This invention concerns a method of attaching a coupling to a pipe end where the pipe is of a malleable material such as polyethylene.

Conventionally, polyethylene pipe is attached to a coupling member such as a flanged tubular end piece by introducing the coupling member into the pipe end and securing with a crimped or otherwise clamped outer band or the like. Alternatively, a ribbed insert may be introduced into the pipe end and then the pipe may be forced onto the ribbed insert and retained by an outer collar.

A problem inherent with polyethylene pipe is that the material of the pipe has considerable ductile properties and may creep under pressure causing the connection between the pipe and the coupling to fail after a period of use.

It is an object of the present injection to provide a more secure pipe coupling for pipes of a malleable material such as polyethylene or the like and which can be used with pipes having a diameter in the region of 60mm to in excess of 300mm, and to meet current legislation the joint must be stronger than the pipe and comply to so- called 'class 1 end constraint' properties and which may be pressure tested, without fail, to pressures in the region of 50 bar.

According to the present invention there is provided a method of attaching a coupling to a pipe end where the pipe is of a malleable material, comprising the steps of providing a tubular insert having an outer diameter substantially equal to the inner diameter of the pipe, a tubular coupling part having an internal diameter substantially equal to the outer diameter of the pipe and including an internal helical track and a helically formed guiding part associated with the tubular part; introducing the tubular insert into an end region of the pipe to support the pipe internally in said end region; introducing the pipe end into the coupling part; and rotationally advancing the coupling part over the pipe end region by helical engagement with the guiding part

such that the material at the outer wall surface of the pipe is caused to form a helical track corresponding to the helical track of the coupling part whereby the coupling part and the end region of the pipe become threadedly interengaged.

Also according to the present invention there is provided a pipe end coupling assembly for a pipe of malleable material, comprising a tubular insert having an outer diameter substantially equal to the inner diameter of the pipe, a tubular coupling part having an internal diameter substantially equal to the outer diameter of the pipe and including a helical track, and a helically formed guiding part associated with the tubular part, the tubular insert being adapted for introduction into an end region of a pipe to be attached to the coupling and the tubular coupling part being adapted for threaded engagement with the helically formed guiding part, thus to enable the tubular coupling part to be advanced over the pipe end and to form a corresponding helical track in the outer wall surface of the pipe end region.

The insert may have an axially directed knurled surface to grip the internal wall surface of the pipe against relative rotation therewith.

The insert may have an externally threaded part to be exposed beyond the pipe end and serving as the helically formed guiding part to become threadedly engaged with the helical track of the coupling part.

The coupling part may have an internal seal ring adapted to seal against the outer wall surface of the pipe.

The helical track of the coupling part may be of rectilinear shape.

The helical track of the coupling part may become engaged with a helical spring also engaged with and captivated by the tubular insert, and serving as said helically formed guiding part

The coupling part may have an outer surface formation to be gripped by a tool for rotating the coupling part relative to the pipe end.

The helical track of the coupling part may have a maximum diameter substantially equal to the outer diameter of the pipe.

The helical track of the coupling part may have a maximum diameter greater than the outer diameter of the pipe.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 illustrates, in perspective and partly in cross-section, a pipe end coupling assembly for connection to a polyethylene pipe end;

Fig. 2 shows the assembly in partly assembled form;

Fig. 3 shows the assembly in a further assembled form;

Fig. 4 shows the assembly in a completely assembled form;

Fig. 5 is a diametrical cross-section of a coupling part of the assembly;

Fig. 6 is a view similar to Fig. 1 of an alternative form of coupling assembly;

Fig. 7 is a view similar to Fig. 2 of the alternative form;

Fig. 8 is a view similar to Fig. 3 of the alternative form;

Fig. 9 is a view similar to Fig. 4 of the alternative form; and

Fig. 10 is a view similar to Fig. 5 of the alternative form.

