US3257501A

US3257501A - Self-cleaning electrical insulator constructions

Info

Publication number: US3257501A
Application number: US104339A
Authority: US
Inventors: Louis E Sauer
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1961-04-20
Filing date: 1961-04-20
Publication date: 1966-06-21
Anticipated expiration: 1983-06-21
Also published as: CH400277A; JPS3916143B1; JPS3923434B1; GB943578A

Description

June 21, 1966 L.. E. sAUER 3,257,501

SELF-CLEANING ELECTRICAL INSULATOR CONSTRUCTIONS Filed April 20, 1961 11 Sheets-Sheet 1 CIRCUMFE TIAL PI VA S FR f4 TO |=5 Fig IA.

THERMOSETTING RESIN l mllllk :5E IO 36 i Mmmm Elw I3 i a $7/ June 21, 1966 l.. E. sAUER 3,257,501

SELF-CLEANING ELECTRICAL INSULATOR CONSTRUCTIONS Filed April 20, 1961 11 Sheets-Sheet 2 wlTNE INVENToR Louis E. Sauer BY f m I MTTO R N EY L. E. SAUER June 21, 1966 1l Sheets-Sheet 3 \THERMOSETTING RESIN Filed April 20, 1961 Fig.4.

Fig.3.

L. E. BAUER June 21, 1966 SELF-CLEANING ELECTRICAL INSULATOR GONSTRUCTIONS ll Sheets-Sheet 4 Filed April 20, 1951 THERMO SETTING RESIN June 21, 1966 L. E. sAUER 3,257,501

SELF-CLEANING ELECTRICAL INSULATOR CONSTRUCTIONS Filed April 2o, 1961 11 sheets-sheet 5 Fig. 5B.

HE ATER Ll E. SAUER June 21, 1966 SELF-CLEANING ELECTRICAL INSULATOR CONSTRUGTIONS l1 Sheets-Sheet 6 Filed April 20, 1961 June 21, 1966 l.. E. sAUER 3,257,501

SELF-CLEANING ELECTRICAL INSULATOR CONSTRUCTIONS Filed April 20, 1961 l1 Sheets-Sheet '7 L- E. SAUER June 21, 1966 SELF-CLEANING ELECTRICAL INSULATOR CONSTRUCTIONS l1 Sheets-Sheet 8 Filed April 20, 1961 Fig.|4.

L. E. SAUER June 21, 1966 SELF-CLEANING ELECTRICAL INSULATOR CONSTRUCTIONS Filed April 20, 1961 l1 Sheets-Sheet 9 .ma F

E TR n MOS MOSETTING N June 21, 1966 L. SAUER 3,257,501

SELF-CLEANING ELECTRICAL INSULATOR CONSTRUCTIONS Filed April 20, 1961 ll Sheets-Sheet 10 June 21, 1966 l.. E. sAUER 3,257,501

SELF-CLEANING' ELECTRICAL INSULATOR CONSTRUCTIONS Filed April 2o, 1961 11 sheets-sheet 11 DRIPPING DECREASES T0 suesTANTlALLY zERo HERE Fig 25 E L g \wATER FLow BEcoMEs o so THIN THAT CURRENT 2 cARRYlNG cAPAc|TY oF wATER DEcREAsEs: U, ARclNG occuRs AT I: LowER voLTAsE 5 |=|3.|-' v6.26 I ,v4.8 v3.13

(D *j o O i X l l l l I |w |=|o |18 |=6 |=5 |-4 H3 |2 |=l AVERAGE CIRCUMFERENTIAL PITCH(RATIO OF LEAD T0 ROOT CIRCUMFERENTIAL DISTANCE )FOR CONSTANT LEAD Fig.27.

l /2 LEAD United States' Patent O M 3,257,501 SELF-CLEANING ELECTRICAL INSULAT() CONSTRUCTIGNS Louis E. Sauer, Sharon, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 20, 1961, Ser. No. 104,339 12 Claims. (Cl. 174-143) This invention relates generally to insulating electrical devices, and, more particularly, to insulating supports, terminal bushings and improved apparatus and methods for constructing the same.

A general object of :the present invention is to provide an improved insulating device, such as an insulating support or a terminal bushing, in which the disadvantages associated with procelain, which is customarily employed for weather-proof shells for such devices, are overcome by a novel material and construction.

Another object of the present invention is to provide an improved terminal bushing, in which the condenser body may be cast of thermosetting resin, and the advantages of abutyl rubber, or like material, may be employed in substitution of the usual porcelain weather casing.

Still a further object of the present invention is the provision of an improved assembly iixture for properly positioning the several component parts of the terminal bushing and for providing for the admission of a suitable casting thermosetting resin thereinto by an improved construction.

Yet a further object of the present invention is the pro- Vision of a rubber-like Weather-proof casing for various voltage ranges composed of a synthetic elastomeric material by the employment of a rubber-like strip having keyed attaching portions to minimize the cost of presses and molding equipment if the casing element were to be molded as a single piece.

Another object ofthe present invention is the provision of an improved terminal bushing, in which the Weather casing and the lower insulating sleeve of the terminal bushing have longitudinally-extending lands, with stepped portions for properly concentrically positioning a plurality of flared tubular condenser tubes into a proper position prior to the admission of a suitable casting thermosetting resin into the interior of the terminal bushing.

Another object of the invention is to provide an improved suspension-type insulator in which the use of an exterior porcelain shell is avoided.

Still another object of the invention is to provide an improved suspension-type insulator of reduced cost and improved strength, and having greater temperature resistance.

Another object is to provide an improved filling-hole construction for the rubber-like weather shell of a terminal bushing, insulator device, or the like involving a splitblister arrangement to cut down the time required for the operation of injecting the thermosetting resin and to prevent the inilux of air bubbles when the filling operation is stopped.

