Composite electrical insulator, method of assembling same and method of manufacturing same

Methods of manufacturing and assembling a composite insulator are provided. At least one metal end fitting is provided having a sleeve portion which defines a bore with a first diameter, d1. An insulator subassembly is then formed. The insulator subassembly includes a rod of electrically insulating plastic material and an insulator sheath covering at least a portion of the outer surface of the rod. An end portion of the sheath has a deformable circumferential ridge formed on the outer surface thereof. This circumferential ridge has a second diameter, d2, which is greater than the first diameter, d1. The insulator subassembly is then inserted into the bore of the metal end fitting with a spacer member interposed between the metal end fitting and at least the circumferential ridge. The spacer member serves to deform the ridge to define a temporary vent for allowing air within the bore to escape. The spacer member is then removed thereby allowing the resilient ridge to return to its original size and shape to form a tight seal between the metal end fitting and the insulator subassembly. The resultant composite insulator has a construction which includes an insulator subassembly including a rod and a sheath covering at least a portion of the outer surface of the rod. The sheath has an end portion and at least one deformable circumferential ridge formed on an outer surface thereof. The composite insulator also includes a metal end fitting having a sleeve portion that surrounds the end portion of the sheath. An end region of the metal end fitting that overlaps the ridge is free from deformation. As a result, it is no longer necessary to crimp the metal end fitting to form a good seal, although the crimping step could be performed if additional tightness is desired.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates generally to the field of composite
 electrical insulators, and more particularly to methods of assembling and
 manufacturing a composite electrical insulator comprising an insulator
 sub-assembly and a metal end fitting, and the resultant composite
 electrical insulator.
 2. Description of the Related Art
 For quite some time composite electrical insulators have been used to
 insulate high tension wires from the towers to which they are anchored.
 Over time this field has become fairly complex as engineers have
 continually improved these insulators. In recent years, it also been a
 priority to improve the ease with which these insulators are produced. For
 example, U.S. Pat. No. 5,563,379 to Kunieda et al., incorporated by
 reference herein, shows, with reference to FIG. 1 herein, a composite
 electrical insulator 100 capable of maintaining good water-tightness
 between a metal fitting 102 and a sheath 104 without an increased clamping
 force. The metal end fitting 102 has a sleeve portion 106 defining a bore
 107 in which the end portion of an FRP rod 108 is received. The FRP rod
 108 is covered by the sheath 104, which has two circumferential ridges 110
 on its outer surface. As shown in FIG. 2A, the circumferential ridges each
 have an outer diameter (d.sub.2). The inner diameter (d1) of the bore 107
 defined by the sleeve portion 106 is greater than the outer diameter (d2)
 of the circumferential ridges 110. In order to prevent water from leaking
 into the space between the sleeve 106 and the ridge 110, as shown in FIG.
 2b, Kunieda et al. crimped the sleeve portion 106 onto the circumferential
 ridges 110 to force intimate contact between the circumferential ridges
 110 and the inner surface of the bore 107 of the metal fitting 102. Once
 assembled, the circumferential ridges 110 served as O-rings which
 prevented the water from penetrating inside the metal fitting 102. That
 is, when the sleeve portion 106 of the metal fitting is applied with a
 moderate crimping force, the circumferential ridges 110 are compressed by
 the metal fitting 102 into conformity with any unevenness on the inner
 surface of the metal fitting 102, thereby maintaining the desired
 water-tightness for a long period.
 However, one problem with manufacturing an insulator according to this
 method is that if there is any variance in the dimensioning of the bore
 107 and the circumferential ridges 110, the ridges 110 may not completely
 contact the inner surface of metal fitting 102. Similarly, any
 eccentricity between the sleeve portion 106 and the bore 107 may result in
 a gap between the sleeve 106 and ridges 110. In either case, there is a
 chance water may leak into the gap between the sleeve 106 and the ridges
 110. This is dangerous since water may possibly penetrate the boundary
 between the FRP rod 108 and the sheath 104, and the electrical insulating
 performance of the insulator will deteriorate so much that electrical
 discharge (i.e., flashover) will occur. As a result, the very function
 these insulators are intended to perform (i.e., insulation) is destroyed.
