Fig. 291a.
“A longitudinal section of the tapering tube end, with the screw-thread as actually formed, is shown full size in [Fig. 291a] for a nominal 21⁄2 inch tube, that is, a tube of about 21⁄2 inches internal diameter, and 27⁄8 inches actual external diameter.
“The thread employed has an angle of 60°; it is slightly rounded off both at the top and at the bottom, so that the height or depth of the thread, instead of being exactly equal to the pitch, is only four fifths of the pitch, or equal to 0.8 × 1/n if n be the number of threads per inch. For the length of tube end throughout which the screw thread continues perfect, the empirical formula used is (0.8D + 4.8) × 1/n, where D is the actual external diameter of the tube throughout its parallel length, and is expressed in inches. Further back, beyond the perfect threads, come two having the same taper at the bottom, but imperfect at the top. The remaining imperfect portion of the screw thread, furthest back from the extremity of the tube, is not essential in any way to this system of joint; and its imperfection is simply incidental to the process of cutting the thread at a single operation.”
The standard thicknesses of the pipes and pitches of thread are as follows:—
STANDARD DIMENSIONS OF WROUGHT IRON
WELDED TUBES.
| DIAMETER OF TUBE. | THICKNESS OF METAL. | SCREWED ENDS. | |||||
| Nominal Inside. | Actual Inside. | Actual Outside. | Number of Threads per Inch. | Length of Perfect Screw. | |||
| Inches. | Inches. | Inches. | Inch. | No. | Inch. | ||
| 1⁄8 | 0.270 | 0.405 | 0.068 | 27 | 0.19 | ||
| 1⁄4 | 0.364 | 0.540 | 0.088 | 18 | 0.29 | ||
| 3⁄8 | 0.494 | 0.675 | 0.091 | 18 | 0.30 | ||
| 1⁄2 | 0.623 | 0.840 | 0.109 | 14 | 0.39 | ||
| 3⁄4 | 0.824 | 1.050 | 0.113 | 14 | 0.40 | ||
| 1 | 1.048 | 1.315 | 0.134 | 11 | 1⁄2 | 0.51 | |
| 1 | 1⁄4 | 1.380 | 1.660 | 0.140 | 11 | 1⁄2 | 0.54 |
| 1 | 1⁄2 | 1.610 | 1.900 | 0.145 | 11 | 1⁄2 | 0.55 |
| 2 | 2.067 | 2.375 | 0.154 | 11 | 1⁄2 | 0.58 | |
| 2 | 1⁄2 | 2.468 | 2.875 | 0.204 | 8 | 0.89 | |
| 3 | 3.067 | 3.500 | 0.217 | 8 | 0.95 | ||
| 3 | 1⁄2 | 3.548 | 4.000 | 0.226 | 8 | 1.00 | |
| 4 | 4.026 | 4.500 | 0.237 | 8 | 1.05 | ||
| 4 | 1⁄2 | 4.508 | 5.000 | 0.246 | 8 | 1.10 | |
| 5 | 5.045 | 5.563 | 0.259 | 8 | 1.16 | ||
| 6 | 6.065 | 6.625 | 0.280 | 8 | 1.26 | ||
| 7 | 7.023 | 7.625 | 0.301 | 8 | 1.36 | ||
| 8 | 8.982 | 8.625 | 0.322 | 8 | 1.46 | ||
| 9 | 9.000 | 9.688 | 0.344 | 8 | 1.57 | ||
| 10 | 10.019 | 10.750 | 0.366 | 8 | 1.68 | ||
The taper of the threads is 1⁄16 inch in diameter for each inch of length or 3⁄4 inch per foot.
WHITWORTH’S SCREW THREADS FOR GAS, WATER, AND HYDRAULIC IRON PIPING.
Note.—The Internal and External diameters of Pipes, as given below, are those adopted by the firm of Messrs. James Russell & Sons, in Pipes of their manufacture.