GB2079663A

GB2079663A - Making corrugated tubes

Info

Publication number: GB2079663A
Application number: GB8024011A
Authority: GB
Original assignee: United Gas Industries Ltd
Current assignee: United Gas Industries Ltd
Priority date: 1980-07-23
Filing date: 1980-07-23
Publication date: 1982-01-27
Also published as: GB2079663B

Abstract

A corrugated tube is formed from a tube 5 of heat shrinkable thermoplastic material by inserting a coil spring 1 or series of discs into the tube and heating the tin be so that it shrinks as shown at 4 into the interstices between the coils of the spring. The spring is held on end supports 3 which may be moved toward one another during the process so that the spring is compressed. <IMAGE>

Description

SPECIFICATION Bellows This invention relates to bellows formed of thermoplastic material and to a method of making such bellows.

The invention provides a method of making bellows comprising placing a tube of heat-shrinkable thermoplastic over an internal support of a shape related to the desired configuration of the finished bellows and applying heat to said tube so that it shrinks onto said support.

Said support may comprise a spiral coil or series of rings with interstices between the coils or rings into which the heat-shrinkable tube shrinks. Preferably the spiral coil is a spring which forms part of the finished bellows and supports it against external pressure.

The method may also include compressing the spiral spring during the heat shrinking step.

The invention also extends to a bellows made by the method described above. The bellows may be adapted for use as a protective covering over metal bellows subject to an aggressive environment, e.g., in buried pipelines for district heating installations.

A specific embodiment of the invention is illustrated in the accompanying drawings, in which: Figure 1 is a side section through a bellowsforming apparatus before the bellows is formed, Figure 2 shows the apparatus of Figure 1 with a formed bellows, Figure 3 shows a further operation on bellows formed on the apparatus of Figures 1 and 2, and Figure 4 is a diagram illustrating the action of the bellows-forming apparatus of Figures 1 and 2.

Referring first to Figure 1, acoiled spiral metal spring 1 is held between end plungers 3. The distance A between the individual coils is set to a predetermined distance: this may be the unstressed distance between the coils or may be set by extending or compressing the spring. A tube 2 of heat-shrinkable thermoplastic material is placed over the outside of the spring 1 and is then subjected to heat to cause it to shrink into the position shown in Figure 2. It will be seen that because the spring resists radial compression the thermoplatic material is only able to shrink fully into the interstices between the coils of the springs so forming bellows convolutions 4.

Reference to Figure 4 shows that the distance A between the coils of the spring is reduced to distance B in order to allow the same length A of tube 2 to take up the convoluted form 4. This may be allowed for by the plungers 3 moving toward one another during the heating process. Alternatively the shrinkage of the material itself draws the spring into a compressed condition. The heat may be applied either by placing the apparatus in an oven or by applying heat from a local source, e.g., hot air or radiant heat.

The material forming the tube 2 is chosen from a well-known group of polymeric and elastomeric materials which shrink when heated. These materials are generally formed by intermolecular crosslinking by radiation of plastics materials, for instance polyolefin based materials. The preferred material is polyolefin.

It will be noted from Figure 2, that the bellows ends 5 are shrunk only to the size of the plungers 3, forming ends of the maximum bellows diameter. If a tubular end of a diameter less than this is required, a further shrinking operation may be performed using the apparatus to Figure 3. Here the bellows is held in a guide 6 which engages the outer diameter of the bellows to maintain it centralized with respect to a smaller diameter tube 7 which extends within the end 5. Further heat applied brings the end 5 down to the smaller diameter 5' defined by tube 7. The diameter 5' may be chosen so that the bellows ends may be fitted to mating parts by further shrinking of the tubular ends onto the mating parts.

In alternative arrangements the spiral spring may be replaced by a series of rings mounted on a central spider or core. The rings or spring may be of material other than metal, e.g., of a plastics material.

The bellows may include the spring or rings as permanent strengthening members increasing the resistance to external pressure on the bellows. It will be appreciated that the spring rate of the spring also has a major effect on the axial compression characteristic of the bellows. However, the springs may be removed by "unscrewing" movement if the bellows are required without them.

One particular use for a plastic bellows as described above is in underground pipe lines, e.g., carrying hot fluids for district heating systems. Such pipes are subject to expansion movements which are commonly taken up by metal bellows which form expandible joints between pipe lengths. Jackets of plastics insulating material in an outercasing of polyethylene or other plastics material surround the pipes.

It has been found that when such pipe lines are buried in the ground, movements of the outer casing in the ground often occur and this can cause failure of joints in the outer casing allowing ingress of water and resulting in damage to the insulation, pipe and metal bellows. The presently described plastics bellows may be used to surround the metal bellows, with a compressible layer of insulating material sandwiched between the metal and plastics bellows, the ends of the plastics bellows being sealed to the ends of the outer casing of the jackets surrounding the pipes. The plastics bellows therefore allow the movement of the outer casings without damage to the joints.

1. A method of making bellows, comprising placing a tube of heat-shrinkable thermoplastic over an internal support of a shape related to the desired configuration of the finished bellows and applying heat to said tube so that it shrinks onto said support.

