http://www.google.com/patents/US2418713 Excellent article porcelain vs tempered glass. Ever wonder why when a glass suspension shatters the head stays intact? Read on and find out this and many more things you didn't know you didn't know. April 1940. J. T. LITTLETON 2,198,734
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- Patented Apr. 30, 1940 UNITED STATES PATENT OFFICE INSULATOR Jesse T. Littleton, Corning, N. Y., assignor, by mesne assignments, to Corning Glass Works, Corning, N. Y., a corporation of New York Application January 31, 1936, Serial No. 61,840
This invention relates to insulators and more particularly to the strengthening of the dielectric parts employed in suspension insulators.
It has generally been recognized that the limited tensile strength of glass has restricted its usefulness as the dielectric element for certain types of electrical insulators. Insulators of the type commonly used to support power cables are required to sustain not only long time heavy mechanical loads but also short time high overloads such as result from heavy ice accumulations on the cables, high winds and the like. The stresses which are thrown into the insulators under such conditions are not uniformly distributed throughout the entire dielectric body, but as a general thing the head is required to carry the full mechanical load while the remaining portions of the dielectric body such as the skirts are substantially free from the stresses imposed by the load. In the construction of high voltage lines, it is common practice to support the cable on a string of suspension type insulators, i. e., a series of insulators in which one is suspended from another in vertical alignment,
It frequently happens that insulators will puncture electrically in the head due to various causes, thereby causing electrical but not necessarily mechanical failure of one or more units. Due to the opacity of many of the dielectrics heretofore employed in the production of suspension insulators, the type of failure above mentioned is very difficult to locate, with the result that serious and costly line service interruptions may result before the trouble is finally corrected.
Ceramic materials such as porcelain have long been known and usedas insulating materials and for some years past have found application as the dielectric employed I in suspension insulators. Due to the relatively low tensile strength of such ceramic materials, however, the loads carried by such insulators have been considerably restricted.
This condition has also existed in glass in spite Another object is to facilitate detection of defective insulators.
Among its features my invention embodies a tempered glass insulator and more particularly a dielectric element for suspension type insulators made from glass and tempered in such a manner that the degree of compression per square millimeter in the surface layers will be greater in certain predetermined parts than in others.
Specifically, my invention embodies a glass insulator in which the degree of compression per square millimeter in the surface layers of the pinhole and the head is greater than that in the skirt.
By Way of illustrating the application of my invention to one type of insulator, I have shown in the drawing a cap and pin type suspension insulator, partly in section, to more clearly illustrate my invention.
The insulator illustrated comprises a body 10 with a head 12 formed with a pin hole II. A pin 14 is fastened in the pin hole by means of a suitable alloy 13. A cap I5 is fitted over the head l2 and is fastened in place by a suitable socket I6 which is formed by pouring a suitable alloy in the space between the head [2 and the cap l5. All this is quite common practice in the fastening of metal to glass.
In carrying my invention into practice, an insulator body, preferably as it comes from the press, is subjected to a treatment by which it is brought to a uniform temperature above the annealing temperature of the glass from which the insulator is made. It is desirable to so heat the glass that the temperature is substantially uniform throughout its entire mass. In order to accomplish this, for instance, when using a glass of composition B1 disclosed in Patent Number 1,304,623 granted to Sullivan and Taylor on the 29th day of May, 1919, I introduce the formed insulator into a temperature equalizing kiln such as is commonly employed in the glass art, and maintained at a temperature of over 550 C. As an alternative, I may submerge the formed insulator into a heating bath composed of molten inorganic salt held at a temperature above 550 C. In order to avoid warpage or change'of shape of the insulator during the time that it is being heated, I provide a. suitable support for the insulator body and after the insulator has reached a temperature of at least 550 C. throughout its mass, it is subjected to a chilling treatment to be more fully hereinafter described by which the. degree of compression per square millimeter in the surface of the head I2 and pinhole H will exceed that in the surfaces of the skirt. Regulation of the degree of temper may be obtained by governing the temperature of the liquid, when a liquid chilling medium is used, and governing the velocity of the chilling medium when a gaseous chilling medium is employed. I have found that a satisfactory degree and distribution of compression in the surface areas may be obtained by subjecting the head and pinhole to a more severe chilling treatment than that employed to temper the skirt. This may be accomplished by introducing a nozzle into the pinhole and projecting a stream of high velocity air onto the walls thereof, and simultaneously directing a. high velocity air blast against the outer side of the head. The skirt may be chilled at a slower rate and thus the compression per square millimeter in the surface of the latter will be of a lesser degree than that introduced into the head. Alternatively, I may employ a nozzle which is introduced into the pinhole and through which a jet of molten inorganic salt held at a temperature of about 250 to 300 C. may be introduced. For this purpose I prefer to use the eutectic mixture of sodium nitrate and potassium nitrate whichmelts at about 220 C. Simultaneously the outer surfaces of the head are sprayed with the same molten salt held at a like temperature while the top and bottom surfaces of the skirt may be chilled by an air blast or jets of molten salt or the like, the properties of which are such that a less severe chill is produced. In other words, any'suitable method of chilling may be employed by which the-heat is more rapidly removed from the surfaces of the head and pinhole than from the surfaces of the skirt.
Obviously for some purposes it is not absolutely essential that the skirt of the insulator be tempered to a lesser degree than the head, though in order to avoid complete destruction of the insulator through chipping or cracking of the skirt such a condition is to be preferred. Also, it is within the scope of my invention to produce an insulator in which the head of the insulator is highly tempered while the skirt may be annealed. This may be accomplished by first tempering the insulator by heating it as above described, chilling it either by employing an air blast or by subjecting it to the molten salt treatment as above described and then after properly thermally shielding the head and pinhole, submitting the insulator to an annealing treatment.
What is claimed is:
1. As a new article of manufacture, a glass line insulator which has been tempered.
2. As a new article of manufacture, a glass line insulator which has been so tempered that predetermined portions are under different compressive stress than other portions.
3. As a new article of manufacture, a tempered glass insulator having a head and a skirt, the degree of temper in the head being greater than that in the skirt.
4. As a new article of manufacture, a glass line insulator having a tempered head.
5. A tempered glass insulator having a head, a'
pinhole therein and a skirt integral with and extending outwardly from the head, the surface of the skirt being under compressive stress to a lesser degree than the surface of the head.
6. A glass insulator body of the suspension type having a head, a pinhole therein and a skirt integral with and extending outwardly from the head, the head being tempered.
'7. A tempered glass insulator body of the suspension type having a head, a pinhole therein and a skirt integral with and extending outwardly from the head, the head being more highly tempered than the skirt.
JESSE T. LITTLETON.