WO2002023076A2

WO2002023076A2 - Plastic pipe adhesive joint

Info

Publication number: WO2002023076A2
Application number: PCT/US2001/028697
Authority: WO
Inventors: Joe T. Minarovic; Elmont E. Hollingsworth
Original assignee: Minarovic Joe T; Hollingsworth Elmont E
Priority date: 2000-09-15
Filing date: 2001-09-14
Publication date: 2002-03-21
Also published as: AU2001290896A1; WO2002023076A3

Abstract

A method of joining two pipe ends, by forming male threads on an end of a first pipe with a steep taper angle, forming matching female threads on an end of a second pipe, and applying adhesive material to the threads before twisting the pipes to fully engage the male and female threads, thereby spreading the adhesive material across adjacent load-bearing surfaces of the threads. The threads have multiple start locations for thread engagement so that the pipe ends may be twisted to achieve full engagement in less than one full relative rotation. The male and female threads are formed with a squeeze angle between matching surfaces of the threads between 0.5 and 5.0 degrees. The finished joint provides immediate leak-tightness, and immediate tensile, compressive and torsional strength and does not protrude outside of the pipe profile.

Description

PLASTIC PIPE ADHESIVE JOINT BACKGROUND OF THE INVENTION

1. Technical Field:

The present invention generally relates to methods and systems used to attach structural members, more specifically, to a joint for connecting conduits, and particularly to a joint that may be used to provide both mechanical attachment and a complete pressure seal for polymeric (plastic) pipe.

2. Description of Related Art:

Many types of conduits are used to provide various utility services, such as telephone, cable, water, waste water, and natural gas. These conduits come in a variety of sizes, shapes and materials. One of the most common forms is a pipe having a circular cross-section, constructed of a polymeric (plastic) material, particularly acrylonitrile butadiene styrene, or polyvinyl chloride. These specific materials are typically used to construct pipes that are relatively rigid.

Conventional distribution systems for these various utilities use polymeric pipes of varying diameters to transport product, or convey signal lines, from suppliers to end users. There is a continual problem regarding leakage in such pipe systems due to inadequate joining of pipe sections. Leakage may be from the inside of the pipe to the outside (e.g., water or natural gas) resulting in the loss of product, or from the outside of the pipe to the inside (e.g., water penetration) which may deteriorate cabling that is protected by the conduit.

Various methods and systems have been devised to join the ends of plastic pipes. One approach is the use of adhesives (see, e.g., US. , Patent Nos. 3,784,235, 3,909,045 and 3,826,521). Joining pipe using adhesives, however, is generally unacceptable due to the difficult adherence characteristics of many polymers, which can result (after a period of time) in cracks in the cured adhesive or in separation of the adhesive from the pipe surface. The use of adhesive or glue (which may be toxic) also often involves a change in the inner diameter or outer diameter of one or both of the pipes, such as with a bell-and-pipe slip joint. Non-uniform pipe diameters can introduce difficulties when installing the pipe. Another technique for fitting pipes together involves the use of electro- fusion couplers (see, e.g., U.S. Patent No. 4,486,650). These couplers overcome the adherence properties of the polyolefin compositions by being formed of the same thermoplastic material as the conduits to be joined. An electrical resistance heating element within the coupler raises the temperature of the material in both the coupler and the adjoining pipe, which then flows together. Upon cooling, the material from the two parts fuses together. As with other prior art techniques, however, electrofusion couplers often do not create a complete seal and the entire process must sometimes be repeated. Electrofusion couplers are also relatively expensive.

A more complicated technique for joining plastic pipe, using microwaves, is disclosed in U.S. Patent No. 5,254,824. That system requires a composite bonding material comprised of a plurality of coated susceptor particles suspended in a matrix. As with the electrofusion couplers, the material of the matrix is adapted to fuse with the material of the pipe at high temperatures. The material is heated by energization of the susceptor particles via, radiant microwave energy. This system is very complicated, however, as it requires a special microwave cavity and power supply, both of which can be very bulky. This technique also presents safety hazard to workers who may be exposed to microwave radiation.