Another object of the inventionis to providea novel configuration of the multiple-thread helical weather sheds for the weather casing of a terminal bushing, insulator device, or the like, involving a range of improved circumferential pitch so that rain water will be swirled helically around the multiple-thread weather sheds instead of cascading downwardly over the edges of the weather sheds, as has been the custom heretofore.

Still a further object of the present invention is to provide new formulations of iilled resins to be injected into terminal bushings, insulator devices, etc.

Another object of the invention is to provide an im- 3,257,501 Patented June 21, 1966 ICC proved terminal bushing of reduced cost with a corresponding reduction of assembly time, inwhich the impulse iiashover level is increased and the voltage gradient accurately controlled in the interior of the bushing.`

Further objects and advantages will readily become apparent upon reading the following specification, taken in conjunction with the drawings, in which:

FIGURES 1A and 1B collectively illustrate a terminal bushing constructed in accordance with the teachings of the present invention, half of the terminal bushing being illustrated in side elevation, and the other longitudinal half of the bushing being illustrated Ain vertical section;

FIG. 2 is a sectional plan view taken substantially along the line II-II of FIG. 1A;

FIG. 3 is a plan view, in section, taken substantially along the line III-III of FIG. 1B; 4

FIG. 4 is a sectional plan view taken substantially along the line IV--IV of FIG. 1B; I

FIGS. 5A and 5B collectively illustrate, fragmentarily in side elevation and partially in vertical section, an irnproved assembly fixture for positioning the component parts of the terminal bushing prior to admission of the casting resin thereinto;

FIG. 6 is a side elevational View, taken on a reduced scale, of the assembly fixture of FIGS. 5A and 5B, and the associated vacuum equipment connected thereto;

FIG. 7 is a side elevational view, partially in vertical section, of the rubber-like weather shell of the present invention;

FIG. S isa sectional view taken substantially along the line VIII-VIII of the rubber-like weather shell of FIG. 7;

FIG. 9 is a top plan view of the rubber-like weather shell of FIG. 7;

FIG. l0 is atop plan view of the bushing cap for the terminal bushing of FIG. 1A;

FIG. l1 is a sectional View taken along the lineXI-XI of the bushing cap of FIG. l0;

FIG. 12 is a side elevational view, partially in vertical section, of one of the condenser tubes formed of-conducting material employed in the improved terminal bushing of FIGS. 1A and 1B;

FIG. 13 is a broken side elevational view of the ground iiange support for the terminal bushing of FIGS. 1A and 1B;

FIG. 14 is a top plan view of the ange plate alone of the flange assembly of FIG. 13;

FIG. 15 is a sectional view taken along the line XV-XV of FIG. 16 showing the rubber-like bushing arc shield provided at the lower end of the terminal bush sulator;

FIG. 20 is a view somewhat similar to FIG. 19, but illustrating the improved insulator Iconstruction and improved electrical stress conditions as compared with FIG. 19;

FIG. 20A illustrates the rubber split dimple member used as a check valve in iilling the insulator of FIG. 20;

FIG. 21 illustrates :a modified post-type insulator'embodying features of the present invention;

FIG. 22 illustrates in perspective the improved terminal bushing of the present invention;

FIG. 23 illustrates a construction of the weather-proof casing involving keyed strips associated with a molding form;

FIG. 24 is a parti-al sectional view taken substantially on the line XXIV-XXIV of FIG. 23;

FIG. 25 illustrates another manner of constructing the Weather-proof casing in sections rather than a single molded piece;

FIG. 26 illustrates a graph of weather-shed pitch against withstand flashover in kv.; and

FIG. 27 diagrammatically illustrates different circumferential pitches. A

Referring to the drawings, and more particularly to FIGS. 1A and 1B thereof, the reference numeral 1 generally designates the improved terminal bushing of the present invention. As shown, the terminal bushing 1 generally comprises an axially-extending conductor stud 2 having the opposite ends 3, 4 thereof threaded. A clamping cap 5, more clearly shown in FIGS. and 11, is threadedly secured, as at 6, to the upper threaded end 3 of the terminal stud 2 and exerts a slight compressive force against a corrugated weather-proof shell 7 having a plurality of helically arranged uanges, or weather sheds 8 :associated therewith.

It will be observed that the weather-proof shell or casing 7 has integrally formed therewith a plurality of longitudinally extending stepped lands 9 having

step portion

9a, 9b, and 9c as shown. FIGS. 7-9 more clearly illustrate the configuration of the molded rubber-like Weather shell 7. In addition, the weather-proof shell 7 has an annular bead 10 Which is disposed within a groove 11 associated with a grounding supporting sleeve 12. The purpose of the bead 1t) :and the groove 11 is to iix the position of the shell 7 relative to the ground iange assembly 12.

With reference to FIGS. 7 and 9 of the drawings, it will be noted that the elastomeric weather shell 7 has a tapered configuration 7a to conform tothe capacitance sections and to provide the requisite surface leakage path. As shown in FIG. 7' the shell 7 has a'threaded opening 7 b. For a particular example, a weather shell 7, such as shown in FIG. 7, had 6 threads, 1% inch pitch between adjacent threads, and a 6% inch lead, that is, the longitudinal travel for one complete turn of one thread.

The lower elastomeric sleeve or casing 17 of the terminal bushing 1 preferably has a threaded opening 17a to accommodate the threaded end 4 of the terminal stud 2.

Elastomeric materials which may be used for molding the upper weather shell 7 and the lower sleeve 17 include the Buna rubbers, neoprene, polyurethane, and polyester rubbers. Buna-S is a copolymer of butadiene and styrene while Buna-N is a copolymer of butadiene :and acrylonitrile. These materials are ordinarily employed in conjunction with various fillers such as carbon black, and inorganic compositions such as clays, hydrated and calcined aluminas, calcium carbonates, and the like. The polyurethane rubbers are commonly reaction products of polyisocyanates land polyesters and/or polyglycols. The neoprene, Buna-N, acrylic, polyurethane and polyester rubber are particularly -oil resistant. As a result, these are preferred for the lower sleeve 17 when submergd within an oil-filled tank of a circuit interrupter or transformer.