2. A method as claimed in claim 1, wherein said internal support is a spiral coil spring.

3. A method as claimed in claim 2, including the step of compressing the spiral spring during the heat shrinking step.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Bellows This invention relates to bellows formed of thermoplastic material and to a method of making such bellows. The invention provides a method of making bellows comprising placing a tube of heat-shrinkable thermoplastic over an internal support of a shape related to the desired configuration of the finished bellows and applying heat to said tube so that it shrinks onto said support. Said support may comprise a spiral coil or series of rings with interstices between the coils or rings into which the heat-shrinkable tube shrinks. Preferably the spiral coil is a spring which forms part of the finished bellows and supports it against external pressure. The method may also include compressing the spiral spring during the heat shrinking step. The invention also extends to a bellows made by the method described above. The bellows may be adapted for use as a protective covering over metal bellows subject to an aggressive environment, e.g., in buried pipelines for district heating installations. A specific embodiment of the invention is illustrated in the accompanying drawings, in which: Figure 1 is a side section through a bellowsforming apparatus before the bellows is formed, Figure 2 shows the apparatus of Figure 1 with a formed bellows, Figure 3 shows a further operation on bellows formed on the apparatus of Figures 1 and 2, and Figure 4 is a diagram illustrating the action of the bellows-forming apparatus of Figures 1 and 2. Referring first to Figure 1, acoiled spiral metal spring 1 is held between end plungers 3. The distance A between the individual coils is set to a predetermined distance: this may be the unstressed distance between the coils or may be set by extending or compressing the spring. A tube 2 of heat-shrinkable thermoplastic material is placed over the outside of the spring 1 and is then subjected to heat to cause it to shrink into the position shown in Figure 2. It will be seen that because the spring resists radial compression the thermoplatic material is only able to shrink fully into the interstices between the coils of the springs so forming bellows convolutions 4. Reference to Figure 4 shows that the distance A between the coils of the spring is reduced to distance B in order to allow the same length A of tube 2 to take up the convoluted form 4. This may be allowed for by the plungers 3 moving toward one another during the heating process. Alternatively the shrinkage of the material itself draws the spring into a compressed condition. The heat may be applied either by placing the apparatus in an oven or by applying heat from a local source, e.g., hot air or radiant heat. The material forming the tube 2 is chosen from a well-known group of polymeric and elastomeric materials which shrink when heated. These materials are generally formed by intermolecular crosslinking by radiation of plastics materials, for instance polyolefin based materials. The preferred material is polyolefin. It will be noted from Figure 2, that the bellows ends 5 are shrunk only to the size of the plungers 3, forming ends of the maximum bellows diameter. If a tubular end of a diameter less than this is required, a further shrinking operation may be performed using the apparatus to Figure 3. Here the bellows is held in a guide 6 which engages the outer diameter of the bellows to maintain it centralized with respect to a smaller diameter tube 7 which extends within the end 5. Further heat applied brings the end 5 down to the smaller diameter 5' defined by tube 7. The diameter 5' may be chosen so that the bellows ends may be fitted to mating parts by further shrinking of the tubular ends onto the mating parts. In alternative arrangements the spiral spring may be replaced by a series of rings mounted on a central spider or core. The rings or spring may be of material other than metal, e.g., of a plastics material. The bellows may include the spring or rings as permanent strengthening members increasing the resistance to external pressure on the bellows. It will be appreciated that the spring rate of the spring also has a major effect on the axial compression characteristic of the bellows. However, the springs may be removed by "unscrewing" movement if the bellows are required without them. One particular use for a plastic bellows as described above is in underground pipe lines, e.g., carrying hot fluids for district heating systems. Such pipes are subject to expansion movements which are commonly taken up by metal bellows which form expandible joints between pipe lengths. Jackets of plastics insulating material in an outercasing of polyethylene or other plastics material surround the pipes. It has been found that when such pipe lines are buried in the ground, movements of the outer casing in the ground often occur and this can cause failure of joints in the outer casing allowing ingress of water and resulting in damage to the insulation, pipe and metal bellows. The presently described plastics bellows may be used to surround the metal bellows, with a compressible layer of insulating material sandwiched between the metal and plastics bellows, the ends of the plastics bellows being sealed to the ends of the outer casing of the jackets surrounding the pipes. The plastics bellows therefore allow the movement of the outer casings without damage to the joints. CLAIMS

4. Apparatus for carrying out the method of any of claims 1 to 3, comprising means for supporting the internal support and for applying heat to the said tube.

5. Apparaus as claimed in claim 4 as appendant to claim 3, comprising also means for compressing the internal support.

6. A bellows made by the method of any of claims 1 to 3.

7. A bellows as claimed in claim 6, formed of a heat shrinkable polyolefin.

8. A bellows as claimed in claim 6 or claim 7, having an internal spring which acts as a strengthening member and which also comprises said internal support in the making of the bellows.

9. A bellows substantially as described herein be fore with reference to the accompanying drawings.

10. Apparatus for making a bellows substantially as described hereinbefore with reference to the accompanying drawings.

11. A method of making a bellows substantially as described hereinbefore with reference to the accompanying drawings.