Threads and Threading - Sportpilot.info

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