Abstract: The present invention is directed to a method of making a hollow fiber membrane contractor comprising the steps of: winding a hollow fiber fabric around a center tube, potting the fabric and the tube together, forming thereby a unitized structure, placing the structure into a shell, potting the structure and the shell together, and forming thereby a cartridge. The invention, also, includes a hollow fiber membrane contactor comprising a unitized structure. The structure includes a center tube, a hollow fiber fabric wound around the tube and a first potting material joining together the fabric and the tube. A second potting material joins together the structure and the shell.

Abstract: A wire bead build-up wheel assembly for a single-wire bead-building process has a disk member with a peripheral groove for the bead wire, and an open core, and a hub that has a head and a threaded shaft. There are thrust bearings between the main disk member and the head of the hub, and between the main disk member and a second annular disk member. An axial sleeve bearing may be positioned over the hub shaft. A suitable retainer keeps the build-up wheel assembly together. The thrust bearings may have radially disposed bearing pins. An associated wire guide chute is formed of a pair of side plates mounted to a base or block.

Abstract: A method of manufacturing a wall thickness program onto an elastomeric tubular component, such as an expandable bladder and/or cover element, for incorporation into a packing device for use in a subterranean well includes steps of contouring upon a surface a diametrical configuration defining a wall thickness program for the tubular component. The diametrical configuration is transferred from the surface onto at least one of the walls of the tubular component as a wall thickness program whereby the transfer diametrical configuration conforms substantially as a mirror image of the program on the contoured surface. An elongated mandrel may be provided as the surface onto which the diametrical configuration of the wall thickness program is contoured, such as by machining. Thereafter, the elastomeric tubular component may be prepared on the mandrel, such as by wrapping elastomeric ribbon in layers therearound as the mandrel is manipulated.

Abstract: An easily manufactured shoe press belt for a papermaking machine comprises a high molecular weight cylindrical elastic member having embedded within it, between its inner and outer walls, a multiple layer base member formed by winding a belt-like woven fabric. The layers of the base member are substantially axially coextensive, reinforce the belt against sideward elongation, and achieve uniform hardness and improved durability.

Abstract: A tubular or ring-shaped core for winding pressure-sensitive tape has a paper board body and a release-coated outermost ply formed of a strip of polymer film having a release coating on its outer surface and a tacky adhesive layer on its inner surface. The strip is wound onto the core with one edge of the strip folded outwardly away from the core so as to expose the tacky adhesive layer on the folded edge. This folded edge is overlapped by the opposite edge of the strip, thereby placing the tacky adhesive layers on the two edges in contact with each other. A firm bond between the overlapping edges is achieved by the adhesive-to-adhesive contact. In a preferred embodiment, the overlapping edge of the strip overlaps slightly beyond the folded edge onto the unfolded outer surface of the strip. This unfolded portion is treated to promote adhesion of the overlapping edge thereto.

Abstract: Disclosed is a process for obtaining a flexible conduit comprising a tube and a helical structure. The process comprises extruding a crosslinkable plastic material in a malleable or formable state, so as to form a profiled strip having, as one piece, a web with two mutually complementary marginal edges, and at least one rib projecting on one side from the web; winding the profiled strip about an axis in a helix with touching turns so that the complementary marginal edges of the profiled strip, belonging respectively to two different turns, form between them, directly, a sealed joint; varying and controlling a winding pitch of the profiled strip, in order to modulate and control a quantity of available and formable plastic material of the tube along the length of the conduit; and crosslinking the wound profiled strip.

Abstract: A method of manufacturing multi-ply tubular containers for food products is provided including the steps of applying an aqueous adhesive to a surface of a paperboard body ply and then heating the aqueous adhesive to evaporate part of the water content and render the adhesive substantially tacky. The body ply and a polymeric liner ply having a moisture barrier layer are then passed through a nip to adhere the liner ply to the body ply and are wrapped around a shaping mandrel to create the tubular container. Accordingly, an advantageous tubular container can be manufactured having a body ply formed of paperboard which is wrapped into a tubular shape to define an inner surface. The body ply defines a predetermined circumferential length before being wrapped which corresponds to one revolution of the body ply when wrapped in a tubular shape. A polymeric liner ply is adhered to the inner surface of the body ply and defines a circumferential length equal to that of the body ply prior to being wrapped.