For outside use, as a protection from the weather, the butadiene-styrene -copolymer rubber and butyl ruber are particularly desirable.

A mounting iiange plate 13 (FIG. 14) is affixed, :as by Welding at 14, to the grounding sleeve 12. As shown in FIG. 1B, the lower end of the grounding sleeve 12 has an annular groove 15 associated therewith, into which is disp-osed a bead 16 integrally formed with the lower insulating sleeve 17 (FIG. 15) preferably formed of a suitable synthetic resilient or elastomeric material, such as a polyester rubber. The bead 1e and groove 15 have the same purpose as noted above, namely fixing the position of sleeve 17 relative to the ground flange assembly 12. It will be observed that the insulating sleeve 17 has a plurality of stepped lands 1S having stepped shoulders 18a, 1gb, which assist in positioning a plurality of concentrically-disposed conducting

condenser tubes

19, 20, which encircle the centrally-disposed conductor stud 2. FIG. 12 more clearly illustrates the configuration of the

metallic tubes

19, 20, which preferably have flared ends 19a or 20a.

Preferably, the upper weather-proof shell 7 is formed from a suitable synthetic resilient elastomeric rubber-like material, such as butadiene-styrene copolymer rubber, neoprene, butadiene-acrylonitrile copolymer rubber, acrylic rubber, butyl rubber, polyurethane rubber or polyester rubber.

In the past, considerable difficulty has been had with the use of porcelain weather-proof casings. As is obvious, porcelain shatters very easily upon impact or when a power arc follows a ashover. Even repeated corona streamers can produce suliicient thermal stress to cause fractures. It is alsoa weak material as relates to shearing stresses. Although porcelain is a good materiall in compression, nevertheless it is poor as far as reslsting tensile stresses is concerned. To overcome the foregoing disadvantages, the rubber-like Weather-proof shell 7, as illustrated in FIGS. 7-9, is employed.

Filling the spaces 2li-23 :adjacent the condenser tubes 1.9, 2?, is a suitable thermosetting casting resin. Many different types of casting resins may be employed.

For example, the resinous epoxy compositions which are employable in this invention may be prepared, in accordance with one preferred procedure, by reacting predetermined amounts of at least one polyhydric phenol or polyhydric alcohol and at least one epihalohydrm 1n an alkaline medium. Phenols which are suitable for use in preparing such resinous polymeric epoxides include those which contain at least two phenolic hydroxide groups per molecule. Polynuclear phenols which have Vbeen -found to be particularly suitable include those wherein the phenol nuclei are joined by carbon bridges, such for exlample as 4,4-dihydroxy-dimet-hylmethane (referred to hereinafter as bis-phenol A), 4,4dihydroxydiphenyl methylmethane and y 4,4dihy'droxy-diphenyl-methane.

While it is preferred to use epichlorohydrin as the epihalohydrin in the preparation of the resinous polymeric epoxide starting material (A) of the present invention, homologues thereof, for example, epibromohydrin, and the like, also may be used advantageously.

In the preparation of the resinous polymeric epoxides, aqueous alkali is employed to combine with the halogen of the epichlorohydrin reactant. The amount of` alkali employed should be substantially equivalent to the amount of halogen present and preferably should be employed in an amount somewhat in excess thereof. Aqueous mixtures of alkali metal hydroxides, such as potassium hydroxide and lithium hydroxide may be employed although it is preferred to use sodium hydroxide since it is relatively inexpensive.

The product of the reaction, instead of being a single simple compound, is generally a complex mixture of glycidyl polyethers, but the principal product may be represented by the formula:

wherein n is an integer of the

series

0, 1, 2, 3, and R represents the divalent hydrocarbon radical of the dihydric phenol. When for any single molecule of the polyether n is an integer, the fact that the obtained polyether is a mixture of compounds causes the determined Value for n, from molecular weight measurement, to be an average which is not necessarily zero or a whole number. Although the polyether is a substance primarily of the above formula, it may contain some material with one or both of the terminal glycidyl radicals in hydrated form.

The simplest polyether is a diglycidyl diether of the dihydric phenol which contains a single divalent aromatic hydrocarbon radical from the dihydric phenol and has two glycidyl radicals linked thereto by ethereal oxygen atoms. More generally, the polyether is of more complex charac- .ter and contains two or more aromatic hydrocarbon radicals alternating with glyceryl groups in a chain which are linked together by intervening ethereal oxygen atoms.

The resinous polymeric epoxide, or glycidyl polyether of a dihydric phenol suitable lfor use for reacting with an epihalohydrin in accordance with this invention has a 1,2-epoxy equivalency greater than 1.0. By epoxy equiv-l alency reference is made to the average number of 1,2- epoxide groups:

contained in the average molecule of the glycidyl ether. Owing to the method of preparation of the glycidyl polyethers and the fact that they are ordinarily a mixture of chemical compounds having somewhat different molecular weights and contain some compounds wherein the terminal glycidyl radicals are in hydrated form, the epoxy equivalency `of the product is not necessarily the integer 2.0. However, in all cases, it is a value greater than 1.0. The 1,2-epoxy equivalency of the polyethers is thus a value between 1.0 and 2.0.

The 1,2-epoxide value of the glycidyl polyether is determined by heating a weighed sample of the ether with an excess of 0.2 N pyridinium chloride in chloroform solution at the boiling point under reflux for two hours whereby the pyridinium chloride hydrochlorinates the epoxy groups to the chlorohydrin groups. After cooling, the excess pyridinium chloride is back-titrated with 0.1 N sodium hydroxide in methanol to the phenolphthalein end point.

Resinous polymers epoxides or glycidyl polyethers suitable for use in accordance with this invention may be prepared by admixing and reacting from one to ten mol proportions ofl an epihalohydrin, preferably epichloroll'tydrin, with from one to three mol proportions of bisphenol A in the presence of at least a stoichiometric excess `of 'alkali based on the amount of halogen.

