Method of Producing a Rod Anchoring Structure

The present invention relates to an anchoring structure and to a method of producing one.

Such structures are particularly intended for use in insulators such as pylon stays as described in the parent application.

A difficulty occurs in the method of manufacture described in the parent application in relation to a wedge element which is moulded between an anchoring fitting in the form of a sleeve and a rod received therein. The wedge element is formed after the rod is received in the sleeve by pouring a substance such as resin into the gap between the rod and the sleeve and allowing the substance to set. Under such circumstances it is difficult to check that the wedge element is homogeneous and that it fills the gap entirely without leaving any spaces or air bubbles. Such spaces reduce the mechanical strength and also reduces the maximum voltage the anchoring structure can withstand. This is of importance in the high tension applications envisaged.

The present invention mitigates this drawback by providing a method which simplifies verification of the wedge element.

The present invention provides a method of producing a rod anchoring structure comprising the steps of fitting the edge of a rod with a surrounding, double-ended wedge element having a neck from which flared ends of increasing cross-sectional area extend in both axial directions along the rod and of so fitting the wedge element in an anchoring fitting that the wedge element is radially compressed by the anchoring fitting even in the absence of an axial load on the rod.

Preferably the anchoring fitting has an initial shape of an open-ended tubular sleeve in which the wedge-bearing end of the rod is received and which is subsquently deformed into an open-ended nozzle-shaped sleeve pressing tightly against the wedge element.

Alternatively the anchoring fitting is formed by winding a filament of suitable material such as metal wire or resin-impregnated organic fibre around the wedge element.

The wedge element may have an axial pretensioning applied thereto during the fitting of the anchoring fitting.

The wedge element may be made of resin which is preferably moulded onto the end of the rod in a mould for producing the desired shape to avoid subsequent machining or trimming. Advantageously the rod is made of resin agglomerated fibres and the wedge element is moulded onto the rod before the resin of the rod has completely polymerized, thereby improving adhesion between the rod and the wedge element.

The present invention also provides a rod anchoring structure produced by the method.

Embodiments of a rod anchoring structure embodying the present invention are described by way of example with reference to the accompanying drawing in which:

FIG. 1 shows a rod with a wedge element thereon;

FIG. 2 shows a sleeve anchoring fitting receiving the wedge element and before being pressed onto it;

FIG. 3 shows the fitting of FIG. 2 after being pressed onto the wedge element;

FIG. 4 shows a fitting analogous to the one of FIG. 3, also comprising a hoop or strapping around the middle part of the sleeve; and

FIG. 5 shows a fitting analogous to the one of FIG. 3, also comprising a thin sleeve of soft ductile metal between the wedge element and the sleeve .

In FIG. 1, a rod 1 made of resin-agglomerated glass fibres is embedded at one end in a wedge element 2 of resin which is full of an aggregate such as grains of silica and/or fibres and which has a double truncated-cone or "diabolo" shape. The wedge element 2 is moulded onto the end of the rod 1 in a two part mould (not shown). After polymerization of the resin mould is opened and the rod 1 together with its fitted wedge element 2 are removed, which makes it easy to verify the condition of the resin of the wedge element 2. To ensure a better connection between the rod 1 and the resin wedge element 2, it is possible to start with an incomplete polymerization of the resin of the rod 1 before the moulding of the wedge element 2 thereto, and then allow the polymerization to finish after the moulding of the aggregated resin.

An anchoring fitting is then applied to the wedge element 2 in order to complete the anchoring of the end of the rod 1. The initial shape of this fitting is shown in FIG. 2. It comprises a cylindrical sleeve 4 which is open at one end and closed at the other end by an end portion 3 which includes an eye for fixing the rod to a support. The bore diameter of the sleeve 4 is slightly greater than the maximum diameters of the wedge element 2. The wedge element 2 is inserted into the sleeve 4 which is than deformed around the neck of the wedge element 2 to become a tight fit. The deformation may be by forging, by a cold working method such as rolling or, when a relatively thin sleeve of ductile metal such as aluminium is used, by isostatic pressing inside a pressurized elastomeric casing or by magnetic crimping. The anchoring element shown in FIG. 3 is thus obtained.

If greater strength is required for the anchoring fitting, the sleeve 4 may be made of steel (or any other metal having high mechanical strength) and should be pressed against the wedge element 2 in the region of the neck and then (or preferably simultaneously) on either side thereof.

The mechanical strength of the anchoring fitting can be further reinforced by strapping the deformed sleeve (FIG. 4). The deformed sleeve can be strapped in the usual way by winding a metal wire round it or even by casting an exterior sleeve 5 made of an alloy having a moderate melting point, such as an aluminium alloy.

To be more certain of preventing any inclusion of air between the wedge element 2 and the bore of the sleeve 4 it is advantageous to cover the preformed resin wedge element 2 with a protective layer 6 made of a soft ductile metal such as aluminium.

One advantage of pressing the sleeve 4 onto the resin wedge element 2 is that the progressive deformation of the sleeve 4 on either side of the neck can be made to provoke a condition of permanent tensile stress with its resultant in the axial direction between the wedge element 2 and the sleeve 4. This stress contributes to pressing the two conical parts of the wedge element 2 against the nozzle-shaped bore of the sleeve 4. When the rod 1 is subsequently subjected to longitudinal traction, there is no displacement of the wedge element 2 in relation to the nozzle for as long as there remain compression stresses at the wedge-nozzle interface on the side on which the traction is applied (i.e. on the side which is progressively releaved of stress by the applied traction). This effect can further be accentuated by subjecting the rod 1 and consequently the resin wedge element 2 to a traction during the forming of the sleeve 4 about the wedge element 2. For this purpose it is advantageous firstly to press the part of the sleeve 4 which is closest to the end of the rod 1, up to the neck, then to apply traction to the rod 1 from its opposite end and then to continue the pressing of the sleeve 4 over the other half of the wedge element 2.

Although the variants of embodiment of the improved structure, as well as their manufacturing methods, which have just been described, appear preferable, it will be understood that various modifications can be made thereto without going beyond the scope of the invention, it being possible to replace some elements or some operations by others which would still fall within the scope of the claims. In particular the anchoring fitting could be formed by winding a filament or thread of suitable material around the wedge element 2. The filament should be tightly bound to provide the required compressive stress in the wedge element. Suitable materials include metal wires or ropes and resin bounded fibres.