US1994291A - Insulator - Google Patents

Description

March 12, 1935. w. A. SMITH 1,994,291

INSULATOR Filed Feb. 2, 1953 Fig.5

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Patented Mar. 12, 1935 UNITED STATES PATENT ()FFIQE INSULATOR Application February 2, 1933, Serial No. 654,828

1 Claim.

5 for an extended period of time without danger of mechanical or electrical failure.

Another object of the invention is to provide a device of the class named which shall be of improved construction and operation.

Other objects and advantages will appear from the following description.

This invention is in the nature of an improvement of the invention of Arthur 0. Austin shown and claimed in his application Serial No. 453,180, filed May 17, 1930.

The invention is exemplified by the combination and arrangement of parts shown in the accompanying drawing and described in the following specification, and it is more particularly pointed out in the appended claim.

In the drawing:

Fig. 1 is an elevation with parts in section showing one embodiment of the present invention.

Fig. 2 is a section of the insulator pin taken on line 2-2 of Fig. 1.

Fig.. 3 is an elevation of the upper end of a modified form of insulator pin.

Fig. 4 is a section on line 44 of Fig. 3.

Fig. 5 is a view similar to Fig. 3 showing another modification.

The dielectric material, such as porcelain, commonly used for insulators for supporting electrical transmission lines, has a high mechanical strength in compression but is relatively weak in tension and shear. To utilize this material to the best advantage, the present invention provides a construction in which the load is transmitted from one insulator fitting to the other through the porcelain member largely by a compressive force. The insulator fittings being made of metal have a different coefficient of expansion and contraction for temperature changes from that of porcelain, and provision is also made to prevent excessive stresses being set up in the porcelain because ofthe differential expansion or contraction of the parts for temperature changes.

In the form of the invention shown in Fig. 1, the numeral designates the metal cap, the numeral 11 the porcelain member, and the numeral 12 the metal pin of a suspension insulator. The porcelain member 11 is preferably provided with a treated sanded surface for engaging the cement by which it is secured to the metal (01. rye-ms) parts, such a surface being described in Patent 1,284,975, granted November 19, 1918 to A. 0. Austin. Cement 13 is interposed between the surface of the dielectric member and the cap, and cement 15 connects the dielectric member to the pin. The upper end of the pin 12 is shaped to form an inverted cone, the surface of which is covered by a helical spring 16 wound about the outer face of the cone. The ends of the springs may be attached in any suitable manner. For this purpose, an opening 17 may be drilled into the pin at the lower end of the conical surface, into which one end of the spring may be inserted. The spring is then wound in successive convolutions about the conical surface until substantially the entire surface is covered; the upper end of the spring being secured to the surface or adjacent turn by solder or other suitable attachment. This not only provides a yielding bearing surface for the pin but also provides roller bearings between the pin surface and the cement 15.

In order to prevent the cement from entering the space within the helical spring and the depressions in the outer surface between the different convolutions, the spring is covered by Wax or other yielding material and the excess wax is scraped off, leaving a smooth conical surface. When the pin is secured in the recess in the dielectric member by the cement 15, this will form a smooth inner bearing surface on the cement upon which the spring 16 may roll to permit a wedging action between the bearing surface of the pin and the bearing surface of the cement. The entire surface of the pin embedded in the cement is coated with this yielding material or otherwise treated to prevent bonding between the cement and the pin. This leaves the portion of the pin below the bearing surface unattached to the cement so that it is free to move in the direction of the axis of the pin. .By this arrangement, the entire force of the load is transmitted by the single tapered bearing surface at the inner end of the pin. The inner surface of the cap is also coated or otherwise treated to prevent bonding between the cap and the cement.

It will be noted that the cap 10 is provided With a wedging bearing surface 17 which is substantially parallel to the conical bearing surface on the pin and disposed in opposed relation thereto. The direction of the force transmitted by the dielectric member and interposed cement is substantially normal to the two opposed bearing surfaces, and the porcelain between these surfaces is thus put under compression. If the two surfaces 16 and 17 were both rigid surfaces, there would be an abrupt termination of the compression in the dielectric member at the edges of the zone transmitting the load and the compressive force would be greatest at the points where the interposed cement is the thinnest. This would bring excess pressure at the upper end of the pin where the edge of the cone approaches closest to the dielectric member and at the lower edge of the cap. The pressure at the lower edge of the cap is relieved by the resiliency of the cap, which tends to spring out at its lower edge, thus tapering off the pressure at the edge of the pressure zone and relieving the pressure where it is the greatest.

The helical spring on the pin accomplishes a similar result because it permits yielding at any point of excess pressure, thus passing the pressure along to the next adjacent spring and so on over the bearing surface, eifecting a distribution of the stress. The reason the pressure is greatest where the cement is the thinnest is because the cement forms a resilient strut, transmitting the pressure, and the total resiliency will of course be proportional to the length of the strut. The porcelain itself is very rigid and has a high modulus of elasticity compared to cement so that the porcelain itself gives but little; and where there is but a short elastic strut between the fitting'and the porcelain, this strut can yield but a small amount so that the greater proportion of the force will be transmitted'to the porcelain at this point. Where the cement strut is longer, it will act like a long compression helical spring and will yield sothat only a small proportion of the force will be transmitted.

If it were not for the yielding surface provided by the spring 16, the force of the load transmitted by the conical bearing surface would be concentrated at the upper edge of the surface where the cement strut is very short. The spring, however, being much more yielding than the cement will greatly offset the difference in length of the cement struts, and effect a much more uniform distribution of the stress.