To prepare the resinous polymeric epoxides which constitute the starting material (A), aqueous alkali, bis-phenol A and epichlorohydrin are introduced into and admixed in a reaction vessel. The aqueous alkali serves to dissolve the bis-phenol A with the formation of the alkali salts thereof. v If desired, the aqueous alkali and bisl) phenol A may be admixed and the epichlorohydrin added thereto, or an aqueous solution of alkali and bisphenol A may be added to the epichlorohydrin. In any case, the mixture is heated in the vessel to a temperature within the range of about 80 C. to 100 C. for a period of time varying from about one-half hour to three hours, or more, depending upon the quantity of reactants used.

Upon completion of heating, the reaction mixture separates into layers. The upper aqueous layer is withdrawn and-discarded, and the lower layer, containing the desired epoxy, is washed with hot water to remove unreacted alkali and halogen salts, in this case, sodium chloride. If desired, dilute acids, for example, acetic acid of hydrochloric acid, may be employed during the washing procedure to neutralize the excess alkali.

The foregoing epoxy resins may be cured in conjunction'with amines, such as those set forth below. Some of the classes of said amines which maybe employed are primary, secondary and tertiary aliphatic amines, aromatic amines, cyclic amines, heterocyclic amines, polyfunctional amines, etc. and specific examples of some of them are methyl benzyl amine, pyridine, alkyl pyridine,

quinoline, N,N dimethyl alpha benzylamine, ethyl morpholine, piperidine, melamine, dialkyl melamine, dicyanodiamide, ethylene diamine, propylene diamine, 1,6 hexamethylene diamine, diethylene triamine, dipropylene triamine, triethylene tetramine, tetraethylene pentamine, 3,3-diethylaminopropylamine, etc.

Examples of some more of the amines which I prefer to employ are the-aliphatic or aromatic amines and particularly those'which are free of substituents reactable with the oxirane ring of glycidyl ether of phenol and which do not have an active hydrogen atom on the amine nitrogen atom. This particular class of tertiary amines I employ when combined with a polybasic carboxylic acid and such combinations are used as hardening agents, although they may be used alone as hardening agents. Examples of some of these are (d) aromatic cyclic amines such as pyridine, alkyl pyridines and quinoline; (e) dialkyl aromatic amines such as dimethyl aniline; 4,4,4 methylidine tris (N, N, dimethyl aniline); 4,4,methylene bis (N,N dimethyl aniline); dimethyl amino methyl phenol; tri(dimethyl amino methyl) phenol; etc.; (f) tertiary alkyl amines such as benzyl dimethyl amine; tributyl amine; N,N dimethyl cyclohexylamine; N,N dimethyl alpha methylbenzylamine; and (g) aliphatic cyclic amines such as phenyl morpholine, ethyl morpholine, methyl morpholine, etc.

A particularly desirable epoxy resin, which gave exceptional test results, was that obtained by use of Epi-Rez 504 supplied by the Jones-Dabney Co. and Epon Curing Agent Z supplied by the Shell Chemical Corporation. The Epi-Rez 504 is a basic epoxy plus about 8% of butylglycidyl ether. The Epon Curing Agent Z comprises 60% metaphenylene diamine, having the structural formula and 40% methylene dianiline which has the structural formula:

(4,4'-diamine diphenyl methane) EXAMPLE I 5 parts by weight of Epi-Rez 504 was admixed with 1 part by weight of Epon Curing Agent Z. This catalyzed resin was admixed with the filler in the following proportions by weight:

Sand-resin-mix #23 (parts by weight) Flint silica-34 mesh (added second) 50 Sand 20/30 (added third) 30 Catalyzed resin (see above) 15l Mica powder-325 mesh (added first) 5 A curing time of two hours at 175 F. temperature was followed by a two hour post cure at 300 F. The reaction is exothermic.

EXAMPLE II The catalyzed resin was prepared in the same manner as discussed above in connection with Example I. It was then admixed with the filler in the following proportions by weight:

Sand-resin-mix #26 (parts by weight) Flint silica-34 mesh (added second) 50 Sand 20/30 (added third) 25 Catalyzed resin (see above) 18 Mica powder 325 mesh (added first) 7 The cure and post cure was as described above in connection with Example I.

' 7 EXAMPLE 111 The catalyzed resin was prepared in the same manner as described above in connection with Example I. It was then admixed with the filler in the following proportions by weight:

Sand-resin-rnix #28 (parts by weight) Flint silica-34 mesh (added second) 55 Sand 20/ 30 (added third) 22 Catalyzed resin (see above) 18 Mica powder-325 mesh (added first) 5 The cure and post cure was as described above in connection with Example I.

EXAMPLE IV Catalyzed resin (see above) 19 Mica powder-325 mesh (added first) (fineness- Flint silica-34 mesh (added second) 75 The cure and post cure was as described above connection with Example I.

In the above examples, the designation sand 20/30 means that 20 mesh, having a screen size 0.0331 inch will pass the sand, Whereas l3() mesh, having a screen size 0.0232 inch, will retain the sand.

The purpose of the iiller is to produce a mix which more nearly matches the thermal coefficient of expansion of the metal parts such as the terminal stud, the bushing fiange, and the condenser tubes (when they are made of metal). The condenser tubes, or parts thereof, although indicated in the drawings as made of metal, for instance aluminum, could, however, Ybe molded o a suitableconducting plastic, such as an epoxy resin with carbon particles as a filler. Bronze powder, aluminum powder, etc. could also be used as the conducting component of the mix. For achieving a condensive effect it is merely necessary to have a high dielectric constant for the condenser tubes, and not necessarily a good conducting material.

