Variable stiffness bellows

This invention relates to a flexible tube that includes a bellows having a plurality of convolutes formed in the wall of said tube and extending above the top surface of the tube, said convolutes being spaced from one another in the axial direction of the tube, wherein at least one of said convolutes comprises two opposing bending sections and two restrained elongation sections positioned between the bending sections, and wherein the height of the bending sections above the top surface of the tube is greater that the height of the restrained elongation sections above the top surface of the tube.

BACKGROUND

1. Field of the Invention

This invention relates to the field of flexible tubes that may be used as air ducts, coolant pipes, fuel tubes, filler necks and the like, such as are used in the automotive industry.

2. Description of the Related Art

Flexible tubes are used, for example, as air ducts in automobiles where the air duct carries air to an engine. The air may pass through a turbocharger, in which case the air flowing through the duct is heated to temperatures of generally 140–160C, but possibly up to 180–200C.

These air ducts are often made by various blow molding techniques. Sequential extrusion blow molding technology is used to produce one-piece air ducts that combine several hard and soft segments.

The internal surfaces of the air duct exposed to the air pressure are under stress. Further, such exposed surfaces create an axial force that causes a longitudinal tension on the air duct because of end-cap effects. As the bellows of the air duct are more flexible than the tube, the bellows tends to elongate more in the longitudinal direction, which is the so-called “pneumatic piston effect”. Expansion of the bellows in the radial direction, called “ballooning”, is relatively low in absolute value compared to the longitudinal elongation of the bellows.

Tensile and shear forces also create constant stress on the air duct which leads to creep of material of the air duct, causing the air duct to elongate over time. This elongation changes the bending stiffness of the air duct, which is why the design of a particular air duct bellows is a case-by-case compromise of various design criteria.

An example of a flexible tube that may be used an air duct is shownFIGS. 1A–1C, where there is shown a flexible tube11that includes a bellows13having a plurality of convolutes15formed in tube11. Convolutes15are, essentially, raised circumferential ridges formed in the surface of tube11.

When fluids, such as air or liquids, move under pressure through a tube, the pressure in contact with the projected axial surface of tube11caused by the end-cap effects creates a force which pulls and deforms bellows13. This force generates material stress which leads to material creep over time, and which can make tube11fall out of its design tolerance. Tube11may then move undesirably and contact other parts which may damage tube11or the parts it contacts. When used in an engine, the duct may contact hot surfaces or sharp angles and be damaged.

The bellows15of flexible tube11provides the same bending stiffness along the Y and Z axes. There is no additional design feature that serves to reduce or control the longitudinal deformation, that is, the deformation along the X axis, when flexible tube11is subject to elevated temperature and pressure due to the compressed air circulating therein.

One way to overcome the problem of deformation of tube11along the X axis is to add two parallel longitudinal ribs to bellows13of flexible tube11. Turning now toFIGS. 2A–2B, there is shown a flexible tube21that includes a bellows23having a plurality of convolutes25formed in tube21. A pair of opposing longitudinal ribs27is formed along bellows23so as to connect convolutes25with one another. The width and height of ribs27may be varied. If rib27is wider or higher than convolutes25, flexible tube21will have a greater tensile strength along the X axis but tube21will have a weaker burst resistance because rib27will tend to unfold or open when subject to high pressure.

Flexible tube21provides bending capability along the Z axis, but reduced bending along the Y axis and reduced elongation along the X axis. The design ofFIGS. 2A–2Bsuffers from the problem that it is difficult to design tube21to get the right compromise of pressure resistance, stiffness and bending properties.

Another way to reduce the longitudinal deformation of a flexible tube is shown inFIGS. 3A–3Cwhere there is shown a flexible tube31that includes a bellows33having a plurality of convolutes35formed in tube31. Opposing sides of convolutes35have a flattened portion37where the outer surface of convolutes35is equal to the outer surface of tube31. This type of flexible tube is the subject of pending patent applications WO 99/22171 and EP 0 863 351 A2.

