Threads and Threading - Sportpilot.info

1832 CENTRALIZING ACME SCREW THREADS
Table 4. American National Standard General Purpose Acme Single-Start Screw
Threads — Pitch Diameter Allowances ASME/ANSI B1.5-1988
Nominal Size
Range a Allowances on External Threads b
Machinery's Handbook 27th Edition
Nominal Size
Rangea Allowances on External Threads b
Class 2G
Above
To and
Including
c , Class 3G, Class 4G,
Class 2G
Above
To and
Including
c , Class 3G, Class 4G,
0 3
⁄16 0.0024 0.0018 0.0012 17 ⁄ 16 19 ⁄ 16 0.0098 0.0073 0.0049
3
⁄16
5
⁄16 0.0040 0.0030 0.0020 19 ⁄ 16 17 ⁄ 8 0.0105 0.0079 0.0052
5
⁄16
7
⁄16 0.0049 0.0037 0.0024 17 ⁄ 8 21 ⁄ 8 0.0113 0.0085 0.0057
7
⁄16
9
⁄16 0.0057 0.0042 0.0028 21 ⁄ 8 23 ⁄ 8 0.0120 0.0090 0.0060
9
⁄16
11
⁄16 0.0063 0.0047 0.0032 23 ⁄ 8 25 ⁄ 8 0.0126 0.0095 0.0063
11
⁄16
13
⁄16 0.0069 0.0052 0.0035 25 ⁄ 8 27 ⁄ 8 0.0133 0.0099 0.0066
13
⁄16
15
⁄16 0.0075 0.0056 0.0037 27 ⁄ 8 31 ⁄ 4 0.0140 0.0105 0.0070
15
⁄16 11 ⁄ 16 0.0080 0.0060 0.0040 31 ⁄ 4 33 ⁄ 4 0.0150 0.0112 0.0075
11 ⁄ 16 13 ⁄ 16 0.0085 0.0064 0.0042 33 ⁄ 4 41 ⁄ 4 0.0160 0.0120 0.0080
13 ⁄ 16 15 ⁄ 16 0.0089 0.0067 0.0045 41 ⁄ 4 43 ⁄ 4 0.0170 0.0127 0.0085
15 ⁄ 16 17 ⁄ 16 0.0094 0.0070 0.0047 43 ⁄ 4 51 0.008 D 0.006 D 0.004 D 0.008 D 0.006 D 0.004 D
⁄ 2 0.0181 0.0136 0.0091
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 per cent 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/ANSI B 1.8-1988.
three-start threads, 75 per cent of these allowances; and for four-start threads, 100 per cent
of these same values.
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, respectively.
GO thread plug gages and taps would be increased by these same values. To maintain
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 per cent
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
applied, 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
percentages could be applied to Class 4G allowances used on Class 3G internally threaded
product. 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: 2.875-0.4P-0.8L-ACME-2G with 50 per cent of
the 4G internal thread allowance.
Centralizing Acme Threads.—The three classes of Centralizing Acme threads in American
National Standard ASME/ANSI B1.5-1988, 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
Copyright 2004, Industrial Press, Inc., New York, NY