FIG. 6 illustrates, in side elevation, an improved assembly fixture 26, which is readily adapted for accurately constructing the improved terminal bushing 1 of FIGS. 1A Iand 1B. The assembly fixture 26 includes a vacuum chamber 27 connected by a pipe 23 to a vacuum pump 29. The vacuum pump 29 may be of a conventional type and preferably pulls a vacuum better than millimeters of mercury.

The vacuum chamber 27 includes a cylindrical member 30, an end plate 31, an upper plate 32, and an upstanding positioning cylinder 33. A suitable extruder 34, having a filling sprew 35 (FIG. 5B), is employed to extrude a thermosetting casting resin mixture 36 into a split dimple 37, more clearly shown in FIG. A of the drawings.V The vacuum pump 29 is employed to extract the air from the interior of the terminal bushing 1, tank 27 and hopper 38 so as to reduce the size and quantity of bubbles inthe resin mix forced Within the bushing 1. As mentioned, the resin mix is pushed into the spaces 21-23 between the

condenser tubes

19, 20.

As shown in FIG. 6, the extruder 34 includes a hopper 38 and a revolving screw 39 driven through a gear box 40. A pulley 41, fixed to a shaft 42 of the gear box 402, is driven by a belt 43. The belt 43 is driven by a pulley 44 afiixed to the shaft 45 of a drive motor 46. A vacuum line 47 leading from the vacuum pump 29 removes air from the hopper 3S and the entering mix 36.

The rotating screw 39 serves to transport the resin mixV through the barrel 43, working it at the same time. The softened plastic emerges from the filling sprew 35 under u pressure and rises upwardly betweenY the concentricallypositioned condenser tubes until it may be seen at 51 in the sight glass 49 (FIG. 5A) emerging from the holes 50 formed at the end of the lower sleeve 17 of the terminal lbushing 1. The reference numeral 51 indicates the excess material emerging from the holes Si) of the sleeve 17 in FIG. 5A.

With reference to FIGS. 5A and 5B, it will be observed that a split wedge-shaped ring 55 is employed to centrally locate the mounting fiange plate 13. Preferably, studs 56, cooperating with nuts 57, serve to clamp the mounting flange plate 13 and the associated grounding sleeve 12 into a proper position prior to the extrusion of the casting resin mix 36 into the terminal bushing 1 by the extruder 34.

It will be observed that the stepped lands 9, 13 serve to maintain the

condenser tubes

19, 20 in a proper position in concentric relationship prior to the admission of the casting resin 36 therebetween within the spaces 21-23.

Nuts 59, serve to maintain the threaded end 3 of the conductor stud 2 into a proper position.

As shown in FIG. 5A, a split ring clamp 61 assists in properly positioning the grounding sleeve 12. In addition, a stud-centering ring 62 is employed for properly locating the threaded end 4 of the conductor stud 2. The window plate 49 permits the external observation of the casting resin 36 during the filling operation as it rises through the bleed holes 50 in the end of rubber sleeve 17.

The manner of constructing the terminal bushing 1 will be evident from the foregoing description of the assembly fixture 26. By way of recapitulation, it will be observed that the Vacuum is first drawn by the vacuum pump 29. Subsequently, the casting resin 36 is admitted into the hopper 38 of the extruder 34.

The drive motor 46 rotates the revolving extruder screw 39 forcing the plastic 36 under pressure through the feed pipe 63 and through the vfilling sprew 35, illustrated in FIG. 5B. The slit 64 in the formed dimple 37 permits the entry of the resin into the interior of the terminal bushing 1 and prevents the back flow of resin mix when the sprew is removed. As shown in FIG. 5B, the dimple 37 is formed at the inner end of the formed filling hole 65 of the rubber-like weathershell 7.

The resin 36 passes Vbetween the spaces 67 between the lands 9, 18 to completely tillV the spaces 21-23 between the

condenser tubes

19, 26. When the casting resin 36 rises to the overfiow point of ring 62, the

operator may externally observe this fact through the transparent upper plate 49, and thus cease the filling operation. The resin 36 is then allowed to cure.

It will be observed that the split di-mple 37 acts as a check valve for filling the rubber weather shells 7. Thus, when molding the rubber weather shells 7 for 'casting resin-filled bushings 1 or insulators, hereinafter described, a tapped hole is formed in the side of the rubber vshell 7 to take the filling sprew 35. After the shell 7 is filled with res-in mixture it must set for 1 to 2 hours until the resin 36 gells sufiiciently to prevent back liow of the resin 36 or entrance of air when the iilling sprew 35 is removed. This ties up the filling equipment and increases clean-up time.

An important feature of the present invention is to mold the blisher 37 over the inside end of the sprew 65 and make a slit 64 in the blister 37 with a knife blade. When the resin mix is pushed in from the extruder 34, the slit 64 opens to admit the resin 36. But when the pressure is reduced, on the external side of the shell 7 by removal of the filling sprew 35, this acts to close the slit 64 and prevent back flow of theV resin because of the arched construction of the blister or dimple 37. This permits immediate removal of the filling sprew 35, requires only one set of iilling equipment, and clean up is required only at the end of the run.

To ensure that the gelling of the resin 36 will start at the lower end of the fixture 26, preferably a heater 63 is employed, as shown in FIG. 5B.

The heater 68 renders the resin mix 36 more fluid so it will more easily ll the spaces 21-23 between the

concentric conducting tubes

19, 20.

Because of the blister valve 37, the terminal bushing 1 together with upper plate 32, heater 68 and cylinder 33 is removed upwardly from the lower vacuum tank 30, and set aside to gell. Another

xture

32, 33 and bushing 1 may be lowered into the Vacuum tank 30 and lilled with resin mix therein, as heretofore described.