Flexible tube31provides bending capability along the Z axis, but reduced bending along the Y axis and reduced elongation along the X axis. Further, bellows33allows tube31to be decoupled, that is, it allows tube31to move independently of other parts of an engine. Decoupling prevents or greatly reduces the shear stress on the end portions of tube31.

However, tube31suffers from the problems that it is difficult to adjust in terms of pressure resistance, bending stiffness and that the bending stiffness in the Z axis is too high due to limitations in the design and the method of manufacturing such flexible tubes31.

Flexible tubes such as those discussed above are made by a blow molding process wherein an extruded parison, or extruded tube, of a polymer material is placed in a tool cavity. Turning toFIG. 4A, there is shown a tool41having a cavity43for making a flexible tube45. A parison47is placed in cavity43, said parison47having a varying thickness so that parison47is thicker in the area nearer to the part of the cavity43that forms convolutes48of tube45.

Air is blown into cavity43and as shown inFIG. 4B, once the blow molding process has been completed, tube45is in contact with tool41, and, due to the internal pressure applied during the transformation, the entire surface of cavity43is covered with polymer. As a result of parison47being thicker in certain portions so as to accommodate convolutes48, the wall thickness distribution of tube45is not uniform. In particular, a flattened portion49of convolute48is thicker than any other portion of flexible tube45.

In general, profiles and shapes close to the initial parison tend to be somewhat thicker than remote ones. The ratio between the location of minimum material expansion, in this case the flattened portion49to the location of the maximum material expansion, the thickness of the outer edge of convolute48, is called the blow ratio and depends on the material used in the blow molding process.

This variation in the wall thickness of flexible tube31has the following negative effects:There is a decrease in the bending capability of tube45along the Z axis (shown for example inFIGS. 3A–3C) because the relatively thicker walls of flattened portions49are stiffer.The extra material present in flattened portions49is not needed and does not reduce stress on the most exposed area of tube45, namely the external surface of convolutes48.

What is needed, therefore, is, a flexible tube having a bellows design that overcomes the problems associated with the bellows of the prior art.

SUMMARY OF THE INVENTION

The present invention is directed to a flexible tube having a bellows that includes a plurality of convolutes formed in the wall of said tube and extending above the top surface of the tube. The convolutes are spaced apart from one another in the axial direction of the tube, and at least one of the convolutes has two opposing bending sections and two restrained elongation sections positioned between the bending sections, where the height of the bending sections above the top surface of the tube is greater that the height of the restrained elongation sections above the top surface of the tube.

The tube is preferably made of a thermoplastic material.

The bending sections are preferably opposite each other at an angle of 150–210°, preferably 180° and, similarly, the restrained elongation sections are preferably opposite each other at an angle of 150–210°, preferably 180°.

DETAILED DESCRIPTION

The present invention is directed to a flexible tube having a bellows that includes a plurality of convolutes formed in the wall of said tube and extending above the top surface of the tube. The convolutes are spaced apart from one another in the axial direction of the tube, and at least one of the convolutes has two opposing bending sections and two restrained elongation sections positioned between the bending sections, where the height of the bending sections above the top surface of the tube is greater that the height of the restrained elongation sections above the top surface of the tube.

The bending sections are preferably opposite each other at an angle of 150–210°, preferably 180° and, similarly, the restrained elongation sections are preferably opposite each other at an angle of 150–210°, preferably 180°.

The inventive flexible tube maintains the benefits of flexible tube31discussed above, namely, a simple design and applicability to a wide range of profiles, while exhibiting improved bending and elongation characteristics. The convolutes in the tube of the invention are higher above the tube surface in the bending sections than in the restrained elongation sections, which design has the beneficial effects of providing:effective restriction of the elongation of the tube in the X axis;a bending ability in the Z axis is as good as or better than the bending ability of flexible tube31;a more uniform wall thickness in the restrained elongation section of the bellows;a pre-defined or pre-set bending path in the restrained elongation sectors.a flexible tube design that may be made by blow molding; anda reduced bending capability along the Y axis.

