EP1617442A1

EP1617442A1 - Electric isolator and method of manufacturing the same

Info

Publication number: EP1617442A1
Application number: EP05014139A
Authority: EP
Inventors: Riccardo Mira D'ercole
Original assignee: Fabbrica Isolatori Porcellana "fip" SpA
Current assignee: Fabbrica Isolatori Porcellana "fip" SpA
Priority date: 2004-07-13
Filing date: 2005-06-30
Publication date: 2006-01-18
Also published as: ITMI20041399A1

Abstract

An electric insulator and a method for manufacturing the same are described, wherein the insulator comprises a body made of a ceramic material and at least one metal element mounted at one end thereof. At least one clamping element that can be externally engaged on at least one length of the engaging portion of the metal element is provided in order to obtain the mounting.

Description

The present invention relates to an electric insulator provided with a body made of ceramic or glass material, and one or more metal elements mounted on the body made of ceramic material, as well as a method for manufacturing such an electric insulator. By "body made of ceramic or glass material" herein is meant any product obtained from inorganic raw materials by means of forming and subsequent thermal treatment.
The electric insulators of this kind must meet strict requirements in terms of coupling between the metal elements and the ceramic body of the insulator.
First of all, the coupling between such elements is required to have a particular mechanical resistance to the tensile and torsional stresses. Further particularly strict requirements concern the tight seal of the coupling, mainly for those applications where a fluid within the insulator (e.g. an oil or a gas such as SF6) must be prevented from leaking outside.
The metal elements that are mounted on the insulator body generally comprise shanks, flanges, caps and the like, which are generally mounted either at one or both ends of a body made of ceramic material, such as a body made of porcelain or other ceramic material.
One of the possible coupling methods currently used in the art provides that a metal element, such as brass, is clearance-fitted to the end of the insulator ceramic body. The gap resulting from the backlash between the metal element and the mounting end of the insulator body is filled with a lead-based alloy brought to the molten state. When the alloy employed according to this process has cooled, the metal element remains fixed to the insulator body so as to ensure tight seal and resistance to tensile and torsional stresses.
While this method allows one to obtain a low-cost, reliable product, it is laborious and can have harmful effects on the operators.
It is further known that the use of heavy metals, mainly lead and its alloys, in industrial productions tends to be more and more restricted in view of the recent international environmental regulations.
For these reasons, a number of alternative manufacturing methods have been provided which allow metal elements to be fastened to the insulator body.
For example, the subject of US patent n. 5,977,487 is an electric insulator made of ceramic material, in which the ends of the ceramic body are specially worked to be coupled by shrink fitting with metal elements provided with flanges.
Another example is the US patent n. 6,064,010, which describes a method for fixing a metal cap to the insulator body made of composite material. The cap is provided with a through hole having the shape of a truncated cone and is shrink fitted to an end of the insulator body that is inserted in the hole. A firm coupling is obtained by means of the interference generated between the cap and the insulator when the cap shrinks upon cooling.
Still another example of prior art is described in the US patent n. 6,693,242, wherein the mounting of the metal elements to the ends of an insulator ceramic body is obtained by means of plastic deformation of a fixing portion of each mechanical element at a circumferential groove formed at each end of the insulator body.
However, the methods provided in the above documents are very complicated to carry out, and consequently, they are not cost-effective.
Furthermore, with similar types of shrink fitting, the tight seal of the coupling is not ensured, unless suitable sealing elements are provided or sealing compounds such as resins or the like are applied. Anyway, it should be noted that the resins employed to this purpose often suffer from ageing problems, i.e. their physical characteristics change over time and may lead to sealing failure.
For example, those resins that are used to obtain the tight sealing of the coupling between the insulator body and metal elements are subjected to the same thermal shocks and thermal cycles as the insulator upon operation, thus resulting in the deterioration of the resins and possible sealing failure.
Accordingly, the object of the present invention is to provide a particularly simple, reliable and feasible low-cost method for making electric insulators having their bodies made of ceramic material and one or more metal elements mounted thereon.
Another object of the present invention is to provide a method for making insulators of the above mentioned type, which is environment-friendly.
According to a first aspect of the present invention, this and other objects are achieved by an electric insulator comprising a body made of ceramic material and at least one metal element mounted at one end thereof, in which the at least one metal element has an engaging portion provided with an inner surface overlapping, either entirely or partially, the outer surface of the corresponding end portion of the insulator body, characterized by comprising at least one clamping element that can be externally engaged on at least one length of the engaging portion of the at least one metal element.
Particularly, the coefficient of thermal expansion of the metal element is greater than the coefficient of thermal expansion of the clamping element. Thus, when the insulator is installed and subjected to heating, the metal element expands more than the clamping element. Consequently, as the temperature to which the insulator is subjected under operating conditions increases, the outwards expansion of the metal element is restricted by the clamping element and advantageously develops only inwards, i.e. towards the insulator body, thereby keeping a firm coupling condition that is capable of ensuring suitable characteristics of mechanical resistance and tight sealing. By cold, or however at room temperature, the volume reduction in the clamping element and metal element increases the clamping to the body made of ceramic material.
The clamping element can consist of a ring, or any substantially ring-shaped element, made from a metallic material such as steel, whereas the metal element to be fixed to the ceramic body can consist of a cap and/or a flange for example made of brass.
The ring-shaped clamping element envelops that portion of the cap and/or flange which is in contact with the insulator body. The inner surface of the clamping element can either be smooth or comprise one or more grooves, one or more threaded portions or any other suitable surface workings. Alternatively, the clamping element can be however ring-shaped with a undulated profile section, in order to facilitate slight elastic deformation, if desired.
In accordance with a second aspect of the present invention, there is provided a method for manufacturing an electric insulator having a body made of ceramic material and at least one metal element mounted at one end thereof, comprising at least one step of mounting the at least one metal element to one of the ends of the insulator body. The metal element has an engaging portion provided with an inner surface that overlaps, either entirely or partially, the outer surface of the corresponding end portion of said insulator body. The method is characterized by mounting at least one clamping element outside of at least one length of the engaging portion of the at least one metal element.
The mounting of the metal element to one of the ends of the insulator body can be carried out either by interference fit, or shrink fitting. In the first case, it is desirable to provide a minimum interference in order to avoid any damage to the body made of ceramic material.
Similarly, the clamping element can be fitted to the metal element by shrink fitting, by interference fit, or still exploiting the elastic deformation of the clamping element that can be suitably shaped to this purpose.
Accordingly, the present invention provides a coupling between ceramic body and metal element(s) resulting very practical to carry out, effective and free of solderings. While the tight seal of the coupling thus obtained can be suitable for a number of applications, sealing means to be interposed between each metal element and the insulator ceramic body may be also provided, if required.
Further aspects and advantages of the present invention will be better understood from the description below, which is intended to be illustrative and nonlimiting, with reference to the annexed schematic drawings, in which:

Fig. 1 is a sectional view of the body of an insulator according to the present invention, without metal elements;
Fig. 2A is a sectional view of a metal cap and a ring according to the invention;
Fig. 2B is a cross-sectional view of a flange and a ring according to the invention;
Figs. 3A and 3B are enlarged sectional views illustrating several possible embodiments of clamping elements according to the present invention;
Fig. 4 is a sectional view of a possible embodiment of an insulator according to the present invention;
Fig. 5 is an enlarged view of a detail from Fig. 4; and
Fig. 6 is an enlarged view of another detail from Fig. 4.

Fig. 1 illustrates the body of an insulator 1 according to the present invention. The insulator 1 is made of a ceramic material, such as porcelain, or glass, glass-ceramic material, and the like. Generally, the insulator 1 can have a through hole, such as in the example illustrated, or be of the solid-core type, and is provided with a first end portion 2 and a second end portion 3. The middle portion 4 of the insulator 1 can provide, as in the case illustrated, a series of ribs 5 in order to elongate the surface conduction path.
The end portions 2 and 3 are generally cylindrical with a circular section as in the embodiment described herein, with respective engaging portions 6 and 7 for the metal elements to be mounted.
However, it should be noted that the principles of the present invention may be as well applied with end portions 2 and 3, as well as the respective engaging portions 6 and 7, having different sections, such as lobed, or other more or less regular shapes. Furthermore, the end portions 2, 3 and engaging portions 6, 7 can provide section variations, such as a slight taper, and can be provided with one or more grooves at the outer surface thereof.
Figs. 2A and 2B illustrate possible embodiments of the metal elements that can be mounted on the insulator 1. Particularly, Fig. 2A shows a metal cap 8 that can be mounted at the end portion 2 of the insulator 1. The cap 8 is provided with a portion 9 suitable to engage the engaging portion 6 by overlapping the same, and an elongate portion 10.
Fig. 2B illustrates for example a flange 11 having an engaging portion 13 that can be mounted on the engaging portion 7 provided at the end 3 of the body of the insulator 1. In several possible embodiments, the metal element comprising the flange 11 may not be provided, such as in the case where the insulator is direcdy inserted on the suitably arranged condenser cover, i.e. with an engaging portion similar to that designated with 13 in Fig. 2B. Also in this case, the insulator may be thus directly fixed to the cover according to the principles of the present invention.
Generally, the metal elements that can be mounted to the body of the insulator 1 may have other shapes than those of the cap 8 or flange 11. It is important that the metal elements are provided with a portion suitable to engage the corresponding engaging portions being provided at the ends 2 or 3 of the insulator body 1.
The cap 8 and/or the flange 11 can be mounted to the respective ends 2, 3 of the body of the insulator 1. In this case, the cap 8 is dilated by increasing the temperature thereof and externally fitted to the end portion 2, possibly with a minimum interference, in order to engage the same portion 2 after it has shrunk subsequent to cooling.
Alternatively, the cap 8 and/or flange 11 can be also mounted with interference to the ends 2 and 3, provided that small differences in diameter are selected which are sufficiently small to avoid damaging the insulator body.
Still referring to Figs. 2A and 2B, after each metal element has been mounted on the body of the insulator 1, a clamping element 12 is externally coupled on at least one length of the engaging portion of each metal element such as to envelop, either entirely or partially, the outer surface of the respective engaging portions.
The coupling of the clamping element 12 can be carried out by shrink fitting, such that the ring 12 is expanded until it reaches a sufficient dimension in order to be mounted overlapped to the outer surface of the engaging portion of the respective metal element to be fixed.
Also in this case, the coupling of the ring can be also carried out by cold, by forcing the ring 12 to overlap the engaging portion of the respective metal element to be fixed.
In any case, according to the principles of the present invention, the metal elements mounted on the body of the insulator 1 have a greater coefficient of thermal expansion than the coefficient of thermal expansion of the clamping element 12. Consequently, any expansion of the elements mounted at the body of the insulator 1 is restricted outwardly by the clamping element 12, and inwardly by the body of the insulator 1, since the latter remains dimensionally stable upon temperature variation. Thereby, as the temperature of the insulator 1 increases, the pressure exerted by the elements mounted on the end portions 2 and 3 will be also increased, which ensures a tight seal and the resistance of the coupling between metal and ceramics to tensile and torsional stresses.