Abstract: A process for the manufacture of a rim preform or rim-element having an axis of revolution by the placing on a preforming matrix pre-assembled fibers along two orientations defining deformable meshes. A winding of a strip of fibers is fastened on a circumference of the preforming matrix with asymmetrical orientation of the fibers, whereupon it is applied progressively until covering the entire surface of the preforming matrix by subjecting it to tensions of substantially circumferential orientation.

Abstract: A method for manufacturing a press fabric for a paper machine includes the attachment of a strip of top laminate layer material to a base fabric using a heat-activated adhesive film. The top laminate layer material may be a woven fabric, a nonwoven mesh, or a thermoplastic sheet material, and, in any case, has the heat-activated adhesive film bonded to one of its two sides. The strip of top laminate layer material and heat-activated adhesive film together form a multi-component strip, which is spiralled onto the outer surface of the base fabric, with the side of the strip of top laminate layer material having the heat-activated adhesive film against the outer surface, in a closed helix, and bonded thereto with heat and pressure. The portions of the multi-component strip overhanging the lateral edges of the base fabric are then trimmed, and a staple fiber batt is needled into and through the top laminate layer formed by the multi-component strip to firmly attach it to the base fabric.

Abstract: A tubular composite container includes a paperboard body ply wrapped into a tubular shape, and a polymer film liner ply wrapped into a tubular shape and adhered to the inner surface of the body ply. The liner strip includes a heat seal layer facing the body ply that is formed of a heat-sealable material, and the inner surface of the liner ply that contacts the mandrel is formed of a hard polymer material whose melting temperature is substantially higher than the sealing temperature at which the heat-sealable material softens and seals to itself. One edge portion of the liner ply is folded inward toward the mandrel, the liner ply is wrapped onto the mandrel such that the folded edge portion overlaps an opposite edge portion of the liner ply, and the heat seal layer on the folded edge portion is heat sealed to the opposite edge portion.

Abstract: A method of resurfacing a traction roll to provide a friction surface to replace a worn surface. A supply reel of metal strip material is continuously unwound from a supply reel and rewound on a take-up reel. Between the reels, the unwound strip passes through a treating station at which a surface of the strip is prepared for coating and is then coated by one or two sprays of various types of thermal spray equipment as the applicator, with a polymer coating sometimes applied as a top coating. The take-up reel with the coated strip is then transported to a facility where the traction roll is located. The traction roll is then prepared for winding of the strip thereonto. The strip is wound helically onto the traction roll and secured thereto by doublesided adhesive tape disposed between and attached to the roll surface and the metal strip. In a modification, the strip is fed through the treating location in contact with an arcuate surface of a mandrel to impart curvature to the strip.

Abstract: It is an object of this invention to provide a tubular film having a high film thickness uniformity and suitable for a fixing film of an image forming apparatus. To achieve this object, a thermoplastic sheet film is so wound that the leading and trailing ends of the film partially overlap each other to form overlapping portions, and the wound film is heated between a core member and a tubular molding member.

Abstract: A paper base having a thin metallization on at least a portion of its surface is folded into a box for accommodating golf balls. The box has an elegant appearance, can be disposed of without problems, and can be effectively manufactured.

Abstract: In the present invention, hollow thermoplastic foam tubes of any desired diameter are easily achieved as well as large sheets or planks of thermoplastic foam material in any width and thickness desired by spirally wrapping and fusing a thermoplastic foam profile having a desired size and shape. By employing a thermoplastic foam extruder to produce a profile having a desired cross-sectional shape or configuration, and advancing the profile onto a rotating support member for being wrapped peripherally surrounding the rotating support and continuously bonding the abutting edges of the profile as the profile is spirally wound, a unique spiral forming and product manufacturing procedure and system is realized. By employing this unique spiral forming process, a hollow cylindrical thermoplastic foam tube is formed on a continuous basis, with the length thereof being controlled only by the needs of the customer.

Abstract: A method of making a fabric Venetian blind comprises helical winding of a narrow, elongated strip, which, when properly overlapped and bonded along longitudinal glue lines, will form the vanes and front and rear facings of the blind. The strips may be formed of a single, uniform material, or of adjacent, longitudinally extending portions of relatively opaque and relatively translucent fabric. An alternative embodiment uses the same winding technique to similarly form a structure of only translucent fabric, but with pockets for subsequent insertion of opaque vanes. Several alternative strip configurations and assembly arrangements are disclosed. The disclosed method and structure may also be used to form products for other applications.

