US3582538A - Prestressed insulator - Google Patents

Abstract

This disclosure relates to porcelain insulators, particularly post insulators. Such post insulators are subjected to cantilever loads which induce tensile bending stresses in the porcelain. Unfortunately porcelain is weak in tension. Accordingly the inventors here teach prestressing the porcelain in compression so that prestress compression counters tension induced by the cantilever loads so that the insulator is enabled to carry greater loads more reliably.

Description

United States Patent [72] Inventors Stanley C. Killian;

Benjamin F. Dewey, both 01' LeRoy, N.Y. [21] Appl. No. 887,433 [22] Filed Dec. 22, 1969 [45] Patented June 1, 1971 [73] Assignee lnterspace Corporation Parsippany, NJ. Continuation-impart of application Ser. No. 787,948, Dec. 30, 1968, now abandoned. This application Dec. 22, 1969, Ser. No. 887,433

[54] PRESTRESSED INSULATOR 5 Claims, 5 Drawing Figs.

[52] U.S. CI 174/158, 174/169, 174/178, 174/179 [51] lnt.Cl ..H0lb l7/l4 [50] Field of Search 174/30, 150,158,165,169,171,176,l77,178,179,180, 182,186,188,189,191,l92,194,195,196, 197, 198, 209

[5 6] References Cited UNITED STATES PATENTS 1,251,097 12/1917 Peck 174/198X 1 H1923 Goddard 1. 174/179UX l/l930 Barfoed 174/182X 6/1930 Schramm 174/179 8/1930 Dunmire 174/179X 10/1939 Austin 174/179X 12/1950 Ludwig et al.. 174/179UX l2/l953 Burrneister..... l74/30X 2/1966 Gonek et al. 174/179X FOREIGN PATENTS 10/1927 France 174/179 3/1964 Great Britain.. 174/30 12/1933 ltaly 174/198 10/1931 Norway 174/198 Primary Examiner--Laramie E. Askin Attorneys-Charles E. Baxley and Thomas E. Tate ABSTRACT: This disclosure relates to porcelain insulators, particularly post insulators. Such post insulators are subjected to cantilever loads which induce tensile bending stresses in the porcelain. Unfortunately porcelain .is weak in tension. Accordingly the inventors here teach prestressing the porcelain in compression so that prestress compression counters tension induced by the cantilever loads so that the insulator is enabled to carry greater loads more reliably.

INSULATING ROD PATENTE'D'JUN 1m 3.582538 INSULATING 00 FIG- I 1N VENTOR.

STANLEY C- KILL/AN BENJAMIN F- DEWEY paw M441 AT RAEYS PRESTRESSED INSULATOR cRoss REFERENCE This application is a continuation-in-part of US. application Ser. No. 787,948 filed Dec. 30, 1968 by the same inventors and now abandoned.

BACKGROUND OF INVENTION It is generally known that cantilever strength of structural materials which are weak in tension can be improved by prestressing such materials in compression, and several practical applications of this principle have been suggested such as prestressed concrete. Past insulators generally have hollow porcelain bodies with a proximal mounting base and a closed distal end adapted to carry line-supporting hardware. Cantilever loading is the most common loading on post insulators, and the ability of post insulators to withstand tensile stresses imposed thereby is vital to its structural integrity.

One approach to improving tensile strength of porcelain has been to reinforce the procelain. But the temperatures at which porcelain is fired are too severe for all but a few exotic materials (which are also economically unfeasible and in some cases conductors) so this approach has been frustrated.

Some manufacturers have tried to increase the strength of post insulators by making them solid instead of hollow; however, they found that the increase of strength wassmall.

SUMMARY OF INVENTION The inventors have solved the problem of improving tensile strength of insulators, particularly post insulators in a novel, facile and unobvious way. Instead of providing reinforcement or a more massive design, they prestressed post insulators in compression to counter tension induced by cantilever loads so that tension stresses are counteracted. Accordingly the objects of the invention include improving the cantilever strength of post insulators to reduce breakage, electrical hazards and maintenance costs as well as accomplishing this with an inexpensive construction that does not reduce the performance capabilities of the insulators which are also otherwise well suited to their intended functions.

These and other objects will be apparent from the specification which describes the invention, its use, operation, and preferred embodiments; from the drawings, which constitute a part of the disclosure and from the subject matter claimed.

DRAWING Throughout the drawing like numericaldesignations refer to like objects. Also, in the drawing:

FIG. 1 shows a partially cutaway side elevation of a post insulator illustrating one embodiment of the invention;

FIG. 2 is a partially cutaway fragmentary side view of a post insulator according to another embodiment of the invention;

FIG. 3 isa partially cutaway fragmentary side view of a post insulator according to still another embodiment of the invention;

FIG. 4 is a sectional view of the insulator of FIG. 1 taken along the line 4-4 thereof; and

FIG. 5 is a partially cutaway fragmentary side view of a post insulator according to still another embodiment of the invention.