The clamping element is preferably ring-shaped, which shape is adapted to be clamped to the outer surface of the metal element to which it is coupled.
In the embodiment from Figs. 2A and 2B the clamping element 12 consists of a circular ring 12 that can be mounted on the engaging portions 9 and 13 of the cap 8 and flange 11, such as illustrated in the subsequent Figs. 4-6.
In Figs. 3A and 3B there are illustrated several possible alternative embodiments of clamping elements suitable to the application according to the present invention.
Particularly, Fig. 3A illustrates a cross-sectional view of an annular clamping element 22 provided with at least one groove 23 arranged in this case on the inner surface. The provision of one or more grooves on the inner or outer surface allows to facilitate the axial elongation of the clamping element thereby reducing the tensile stress to which it is subjected after application. As an alternative or in combination with the provision of grooves, the clamping element can further have one or more threaded surface portions. In the embodiment from Fig. 3A, there are provided two surface portions 24 and 25 that have opposed threads. The threaded surfaces 24 and 25, which are preferably arranged at least at the axial ends, allow to increase the resistance to torsional stress of the clamping element.
The cross section of Fig. 3B illustrates an annular clamping element 32 with undulated profile section. This particular embodiment allows a slight elastic deformation of the clamping element which can be mostly useful when the latter is mounted without shrink fitting.
In any embodiment, the clamping element envelops the engaging portions of the cap 8 and flange 11 with the body of the insulator 1 by exerting a pressure that contributes in strengthening the coupling of the elements 8 and 11 with the body of the insulator 1.
Different materials can be used for manufacturing the metal elements and the clamping element.
In several prototypes actually manufactured according to the embodiments shown in Fig. 4 to 6, the cap 8 and the flange 11 were made of OT 58 brass. Other metal materials can be used which are particularly suitable for use in the field of electrical insulators, metals or alloys preferably with non-magnetic characteristics, such as stainless steel and the like.
For example, UNI52SiCrNi5 spring steel (not non-magnetic) and X5CrNi18 10 shot pin steel (AISI 304) has been used for the prototype clamping elements. The latter has proved to be preferred as compared with the former, due to its non-magnetic characteristics, because it maintains the mechanical characteristics in a wide temperature range, and is also easily available on the market. In any case, other types of similar materials can be selected provided they have a lower coefficient of thermal expansion than the metal element to be mounted and the desired temperature resistance characteristics.
The coupling obtained using the clamping element 12 has proved effective, simple to carry out and stable in a wide temperature range.
According to a possible embodiment, the tight seal of the coupling can be further ensured by interposing sealing means between the metal elements mounted at the body of the insulator 1.
For example, in some of the prototypes, there has been provided a gasket 14 made of graphite, which can be seen in Figs. 4 and 5. This material has been selected because its characteristics are suitable to this application, since it is non-magnetic, atoxic and stable even at high temperature. Particular types of graphite can even withstand temperatures higher than 400°C and have a coefficient of thermal expansion that is numerically comparable to that of porcelain, with a low limit of elasticity. However, other materials can be used as well, according to the type of application, for example silicone or other materials that are already used in the field for this purpose.
The shape of the gasket can also be other than that illustrated therein and can also be arranged in a different location. By way of example, a gasket 14' being interposed between the upper part of the insulator and the cap is shown in Figs. 4 and 5.
While the embodiment illustrated in Figs. 4-6 is of the type generally intended for being installed on condensers, it is understood that the principles of the present invention may be as well applied to any type of insulator, both for electric lines, such as electric aerial lines, railroad electric lines, and the like, and equipment in general, such as disconnectors, switches, lead-in insulators, stand-off insulators, or the like.