Referring now to Figs. 1 to 5, a coupling assembly for a polyethylene pipe 10 an end region of which is shown, comprises a tubular insert generally indicated at 11 having a portion 12 with an axially directed knurled surface adapted to grip the internal wall surface of the pipe end 10. The insert 11 also comprises a guiding part 13 having a recessed helical track 14 and a frosto-conical front end 15. The outer diameter of the guiding part 13 is substantially equal to the outer diameter of the pipe end 10.

Also forming part of the coupling assembly is a tubular coupling part generally indicated at 16 having a flanged front end 17 with apertures 18 to receive fixing bolts for attachment of the coupling to a similarly formed coupling or other flat surface. The tubular coupling part 16 is adapted to receive the insert 11 and is thus axially aligned therewith prior to assembling the coupling. Internally of the coupling part 16 is a sealing ring 19 and a helical track 20 extending inwardly of the tubular passage through the coupling part whereby the maximum internal diameter of the coupling part 16 is substantially equal to the outer diameter of the pipe 10 while the track 20 is of lesser diameter than the pipe.

The helical track 20 is of rectilinear form, preferably of rectangular section, for a purpose to be described.

On the outer cylindrical wall of the coupling part 16 is a knurled formation 21 to be gripped by a tool for rotating the coupling part relative to the pipe end.

Fig. 2 shows the insert 11 introduced into the pipe end 10 and a tool 22 positioned around the pipe end region to grip the pipe end and to force it into gripping relationship with the knurled part 12 of the insert 11. The insert is driven into the pipe until the end thereof abuts the guiding part 13 of the insert.

In Fig. 3 it can be seen that a tool 23 is clamped around the knurled part 21 of the coupling part 16 and by rotation of the tool 23 and thus the coupling part, in the direction of arrow 24, the coupling part is rotatably advanced onto the pipe end with the helical tracks 14 and 20 interengaged in a threaded relationship whereby the part 13 of the insert forms a guide to cause the coupling part 16 to advance rotationally onto the pipe end region.

Referring now to Fig. 4 it will be seen that as the coupling part 16 is further advanced rotationally onto the pipe end the outer wall surface of the pipe is caused to conform to the helical track 20 of the coupling art so that a corresponding recessed helical track 25 is foπned in the outer wall of the pipe whereby the coupling part and the end region of the pipe become threadedly interengaged. The coupling part is fully advanced onto the pipe when the frosto-conical part 15 of the insert 11 bears against a corresponding tapered internal surface of the coupling part, and the coupling is fully assembled.

In this condition, the seal ring 19 forms a seal against the external surface of the pipe and the tools 22 and 23 may be removed.

The nature of polyethylene pipe is that its surface has low frictional properties and therefore acts as a lubricant to the advancing helical track 20. Because the polymer of the pipe is positively reatained captively in the rectangular-sectioned rectilinear valleys of the helical track 20, there is no tendency for the polymer to "flow" and creep from the assembly and so there is no tendency for the coupling to become loose. The assembled coupling has a greater strength than that of the pipe

itself and complies to class 1 end constraint properties. The coupling has been pressure tested to 50 bar to verify its integrity.

Typically, such a coupling may be applied to polyethylene pipe having an outer diameter in the region of 63mm to one as great as 315mm. These are standard pipe sizes for liquid and gas pipes.

For example, in a pipe coupling to accommodate a 63mm outer diameter polyethylene pipe the depth of the helical track 20 may be 3mm thus closing to a minimum diameter of 60mm whilst the pitch of the track may be 8mm with an axial extent of 2mm, and the coupling closing to a front end diameter of 45mm. The seal ring 19 occupies an axial length of 6mm. The wall of the coupling part where it overlies the pipe end has a thickness in the region of 20mm.

Referring now to Fig. 6, in an alternative embodiment, the insert generally indicated at 30 comprises a plain- walled section 31 at its front end and a knurled section 32 at its rear end similar to the knurled formation 12 in Fig. 1.