A simpler approach to joining pipes is the use of pipe threading, i.e., where one pipe (with exterior male threads) screws into the end of the second pipe (with interior female threads). While threading is an acceptable way of joining certain kinds of pipe, particularly metal pipe, it is again much less useful in joining plastic pipe. Plastic threads do not have the strength of metal threads, and are more susceptible to stripping. Also, when a straight thread is used to connect two cylinders, a deep shoulder is required to accommodate each thread (male and female), which accordingly decreases the longitudinal strength of the cylinder wall. To avoid this problem, it is necessary to alter either the inner diameter or outer diameter of one of the pipes, which may lead to the difficulties mentioned above.

An alternative molded plastic cylinder thread design is disclosed in NASA Tech Brief 71-10336. That design uses a specific, molded Spiral Buttress Thread with a 30 degree taper. With the Spiral Buttress design, no shoulder is required, removing the concerns regarding compromised wall thickness. The Spiral Buttress Thread is generally easy to start (that is, to align the male and female threads) and may, depending upon the initial relative axial orientation of the two pipe ends, be fully engaged in less than one relative rotation of the pipe ends, regardless of their diameter. This design, however, still does not provide a sufficient mechanical connection or a reliable high pressure seal. It would, therefore, be desirable to devise an improved pipe joint which both mechanically affixes two pipe ends and provides a stronger pressure seal. It would be further advantageous if the improved joint could be quickly completed without the need for special tools or equipment.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide an improved joint for connecting cylindrical polymeric pipes.

It is another object of the present invention to provide an improved joint that does not protrude outside of the pipe profile.

It is still another object of the present invention to provide an improved joint that has immediate strength sufficient to withstand handling and installation.

It is yet another object of the present invention to provide such a joint that imparts sufficient mechanical strength and sealing required for high pressure distribution of natural gas.

The foregoing objects are achieved in a method of joining two pipe ends, generally comprising the steps of forming a plurality of male tlireads on an end of a first pipe member, wherein the male threads have a steep taper angle, forming a plurality of female threads on an end of a second pipe member, wherein the female threads have the steep taper angle and are adapted to. engage the male threads, applying adhesive material to a selected one of the male and female threads, aligning male threads with the female threads as the end of the first pipe member is brought into contact with the end of the second pipe member, then twisting the pipe members to fully engage the male and female tlireads, and thereby spread the adhesive material across adjacent load-bearing surfaces of the threads. Thereafter, the adhesive material is allowed to cure. The invention is particularly suited for use with polymeric pipe, and the threads can either be molded into the pipe ends, cut using a computer-controlled lathe or cut with other specialty thread cutting machines. The threads have multiple start locations for thread engagement. The pipe ends may be twisted to achieve full engagement in less than one full relative rotation of the pipes. The finished joint thereby formed provides immediate low pressure leak-tightness and immediate tensile, compressive and torsional strength.

The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well, as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:

Figure 1 is a perspective view of one embodiment of a male pipe end constructed in accordance with the present invention;

Figure 2 is an elevational view of the male pipe end of Figure 1;

Figure 3 is a plan (axial) view of the male pipe end of Figure 1;

Figure 4 is a detail of Figure 2 depicting trie tapered design of the threads of the male pipe end; and

Figure 5 is a sectional view illustrating a completed joint in accordance with the teachings of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

With reference now to the figures, and in particular with reference to Figures 1 through 3, there is depicted one embodiment 10 of a pipe constructed in accordance with the present invention. Pipe 10 is cylindrical with a generally circular cross-section, and has an end 12 provided with male threading 14. The hollowed interior of the pipe may be used to convey a particular fluid product, or may accommodate other conduits e.g., telecommunications lines.

Pipe 10 may be constructed of any durable material, preferably a rigid polymer such as polyvinyl chloride (PVC) or acrylonitrile butadiene styrene (ABS). The specific dimensions of pipe 10 may vary considerably, depending upon the application. For example, the outer diameter of pipe 10 might be as small as half an inch, or as large as six feet. The length of pipe 10 may similarly vary, from a couple of feet to a hundred feet or more.

Threading 14 is tapered across the complete thickness of pipe wall 16. The taper angle is relatively steep, preferably in the range of 20_-60_. In the illustrative embodiment shown in the figures, the taper is approximately 35_. With this taper, the threads are not capable of cross-threading. When assembled with a corresponding female threaded pipe end, the joint is self-centering and self- rounding.