There is no particular limitation on the material from which the tube of the invention is made provided that the tube is flexible. If blow molded, the flexible tube and bellows may be made from any blow-moldable thermoplastic resin. The term “thermoplastic resin” includes synthetic polyamides, polyesters, polyacetals, block polyester ether copolymers, ethylene propylene diene elastomer (EPDM), olefins such as polypropylene, as well as mixtures or blends thereof.

The invention is illustrated inFIGS. 5A–5Cwhere there is shown a flexible tube51that includes a bellows53having a plurality of convolutes55formed therein spaced at an interval from each other in the axial direction of flexible tube51. Convolutes55, which are raised circumferential ridges formed in the surface of tube51as an integral part thereof, have bending sections57, restrained elongation sections59, and transition sections61that connect bending sections57to restrained elongation sections59.

Bending sections57are preferably opposite each other at an angle of 150–210°, preferably 180°. Similarly, restrained elongation sections59are preferably opposite each other at an angle of 150–210°, preferably 180°.

Bending sections57are approximately the same height above the outer surface of tube51, and likewise restrained elongation sections59are approximately the same height above the outer surface of tube51with bending sections57having a height above the outer surface of tube51greater than the height of restrained elongation sections59above the outer surface of tube51.

Turning toFIGS. 6A–6B, there is shown a tool71having a cavity73that includes a bending section cavity75and restrained elongation section cavity77. A parison79is placed in cavity73, said parison79having a varying thickness. As is shown inFIG. 6B, after parison79has been blow molded, there is formed a flexible tube81having a bending section83and an restrained elongation section85. The wall thickness distribution in flexible tube81is more uniform than the wall thickness distribution in flexible tube45shown inFIG. 4B, especially as it relates to the area of restrained elongation section85, which improves the bending capability of flexible tube81while concurrently adequately restraining the elongation thereof.

The length (in the radial direction), the width (in the axial direction) and the cross-sectional shape of bending sections57, restrained elongation sections59and transition sections61of convolutes55may each be varied individually or together to modify the elongation and bending properties of flexible tube51. Further, the angle of the junction of restrained elongation sections59and transition sections61may be varied, as may be the angle of the junction of bending sections59and transition sections61.

For example, inFIGS. 7A–7C, there is shown another embodiment of the invention which is similar to that shown inFIGS. 5A–5C, except that the shape of the convolutes has been modified. Turning toFIGS. 7A–7C, there is shown a flexible tube61athat includes a bellows53ahaving a plurality of convolutes55aformed in tube51awherein the convolutes have opposing bending sections57a, opposing restrained elongation sections59a, and transition sections61athat connect bending sections57ato restrained elongation sections59a.

In the case of flexible tube51a, near the surface of tube51aconvolutes55aare narrower in the area of restrained elongation sections59athan in the area of bending sections57a, and the width of transition sections61anear the surface of tube51ais tapered moving from bending sections57ato restrained elongation sections59a.

Another embodiment of the invention is shown inFIGS. 8A–8D, there is shown a flexible tube91that includes a bellows93having a plurality of convolutes95formed in tube91wherein the convolutes have opposing bending sections97, opposing restrained elongation sections99, and transition sections101that connect bending sections97to restrained elongation sections99. In this embodiment, restrained elongation sections99include two radially-spaced convolutes103which are attached at both ends to transition sections101. This embodiment of the invention has an improved wall thickness due to the additional contours formed on the inside of tube91by convolutes103in restrained elongation section99.

The shape and size of convolutes103in restrained elongation sections99may be varied to modify the bending and elongation characteristics of flexible tube91.

For example, inFIGS. 9A–9Cthere is shown a flexible tube91athat includes a bellows93ahaving a plurality of convolutes95aformed in tube91ahaving opposing bending sections97a, opposing restrained elongation sections99a, and transition sections101a. Restrained elongation sections99ainclude two radially-spaced convolutes103awhich are attached at both ends to transition sections101aand which extend into transition sections101a. Further, convolutes103aare tapered so as to vary in width moving from the center of a restrained elongation section99ato a transition section101a. The modified convolutes103amodify the bending and elongation characteristics of flexible tube91acompared to flexible tube91.