 Such water leakage can also cause rusting of the inner surface of metal
 fitting 102, which in turn relaxes the crimping force between the
 rod/sheath insulator subassembly and metal fitting 102.
 The only way to ensure a good fit between the sheath and the metal fitting
 and thus guard against such water leakage is to ensure extremely precise
 dimensional control of the circumferential ridges 110 and the inner
 surface of the metal fitting 102. The former requires precisely machined
 molds, and the latter requires precise machining of the metal end fitting.
 Both complicate the manufacturing process and increase cost.
 Additionally, because the outer diameter (d2) of the circumferential ridges
 110 is less than the inner diameter of the bore defined by the metal
 fitting 102, that portion of the metal fitting 102 overlapping the
 circumferential ridge 110 must be crimped to compress the ridge 110 and
 form a good seal. This crimping step is in addition to the crimping step
 used to plastically deform the metal fitting 102 around the FRP rod 108.
 It would be desirable to eliminate this second crimping step to make the
 insulator easier and cheaper to assemble.
 Thus, there is a clear need in the industry for a composite electrical
 insulator which is more easily and securely assembled to a metal end
 fitting member. By eliminating the associated need for high precision
 dimensional control and two crimping steps, manufacturing time and expense
 could be significantly reduced.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to overcome the above-discussed
 drawbacks associated with prior art assembly methods.
 It is a further object of the present invention to eliminate the need for
 precise dimensional control of components used to assemble an insulator.
 It is yet a further object of the present invention is to simplify
 manufacturing by eliminating the necessity of the second crimping step in
 assembling a metal end fitting and an insulator sub-assembly.
 In order to alleviate the need for precise dimensional control of the
 components of the insulator and to eliminate the second crimping step, the
 inventor tried making the diameter (d2) of the circumferential ridge
 greater than the inner diameter (d1) of the bore in the metal end fitting
 so that the circumferential ridge would form a seal with the inner surface
 of the metal end fitting without crimping that portion of the metal end
 fitting that overlaps the ridge.
 However, by solving one problem another was created. When the insulator
 subassembly was forced into the bore of the metal end fitting, any air
 present in the cavity became trapped, since the diameter of the ridge (d2)
 was greater than the inner diameter (d1) of the metal end fitting. The
 trapped air was compressed by insertion of the insulator subassembly and
 acted as a counter force to push the subassembly back out of the metal end
 fitting. That is, once the force being used to insert the insulator
 subassembly was removed, the air pressure inside the bore forced the
 insulator sub-assembly out of the bore.
 The inventor considered putting a vent in the bottom of the metal end
 fitting to allow any trapped air to be forced out of the cavity upon
 insertion of the subassembly. However, such a vent created additional
 manufacturing steps , in that it had to be formed in the metal end fitting
 and then sealed to prevent water leakage. The sealant material would
 likely break down over time and allow water to enter the interior of the
 metal end fitting, causing it to rust and destroy the crimping strength of
 the fitting on the FRP rod, and leading to flashover, as discussed
 earlier.
 To overcome the problem of trapped air, the inventor inserted a spacing
 member on top of and across the circumferential ridge(s) of the sheath
 during insertion of the rod/sheath insulator subassembly into the metal
 end fitting. The spacing member deforms the ridge, which is resilient, and
 provides a temporary venting passageway to allow the air in the cavity to
 escape when the insulator subassembly is forced into the cavity of the
 metal end fitting. Once the air under pressure in the cavity escapes, the
 spacing member is removed. The resilient ridge then returns to its
 original size and shape to form a tight seal between the metal end fitting
 and the insulator subassembly.
 The spacing member can be of any shape which will temporarily deform the
 ridge(s) and allow air to escape from the cavity during the insertion
 step. For instance, the spacing member could have a hollow tubular
 construction for allowing the air to vent through the spacing member.
 Alternatively, the spacing member could simply be a cord or wire of
 sufficient diameter to allow air to vent around the cord or wire and out
 of the cavity.
