Machinery's Handbook, 31st Edition
2086
WHITWORTH THREADS WHITWORTH THREADS
British Standard Whitworth (BSW) and British Standard Fine (BSF) Threads The BSW is the Coarse Thread series and the BSF is the Fine Thread series of British Standard 84:1956—Parallel Screw Threads of Whitworth Form. The dimensions given in the tables on the following pages for the major, effective, and minor diameters are, respec tively, the maximum limits of these diameters for bolts and the minimum limits for nuts. Formulas for the tolerances on these diameters are given in the table below. Whitworth Standard Thread Form.— This thread form is used for the British Standard Whitworth (BSW) and British Standard Fine (BSF) screw threads. More recently, both threads have been known as parallel screw threads of Whitworth form. With standardiza tion of the Unified thread, the Whitworth thread form is expected to be used only for replacements or spare parts. Tables of British Standard Parallel Screw Threads of Whit worth Form will be found on the following pages; tolerance formulas are given in the table below. The form of the thread is shown by the diagram. If p = pitch, d = depth of thread, r = radius at crest and root, and n = number of threads per inch, then . . . . cot d p p n r p n 3 1 27 30 0 640327 0 640327 = = ° ′ 0 137329 0 137329 # = ' ' = =
p
It is recommended that stainless steel bolts of nominal size 3 ∕ 4 inch and below should not be made to Close Class limits but rather to Medium or Free Class limits. Nominal sizes above 3 ∕ 4 inch should have maximum and minimum limits 0.001 inch smaller than the values obtained from the table.
55°
r
d
r
Tolerance Classes : Close Class bolts. Applies to screw threads requiring a fine snug fit, and should be used only for special work where refined accuracy of pitch and thread form are particularly required. Medium Class bolts and nuts. Applies to the better class of ordinary interchangeable screw threads. Free Class bolts. Applies to the majority of bolts of ordinary commercial quality. Normal Class nuts. Applies to ordinary commercial quality nuts; this class is intended for use with Medium or Free Class bolts. Table 1. Tolerance Formulas for BSW and BSF Threads Class or Fit Tolerance in inches a (+ for nuts, − for bolts) Major Dia. Effective Dia. Minor Dia.
. T 3 2 001 + . T . T 2 3 001 +
. T 3 2 0013 +
p
p
Close
2 ∕
3 T
Bolts
p 001 +
. T
p 002 +
Medium
T
. T 2 3 002 +
p
p
Free
3 ∕ 2 ∕
2 T 3 T
0.2 p + 0.004 b 0.2 p + 0.005 c 0.2 p + 0.007 d
Close
… … …
Nuts
Medium Normal
}{
T
3 ∕
2 T
a The symbol , where D = major diameter of thread in inches; L = length of engagement in inches; p = pitch in inches. The symbol p signifies pitch. . 0002 . 0003 0005 . T D L p 3 = + +
b For 26 threads per inch and finer. c For 24 and 22 threads per inch. d For 20 threads per inch and coarser.
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