Specification for Standard Thread for Pins and Insulators

SPECIFICATION FOR STANDARD THREAD FOR PINS AND INSULATORS

This subject was investigated by the Power Distribution Committee, in connection with the preparation of specifications for overhead materials, in the period 1912-1915, and much data was collected, but only general conclusions were reached.

In 1915, however, this matter was assigned as a specific subject, and at the invitation of the President of the Engineering Association a Joint Committee was formed consisting of representatives from:

The American Institute of Electrical Engineers,

The American Railway Engineering Association,

The American Telephone and Telegraph Company,

The National Electric Light Association,

The Railway Signal Association,

and this Association. The Joint Committee has also had the hearty co-operation of the Porcelain Section of the Associated Manufacturers of Electrical Supplies, and here records its appreciation of such assistance.

The first insulators had smooth pin holes, and were forced upon their pins, paper, cloth or similar material often being used to secure a tight fit.

In 1865 a Yankee genius suggested that insulator and pin be threaded, and submitted to the Brookfield Glass Company a whittled threaded pin as a model. The Western Union Telegraph Company at first rejected the plan, but later accepted it, when the model was used as a pattern and two bronze pins were cast from it to be used as standards; one of these is still in the possession of the Western Union Telegraph Company, the other, in the possession of the Brookfield Glass Company, and all American telegraph and telephone line insulators are made to fit. This standard has a diameter of one inch measured at the bottom of the groove at the closed end of the insulator; there are four threads per inch, and the taper is such that the diameter increases at the rate of 1.111 inches per foot of length (almost 0.0926 in. per inch). The original thread had a flat valley 1/8 in. wide and a semi-circular ridge of 1/8 in. diameter, and the cast iron bland used in making glass insulators has the reverse of this form when new. The pin hole and thread is formed by screwing this bland into the melted glass in the mould; the glass solidifies at once and retains the exact shape of the bland but the intense heat and the friction rapidly cuts away the corners until it finally is making a thread which has ridge and valley alike.

The shape of the thread in the pin — a semi-circular groove with a land of equal width — is that which is easiest to cut on a wood lathe; the pitch gives a sturdy section; the taper however is hard to explain, the most reasonable suggestion being that it was intended to be 3/32 in. per inch (1.125 in. per foot) but that the exact measurement of the bronze master die was taken, the loss of the difference (0.014) having occurred either in the reproduction, through irregular shrinkage in the model, or by an actual error in making the latter. Whatever the facts, the 1.111 inch taper is now standard for all telephone and telegraph insulators in this country, and at the outset it was recognized that the "Western Union Thread" would have to remain the standard for such purposes.

High tension insulators abroad have all kinds of pin holes and threads; in this country, however, there are but two sizes, named for the nominal diameter of the end of the pin fitting them, and both the "One Inch" and "1-3/8 Inch" have a pitch of four threads to the inch and a common taper increasing the diameter by 1/16 inch per inch of length (0.75 inch per foot instead of 1.111 inch per foot as in the case of the Western Union Thread). In 1912 there were in use several forms of thread, those for pins being semi-circular or V grooves with lands, or portions of the original surface between grooves, of varying widths, the sum of the two being in each case 1/4 inch. The thread in the insulators also varied but was one of the forms found in glass insulators. Apparently these "standards" were originally obtained by copying the thread actually found in a glass insulator, one maker getting one made with a new bland, another one made by a bland ready to be rejected, and others at intermediate stages, all of them however proving perfectly satisfactory in service. Porcelain insulator pin holes are made as in the case of glass insulators, by a metal bland which is screwed into the plastic clay and screwed out again, but while this compression does stiffen up the clay materially there is not the "freezing" action of the comparatively cold bland on the hot glass, and there is a greater tendency of the clay to deform the hole as the bland goes out, while the smaller taper makes any deformation of greater effect than in glass. Moreover, even though the hole be left perfect in the clay, on firing there is very apt to be a change in shape, while if the hole is glazed, beads and points are likely to be formed.

Telegraph and telephone insulators almost without exception, are screwed on all-wood pins or on metal pins with wood tops or cobs; other pin type insulators may be screwed on all-wood pins, metal pins with wood cobs, or on all-metal pins, but in the latter case with very few exceptions the threads are of lead or of a spring type, the more usual method where all metal pins are used being to cement or sulphur on the insulator, directly to the pin if small, or to a cast iron or pressed steel thimble which in turn screws to the base of the pin if the insulator is large. The large and reasonably uniform demand for wood pins for telegraph service allows the makers to give more care to dimensions for the same money than in the case of other pins, which are neither standardized nor for which is there any regular market. And the further fact that practically all wooden pins are made, usually as a side issue, in little wood turning plants throughout the South, none of which has an output of any quantity, makes it next to impossible to maintain close control over the product.

In spite of all these troubles and variations in thread form, the Committee was unable to find any case where there had been trouble in the fit of wood pins and porcelain insulators where the material had been supplied by reputable dealers, the one or two cases of trouble with metal pins being due to an attempt to screw insulators on to rigid metal pins, with results that should have been foreseen.

Your Committee found that there was a growing tendency to use Lee or similar type metal pins cemented in the insulator, in place of wood pins, so that the importance of accuracy in the thread is constantly decreasing, while the manufacturers have come to what is practically a standard design. Under the circumstances it seems wise to standardize only along general lines, establishing a performance requirement with a test guage [sic] gauge rather than exact thread contours and dimension. Such a standard has been practically agreed upon, but at the writing of this report it has not had complete and final sanction of all concerned; it is hoped however that a final report and recommendation can be made on the floor of the Convention.