 To carry out the objects described above, methods of manufacturing and
 assembling a composite insulator are provided. According to these methods
 at least one metal end fitting is provided having a sleeve portion which
 defines a bore with a first diameter, d1. An insulator subassembly is then
 formed. The insulator subassembly includes a rod comprising an
 electrically insulating plastic material, and an insulator sheath covering
 at least a portion of the outer surface of the rod. An end portion of the
 sheath has a deformable circumferential ridge formed on the outer surface
 thereof. This circumferential ridge has a second diameter, d2, which is
 greater than the first diameter, d1. The insulator subassembly is then
 inserted into the bore of the metal end fitting with a spacer member
 interposed between the metal end fitting and at least the circumferential
 ridge. The spacer member serves to deform the ridge to define a temporary
 vent for allowing air within the bore to escape. The spacer member is then
 removed thereby allowing the resilient ridge to return to its original
 size and shape to form a tight seal between the metal end fitting and the
 insulator subassembly.
 As a result, the resultant composite insulator has a construction which
 includes an insulator subassembly including a rod comprising an
 electrically insulating plastic material and a sheath covering at least a
 portion of the outer surface of the rod. The sheath has an end portion and
 at least one deformable circumferential ridge formed on an outer surface
 thereof. The ridge has a second diameter, d2. Preferably, the sheath is
 made of a resilient and electrically insulating material. The composite
 insulator also includes a metal end fitting having a sleeve portion
 defining a bore having a diameter, d1, that is less than the second
 diameter, d2. The metal end fitting surrounds the end portion of the
 sheath, and an end region of the metal end fitting that overlaps the ridge
 is free from deformation. As a result, it is no longer necessary to crimp
 the metal end fitting to form a good seal, although the crimping step
 could be performed if additional tightness is desired.
 Additional objects, advantages, and other novel features of the invention
 will become apparent to those skilled in the art upon examination of the
 detailed description and drawings that follow.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 To carry out the objects described above, methods of manufacturing and
 assembling a composite insulator 1 are provided.
 As shown in FIG. 3A, at least one metal end fitting 1 is provided having a
 sleeve portion 2 which defines a bore 3 with a first diameter, d1. The
 metal end fitting 1 may comprise a high tension steel, aluminum, ductile
 iron or other appropriate metal, which has been plated by zinc, for
 example.
 The insulator subassembly 4 is then formed. The insulator subassembly 4
 includes a rod 5 comprising an electrically insulating plastic material,
 and an insulator sheath 6, covering at least a portion of the outer
 surface of rod 5.
 Preferably, rod 5 is made of a fiber reinforced plastic material. This
 fiber-reinforced plastic material may comprise knitted or woven fibers or
 bundles of longitudinally oriented fibers, such as glass fibers or other
 appropriate fibers having a high moduluses of elasticity, and a
 thermosetting type synthetic resin, such as epoxy resin, polyester resin
 or the like, impregnated in the fibers as a matrix resin. Such fiber
 reinforced plastic material offers superior mechanical strength and
 improved resistance to tensile, bending, torsional, and compressive
 forces. This fiber reinforced plastic material also exhibits an excellent
 weight to strength ratio.
 According to one embodiment, when forming the insulator subassembly 4, rod
 5 is placed in a mold and sheath 6 is molded around rod 5. Preferably, the
 end portion 7 of sheath 6 terminates in a generally frustoconical free end
 having a radially innermost surface region 8 which is axially depressed.
 This axially depressed surface region at the free end of sheath 6 serves
 to positively prevent separation of sheath 6 from rod 5 upon thermal
 expansion or cooling shrinkage of sheath 6. Preferably, sheath 6 is made
 of a resilient and electrically insulating material, such as silicone
 rubber (preferably) and ethylenepropylene rubber since these materials
 offer superior weatherability and anti-tracking characteristics. The
 insulating sheath 6 should also include a series of shed portions 9 which
 are axially spaced from one another to preserve a desired surface leakage
 distance.
 The end portion 7 of sheath 6 should also include at least one deformable
 circumferential ridge 10 formed on the outer surface thereof to provide a
 tight seal with the inner surface of metal fitting 2 defining bore 3. The
 outer surface of sheath 6 may include a plurality of circumferential
 ridges 10 axially spaced from one another by a predetermined distance.