PREFERRED EMBODIMENTS The invention will be described here in terms of a post insulator, although it will be clear that the inventive concept applies to other insulator applications wherein tension in the porcelain becomes a problem. Further, prestressing means are positionable conveniently in a hollow open at a proximal base and of the insulator and extending toward a closed distal end thereof, but it will be equally clear that other arrangements for providing prestressing compression can be provided.

Generally, with regard to a hollow post insulator, cantilever strength is increased according to this invention by anchoring the head of a rod inside the hollow body of the insulator near its closed end, providing a base engaging the body and through which the rod penetrates, and turning a nut on the rod to draw the base against the body for compressively prestressing the portion of the body between the head of the rod and the base to increase the cantilever strength of such portion. The rod can be anchored in place by washers arranged to interlock with the inside surface of the insulator near its closed end or by cement filling the space between the inside surface of the hollow body and the head of the rod. The rod is preferably formed of a dielectric material such as resin-bonded glass fibers, and the insulator body is preferably formed of porcelain in a conventional manner.

Referring particularly to FIG. 1, a post insulator according to the invention may have a conventional body 10 which is made of porcelain, glass, or other dielectric material. Body 10 can have a variety of known shapes common to post insulators and has closed distal end 11, hollow bore 12, and open proximal end 13. It will be understood that post insulators can be mounted horizontally, vertically, or otherwise. A clamp or holding means 14 is secured as by cementing to distal end 11 to secure a power line in place at the distal end of the insulator. Line-holding means other than the illustrated clamp 14 are usable within the spirit of the invention. Base 15 is secured over open bottom 13 of body 10, and base 15 cooperates with known equipment to hold the insulator on a pole, tower, substation, or the like. Base 15 is preferably formed of a highstrength material and is joined to the bottom ofinsulator 10 by cement 16.

Cantilever strengthening of body 10 according to the invention is accomplished generally by rod or bolt 20, the head end of which is anchored in place near the closed distal end 11 of body 10 and the bottom end of which is threaded to receive nut 21. Nut 21 is tightened on the threaded end of rod 20 against base 15 to apply a predetermined compressive prestress to body 10, and such compressive prestress greatly increases the cantilever strength of body 10. Bolt 20 is formed preferably of a dielectric material of substantial tensile strength such as resin-bonded glass fibers.

Anchoring of the head end of bolt 20 in place near the distal end is accomplished in several ways according to the invention. One preferred arrangement is best illustrated in FIG. 1 and 4. A plurality of uniformly spaced ledges 25 are formed around the inside of hollow bore 12 near distal end 11. Ledges 25 extend inward and form a plurality of abutments in one plane near distal end 11. Washer 26 is notched peripherally to fit between ledges 25 when positioned as illustrated in FIG. 4. Head 22 on bolt 20 is held above the distal surface of washer 26 with the shaft of bolt 20 extending downward through central apertures in washer 26 and base 15. After head 22 and washer 26 are inserted between ledges 25, washer 26 is turned a few degrees to bring its radially outermost portions into registry with ledges 25; in such position, washer 26 is held by ledges 25 from axially moving toward base 15. Nut 21 is then tightened onto bolt 20 to draw base 15 into compressive engagement with body 10, and body 10 is compressively prestressed by the desired amount in the region between base 15 and ledges 25.

An alternate arrangement for anchoring bolt 20 in place near distal end 11 is shown in FIG. 2. Radially enlarged head 23 on bolt 20 is secured in place by cement 24 filling the upper end of hollow bore 12 and securely engaging head 23 and the inside of bore 12.

Still another alternate arrangement for anchoring bolt 20 near distal end 11 is illustrated in FIG. 3. Annular groove 27 is formed around the inside of hollow bore 12, and an expansible washer 28 is expanded into groove 27 by turning bolt 20 after washer 28 has been inserted into the insulator and into registry with groove 27. Head 29 of bolt 20 disposed above washer 28 is held from axial movement toward base 15.

Still another alternate arrangement for forming the distal end of a post insulator and anchoring the head of the bolt in place is shown in FIG. 5. Distal end 32 of insulator body 31 is given a shape similar to the head of a suspension insulator, and cap 33 is fitted over distal end 32 and cemented in place in a similar manner to the fitting and cementing of a cap on a suspension insulator, thus providing a suspension arrangement for tensile loading. Cap 33 differs from a suspension cap by having included thereon hardware for securing a line in place. Recess 36 inside distal end 32 is slightly larger than head 34 of bolt 35, and cement 37 fills recess 36 and surrounds head 34 to anchor bolt 35 securely in place, deeply inside distal end 32. Tensioning of bolt 35 then compressively prestresses body 31 for strengthening thereof.

