It may be noted that the pins with a central iron bolt only half an inch in diameter, that were used on the 33,000-volt Santa Ana line, were said to have caused no burning of their cross-arms in those cases in which the wooden threads about the top of the central bolt were burned through.
Another possible trouble with iron pins is that they, by their greater rate of expansion than glass or porcelain, will break their insulators. Such results can readily be avoided by cementing each iron pin into its insulator, instead of screwing the insulator onto the pin. Iron pins will, no doubt, cost somewhat more than those of wood, but this cost will in any event be only a small percentage of the total investment in a transmission line. Considering the cost of the renewals of wooden pins, there seems little doubt that on a line where the voltage and other conditions are such as to result in frequent burning, iron pins would be cheaper in the end.
Iron pins have already been adopted on a number of high-voltage lines. Not only iron pins, but even iron cross-arms and iron poles are in use on a number of transmission lines. On a long line now under construction in Mexico, iron towers, placed as much as 400 feet apart, are used instead of wooden poles, and both the pins and cross-arms are also of iron. The 75-mile line from Niagara Falls to Toronto is carried entirely on steel towers.
The Vancouver Power Company, Vancouver, British Columbia, use a pin that consists of a steel bolt about 12 inches long fitted with a sleeve of cast iron 4-1/2 inches long to enter the cross-arm, and a lead thread to screw into the insulator. On the 111-mile line of the Washington Power Company, of Spokane, which was designed to operate at 60,000 volts and runs to the Standard and Hecla mines, a pin consisting of a steel bar 1-1/8 inches in diameter, with a cast-iron shank 2-1/16 inches in diameter to enter the cross-arm, and with the lead threads for the insulator, is used.
[Illustration: FIG. 92.--Iron Pins on Spier Falls Line.]
On the network of transmission lines between Spier Falls, Schenectady, Albany, and Troy, in the State of New York, the insulators are supported on iron pins of two types. One of these pins, used at corners and where the strain on the wire line is exceptionally heavy, is made up of a wrought-iron bolt 3/4-inch in diameter and 16-1/2 inches long over the head, and of a malleable iron casting 8-3/4 inches long. This casting has a flange of 5 by 3-3/4 inches at its lower end that rests on the top of the cross-arm, and the bolt passes from the top of the casting down through it and the cross-arm. Threads are cut on the lower end of the bolt, and a nut and washer secure it in the cross-arm. The total height of this pin above the cross-arm is 9-1/4 inches.