Claims

An electric insulator comprising a body made of ceramic material and at least one metal element mounted at one end thereof, wherein said at least one metal element has an engaging portion provided with an inner surface overlapping, either partially or entirely, the outer surface of the corresponding end portion of the body of said insulator, characterized by comprising at least one clamping element that can be externally engaged on at least one length of the engaging portion of said at least one metal element.
The insulator according to claim 1, wherein the coefficient of thermal expansion of said at least one metal element is greater that the coefficient of thermal expansion of said at least one clamping element.
The insulator according to claim 1, wherein said at least one clamping element is made of metal.
The insulator according to claim 1, wherein said at least one clamping element comprises a smooth inner surface.
The insulator according to claim 1, wherein said at least one clamping element comprises a worked inner surface.
The insulator according to claim 5, wherein the inner surface of said at least one clamping element comprises one or more grooves.
The insulator according to claim 5, wherein the inner surface of said at least one clamping element comprises one or more threaded portions.
The insulator according to claim 7, wherein the inner surface of said at least one clamping element comprises at least two portions threaded in the opposite directions.
The insulator according to claim 1, wherein said at least one clamping element is substantially ring-shaped.
The insulator according to claim 1, wherein said at least one clamping element has a cross-section with an undulated profile.
The insulator according to claim 1, wherein said at least one metal element includes a cap mounted at one of the ends of the insulator body.
The insulator according to claim 1, wherein said at least one metal element includes a flange mounted at one of the ends of the insulator body.
The insulator according to claim 1, wherein said at least one clamping element is mounted on said at least one metal element by shrink fitting.
The insulator according to claim 1, wherein said at least one clamping element is mounted on said at least one metal element by means of interference fit.
The insulator according to claim 1, wherein sealing means are provided interposed between the insulator body and said at least one metal element.
A method for manufacturing an electric insulator having a body made of ceramic material and at least one metal element mounted at one end thereof, comprising at least one step of mounting said metal element to one of the ends of the body of said insulator, said at least one metal element having an engaging portion provided with an inner surface overlapping, either partially or entirely, the outer surface of the corresponding end portion of the body of said insulator; characterized by mounting at least one clamping element outside of at least one length of the engaging portion of said at least one metal element.
The method according to claim 16, wherein the coefficient of thermal expansion of said at least one metal element is greater than the coefficient of thermal expansion of said at least one clamping element.
The method according to claim 16, wherein said at least one clamping element is made of metal.
The method according to claim 16, wherein the mounting of said at least one metal element to one of the ends of the body of said insulator is carried out by interference fit.
The method according to claim 16, wherein the mounting of said at least one metal element to one of the ends of the body of said insulator is carried out by shrink fitting.
The method according to claim 16, wherein the mounting of said at least one clamping element to said at least one metal element is carried out by shrink fitting.
The method according to claim 16, wherein the mounting of said at least one clamping element to said at least one metal element is carried out by interference fit.
The method according to claim 16, wherein the mounting of said at least one clamping element to said at least one metal element is carried out by elastic deformation of said at least one clamping element.
The method according to claim 16, wherein the interposition of sealing means is provided between the insulator body and said at least one metal element.