. Electric railway journal . p type, and the com-pression or barrel type. The spool and the loop types are thesimplest and oldest, but both are limited in mechanical strengthowing to the fact that the load is applied between a cable andpin at right angles, and two cables at right angles, throws a comparatively small volume of porcelain indirect compression, the larger part of the load producing bend-ing stresses. The spool type insulator is further limitedmechanically by the possible bending strength of the pin, andelectrically, by the amount of surface insulation possible. I
. Electric railway journal . p type, and the com-pression or barrel type. The spool and the loop types are thesimplest and oldest, but both are limited in mechanical strengthowing to the fact that the load is applied between a cable andpin at right angles, and two cables at right angles, throws a comparatively small volume of porcelain indirect compression, the larger part of the load producing bend-ing stresses. The spool type insulator is further limitedmechanically by the possible bending strength of the pin, andelectrically, by the amount of surface insulation possible. Ifthe pin fits the hole in the insulator snugly, and the hole isof uniform diameter, the least bending of the pin will throwa bending stress on the insulator and break it. If a metalsleeve be cemented in the pin hole to distribute the load alongits length, the expansion of the metal with heat may burst theporcelain. Practical design has settled down to a form of pinhole smallest in diameter under the wire groove and enlarging. Compression Insulator. toward each end. This allows the pin to bend without pressingon the ends of the pin hole. The problem then becomes oneof designing a pin of such length as to allow the desired insula-tion, and of such diameter as to give the desired bendingstrength. Several forms of spool strain insulators have beendesigned, respectively for 1200-volt and 6600-volt service, andalso for higher voltages. These forms of spool strain insu-lators have an ultimate strength of from 9000 lb. to 12,000 lb. The ideal porcelain strain insulator would have the load-bearing portion of the porcelain made with two exactly par-allel surfaces held between two exactly parallel and absolutelyrigid plates. The accompanying cut shows a form of compres-sion insulator which approaches this condition. It consists ofa porcelain bushing with an undercut head portion, so that twoapproximately parallel bearing surfaces are obtained. Thehead of the bolt passing through the bushi
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