There are also multiple sets of threads, i.e., multiple start or lead-in locations to begin engagement with corresponding female threads. In the depicted embodiment, there are four start locations 18a-18d. Threading 14 may thereby become fully engaged with corresponding female threading with less than one relative rotation (<360_) of pipe 10. The multiple thread sets may also be seen in Figure 2, where a single thread 20 is shaded to show its starting and ending locations. In this particular embodiment, full thread engagement may be accomplished with as little as a 30_ turn and a maximum of a 120_ turn. The 90_ difference is the rotation space between the starts of engagement.

The present invention uses a thread poorly adapted for a ordinary thread joint in plastic to form an adhesive joint of superior quality. The torque required to loosen the thread joint is a small percentage of the tightening torque. The rotation required to loosen the thread joint is very small. The result of over tightening the joint is compressing the end of the pipe with the male tliread and expanding the end of the pipe with the female thread.

The foregoing adhesive joint design presents several advantages when compared to previous joint designs. Adding adhesive to the threads before tightening them produces a superior joint that enhances to ability of the adhesive make and seal a good joint. This tapered threading provides a large surface area compared to the cross-section of the pipe itself. The joint is completed with less than one twist of the pipe, such that the adhesive does not have sufficient time or distance to seize, gall or be scraped off before the joint makes up. The threads need only be lightly tightened to make a satisfactory joint. The male and female threads radially approach each other faster than .008 inches per inch of circumferential travel. The squeeze angle or Arctangent of the rate of radial approach of the surfaces is larger than a half of a degree. This angle is a function of the taper angle of the threads, thread pitch, the diameter of the pipe, and the number of thread starts. The invention contemplates a squeeze angle in the range of 0.5_ to 5_. This small angle facilitates uniform application of adhesive across the load- bearing surfaces of the tlireads. The threads act as a self-clamping joint while the adhesive sets. The threads provide sufficient strength to the joint so that it may be handled and installed before the adhesive sets. The lack of extension beyond the pipe profile decreases the size of hole, trench or tunnel required for installation and does not impede flow of product through the pipe or restrict or the conduits that can be installed within it.

Different types of adhesive may be used, with the joint. The adhesive is preferably non-toxic, strong and quick setting. It need not be gap-filling or crack filling. Solvent glues such as "airplane" glue, and solvent glues that dissolve part of the pipe and then cure to make a single, integral material joint between the pipes being joined may be used, but are relatively slow in their curing times. Although days may be required for complete solvent evaporation, the initial mechanical strength of the joint allows nearly immediate installation and low pressure testing to confirm basic leak-tightness. This advantage of the invention compensates for the drying time of these glues.

Chemical reacting glue such as epoxy, urethane or cyanoacrylate may also be used with this joint. In these cases, the initial mechanical strength may not be the most important feature but the self-clamping of the joint and the thin glue line contribute to a high-strength and convenient joint.

The adhesive is preferably placed completely arpund the periphery of the male thread with a swab, sponge or paint brush. The mating thread design nominally calls for a zero gap between the threads. _rl Tightening.the threads forces the gap closed. This squeezes out excess adhesive. from the joint. The flow of excess adhesive out of the joint assists and assures the even distribution of adhesive throughout the joint. The small gap of the joint design minimizes the adhesive required and maximizes the strength of the adhesive bond between the two surfaces to be bonded.

The minimizing of the amount of adhesive used minimizes cost, minimizes exposure of adhesive to the person applying the adhesive, minimizes clean up and usually lowers the setup time of the adhesive. The minimizing the amount of adhesive gap to be filled increases bond strengths and maximizes chemical and corrosion resistance of the joint,

An exemplary thread profile is shown in detail in Figure 4. The threads have a stepped appearance (frusto- triangular) in which the tips or edges 22 are sheared at the included angle. In this embodiment, the effective included angle between thread flanks is approximately 60_. A complementary geometry is formed at the inner apex of a thread by landing portions 24 whose surfaces are generally parallel with a taper direction of the threading (or radiused). This geometry reduces any incomplete filling of the adhesive material in the interstices between adjacent threads. The nominal pitch (distance between adjacent threads) is picked to result in three to six full threads in the wall thickness. The foregoing thread specifications may be achieved using molding techniques (e.g.,. injection molding), or by cutting with a computer-controlled lathe or milling machine, or with a cam-controlled, specially-designed threading machine.

