Machinery's Handbook, 31st Edition
2056 ACME SCREW THREADS Limiting Dimensions: Limiting dimensions of General Purpose Acme screw threads in the recommended series are given in Table 2b and Table 2c. These limits are based on the formulas in Table 2a. For combinations of pitch and diameter other than those in the recommended series, the formulas in Table 2a and the data in Table 4 and Table 5 make it possible to readily determine the limiting dimensions required. A diagram showing the disposition of allowances, tolerances, and crest clearances for General Purpose Acme threads appears on page 2055. Stress Area of General Purpose Acme Threads: For computing the tensile strength of the thread section, the minimum stress area based on the mean of the minimum pitch diameter d 2 min. and the minimum minor diameter d 1 min. of the external thread is used: . . . min min d d 31416 4 Stress Area 2 1 2 = + a k where d 2 min. and d 1 min. may be computed by Formulas 4 and 6, Table 2a or taken from Table 2b or Table 2c. Shear Area of General Purpose Acme Threads: For computing the shear area per inch length of engagement of the external thread, the maximum minor diameter of the internal thread D 1 max., and the minimum pitch diameter of the external thread d 2 min., Table 2b or Table 2c, or Formulas 12 and 4, Table 2a, are used: . . . . . max tan min max D n d D 31416 05 14 2 1 Shear Area 1 2 1 ° = + − ^ h : D Acme Thread Abbreviations.— The following abbreviations are recommended for use on drawings and in specifications, and on tools and gages: ACME = Acme threads G = General Purpose C = Centralizing P = pitch L = lead LH = left hand Designation of General Purpose Acme Threads.— The examples listed below are given here to show how General Purpose Acme threads are designated on drawings and tools: 1.750-4 ACME-2G indicates a General Purpose Class 2G Acme thread of 1.750-inch major diameter, 4 threads per inch, single thread, right hand. The same thread, but left hand, is designated 1.750-4 ACME-2G-LH. 2.875-0.4 P -0.8 L -ACME-3G indicates a General Purpose Class 3G Acme thread of 2.875-inch major diameter, pitch 0.4 inch, lead 0.8 inch, double thread, right hand. Multiple Start Acme Threads.— The tabulated diameter-pitch data with allowances and tolerances relate to single-start threads. These data, as tabulated, may be and often are used for two-start Class 2G threads but this usage generally requires reduction of the full working tolerances to provide a greater allowance or clearance zone between the mating threads to assure satisfactory assembly. When the class of thread requires smaller working tolerances than the 2G class or when threads with 3, 4, or more starts are required, some additional allowances or increased tolerances or both may be needed to ensure adequate working tolerances and satisfactory assembly of mating parts. It is suggested that the allowances shown in Table 4 be used for all external threads and that allowances be applied to internal threads in the following ratios: for two-start threads, 50 percent of the allowances shown in the Class 2G, 3G and 4G columns of Table 4 ; for three-start threads, 75 percent of these allowances; and for four-start threads, 100 percent of these same values.
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