SCREW THREADS. At a meeting of the American Institute of Mining Engineers, held at Chattanooga in 1885, Major William R. King read a paper on the subject of screw threads, in which he took the ground that the ordinary thread was cut too deep into the iron, and, consequently. the bolt was weakened more than was necessary, and he proposed to remedy the evil by increasing the number of threads per inch, thereby reducing the depth of the thread.
Mr. John L. Gill read a paper before the Franklin Institute, in November, 1887, in which he advocated a thread formed part square and part V, The form of this thread is shown in comparison with the Whitworth, the Sellers, and the old V threads, in the cut, Fig. 1. He found that a thread might be made in this way in which the altitude was not dependent upon the pitch of the thread, and that the altitude could be made in proportion to the diameter of the bolt. Making the altitude 1-6 of an in. high for each i of an in. in diameter, would reduce the cross-section of the bolt uniformly 1535 per cent. on all sizes. On this basis Mr. Gill made a table of sizes from in. to 6 in. in diameter, without reference to the pitch of the threads, and then made a diagram to determine the pitch and the angle of the receding side. He used the same number of threads on the smaller sizes as the Sellers, but on some sizes a different number.
The resisting side of the thread is made at an angle of 90° to the axis, and the receding side at an angle of 45°, the top and bottom of the threads parallel to the axis of the bolt. The flat surface is found by subtracting the altitude from the pitch, and dividing the remainder by two. The iron was of a very good quality, having a breaking strength of over 53,000 lbs. per square inch.
The following table shows the size proposed by Mr. Gill : Mr. Gill made settle tests to determine the strength of bolts made with his thread, as com pared with bolts of the same iron with the Sellers' thread, with an elastic limit of from 63 to 68 per cent. of the breaking load. It was very ductile, the elongations averaging over 21 per cent. in 10 in. The nuts were from common stock and were excellent. as not one of them showed any tendency to give way in the thread. Six specimens of each size, 1-in., and 1-in., all 20 in. long, were testea to determine the quality of the iron. Six specimens of bolts of each size. 4-in., and 1-in., having the Sellers thread, and six specimens of each size, 4-in., and 1-in., having the new thread, were also tested. An abstract of the results is shown forth in the following table : Experiments were made to determine how thin a nut would have to be before the thread would strip. On a 1-in, bolt having the Sellers thread, a nut the thickness of of the eter was found as likely to strip the thread on the bolt as to break the bolt. The thread will never strip in the nut if of a good quality, as the circumference at th3 bottom of the thread on the bolt is much less than the circumference of the thread at the base inside of the nut. On the Gill bolt a nut was required to be as thick as of the diameter. At that thickness of the nut the bolt both broke and stripped, while at the bolt broke, and at -85 the thread stripped ; so if the nuts are made of the same thickness as the diameter of the bolt, there will be a margin of 11 per cent. in favor of the bolt breaking instead of stripping.
Scotching : see Rope-making Machine.