg section, which conically lapering section extends essentially over the entire thickness of the wall of said socket.

It has been found that the sockets or stop-ends known from the prior ut ara in no way suitable for fitting a coupling with screw thread on pipe sections which are used in greenhouses. After all, such pipe sections are of appreciable length and appreciable weight.

A weight of approximately 25 kg and a length of 6 metres may be mentioned by way of example. If one pipe is provided with a socket or coupling piece it is impossible to fit the other pipe in a socket according to the prior at This ia possible by use of the lead-in section according to the invention. Accurate centring is achieved by means of the appreciable length of the accomodation. Sliding into one another in the accomodation is relatively simple.

Sealing ia in any event produced by the engagement of the conically tapenog section of the socket and the corresponding pipe section. Further sealing means can optionally be present Such scale can, for example, comprise a seal acting on the end of the pipe. It iy

also possible to fit one or moro 0-naga between the pipe and tho BCaomodation.

With the construction deaciibed above a smooth transition between socket and pipe is enauied, as a result of which pipe trolleys and the like can be moved over south constructions without inconvenient jolting.

However, it must be understood that the joint described here also ha$ other applications. For example, consideration can be given to coupling piaces for railiags and the like where a tooth transition is likewise of importance.

The socket described above can be constructed as a single socket This can have a stopped for closing off a pipe aection. Furthermore it is possible to fit anther facilitiea on the end face, for joining to other systems.

It is, however, also possible to integrate two or more such sockets. Coupnng pieces, angle pieces, T-connectors, etc. are produced in this way. With such embodiments it is advantageous if stops am present which limit the maximum extent to which the pipe can be screwed in. That is to say, apart from the stop fanned by me conically tapering section there is a further stop for the pipes at the entry to the accomodation. It will be understood that in connection with production tolerances the stops located inside tiC coupling piece will not always be functional.

If there is more man one thread, such as, for example, in the coupling piece, it is preferable to use different types of thread. If a left-hand and a right-hand thread are used in a straight coupling piece and the pipe sections are provided with corresponding left-hand and right-hand thread, it dés to push me pipes towards one another when inserting me coupling piece according to the invention between them-Aa a result of the accus centrin with the accomodaiion according to the invention, it is possible to start the join between the two pipe sections via the coupling piece by exerting some fi~ pressure and tuning me coupling piece-Further turning of the coupling piece can provide a firm joint between the pipe sections. In order to obtain the comptetely smooth appearance of the socker/pipe combination that has been described above, it is proposed according to the invention to provide me pipes concerned with a stepped-down, tapering end. If this is produced by butting, for example, the t of (be wall at the end will as a remit automatically increase compared with the wall thiclmeaa of the remainder of the pipe. This inmwe in wall iMcImess is advantageous because the thread has to be made in is s section. To keep the thread of restricted depth the use of a relatively fine thread, such as a metric fine thread, is proposed. Then) is a transition section between the section of smaller

pipe diameter and the main diameter. According to the invention this is preferably coopted corrEspondingly to the conical section by the accomodabion of the socket. By this means it is possible to achieve a joint between the socket and the pipe section concerned that not only provides a very good seal but also provides very good clamping. It has been found that after the socket aod the pipe section constructed is this way have been screwed tight by hand unscrewing by hand is no longer possible. Toole are needod for thia.

Aids for such tools can, of course, be present for final tightening. One example is a hole in which a hook spanner can be inserted in Oldo-to turn the socket. If a coupling piece is used, such a blind hole ia preferably made at the location of the shoulder so that the weakening of the material as a result of the hole is not important.

The conical angle of the opening in the accomodation is preferably between 15 and 45° and more particularly i approximately 30°.

Other Sang means can also be used instead of the abovementioned screw thread, In this context any constmctton possible in the date of the art, such as gluing, is conceivable.

