Machinery's Handbook, 31st Edition
BUTTRESS THREADS
2081
Table 3. American National Standard Buttress Inch Screw Thread Symbols and Form Thread Element Max. Material (Basic) Min. Material
Pitch
p
Height of sharp-V thread
H = 0.89064 p
= 0.6 p
Basic height of thread engagement
h
Root radius (theoretical)(see footnote a ) r
= 0.07141 p = 0.0826 p = 0.0826 p = 0.0928 p
Min. r Min. s Min. s Min. S
= 0.0357 p
Root truncation
= 0.5; Max. s = 0.0413 p = 0.5; Max. s = 0.0413 p
s s
Root truncation for flat root form Flat width for flat root form
= 0.0464 p
S
Allowance
(see text)
G
Min. h e
Height of thread engagement
h e = h − 0.5 G
= Max. h e − [0.5 tol. on major dia. external thread + 0.5 tol. on minor dia. internal thread].
Crest truncation
= 0.14532 p
f
Crest width
F = 0.16316 p
Major diameter
D
D n = D + 0.12542 p
Max. D n
= Max. pitch dia.of internal thread + 0.80803 p
Major diameter of internal thread
Min. D s
Major diameter of external thread
D s = D − G
= D − G − D tol.
Pitch diameter
E
Max. E n
Pitch diameter of internal thread (see footnote b ) Pitch diameter of external thread (see footnote c )
E n = D − h
= D − h + PD tol.
Min. E s
E s = D − h − G
= D − h − G − PD tol.
Minor diameter
K
Minor diameter of external thread
K s = D − 1.32542 p − G Min. K s
= Min. pitch dia. of external thread − 0.80803 p
Min. K n
Minor diameter of internal thread Height of thread of internal thread Height of thread of external thread Pitch diameter increment for lead Pitch diameter increment for 45 ° clearance flank angle Pitch diameter increment for 7 ° load flank angle
K n = D − 2 h h n = 0.66271 p h s = 0.66271 p
= D − 2 h + K tol.
Δ El
Δ E α 1
Δ E α 2
Length of engagement L e a Unless the flat root form is specified, the rounded root form of the external and internal thread shall be a continuous, smoothly blended curve within the zone defined by 0.07141 p maximum to 0.0357 p minimum radius. The resulting curve shall have no reversals or sudden angular variations, and shall be tangent to the flanks of the thread. There is, in practice, almost no chance that the rounded thread form will be achieved strictly as basically specified, that is, as a true radius. b The pitch diameter X tolerances for GO and NOT GO threaded plug gages are applied to the internal product limits for E n and Max. E n . c The pitch diameter W tolerances for GO and NOT GO threaded setting plug gages are applied to the external product limits for E s and Min. E s .
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