When the resin 36 has gelled, the terminal bushing 1 in the assembly fixture 26 will be placed in an oven for post curing. A

It is to be clearly understood that the present invention is not confined solely to the manufacture of terminal bushings, such as Iillustrated in FIGS. 1A and 1B of the drawings. FIG. 17 illustrates an application of the invention to a suspension-type insulator 70. The external weather shell 71 may be molded or otherwise formed from a resilient, rubber like or elastomeric insulating material, such as a natural rubber or a synthetic rubber of the butyl-type. Preferably aluminum oxide'trihydrate is used as a filler to obtain non-tracking properties of the shell. This may be used also for the shell 7 of the terminal bushing of FIG. lA.

Disposed within the weatherproof shell 71 is the resin mix 36. Corrugated pin supports 72, 73 are molded within the thermosetting resin 36.

FIG. 18 illustrates a stud-type bushing 77 utilizing an outer rubber-like shell 78 and a longitudinally extending terminal stud 79. A bead 80 of the shell 78 extends within a groove 81 of a flange support 82.

FIG. 19 illustrates the concentrated stress condition existing below the socket cap 86 of a suspension-type insulator 87. The lines 88-96 indicate equipotent-ial surfaces in percentages of voltage across the insulator when the insulator 87 is operating under high voltage. As shown, a steel ball bolt 98 is secured by hydraulic cement 99 within the porcelain shell 100.

When such suspension insulators 87 are operated in contaminated and/or foggy atmosphere more porcelain skirts must be added to increase flash-over voltage. This may increase the weight 50% over the standard and about doubles the cost.

The improved suspension insulator 104 of the present invention, shown in FIG. 20, is suitable for example, for 25,000 lbs. The loading equ-ipotential lines in percentages indicate much less stress than was present in the conventional suspension insulator of FIG. 1'9. The weather shell 107 is molded of non-tracking rubber with multi-helix sheds 108. These sheds 108` provide a washing action and the maximum leakage stress is approximately 1A that of the porcelain. 'I'he wet or contaminated flashover characteristic will be equivalent to Foggtype insulators at one-third the weight and one half the dia-meter.

It will be noted in FIG. 20 that a split dimple grommet 1091 may be used in the lilling operation in conjunction with a lower mold member 110. The blister grommet is placed in a hole 111 provided in the socket coupling 112. The function of the blister grommet 109 is the same as the blister 37 utilized in the weather shell 7 of FIG. 5B.

FIG. 2l illustrates a support insulator 113 having an external rubber shell 114. A lower support flange 113a is provided as Well as the upper threaded pin support 113b.

It will be noted from the foregoing description, thatV the weatherproof shell 7 has been indicated as a onepiece item. For the higher voltages, and to avoid excessive mold costs,.it may be desirable to use the construction set forth in FIGS. 23 and 24 of the drawings.

`An extruded non-tracking rubber strip 115 having a crosssteps of the condenser tubes.

10 117. The form 117 has the shape of the interior surface of the bushing shell desired. Two or more strips 115 may be wound in parallel, as shown, about the form 117 to give optimum lead to circumference ratio for higher voltage wet weather strength. Toprovide centering and locating lands for the condenser tubes, pre-molded stepped strips 120'having coated mating surfaces 121 would be* placed in axial grooves 122 in the form 117 and become an integral part of the completed shell when the lwhole is baked to set 4the epoxy adhesive coating.

As shown in FIG. 25, -another solution is to mold

sections

130, 131 of increasing diameter to conform to the Each strip of voltage class would require one or two additional molds, but no increase in press size. The molding cost is, of course, higher than that for the construction of FIGS.4 23 and 24, but much less assembly labor is required and no winding form is needed. Both of the foregoing methods avoid the considerable tooling cost involved if each voltage class were to be molded as a single shell.

To assist in inducing swirling action of rain water, melted ice or snow there is preferably provided multihelical threads 8 of configuration shown in FIGS. lA, 7, 9, 2l and 22 of the drawings. With a tapered bushing shell 7, to keep the lead constant for ready removal from the mold for the shell 7, the circumferential pitch of the flange threads 8 varies from 1:5 to 1:3. For the same shell 7, as shown in FIG. 1A, lthe pitch may vary from say 1:4 at the top to 1:5 at the bottom, this occurring, of course, due to the desirability of providing a constant lead'for the threads 8.

FIG. 26 shows the marked increase of withstand voltage in KV as a function of circumferential pitch. FIG. 2-7 illustrates different approximate circumferential pitches.

It has been experimentally proven that ratios of lead to root circumferential distance, herein called circumferential pitch for the threads 8 lying within the range 1:3 to 1:5 give marked improvement as shown in FIG. 26.

Since the outside diameter of the bushing varies over its length as a result of the frus-to-conical shape, the ratio of lead to root circumferential distance is an average, as shown in- FIG. 27.

It will be apparent from an observation of the geometry of the bushing that the ratio of lead to root circumferential distance is a measure of the slope of the shed surfaces which extend transverse to the axis of the bushing. If the slope is not great enough, rain water does not run down the shed'surface but drips over the edges; the lresult of substantial dripping is that the surface of the shed is not cleaned and the voltage withstand of the wet bushing is decreased. On the other'hand, if the slope of the shed is too great, the water flow becomes so thin that the current carrying capacity of the water decreases, and arcing occurs at a lower voltage.

I have discovered and verified by experimental tests that ratios of lead to root circumferential distance within the range 1:3 to 1:5 provide slopes which are neither too great nor too small; the curve'of FIG. 26 illustrates this. The voltage withstand (wet) falls off for ratios outside of this range.

As previously stated, this feature of the invention is applicable to the insulators of FIGS. 17, 20 and 2l as well as to the bushings of FIG. 1A and FIG. 18.

In FIG. 1A, the potential difference across the shell of the bushing is applied between clamping cap 5 and ground support sleeve 12. l

It will be apparent from a careful inspection of FIG. 1A `and the measurements Iand dimensions thereof, that the distance between these, measured along the root of each thread or shed, and measured in an axial direction along the surface of the shell and following the exact contour, that is, over the top and bottom surfaces of each shed and across the surfaces of the root portions, are equal. These constitute two leakage paths while the insulator or bushing is dry, and maximum leakage resistance for the insulator or bushing is provided by having these distances substantially equal.