According to a particular embodiment of the invention snap-fit means or the like are used for this purpose. That is to say, the pipe is provided with some sort of resilient for example extending outwards in a gmove in the pipe, which ring is able to snap into a corresponding recess in the interior of the socket. Such snap-fit means can be arranged either upstteam or downstream or in the accomodation component.

The invention also relates to a method fipr the production of a pipe string, comprising the provision of at least three pipe sections joined to one another by at least two coupling pieces, wherein in order to tighten the joint from one end a rotary force is applied to the string while restraining at least one of said coupling pieces.

With the aid of the invention it is possible to use pipes and coupling pieces which have been provided beibrehand with a complete corrosion-resistant coating. Further treatment is not required after fitting. Moreover, it will be understood that the production of joints between two pipes can be earned out by an unnamed ps0v under any conditions, both indmrs and out-This is in cootntat to the welding described above. Moreover, it will be understood that it angle pieces, T-pieces and the like are used, in combination with the pipe sections, any desired embodiment of pipeline systems can be produced. That is to say, U-bends, cross-overs, couplings and similar constructions that can be envisaged in the state of the art can be produced using the coupling pieces/sockets according to the invention.

The invention will be explained in more detail below with reference to illustrative

embodiments shown in the drawing. In the drawing: Fig. I shows, diagrammatically in cross-section, a coupling piece and pipo before fitting ; Fig. 2 shows two pipes and the coupling piece according to Fig. 1 during fittimg, Fig. 3 W coupling piece and pipes after fitting ; Fig. 4 shows a atop-end ; Fig. 5 shows an angle piece according to the invention ; and Fig. 6 shows a further variant of the construction according to the invention.

In Fig. 1 a pipe is indicated by 1. Such a pipe can be used, for example, for heating purposes. This pipe can have any length and diameter customary in the state of the art, of, for example, 6, 9 or 10 m and l0-120 tnm, rapectivcly. The section of the pipe that has the main diameter is indicated by 2. Via a tranaitifm section 3 the end of the pipe is provided with a Zeddown section of smaller diameter, which section is indicated by 4. The w thickccas oS for example, 2. 25 mm is locally somewhat palat and a screw thread 5 of low thread depth, for example metric fine thread with a pitch of, for example, 1.5 mm, is made in mis thicker wall. Grooves 16 for receiving O-rings 24 ate mie in me stopped- down section 4.

Put of the coupling piece according to the invention is indicated by 7 in FiS 1. Tnis coupling piece consists of a lead-in section or omoion 8 of length a. which mcrgm into a conical section 9 towards the free end. At the other end the lead-in section or the accomodation is provided with a screw thread 10 of length b. Thread 10 is followed by a thickening in the material, or should=, 12 in which a hole 13 has been made in which a hook spanner can engage, A further screw thread 11 has been made adjoining this shoulder.

This thread is followed by a further lead-in section or accomodation 8, which is adjoined by a conical section 9. In the embodiment shown in Figs 1-3 the winding of thread 10 is opposite to that of thread 11. Obviously, thread 10 corresponds to thread 5 on pipe 1. A thrust washer 14 and a seal 15, bearing against the latter, are bagad at the end of thread 10.

It is show in Fig. 2 that a further pipe 6 is also present-This ia, ofcoume, provided at the end with thread that cotonna to thread 11.

It can be seen from Fig. 2 that accurate centring of the threaded section 5, that is to say the end of the pipe with respect to the thread, and of threaded section 11 i5 posuble with the aid of the lead-in section or accomodation 8. This is important because the weight

of the pipes accenting to the invention is often appreciable. If them were no such centting facility it is virtually certain that attachment of the coupling piece to the pipe section concerned would not be sttaight, which would result, on the one hand, in leakage and, on the other hand, in destruction of the threaded section.

Fig. 3 shows the final situation in which the coupling piece and the pipes 1, 6 are firmly joined to one another. It can be mu from this figure that the conicity of the opening indicated by 9 corresponds to the conicity of the transition section 3. A sealing effect is provided as a result Stop 12 can optionally serve for further sealing, for example by fitting an 0-ring or other sealing means therein.