The wedging surface both at 16 and 17 permits adjustment of the parts under the force of the load to compensate for unequal expansion and contraction of the metal and dielectric members caused by temperature changes. If the pin contracts, it will move downwardly, thus compensating for the relative change in size between the pin and dielectric. If the pin again expands, the outwardly expansive force will tend to raise the pin in its seat to prevent bursting of the dielectric member. The movement of the pin under this wedging action is greatly facilitated by the roller bearing formed by the helical spring. The roller bearing surface maintains its eifectiveness for an indefinite period and in this respect has a great advantage over any lubricating material disposed between the surfaces, which is apt to deteriorate in time or to change its characteristics so as to change the coefiicient of friction between the wedging surfaces. It is quite apparent that if the pin should be drawn into its seat during cold weather and should fail to return when the temperature rises, the expansion of the pin would produce a bursting action tending to destroy the dielectric member. The helical spring prevents this danger, in addition to preventing concentration of stress.

The bearing surface of the pin may be shaped to compensate somewhat for the difference in the length of the resilient cement strut by changing the angle of the surface so as to vary the compressive force transmitted to the cement for different portions of the conical surface.

In Fig. 3 the bearing surface is made convex in the form of an arc. It will be apparent that when a load is placed on the pin, tending to draw it into its seat, there will be a much greater movement normal to the bearing surface in the direction of the arrow at the point 18 than there will be in the direction of the arrow at the point 19. There will, therefore, be a much less compression on the cement strut at the upper edge of the bearing surface than at the lower edge, but since the strut at the upper edge, being shorter, yields a much less amount than the longer strut at the lower edge, the lesser force at the upper edge will be more completely transmitted to the dielectric member than will the greater force at the lower edge of the bearing surface. In this way, the distribution of stress transmitted to the dielectric member will be equalized over the surface of the dielectric member.

Instead of an are shaped bearing surface, the bearing surface may be divided up into conical sections having different angles, as shown in Fig. 5, in which the bearing surface is broken up into two steps 20 and 21; the upper step 20 having a steeper angle than the lower step 21. While the form of bearing surface shown in Figs. 3 and 5 tends to equalize the stress on the dielectric member, it has the disadvantage that it distorts the direction of the stress so that a portion of the stress is directed toward the part of the dielectric member below the edge of the cap 10, at which point there is no abutment for opposing this stress. This tends to produce a shear in the porcelain member for which the dielectric material is ill adapted.

It should be noted that the entire bearing surface of the pin is located at the upper end of the pin on a single wedge shaped step. This aids in preventing stress in the dielectric member from being directed to a portion of the dielectric member below the lower edge of the cap; and where a single conical surface is used, the entire stress may be practically confined to the portion of the dielectric member backed up by the bearing surface 17 of the cap. Where a pin is fixed to the cement throughout the portion thereof within the opening in the dielectric member so that stress is transmitted to the cement throughout this portion of the pin, either thepin, the stress transmitted by the lower portion of the pin will be imparted to the portion of the dielectric member not supported by the bearing surface of the cap, which creates undue shearing stress in the dielectric material. This tendency is magnified by pin stretch, which tends to concentrate the force transmitted at the lower end of the pin. A single wedging surface at the upper end of the pin avoids this tendency and produces an insulator having much greater mechanical strength than where the force is distributed over the entire portion of the pin disposed in the recess in the dielectric member. The advantages thus obtained, however, are largely neutralized if the rigid surby distributed bearing surfaces or by bonding to face of the" pin is permitted to bear upon the cement because of the concentration of stress at the upper portion of the tapered bearing surface where the cement strut is short. By combining the single wedging surface and the resilient roller bearing, the advantage of the single step pin is secured and the disadvantage is oifset so that insulators of mechanical strength heretofore impossible have been obtained, and by the use of the yielding roller bearing surface, the danger of failure of such insulators after a lapse of time is avoided.

In some instances, it is desirable to hold the pin from rotation in the cement. This may be done in a number of ways. In Figs. 1 and 2 the pin is shown as provided with flat faces at 22 and 23 to prevent rotation in the cement, while in Fig. 3 the pin is provided with an elliptical portion 24 which serves a similar purpose.

I claim:

An insulator comprising a dielectric member having a boss thereon provided with a recess, a cap surrounding said boss and having a conical bearing surface adjacent the rim of said cap, cement interposed. between said boss and cap and bonded to said boss but unbonded to said cap, a pin disposed in said recess and having a single conical bearing surface at its inner end, a helical spring roller wound in a plurality of convolutions on said conical bearing surface, said pin having a recess therein for receiving the end of said spring roller to hold the end of said roller in position, cement interposed between said pin and dielectric member and bonded to the surface of said dielectric member but unbonded to said pin, said cement having a smooth conical bearing surface thereon conforming to the bearing surface on said pin but spaced therefrom to accommodate said spring roller between said pin and cement, a portion of said pin in said cement being non-circular to prevent rotation of said pin, said pin and cap being sufliciently overlapped that substantially the entire force of the load on said pin is directed toward the bearing surface of said cap to impart maximum strength to said insulator, said spring roller acting to distribute the load on the bearing surface of said pin and to prevent concentration thereof' at the extremity of the pin where the thickness of the cement between said pin and dielectric member is least.

WILLIAM A. SMITH.

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