This feature of my invention may also be a characteristic of the insulator embodiments of FIGS. 17, 20 and 21 as well as of the bushings of FIGS. 1A and 18.

A further inspection of the bushing of FIG. 1A reveals stili another feature of my invention; by employing a multiple thread helix configuration, I keep the leakage paths along the roots of the threads or sheds segregated from each other, and so provide a bushing in which -a higher voltage is required to ash the leakage current into an open arc. A discussion of the basic principles involved in this feature of my invention may be had by reference to Patent No. 2,933,553 issued April 19, 1960, to M. Zuhlke for High-Tension Insulator `and assigned to the assignee of the instant invention. This feature of my invention may be employed in the insulator embodiments of FIGS. 17, 20 and 21, as well as the bushings of FIGS. 1A and 18.

From the foregoing description it will be apparent that there is provided an improved terminal bushing and an improved assembly fixture therefor. The elimination of porcelain as the weather shell renders a bushing adaptable for rough use, and the rubber has been found to be particularly useful having non-tracking characteristics. The assembly time is shortened by using the evacuating apparatus 26, such as illustrated Vin FIG. 6. Also the constructions and ytechniques may be applied to other insulator forms and types, as described above.

Although there has been illustrated and described particular insulating structures and assembly fixtures for constructing the same, V-it is to be clearly understood that the same were merely for the purpose of illustration, and that changes and modifications may readily be made therein by those skilled in the -art without departing `from the spirit and scopeof the invention.

I claim as my invention:

l. In a condenser bushing having a conductor stud passing therethrough and at least one condenser tube composed of conductive material surroundingthe stud over at least a portion of its length `and spaced therefrom,

in combination, a core portion composed at least partially of a thermosetting resin and filling the space hetween the stud and the condenser tube, and an external lweatherproof casing means composed of a synthetic elastorneric material encompassing at least a part of said core portion, the casing means providing at least some support for the condenser tube, means including the casing means completely covering the condenser tube, the casing means including a shell having helical sheds thereon, the sheds having preselected slopes which optimize the Wet voltage withstand of the bushing,

2. A post-type supporting insulator including a pair of spaced metallic fittings, a core portion composed at least partially of a thermosetting resin, said spaced pair of metallic fittings being embedded within said thermosetting resin, and an outer weatherproof casing composed of a synthetic elastomeric material and surrounding -at least part of said core portion, the weatherproof casing having at least one helical shed on the outer surface thereof, the shed having a preselected slope which optimizes the wet voltage withstand of the insulator, the shed having a preselected width whereby the root distance between the pair of spaced metallic fittings is substantially equal to the distance between the tpair of spaced'metallic fittings measured axially along the surface of the casing and following the contour of the casing.

3. The combination in a terminal bushing of an axiallypositioned terminal stud, at least one condenser tube composed of conductive material surrounding the stud over at least a portion of its length and spaced therefrom, an

Vinsulating core portion surrounding said terminal stud and composed at least partial-ly of a thermosetting resin, and an outer weatherproof shell composed of a synthetic elastomeric material, the shell providing at least some supp-ort for the condenser tube, means including the shell completely covering the condenser tube, the shell having helical sheds thereon, the sheds having .preselected slopes which optimize the wet voltage withstand of the bushing.

4. An electrical insulator casing composed of insulating material, the exterior surface `of said insulator casing having a continuous fiange, the continuous flange projecting in a helical manner from the exterior surface of the casing, Iand the circumferential pitch being within the range of 1:5 to 1:3, where circumferential pitch is the ratio of the lead to fthecircumferential distance measured along the helix, for a constant lead.

5. An electrical insulator casing `generally frustoconical in shape and composed of insulating material, the exteri-or surface `of said insulator casing having a plurality of spaced continuous liange threads extending in a helical manner upon the external surface of the insulator casing, and the circumferential pitch of lthe flange threads averaged over the length of the frus-to-conical casing from the end of the large diameter to the end of small diameter thereof being within the range `of 1:5 to 1:3 where circumferential pitch is the ratio of the lead to the circumferential distance measured along the helix, for a constant lead.

6. The combination in a terminal bushing of an axially disposed conductor stud and a weatherproof outer casing, at least one thread-like convolution on the external surface of lthe weatherproof outer casing, and the circumferential pitch of the threadlike convolution being within the range of 1:5 to 1:3, Where circumferential pitch is the ratio of the lead to the circumferential distance measured along the convolution, for a constant lead.

7. A terminal bushing having an axially-disposed conductor stud, a pair of spaced axially-aligned sleeve-like casing elements formed of an elastomeric material, a ground support sleeve disposed between the casing elements and axially aligned therewith, said pai-r of sleevelike casing elements having stepped lands extending longitudinally thereof, one or more belled-end ltubular condenser elements supported upon the shoulder portions of said stepped lands, .and a thermosetting resin disposed in the spaces :about the belled-end condenser elements, one of the casing elements having Ia plurality of helical sheds on the outer surface thereof, the sheds having preselected slopes to optimize the wet voltage withstand of the last-named casing element, the sheds having preselected widths whereby the leakage distance measured along the root of `a shed is substantial-ly equal to the leakage distance measured axially along the outer surface of the last-named casing element and following the contour lof the outer surface.

8. A weatherproof casing shell for a terminal bushing having interiorly-disposed longitudinally-extending stepped lands adapted to support iconcentrically arranged condenser tubes, and said casing shell having a helically disposed continuous weather shed, the weathershed including a plurality of spaced helical sheds having preselected slopes which optimize the Wet volta-ge withstand of the shell, the Ihelical sheds having preselected widths whereby .the leakage distance measured along the root of a shed is substantially equal to the leakage distance measured axially Ialong the outer surface and following the contourV of the surface of the weathershed.

