Iron pins are either screwed or cemented into their insulators, but the cemented joint is much more desirable, because where a screw joint is made the unequal expansion of the iron and the glass or porcelain is apt to result in breakage of the insulator. Where cement is used, both the pins and insulators should be threaded or provided with shoulders of some sort, so that, although the shoulders of threads do not come into contact with each other, they will, nevertheless, help to secure a better hold. Pure Portland cement, mixed with water to a thick liquid, has been used with success, the insulator being placed upside down and the pin held in a central position in the hole of the insulator while the cement is poured in. Another cement that has been used for the same purpose is a mixture of litharge and glycerin. Melted sulphur is also available.
The same forces that tend to lift an insulator from its pin tend also to pull the pin from its socket in the cross-arm or pole top. With wooden pins the time-honored custom has been to drive a nail into the side of the cross-arm so that it enters the shank of the pin in its socket. This plan is good enough so far as immediate mechanical strength is concerned, but is not desirable, because it is hard to remove a nail when a pin is to be removed, and also because the rust of the nail rots the wood. A better plan is to have a small hole entirely through each cross-arm and insulator pin at right angles to the shank of that pin in its socket, and then to drive a small wooden pin entirely through from side to side.
Some of the important factors affecting the strains on insulator pins vary much on different transmission lines, as may be seen from the following table of lines on which wooden pins are used. On the older line between Niagara Falls and Buffalo, the regular length of span is 70 feet, and each copper conductor of 350,000 circular mils is attached to its insulator 7.5 inches above the cross-arm. On the new line the length of span is 140 feet, and each aluminum conductor of 500,000 circular mils is attached to its insulator 10 inches above the cross-arm.
+----------------------------+-------------+-----------+-------------+ | |Circular Mils|Feet Length| Inches from | |Location of the Lines. | of Each | of Span | Wire to | | | Conductor. | Between |Shank of Pin.| | | | Poles. | | +----------------------------+-------------+-----------+-------------+ |Colgate to Oakland | [B]133,100 | ... | 13 | |Electra to San Francisco | [A]471,034 | 130 | 15 | |Cañon Ferry to Butte | [B]105,600 | 110 | 13-1/2 | |Shawinigan Falls to Montreal| [A]183,750 | 100 | 16-1/4 | |Niagara Falls to Buffalo | [B]350,000 | 70 | 7-1/2 | |Niagara Falls to Buffalo | [A]500,000 | 140 | 10 | |Chambly to Montreal | [B]133,100 | 90 | 8-1/2 | |Colgate to Oakland | [A]211,600 | ... | 13 | +----------------------------+-------------+-----------+-------------+ [A] Aluminum conductors. [B] Copper conductors.