Another embodiment of the invention is shown inFIGS. 10A–10Cwhere there is shown a flexible tube111that includes a bellows113having a plurality of convolutes115formed therein spaced at an interval from each other in the axial direction of flexible tube111. Convolutes115, which are raised circumferential ridges formed in the surface of tube111as an integral part thereof, have opposing bending sections117, opposing restrained elongation sections119, and transition sections121that connect bending sections117to restrained elongation sections119. Convolutes115, when viewed from the axial direction, are elliptically-shaped. Convolutes115are characterized in being relatively flat in restrained elongation sections119and having a relatively long transition section121that connects flat restrained elongation sections119to bending sections117.

The height of convolutes115in bending sections117, restrained elongation sections119, and transition sections121may be varied provided that the overall shape of convolutes115remains elliptical. The width (in the longitudinal direction) of convolutes115may be varied to modify the bending and elongation characteristics of flexible tube111.

For example, inFIGS. 1A–11C, there is shown a flexible tube111athat includes a bellows113ahaving a plurality of convolutes115ahaving opposing bending sections117a, opposing restrained elongation sections119a, and transition sections121a, which convolutes115a, when viewed from the axial direction, are elliptically-shaped. Convolutes115aare narrower in the region of restrained elongation sections119athan in bending sections117a, with the width of transition sections121abeing tapered from the width of bending sections117ato restrained elongation sections119a.

While the above embodiments of the invention have bellows wherein the shape of each of the convolutes of the bellows is the same, the flexible tubes of the present invention may include convolutes of varying shapes.

For example, turning toFIGS. 12A–12Cthere is shown a flexible tube131that includes a bellows133having convolutes135a–fformed therein spaced at an interval from each other in the axial direction of flexible tube131. Convolutes135a–fcomprise opposing bending sections137, opposing restrained elongation sections139, and transition sections141that connect bending sections137to restrained elongation sections139. The length, in the radial direction, of restrained elongation sections139increases from convolute135ato convolute135f. Similarly, the length, in the radial direction, of bending sections137increases from convolute135ato convolute135f. Varying the length of restrained elongation sections139and bending sections137allow the bending and elongation characteristics of flexible tube131to be modified as desired.

The varying radial length of restrained elongation sections139of tube131improves the stress distribution in bending of tube131. For example, if the end of tube131near convolute135ais fixed and the other end of tube131is pulled in a radial direction, force and stress will be applied to convolutes135. However, relatively more stress will be applied to convolute135athan convolute135fand convolutes nearer the fixed end will deform more than those close to the load side. The design ofFIGS. 12A–12Cdistributes more evenly the bending stress over bellows133.

While not exhibited inFIGS. 5–11, the length of the transition sections of the convolutes may be varied from convolute to convolute, and the width, in the axial direction of any or all of the bending section, transition section and/or restrained elongation section may be varied from convolute to convolute in the bellows of a flexible tube.

The majority of applications and developments in blow molded flexible tubes, coolant and fuel pipes are made around tubes, or almost circular profiles. However, there is no limitation on the cross-sectional shape of the convolutes that may be present in the flexible tubes of the present invention. The convolutes in the bending sections, transition sections and/or restrained elongation sections may be triangular, rectangular, trapezoidal or any other shape, or mixtures of such shapes, as long as such shape or shapes provide the benefits of this invention.

The bending sections of the convolutes of this invention are preferably opposite each other at an angle of 150–210°, preferably 180°. Similarly, the restrained elongation sections of the convolutes of this invention are preferably opposite each other at an angle of 150–210°, preferably 180°.

A preferred method of manufacture of the flexible tubes of this invention is blow molding, such as extrusion blow molding, co-extrusion blow molding or sequential blow molding.

The flexible tubes of the invention may be used for air-conduction parts and liquid lines in automobiles or as water lines for washing machines.