The invention includes many alternative embodiments. For example, pipe 10 may be provided with male threading at one end, and female threading at the other end. In this manner, a large number of such pipes may easily be joined from end to end. The joint does not require any kind of coupling,' and can be applied to other types of pipe members besides straight, such as reducers, adapters, sex changers and caps. Other alternative embodiments include variations in thread profiles, thread pitch, number of starts, taper angles, and left-handed threads.

With further reference to Figure 5, a second pipe 28 having female threading is shown engaged with pipe 10. The joint 30 formed thereby is further secured by the thin adhesive layer 32. The joined piping can be immediately handled and installed after make up. The joint maintains the same inner and outer diameters of the terminal portions of the piping. Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention, will become apparent to persons skilled in the ail upon reference to the description of the invention. For example, while application of the present invention has been described with reference to utility conduits, the invention may be used in other applications, such as forming piping for pneumatic delivery systems. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined in the appended claims.

Claims

1. A method of joining two pipe members, comprising the steps of: forming a plurality of male threads on an end of a first pipe member, the male threads having a steep taper angle; forming a plurality of female threads on an end of a second pipe member, the female threads having the steep taper angle, and adapted to engage the male threads; applying adhesive material to at least one of the threads; aligning the male threads with the female threads as the end of the first pipe member is brought into contact with the end of the second pipe member; twisting the pipe members to fully engage the male and female threads, and thereby spread the adhesive material across adjacent load-bearmg surfaces of the threads; and curing the adhesive material.

2. The method of Claim 1 wherein said forming steps are accomplished by cutting the male and female threads into the respective ends of the first and second pipe members.

3. The method of Claim 1 wherein said forming steps form the steep taper angle in the range of 20_-60_.

4. The method of Claim 1 wherein the first pipe member is generally straight and has a circular cross-section, and further comprising the step of forming a plurality of matching female threads on a second end of the first pipe member.

5. The method of Claim 1 wherein said forming steps form the male and female threads having a spiral buttress profile.

6. The method of Claim 1 wherein said forming steps form the male and female threads having multiple start locations for thread engagement.

7. The method of Claim 1 wherein said twisting step achieves full engagement in less than one full relative rotation of the pipe members.

8. The method of Claim 1 wherein said forming steps form the male and female threads with a squeeze angle between matching surfaces of the male and female threads as the threads are brought into engagement of between 0.5 degrees and 5.0 degrees.

9. An article of manufacture comprising a hollow member having at least one generally tubular end, said end having a plurality of threads thereon, said threads having:

a steep taper angle in the range of 20_-60_ and multiple start locations for thread engagement.

10. The article of Claim 9 wherein said threads have a pitch such as to result in 3 to 6 full threads across a thickness of a wall of said hollow member.

11. The article of Claim 9 wherein: said threads further have a spiral buttress profile.

12. The article of Claim 9 wherein: said tubular end has a wall with a thickness; and said threads are tapered across substantially the complete thickness of said wall.

13. The article of Claim 9 wherein: said threads are female threads formed along an inner surface of said end.

14. The article of Claim 9 wherein said threads are male threads formed along an outer surface of said end.

15. The article of Claim 14 wherein: said hollow member is a generally straight pipe having a circular cross- section, said end having said male threads being a first end of said pipe; and said pipe has a second end with female threads formed along an inner surface of said second end, said female threads having a taper angle, and multiple start locations which match with said male threads.

16. The article of Claim 15 wherein said male and female threads provide a squeeze angle between matching surfaces of said male and female threads of between 0.5 degrees and 5.0 degrees.

17. A pipe joint comprising: a first pipe member having an end, said end of said first pipe member having a plurality of male threads with a steep taper angle; a second pipe member having an end, said end of said second pipe member having a plurality of female threads with the steep taper angle, in engagement with said male threads; and an adhesive material spread across adjacent load-bearing surfaces of said male and female threads.

18. The pipe j oint of Claim 17 wherein the taper angle is in the range of 20_-60_.

19. The pipe joint of Claim 17 wherein said male and female threads have a spiral buttress profile.

20. The pipe joint of Claim 17 wherein said male and female threads have multiple start locations for thread engagement.

21. The pipe joint of Claim 17 wherein said threads are cut into said respective ends of said first and second pipe members.

22. The pipe joint of Claim 17 wherein matching surfaces of said male and female threads provide a squeeze angle of between 0.5 degrees and 5.0 degrees.