Machinery's Handbook, 31st Edition
ACME SCREW THREADS 2061 Table 4. American National Standard General Purpose Acme Single-Start Screw Threads—Pitch Diameter Allowances ANSI/ASME B1.5-1997 (R2014) Nominal Size Range a Allowances on External Threads b Nominal Size Range a Allowances on External Threads b
Class 2G c , . D 0008
Class 2G c , . D 0008
Class 3G, . D 0006
Class 4G, . D 0004 Above
Class 3G, . D 0006
Class 4G, . D 0004
To and Including
To and Including
Above
0
0.0024 0.0040 0.0049 0.0057 0.0063 0.0069 0.0075 0.0080 0.0085 0.0089 0.0094
0.0018 0.0030 0.0037 0.0042 0.0047 0.0052 0.0056 0.0060 0.0064 0.0067 0.0070
0.0012 0.0020 0.0024 0.0028 0.0032 0.0035 0.0037 0.0040 0.0042 0.0045 0.0047
0.0098 0.0105 0.0113 0.0120 0.0126 0.0133 0.0140 0.0150 0.0160 0.0170 0.0181
0.0073 0.0079 0.0085 0.0090 0.0095 0.0099 0.0105 0.0112 0.0120 0.0127 0.0136
0.0049 0.0052 0.0057 0.0060 0.0063 0.0066 0.0070 0.0075 0.0080 0.0085 0.0091
1 9 ∕ 16
1 7 ∕ 16 1 9 ∕ 16
3 ∕ 16 5 ∕ 16 7 ∕ 16 9 ∕ 16
1 7 ∕ 8 2 1 ∕ 8 2 3 ∕ 8 2 5 ∕ 8 2 7 ∕ 8 3 1 ∕ 4 3 3 ∕ 4 4 1 ∕ 4 4 3 ∕ 4 5 1 ∕ 2
3 ∕ 16 5 ∕ 16 7 ∕ 16 9 ∕ 16
1 7 ∕ 8 2 1 ∕ 8 2 3 ∕ 8 2 5 ∕ 8 2 7 ∕ 8 3 1 ∕ 4 3 3 ∕ 4 4 1 ∕ 4 4 3 ∕ 4
11 ∕ 16 13 ∕ 16 15 ∕ 16 1 1 ∕ 16 1 3 ∕ 16 1 5 ∕ 16 1 7 ∕ 16
11 ∕ 16 13 ∕ 16 15 ∕ 16 1 1 ∕ 16 1 3 ∕ 16 1 5 ∕ 16
All dimensions in inches. It is recommended that the sizes given in Table 3 be used whenever possible. a The values in columns for Classes 2G, 3G, and 4G are to be used for any size within the nominal size range shown. These values are calculated from the mean of the range. b An increase of 10 percent in the allowance is recommended for each inch, or fraction thereof, that the length of engagement exceeds two diameters. c Allowances for the 2G Class of thread in this table also apply to American National Standard Stub Acme threads ASME B 1.8-1988. These values will provide for a 0.25-16 ACME-2G thread size, 0.002, 0.003, and 0.004 inch additional clearance for 2-, 3-, and 4-start threads, respectively. For a 5-2 ACME-3G thread size the additional clearances would be 0.0091, 0.0136, and 0.0181 inch, respec tively. GO thread plug gages and taps would be increased by these same values. To main tain the same working tolerances on multi-start threads, the pitch diameter of the NOT GO thread plug gage would also be increased by these same values. For multi-start threads with more than four starts, it is believed that the 100 percent allowance provided by the above procedures would be adequate as index spacing variables would generally be no greater than on a four-start thread. In general, for multi-start threads of Classes 2G, 3G, and 4G the percentages would be ap - plied, usually, to allowances for the same class, respectively. However, where exceptionally good control over lead, angle, and spacing variables would produce close to theoretical values in the product, it is conceivable that these percentages could be applied to Class 3G or Class 4G allowances used on Class 2G internally threaded product. Also, these percent- ages could be applied to Class 4G allowances used on Class 3G internally threaded prod- uct. It is not advocated that any change be made in externally threaded products. Designations for gages or tools for internal threads could cover allowance requirements as follows: GO and NOT GO thread plug gages for: 2.875-0.4 P -0.8L-ACME-2G with 50 percent of the 4G internal thread allowance. Centralizing Acme Threads.— The three classes of Centralizing Acme threads in Amer ican National Standard ANSI/ASME B1.5-1997 (R2014), designated as 2C, 3C, and 4C, have limited clearance at the major diameters of internal and external threads so that a bearing at the major diameters maintains approximate alignment of the thread axis and prevents wedging on the flanks of the thread. An alternative series having centralizing control on the minor diameter is described on page 2072 . For any combination of the three classes of threads covered in this standard some end play or backlash will result. Classes 5C and 6C are not recommended for new designs.
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