The 21⁄4-inch diameter of shank in the pins on the new Niagara line goes with a length of only 73⁄4 inches in the stem. The new Niagara pin is thus almost exactly twice as strong as the proposed pin, since the strength of a pin where the shank joins the stem varies inversely as the length of the stem, all other factors being the same.
Pins on the Shawinigan Falls line have a shank 23⁄4 inches in diameter, with a length of 131⁄2 inches in the stem; but the largest of the proposed pins, that with a stem 19 inches long, has a diameter of only 21⁄2 inches in the shank.
It is hardly too much to say in the interest of good engineering that the wooden pin of about 5 inches length of stem and 11⁄2 inches diameter of shank, as well as all longer pins of no greater strength, should be discarded for long transmission lines of high voltage. These pins have done good service on telegraph and telephone lines, and on local lighting circuits of No. 6 B. & S. gauge wire or smaller, and they may well be left for such work.
To meet the conditions of transmission work a change in both the shape and size of pins is necessary. In the first place, the shoulder on pins where the shank and stem meet, that relic of telegraph practice, should be entirely discarded. This change will save considerable lumber on pins of a given diameter at the shank, and will increase the strength of the pin by avoiding the sharp corner at the junction of the shank and stem.
Another change of design should leave an excess of strength in the stem of the pin, to provide for deterioration of the wood, and particularly for charring by current breakage. This increase of diameter and strength near the top of the pin will cost nothing in lumber, for the wood is necessarily there unless it is turned off. The shank of each pin should be proportionately shorter than in the older type, and the pin hole should be bored only part way through the cross-arm. A saving in lumber for pins and for cross-arms will thus be made, since the size of the cross-arm may be less for a given resistance to splitting.
With these changes in general design the pin is a simple cylinder in the shank, with a gentle taper from the shank to form the stem. An example of this design, which might well serve as a basis for a line of standard pins, would be a pin 2 inches in diameter and 31⁄2 inches long in the shank, and tapering for a length of 5 inches from the shank to form the stem, with a diameter of 11⁄2 inches at the top. The hole in a cross-arm for this pin should be 31⁄2 inches deep, and this, in an arm 43⁄4 inches deep, would leave 11⁄4 inches of wood below the pin. From the lower end of the pin hole, a hole 1⁄4-inch in diameter should run to the bottom of the cross-arm to drain off water. A line of longer pins designed to resist the same line pull as this short one would be strong enough for small conductors, say up to No. 1 B. & S. gauge wire.
For larger wires, long spans and sharp angles in a line, a pin 21⁄4 inches in diameter and 41⁄2 inches long in the shank, tapering for 5 inches to a diameter of 13⁄4 inches at the top, or longer pins of equal strength, should be used.
Where the pin holes do not extend through the cross-arm there is no need of a shoulder on the pin to sustain the weight of the line wire. In the cross-arm on the new Niagara Falls line each pin hole is bored to a depth of 5 inches, leaving 1 inch of wood below the hole. On the line from Electra to San Francisco the depth of each pin hole is again 5 inches, and the depth of the cross-arm 6 inches.
The pins for use on the Electra line were kept for several hours in a vat of linseed oil at a temperature of 210° F. The pins for the Shawinigan line were boiled in stearic acid. All wooden pins should be treated chemically, but the object of this treatment should be to prevent decay rather than to give them any particular insulating value.
The lack of strength in wooden pins and their destruction in some cases by current leakage are leading to the use of iron and steel pins. Such a pin, in use on the lines of the Washington Power Company, of Spokane, Wash., is made up of a mild steel bar 171⁄2 inches long and 11⁄8 inches in diameter, cast into a shank at one end, so that the total length is 18 inches. The cast-iron shank has a diameter of 21⁄16 inches, with a shoulder of 21⁄2 inches diameter at its upper end. To prevent the pin from lifting out of its hole a small screw enters the top of the cross-arm and bears on the top end of the shank. Above the cast-iron shank the length of the steel rod is 12 inches, and starting 3⁄4 inch down from its top a portion of the rod 3⁄4 inch long is turned to a diameter of one inch.