Abstract: The disclosed method can manufacture a thick-walled cylindrical part of an excellent quality which is free from the interlayer separation, by using a fiber reinforced plastic composite material. The fiber reinforced plastic composite material having a small thermal expansion coefficient is laminated in the circumferential direction of a mandrel; the laminated fiber reinforced plastic composite material layer is heat-cured to form an auxiliary member of the mandrel; a plurality of sorts of fiber reinforced plastic composite materials having a different elastic modulus, respectively are laminated on the formed auxiliary member; and the laminated fiber reinforced plastic composite materials are all heat-cured.

Abstract: The invention relates to a method for manufacturing a pipe fitting, said method comprising manufacturing a preform (1) of plastic, and forming an additional layer (3) on top of it. According to the invention, the additional layer (3) is formed by winding a film on the preform until the desired total wall thickness is reached, and the film is wound using at the same time pretensioning in a desired direction. The invention further relates to a pipe fitting manufactured by this method.

Abstract: A method and apparatus for making reusable paint. rollers comprised of a core formed from thermoplastic material. The thermoplastic material is formed into a core about a stationary mandril. An adhesive is then applied to the core whereupon a cover is bonded thereto. In a first embodiment the core is formed by winding a plurality of thermoplastic strips about the mandril. In a second embodiment the core is formed by applying liquefied thermoplastic material to a driven belt which transfers the thermoplastic material to the mandril.

Abstract: An invention is disclosed which relates to a process for producing a glass reinforced potable water pressure pipe having an ultra thin inner layer of thermoplastic resin that is made via a continuous mandrel concept and which is ideally suited for use with potable water.

Abstract: Methods and apparatus for making paint roller covers by spirally wrapping a strip of hot thermoplastic film onto an exterior surface of a thermoplastic tubular form and spirally wrapping fabric strip having a thermoplastic backing onto the spiral wound strip of thermoplastic film while the thermoplastic film is still sufficiently hot to cause the thermoplastic film to flow into interstices in the fabric backing and permanently bond the fabric strip to the exterior surface of the tubular form. The tubular form may either comprise a plurality of preformed tubes or be formed by spirally wrapping a continuous length of core strip material around a mandrel while maintaining the adjacent edges of the core strip material in closely spaced, nonoverlapping relation to form a spiral seam between the core strip edges.

Abstract: An apparatus is disclosed for making a conduit or pipe. An extruder is provided for producing a strip of continuous thermoplastic. A winding drum receives the strip and helically winds the strip around the drum's perimeter to form a conduit having an inner diameter corresponding to the outer diameter of the drum. The drum includes a core tube surrounded by a plurality of rollers which define the outer diameter of the drum. The rollers rotate to advance the strip around the drum. A pair of detachable plates are provided for supporting said rollers. Each plate has a plurality of support structures for receiving and removably supporting each of said rollers at either end, at a desired radial distance from the drum axis, in order to define the drum diameter. The pair of plates is part of a plurality of pairs of plates, with each pair being of a different diameter to manufacture pipe or conduit of a variety of different diameters.

Abstract: A method and apparatus for covering a roll core with a polymeric material, preferably a high performance thermoplastic material, is disclosed. The method includes first providing a cylindrical roll core having two ends, a length, and an outer surface. A spacer assembly is attached proximate one end of the roll core, the spacer assembly having a circumference greater than that of the roll core. After preheating to a desired temperature, the roll core is then placed within an apparatus in a substantially vertical orientation and held therein by suitable fixtures, e.g., opposed universal chucks. A length of mold tape is helically wound over the length of the roll core in a spaced-apart relationship therewith to define an application zone between the mold tape and the roll core outer surface. A filament formed of the polymeric material is extruded within the application zone and helically wound over the roll core outer surface so that the roll core is covered with the material.

Abstract: The invention consists of bonding a strip of pile fabric to a plastic tube which is mounted over a mandrel, by an adhesive substance. The adhesive substance may take many forms, and heat derived from a multitude of heat sources, such as infra red, radio frequency, open flame, quartz, induction, ultrasonic or a plasma jet may be used to ensure a good bond between the plastic tube and the pile fabric.