By way of design, for an embodiment such as shown in FIG. 2, having a procelain body and a polyester-resin-bonded glass fibers the amount of compressive prestressing that can be imparted to the porcelain by a prestressing system according to this invention depends upon the section and tensile strength of rod 20 and the strength-in shear and shearing area of the cement 24 between head 23 and the inside of bore 12. With a safe working load of 50 percent of ultimate tensile strength for a l'h-inch diameter rod, it could impart the following compressive stress and thereby greater breaking strength to the porcelain:

50 percent working load =50,000 p.s.i.

Cross-sectional area of rod =1.77 in.

Load-bearing capacity of rod =88,500 pounds Cross-sectional area of porcelain =3 3.2 in. Compressive stress developed in porcelain =2,660 p.s.i.

Given an ultimate strength of 7,500 p.s.i. for high-strength porcelain and 5,000 p.s.i. for ordinary porcelain, the above prestressing in compression represents about 33 percent to 50 percent increase in breaking strengths, respectively. The shearing load in cement 24 which holds rod 20 to the inside of bore 12 would be as follows:

Applied load =88,500 pounds Load bearing area (cylindrical) =rD h D=3 12-inch, h=6 inch A=66 in.

Shearing stress =1,340 p.s.i. =22 percent of ultimate shear strength of cement which is well within acceptable safety tolerances.

Comparative tests to failure were performed for conventional and prestressed post insulators respectively of 90-inch lengths (three segments of 30 inches each) and l3%-inch lengths. In Table 1 set forth below, results for the 90-inch post insulators are shown as la and lb for porcelain insulator prestressed with resin-bonded glass fibers and an unprestressed insulator respectively. Results for the 13 riainch insulators are shown as 2 a and 2 b for a porcelain insulator prestressed with a steel rod and an unprestressed insulator respectively.

TABLE] Test 1 a: -inch post insulator with rod tightened to about ft. lb. on hexagon nut outside mounting base. Breaking load =2,500 lbs. at 90 inches.

Test 1 b: Identical post insulators as Test 1 a but not prestressed.

Breaking loads between 1,500 lbs. and 1,800 lbs. at 90 inches. Comparing Tests 1 a and l bindicates that there is at last a 2,500 1,800 or 1.39 breaking load relationship; i.e. a 39 percent increase in loadbearing capacity with the prestressed post insulator over the identical unprestressed insulator.

Test 2 a: 13% inch post insulator broke 9 /inches form distal end at point of application of load.

Breaking load =4,50O lbs.

Test 2 b: identical post insulators as Test 2 a but not prestressed.

Breaking loads between 3,000 lbs. and 3,200 lbs. again at 9% inches from distal end.

Comparing Test 2 a and 2 b indicates that there is at least a 4500/3200 or 1.41 breaking-load relationship; i.e. a 41 percent increase in load-bearing capacity with the prestressed post insulator over the identical unprestressed insulator.

What we claim is:

1.- A cantilever-strengthened post insulator of a dielectric material which is relatively weak in tension and comprising: an oblong body of the dielectric material formed with a hollow extending longitudinally inward from one end thereof, said body being open at one end and closed at the opposite end, a mounting base covering said open end and arranged for rigidly mounting said insulator to extend away from a support, a cap secured rigidly to said closed end of said body and including means for securing an electric conductor, a rod axially disposed within said hollow and having a head fixedly secured within the closed end of said body and having a free threaded end penetrating through and extending beyond said mounting base, a nut threadably engaged on the protruding free end of said rod for securing said mounting to said body and for applying tension to said rod for subjecting said body to a predetermined compression of a magnitude which is a substantial portion of the compressive strength of said dielectric.

2. The insulator of claim 1 with said mounting base including a central recessed portion fitted within said open end, said rod extending through said recessed portion, said nut situated in said recessed portion.

3. The insulator of claim 1 with said body made of porcelain.

4. The insulator of claim 1 with said rod made of a dielectric material.

5. The insulator of claim 4 with said rod made of resinbonded glass fibers.

Claims (5)

1. A cantilever-strengthened post insulator of a dielectric material which is relatively weak in tension and comprising: an oblong body of the dielectric material formed with a hollow extending longitudinally inward from one end thereof, said body being open at one end and closed at the opposite end, a mounting base covering said open end and arranged for rigidly mounting said insulator to extend away from a support, a cap secured rigidly to said closed end of said body and including means for securing an electric conductor, a rod axially disposed within said hollow and having a head fixedly secured within the closed end of said body and having a free threaded end penetrating through and extending beyond said mounting base, a nut threadably engaged on the protruding free end of said rod for securing said mounting to said body and for applying tension to said rod for subjecting said body to a predetermined compression of a magnitude which is a substantial portion of the compressive strength of said dielectric.

2. The insulator of claim 1 with said mounting base including a central recessed portion fitted within said open end, said rod extending through said recessed portion, said nut situated in said recessed portion.

3. The insulator of claim 1 with said body made of porcelain.

4. The insulator of claim 1 with said rod made of a dielectric material.

5. The insulator of claim 4 with said rod made of resin-bonded glass fibers.

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