9. A terminal bushing including a shell composed of insulating material having an axial bore of non-uniform diameter therethrough, a conduct-or stud extending through said bore, one end of ythe stud extending from the adjacent end of the shell and adapted to have an electrical connecion made thereto, the surface of the bore being formed with a plurality of similar longitudinallyextending stepped lands at spaced intervals around the 'periphery thereof, the lands being so constructed that the diameter of the space between the inner surfaces of the lands progressively increases the farther the steps are from the extended end of the stud, a plurality of con- 13 centric condenser tubes of progressively increasing diameters and composed of conductive material disposed at least partially within the shell, the condenser tube of smallest diameter being the longest with the lengths of the tubes progressively decreasing as the diameters increase, at least one end of each condenser tube being ared with the yliared 4portion abutting .against an anformed with a plurality of similar longitudinally exnular shoulder formed at a step in the lands, a thermosetting resin rillingvthe spaces between the lands and nlling the spaces between the concentric condenser tubes, and 4insulating means disposed in predetermined position with respect to the shell, the insulating means having the conductor stud passing therethrough and supporting the other ends of the condenser tubes, the shell having a plurality of helical sheds thereon, the slopes of .the sheds being preselected to optimize the wet voltage withstand `of the shell. n

10. A .terminal bushing according to claim 9 in which the shell is additionally characterized as being made of an elastomeric material resistant to carbon tracking.

11. A terminal bushing including a shell composed of insulating material having an `axial-bore of non-uniform diameter therethrough, -a conductor stud extending through said bore, yone end of the stud extending from the adjacent end of the shell and adapted to have an electrical connection rnade thereto, the surface of the bore being formed with ya plurality of similar longitudinally extending stepped lands at spaced intervals around the periphery thereof, the lands being so constructed that the diameter of the space between the inner surfaces of the lands progressively increases the ,farther the steps are from the extended end of the stud, a plurality of concentric condenser tubes of progressively increasing diameters `and ycomposed `of conductive material disposed at least partially within the shell, the condenser tube of smallest diameter being the longest with the lengths of the tubes progressively decreasing las the diameters increase, at least lone end `of each condenser tube being flared with the dared portion abutting against an annular shoulder tor-med at a step in 'the lands, a Igrounding sleeve disposed adjacent a portion of the length of the shell and being spaced from the condenser tube of greatest diameter, a thermosetting resin filling the spaces between the lands and filling the spaces between the concentric condenser tubes, and insulating means disposed adjacent the grounding sleeve, the insulating means having the conductor stud passing therethrough, the insulating means supporting the other ends of the condenser tubes, the shell havin-g a plurality yof helical sheds thereon, the slopes of the sheds being preselected to optimize the wet voltage withstand of the shell.

12. A terminal bushing including a shell composed of insulating material having .an axial bore of non-uniform diameter therethrough, the shell having helical ytins on the exterior surface thereof, a conductor stud extending through said bore, one end of the stud extending from the adjacent end of the shell, the surface of the bore being tending stepped lands at spaced intervals around the periphery thereof, the lands being so constructed that the diameter of the space between the inner surfaces of the lands progressively increases the vfarther the steps are from the extended end of the stud, a plurality of concentric condenser tubes .of progressively increasing diameters and composed of conductive material disposed at least partially within the shell, the condenser tube of smallest dia-meter being the longest with the lengths of the tubes progressively decreasing as the diameters increase, at least one end of each condenser tube being flared with the flared portion abutting -against an annular shoulder formed -at a step in the lands, a thermosetting resin filling the space between the lands and filling 4the spaces between the concentric condenser tubes, and insulating means disposed in predetermined position with respect to the shell, the insulating means having the conductor stud extending therethrough, the insulating means supporting the other ends of the condenser tubes, the helical ns of the shell having predetermined slopes which optimize the wet voltage withstand of the shell.

References Cited by the Examiner UNITED STATES PATENTS 1,304,283 5/1919 Eby 174-142 X 1,913,596 6/1933 Jansson 174-31 1,942,284 2/ 1934 Halton 174-211 2,155,848 4/1939 Taylor 174-212 X 2,255,184 9/ 1941 Osenberg 264-262 2,304,461 12/ 1942 Knowles 18-30 2,390,821 12/ 1945 Willcox 264-139 2,507,694 5/ 1950 Cox 174-84 2,713,381 7/ 1955 Seck 29-453 2,766,484 10/ 1956 Sanderson 18-30 2,776,332 1/1957 Von 'Cron 174-211 2,911,683 1l/l959 Palermo et al 174-76` X 2,953,629 9/ 1960 Lapp 174-143 2,987,812 6/1961 Donaldson 29-453 3,001,005 9/ 1961 Sonnenberg 174-209 X FOREIGN PATENTS 559,581 7/ 1958 Canada.

119 1913 Great Britain. 766,230 1/ 1957 Great Britain. 394,787 5/ 1924 Germany.

Designs, General Electric Review, May-July 1956,

pages 24-27. I

BERNARD A. GILHEANY,Primary Examiner.

BENNETT G. MILLER, JOHN P. WILDMAN,

Examiners.

LARAMIE E. ASKIN, Assistant Examiner.

Claims

8. A WEATHERRPROOF CASING SHELL FOR A TERMINAL BUSHING HAVING INTERIORLY-DISPOSED LONGITUDINALLY-EXTENDING STEPPED LANDS ADAPTED TO SUPPORT CONCENTRICALLY ARRANGED CONDENSER TUBES, AND SAID CASING SHELL HAVING A HELICALLY DISPOSED CONTINUOUS WEATHER SHED, THE WEATHER SHED INCLUDING A PLURALITY OF SPACED HELICAL SHEDS HAVING PRESELECTED SLOPES WHICH OPTIMIZE THE WET VOLTAGE WITHSTAND OF THE SHELL, THE HELICAL SHEDS HAVING PRESELECTED WIDTHS WHEREBY THE LEAKAGE DISTANCE MEASURED ALONG THE ROOT