Abstract: A method of installing an optical cable on a conductor of a high-voltage line comprises helically wrapping the cable and conductor with an adhesive tape.

Abstract: A method of manufacturing multi-ply tubular containers for food products is provided including the steps of advancing a continuous body ply formed of paperboard towards a shaping mandrel and advancing a continuous polymeric liner ply adjacent to one surface of the paperboard body ply. The polymeric liner ply includes a moisture barrier layer and an adhesive layer defining one surface of the liner ply, wherein the adhesive layer includes a polymeric adhesive which is activated at a predetermined activation temperature. The body ply is heated to a temperature above the activation temperature of the adhesive, and the body ply and the liner ply are then passed in face-to-face contact through a nip to adhere the liner ply to the body ply. The body ply and liner ply are then wrapped around a shaping mandrel to create the tubular container.

Abstract: A flexible double-walled plastic tubing product includes a helically wound support bead between inner and outer walls, and an helically wrapped electrical resistance heating conductor is disposed adjacent to the inner wall in good heat transfer relation to fluid flow within the tubing product. Fluid flow within the tubing product is insulated against heat loss to ambient by the helical support bead and a trapped “dead-air” space between the walls of the tubing product. Apparatus and method for making the tubing product are disclosed.

Abstract: A method of manufacturing a monolithic composite wing without using mechanical fasteners is described. The process begins with the formation of a center wing box in combination with a pair of spars, riblets and a pair of skin-molds including the wrapping and binding of the box by means of resin impregnated composite tapes. Next, additional cells are adjoined contiguously on either side of the current framework and an overlap wrapping and bonding process is continued around the current framework. The overlap wrapping and binding procedure provides increased torsion stiffness and reduced structural weight. All cells up to the leading and trailing edges will be included in the assembly process. Conduits to convey fuel, hydraulic fluid and electrical wiring will also be installed in designated cells. Finally, the completed wing will be cured in an autoclave under uniform pressure and temperature.

Abstract: A three-dimensional fiber structure is formed by superposing layers of felt and by bonding them together by needling, then the structure is compressed to obtain a preform having a fiber volume fraction of not less than 20%. The preform is held in its compressed state by being impregnated with a liquid composition containing a bonding agent capable of bonding the fibers of the preform together. Thereafter, the preform consolidated in this way is densified, e.g. by chemical vapor infiltration.

Abstract: The invention relates to a method and a device for the manufacture of paint rollers by winding a textile fabric (3) onto a plastic tube (2) along a spiral-shaped line. According to the invention, a number of tube elements (6), having a length suitable for a paint roller, are slipped onto a revolving mandrel (1). The tube elements are clamped together so as to engage one another and so as to allow them to be rotated together with the mandrel (1). The mandrel is rotated at the same time as the fabric is would on and attached to the tube elements, after which the fabric is cut at the joints (8) between the elements. Due to the invention, the cutting can be performed in the same machine without the risk of damaging the mandrel. As the cutting is made through the fabric only, this entails much less knife wear and is less energy-consuming. Preferably, the tube elements have a toothing at their axial ends for mechanical interconnection at the joints between the tube elements and with the mandrel (1).

Abstract: In a method of making rolls or strips of material having no flaws except joints a supply of material, such as a roll of fabric is inspected for flaws. If any flaws are discovered portions of the material containing the flaws are cut away. The material is attached together after each portion is removed to create a joint for each reattachment. The portions that are cut away are sized so that the joints will be separated by a multiple of a selected distance from one another. The resulting material can then be fabricated into a panel of window covering material in which all the joints are co-linear along a perpendicular to the surface of the material. The joints are positioned so that they will be on a cut line for subsequent processing. In one embodiment the material is wound into a tubular structure that is cut along the joints to form a panel of window covering material.

Abstract: A method of manufacturing paint rollers includes the steps of extruding a cylindrical plastic core through a rotating extruder head, and securing an absorbent sheet material onto an outer surface of the core in a continuous process.

Abstract: A method of manufacturing a core, especially a spiral core, from superimposed plies of board produced by winding, gluing, and drying them, is provided. The method is practiced so that the moisture content of at least some of the board plies entering the winding stage differ from each other in order to provide a stepwise moisture structure within the core wall. This decreases unfavorable stresses which are produced in the drying stage if a core is manufactured from plies having equal moisture content.