The enlarged guiding part 13 of Fig. 1 is replaced by a shorter non-helical flange 33 bearing a stud 34 on its front face part way round its circumference.

A helical spring 35 of rectangular section is provided and has a hook 36 to become engaged on the stud 34 as will be described. Spring 35 is adapted to be seated in an annular recess 37 in the coupling part generally indicated at 38. In this example, the coupling part has a recessed helical track 39 of rectilinear form having a maximum diameter greater than the pipe 10 and a minimum diameter substantially equal thereto. Once again, a seal ring 40 is received within an annular recess in the inner open end of the coupling part 38. Otherwise, the parts of the coupling are the same or similar to those of Figs. 1 to 5.

Assembly of the coupling is commenced as shown in Fig. 7 by introducing the insert 30 into the pipe 10 so that the pipe end abuts the flange 33. A similar tool 22 is placed around the pipe over the knurled part 32 of the insert.

The spring 35 is introduced into its recess 37 within the coupling part 38 and then, as shown in Fig. 8 a tool 23 is similarly clamped around coupling part 38 and serves to drive the latter onto the pipe end rotationally as shown by arrow 24.

During the initial part of such rotation the hook 36 of spring 35 becomes engaged on the stud 34 of the insert 30 and, as can be seen from Fig. 9, as the coupling part is advanced onto the pipe the spring engages progressively with the helical track 39 in which it is received and thus guides the coupling part onto the pipe. As the coupling part is advanced, the spring becomes threadedly engaged with the outer surface of the pipe 10 and forms a corresponding rectilinear helical track thereon, much as did the helical track 20 in Figs. 1 to 5.

When the coupling 38 is fully engaged upon the pipe the tools 22 and 23 may be removed and the coupling assumes its fully assembled condition. In this case, the material of the outer wall of the pipe has been caused to conform to the helical track formed by the spring 35 to form the corresponding track in the wall of the pipe which is thus threadedly interengaged with the coupling part. Again, the material of the pipe is retained within the rectilinear recess of the threaded engagement of the coupling part and the pipe end to form a tight connection.

Fig. 10 illustrates, in diametrical cross-section, the coupling part 38 with its recessed helical track 39 and its annular recess 37 for receipt of the spring 35. The part 38 is similarly dimensioned to that of coupling part 16 of Figs. 1 to 5 with an 8mm pitch in the helical track 39 and an axial length of pitch in the region of 2.5mm to receive the thickness of the coils of the spring 35 as they are received within the track.

During formation of the corresponding helical track in the pipe end region, the ductile nature of the material of the pipe acts, to some extent, as a lubricant to assist the rotational engagement of the coupling part and the formation of a helical groove in the pipe. Thus, the tools 22 and 23 may be operated easily by hand requiring no machinery to form the coupling. This is of considerable advantage when the coupling is formed in a hostile environment and in a confined space. Both tools 22 and 23 are of a kind which may be introduced laterally over the respective parts of the coupling without needing to be introduced axially, and the radial extent of the tools need not be more than about four or five times that of the coupling.

While the inserts and coupling parts are preferably of steel or an alloy, in some applications they may be of a rigid and durable plastics material. The cross- section of the helical tracks 20 and 39 is preferably rectilinear but may be other shapes such as trapezoidal, provided that the walls of the helical track in the coupling part are not substantially greater than 90° with respect to the base of the valley defined thereby.

It will be appreciated that in the embodiments of Figs. 1 to 5, the part 13 of the insert 11 with its helical track 14 serves as a guide to direct the coupling part 16 onto the pipe 10, and in the embodiment of Figs. 6 to 10, the spring 35 in its track 39 forms the necessary guide. These guides ensure the formation of the corresponding thread or helical track in the wall of the pipe which in turn provides the necessary interengagement of the coupling with the pipe to prevent the coupling from becoming loosened in use.