It can be seen Dom Fig. 3 that a completely smooth whole is obtained. Furthermore, it can be seen from this figure that the various material thicknesses are so chosen that the joint between the two pipea 1 and 6 is not the weakest link in the pipe string. After the joint has been produced, sealing takes place, both by means of the sealing ring 15 and the Sent end of the stepped down section 4 provided with thread 5 and by means of the two 0-rings 24 and the lead-in section 8. It will be understood that one or more of these scab rond bue omitted. Should 12 is ukewise not needed in all embodiments. If no sealing whatsoever ig required, for example when producing railing construcåons, oU these seats can be omiited, Similarly, other sealing constrttctione, such as employing tape, liquid sealant and the like, can be usent Fig. 4 a stops which its eniet by 17. Thé stop-end is likewise provided with a accomodation or iead-in secnon 18, delinuted at one end by a conical section 19 and delimited at the other end by a screw thread 20. In contrast to the previous embodiment, this thread 20 is adorned not by a stop but by an end plate 21 in which a hole 22 has been drilled for coupling to further joints. Of course, the various features can also be constructed so that they are closed off.

Fig. 5 shows an angle piece. This is indicated in its entirety by 27 and consists of two socket components that are in f ! uid communication with one another. 2B indicates the lead-in or accomodanon section that at one end opens into a conical section 29 and at the other zend into a screw thread 30 and 31, respectively. With this embodiment thread 30 can be of opposite pitch to thread 31. Coupling of two pipe sections takes place simply by moving said two pipe sections towards one another incorporating the coupling piece as shown in Fig. 2 between them. A firm joint between the pipe sections can then be provided by turning coupling piece 7. It is, of course, also possible to make one of the pipe sections

tut, in which case the coupling piece concerned has to be restrained. In this way it is possible to couple a ting of pipes by, from one end, driving the pipe in totuy motion and successively restraining various coupling pieces.

In the latter case it, of coune, also possible to make all threads cithcr left-handed or right-handed. With this arrangement it is no longer necessary to rest the coupling pieces when assembling a pipe sting. It will be understood that the 90"angle shown is one example only.

A anther variant of the present invention is shown in Fig. 6. In this figure the pipe is indicated by 41 and. the coupling piece by 37. The conical section described above ? present in this embodiment also and is indicated by 39 for the conical coupling piece and by 43 &r the pipe. In this embodiment the coupling piece 37 is of smooth construction except for an annular recess 38. The shape thereof corresponds te the shape of a resilient ring 32 that is attached in some way or other to the stepped-down secion 34 of pipe 41. In the embodiment shown here, coxing takes place by making an annular groove 33 in which a compressible element such a an O-ring 35 is fitted. The resilient ring 32, on the other hand, is provided with an intetnal gmove 36 for receiving the O-ring 35. As a result the résilient ring 32 can be pressed inwards in order to move it past the smallest internal diameter of the coupling piece 37 and when it wachs annular recess 38 it snaps into this =am 38 under its own resilient ft= for the force of the 0-ring 35 and coupling is achieved. Sealing takes place with the aid of 0-rinp 40. A lead-in section 42 is also present, as a result of which centring of the stepped-down section 34 with respect to the socket is possible.

It can be seen from this latter variant that many modifications are possible without going beyond the concept of the present invention. Of course, instead of the canning section 42, the external diameter of the stepped-down section 34 can, as in the previous embodiments, have been adapted essentially to correspond to the internal diameter of the coupling piece 37. Similarly, in the embodiment according to Fig. 6 the 0-rings can be replaced by a diftctent eel. The resilient ring 32 can also be arranged in a diffamt position in the stepped-down section 34, for example instead of the ce section 42. As in the case of the embodiments described above, a stop shoulder can be present.

These and further modifications, for the stop-ends and angle pieces as well, are obvious to those stalled in the art aft reading the above description and are within the scope of the appended claims.