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1826 ACME SCREW THREADS ANSI General Purpose Acme Thread Form ASME/ANSI B1.5-1997, and Stub Acme Screw Thread Form ASME/ANSI B1.8-1988 (R2001) Fig. 1. General Purpose and Stub Acme Thread Forms Formulas for Basic Dimensions of General Purpose and Stub Acme Screw Threads General Purpose Stub Acme Threads Pitch = P = 1 ÷ No. threads per inch, n Pitch = P = 1 ÷ No. threads per inch, n Basic thread height h = 0.5P Basic thread height h = 0.3P Basic thread thickness t = 0.5P Basic thread thickness t = 0.5P Basic flat at crest F cn = 0.3707P (internal thread) Basic flat at crest F cs = 0.3707P − 0.259 × (pitch dia. allowance on ext. thd.) F rn = 0.3707P − 0.259 × (major dia. allowance on internal thread) F rs = 0.3707P − 0.259 × (minor dia. allowance on ext. thread − pitch dia. allowance on ext. thread) Machinery's Handbook 27th Edition Basic flat at crest F cn = 0.4224P (internal thread) Basic flat at crest F cs = 0.4224P − 0.259 × (pitch dia. allowance on ext. thread) F rn = 0.4224P − 0.259 × (major dia. allowance on internal thread) F rs = 0.4224P − 0.259 × (minor dia. allowance on ext. thread − pitch dia. allowance on ext. thread) Pitch Diameter Allowances and Tolerances: Allowances on the pitch diameter of General Purpose Acme threads are given in Table 4. Pitch diameter tolerances are given in Table 5. The ratios of the pitch diameter tolerances of Classes 2G, 3G, and 4G, General Purpose threads are 3.0, 1.4, and 1, respectively. An increase of 10 per cent in the allowance is recommended for each inch, or fraction thereof, that the length of engagement exceeds two diameters. Application of Tolerances: The tolerances specified are designed to ensure interchangeability and maintain a high grade of product. The tolerances on diameters of the internal thread are plus, being applied from minimum sizes to above the minimum sizes. The tolerances on diameters of the external thread are minus, being applied from the maximum sizes to below the maximum sizes. The pitch diameter (or thread thickness) tolerances for an external or internal thread of a given class are the same. The thread thickness tolerance is 0.259 times the pitch diameter tolerance. Copyright 2004, Industrial Press, Inc., New York, NY
Machinery's Handbook 27th Edition ACME SCREW THREADS 1827 Limiting Dimensions: Limiting dimensions of General Purpose Acme screw threads in the recommended series are given in Table 2b. 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 Tables 4 and 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 1826. 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 max. of the external thread is used: Stress Area 3.1416 where d2 min. and d1 max. may be computed by Formulas 4 and 6, Table 2a or taken from Table 2b. 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 D1 max., and the minimum pitch diameter of the external thread D2 min., Table 2b or Formulas 12 and 4, Table 2a, are used: d2 min. d1 max. + ⎛------------------------------------------- ⎞ ⎝ 4 ⎠ 2 = Shear Area 3.1416D1 max. 0.5 n 141 = [ + tan ⁄ 2 ° ( D2 min. – D1 max. ) ] 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. 2.875-0.4P-0.8L-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 per cent of the allowances shown in the Class 2G, 3G and 4G columns of Table 4; for Copyright 2004, Industrial Press, Inc., New York, NY
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