Abstract: A method for winding a continuous material onto a supporting surface, the method being characterized more precisely by the following process steps:unwinding a material (6) from a material supply (7) and winding said material onto a supporting surface (5);the pendulum-type support (17) of the material (6,7) during the advance, permitting an automatic adjustment of a winding angle .alpha.;maintaining the tensile stress during the winding of the material (6);the cleaning and conditioning pretreatment of the material (6) carried out between unwinding and winding; andthe coating with adhesive carried out between unwinding and winding.

Abstract: A gas-tight tube out of composite cardboard for cans is formed by welding the polymer coatings of neighboring layers, a linearly-focused halogen light source being employed. A marginal strip of the composite material is heated from the side of the material opposite to that having the polymer coating to be melted and welded. In addition, a layer which absorbs and transforms the halogen light into heat can be provided in the area of the marginal strip on the side thereof on which the halogen light is focused. An apparatus for carrying out the process and suitable composite materials are described.

Abstract: The invention provides a process for producing a tubular membrane assembly comprising helically winding at least one strip of fibrous material on a mandrel to produce at least a single ply tubular support member for a semi-permeable membrane, characterized by passing the strip through a heated section of the mandrel during the helical winding thereof to flatten and smooth fibers protruding along the cross-sectional width of the strip, whereby a tube with a smooth inner bore along its entire length is formed.

Abstract: The invention relates to a fishing rod with preinserted fishing line guide members. A prepreg formed of a high-strength fiber impregnated or mixed with resin is wound to form the rod pipe. A plurality of wide fishing line guide members each having a flat-shaped section are fixed integrally to the inner peripheral surface of the rod pipe, the inner peripheral side portions of the wide fishing line guide members are respectively projected inwardly from the rod pipe inner peripheral surface, and the outer portions of the rod pipe existing in the wide fishing line guide fixed portions are respectively bulged outwardly and widely. The fishing line guide members are strongly fixed to the inner peripheral surface of the rod pipe without lowering the strength of the rod pipe.

Abstract: A two-ply core for core wound paper products. The inner ply of the core has more resistance to compression than the outer ply. The outer ply may have more resistance to tension than the inner ply. The two-ply core is suitable for use with core-wound paper products and particularly those which have been diametrically compressed. The core may comprise two plies of differing basis weights, wherein the inner ply has a greater basis weight than the outer ply.

Abstract: A method of making a fabric Venetian blind comprises helical winding of a narrow, elongated strip, which, when properly overlapped and bonded along longitudinal glue lines, will form the vanes and front and rear facings of the blind. The strips may be formed of a single, uniform material, or of adjacent, longitudinally extending portions of relatively opaque and relatively translucent fabric. An alternative embodiment uses the same winding technique to similarly form a structure of only translucent fabric, but with pockets for subsequent insertion of opaque vanes. The disclosed method and structure may also be used to form products for other applications.

Abstract: The disclosed method can manufacture a thick-walled cylindrical part of an excellent quality which is free from the interlayer separation, by using a fiber reinforced plastic composite material. The fiber reinforced plastic composite material having a small thermal expansion coefficient is laminated in the circumferential direction of a mandrel; the laminated fiber reinforced plastic composite material layer is heat-cured to form an auxiliary member of the mandrel; a plurality of sorts of fiber reinforced plastic composite materials having a different elastic modulus, respectively are laminated on the formed auxiliary member; and the laminated fiber reinforced plastic composite materials are all heat-cured.

Abstract: A process for the manufacture of a rim preform or rim-element having an axis of revolution by the placing on a preforming matrix pre-assembled fibers along two orientations defining deformable meshes. A winding of a strip of fibers is fastened on a circumference of the preforming matrix with asymmetrical orientation of the fibers, whereupon it is applied progressively until covering the entire surface of the preforming matrix by subjecting it to tensions of substantially circumferential orientation.

Abstract: A method and apparatus for manufacturing tubular articles from fiber reinforced thermoplastic sheets. A horizontal conveyor feeds the sheets to a vertical wrapping belt. The sheets are heated on the conveyor to a molten state and fed to a mandrel rotated by the belt. The molten sheets are wrapped around the mandrel and cooled by the action of the belt to form an integral structure.