 Preferably, all circumferential ridge(s) 10 have a semi-circular
 cross-section, although any suitable cross-section could be used.
 Each circumferential ridge 10 has a second diameter, d2, which is greater
 than first diameter, d1. By making the diameter (d2) of the
 circumferential ridge 10 greater than the inner diameter (d1) of the bore
 3 in metal end fitting 1, the circumferential ridge 10 forms a seal with
 the inner surface of the metal end fitting 1 without crimping that portion
 of metal end fitting 1 that overlaps the ridge 10. Additionally, providing
 this positive seal between the ridge 10 and the metal end fitting 1
 eliminates the need for precise dimensional control of the components of
 the insulator. Accordingly, a good tight water seal can be formed without
 the need for precise dimensional control over the components of the
 insulator and without the need for a second crimping step to compress the
 circumferential ridges.
 The insulator subassembly 4 is then inserted into the bore 3 of metal end
 fitting 1. As shown in FIGS. 3B and 4, in order to prevent any air present
 in bore 3 from becoming trapped when the insulator subassembly 4 is forced
 into the bore 3 of metal end fitting 1, a spacer member 20 is interposed
 between the metal end fitting 2 and the circumferential ridges 10. Since
 end portion 7 of sheath 6 can have one or more ridges 10, it should be
 recognized that spacer member 20 must rest on top of and across each
 circumferential ridge 10 in order to provide a venting passageway during
 insertion of the subassembly 4 into the metal end fitting 1. It is the
 spacer member 20 which serves to deform the resilient ridge(s) 10, to
 allow air in bore 3 to escape when the insulator subassembly 4 is forced
 into bore 3.
 The spacer member 20 should be made of a material that will not deform when
 interposed between metal end fitting 1 and circumferential ridge 10.
 Preferably, spacer member 20 should be made of nylon to prevent damage to
 ridge 10 when spacer member 20 is removed. Moreover, spacer member 20 can
 be of any shape which will temporarily deform ridge(s) 10 and allow air to
 escape from bore 3 during the insertion step. The spacer member 20 may
 have either a hollowed or solid cross-section. For instance, the spacer
 member 20 could have a hollow tubular construction for allowing the air to
 vent through the spacer member. Alternatively, the spacer member could
 simply be a cord or wire of sufficient diameter to allow air to vent
 around the cord or wire and out of the cavity. In preferred the
 embodiments, the cross-sectional shape of the spacer member is round to
 prevent ridge 10 from tearing. However, any shape could be used so long as
 it has an adequate radius of curvature to allow air present within bore 3
 to escape, and does not damage ridge 10.
 Once the air under pressure in the cavity escapes, spacer member 20 is
 removed. The resilient ridge 10 then returns to its original size and
 shape to form a tight seal between metal end fitting 1 and the insulator
 subassembly 4.
 As should be clear from the above description, the resultant composite
 insulator has a construction which includes an insulator subassembly 4
 including a rod 5 comprising an electrically insulating plastic material
 and a sheath 6 covering at least a portion of the outer surface of rod 5.
 The sheath 6 has an end portion 7 and at least one deformable
 circumferential ridge 10 formed on an outer surface thereof. The ridge 10
 has a second diameter, d2. The composite insulator also includes a metal
 end fitting 1 having a sleeve 2 portion defining a bore 3 having a
 diameter, d1, that is less than the second diameter, d2. The metal end
 fitting 1 surrounds the end portion 7 of sheath 6, and an end region 11 of
 the metal end fitting 1 that overlaps ridge 10 is free from deformation
 (i.e., the second crimping step is unnecessary). However, it should be
 recognized that end region 11 could be crimped to improve water tightness.
 While the present invention has been described with reference to certain
 preferred embodiments, they were given by way of examples only. Various
 changes and modifications may be made without departing from the scope of
 the present invention as defined by the appended claims.
 For instance, the tightness between sheath 6 and metal fitting 1 may be
 improved by filling the gap therebetween with a sealant resin, such as
 silicone rubber.