Abstract: A homogeneous mixture of a base and a binder material or fiber that is compressed to form a mat or sheet of selected porosity. The binder material has at least a surface with a melting temperature lower than that of the base fiber. The sheet is formed into a selected geometric shape and thermally fused to bind the base fiber into a porous filter element. The preferred shape is a helically wound tube of plural sheets, each sheet being self-overlapped and compressed to overlap another sheet. Each sheet preferably heated and compressed individually and the sheets may be selected to have different porosities and densities. The binder fiber is selected from the group consisting of thermoplastic and resin, and the base fiber is selected from the group consisting of thermoplastic and natural.

Abstract: The electric isolator (1) is manufactured from a support (2) made of laminated material and a continuous band made of elastomeric material (3) wound in several contiguous successive turns around the support on an outer face (20) thereof so as to make an isolating coating in the form of a plurality of annular flanges (16, 17, 18). The continuous band is profiled so as to have, in section, a basis used as a contact surface with the support outer surface, an upper face provided with at least one flange extending transversally to the basis, a first and a second side surfaces which continue the basis by forming a certain angle therewith.

Abstract: A production method of a golf club shaft made of a fiber-reinforced resin according to the present invention comprises the steps of driving and rotating a shaft substrate made of an uncured fiber-reinforced resin on the axis thereof, moving a plurality of creel stands in parallel in the axial direction of the shaft substrate, and feeding out tapes from a plurality of the creel stands and winding and laminating the tapes simultaneously and spirally on the outermost layer of the shaft substrate for integrally molding them while the rotating speed of the shaft substrate and the moving speed of the creel stands are being controlled simultaneously.

Abstract: A core for core wound paper products. The core is made by wrapping dual plies in a spiral pattern and adhering the plies together. The edge of one ply overlaps the ply gap of the other ply, preventing a single ply thickness from occurring anywhere on the core. Alternatively, the edge of each ply may overlap the ply gap of that respective ply. In yet another embodiment, the overlap may be formed by a separate ply applied to either ply.

Abstract: An elongate member is coated with a coating, preferably by coextrusion, and the coated elongate member is wound in a helical manner around a mandrel. The coated elongate member preferably has a square cross-sectional shape so that adjacent portions of the coated elongate member engage one another when the coated elongate member is wound around the mandrel. The coated elongate member is then heated so that the coating on adjacent portions of the coated elongate member fuse together to form an integral strcucture. Another layer of material may be provided on the radially inner or outer wall of the coated elongate member.

Abstract: A hollow and cylindrical laminated molded product which is constituted by mutually overlapped joining of edge sides in a longitudinal direction of at least one or more elongated laminated articles formed by layered lamination of fiber-reinforced thermoplastic resin plates containing reinforcing fibers of not less than 30% and not more than 85% in volume content, and a method of producing the above-mentioned laminated molded product using a simple molding tool.

Abstract: Method and apparatus for preparing beverage transmission lines to form bundles that convey a drink product to a remote location includes the step of providing a plurality of spools that include a wound length of beverage conveying tubing each with a free end portion and unwinding the beverage tube from each spool by advancing the free end portion. The free end portions are transmitted through a plurality of templates that gather the tubing together. The tubing is then choked into a tight bundle while being continuously advanced at a set desired speed and along a generally linear path. The bundle is then spirally wrapped as it is being transported. The spiral wrapping can be achieved with a rotating spool that surrounds the bundle.

Abstract: A belt for use on a long nip press for dewatering a fibrous web includes a base assembled by spirally winding a prepared structure strip in a plurality of non-overlapping turns. Successive turns are abutted against and joined to those previously wound by sewing or otherwise bonding along the continuous spiral seam thus formed. The prepared structure strip may be a fabric strip woven from lengthwise and crosswise yarns, which may be monofilament yarns of a synthetic polymeric resin. The fabric strip may be of either a single- or multi-layer weave. At least one side of the base, namely, that side which will be on the inside of the belt in its endless loop form, and which slides over the arcuate pressure shoe component of the long nip press during its operation, is coated with a polymeric resin, such as polyurethane, to render it impervious to liquids, and especially to the oil used to lubricate the surface of the arcuate pressure shoe.

Abstract: A method of forming a hollow elongated or tubular body consisting of the steps of winding one or more strips of material in self-overlapping fashion to produce a multi-layer tubular structure, the or each strip prior to winding being pre-formed to provide a transverse cross-section having at least one step which, in each convolution of the strip accommodates the overlapping portion of the next convolution.