UNIFIED SCREW THREADS
American Standard for Unified Screw Threads
American Standard B1.1-1949 was the first American standard to cover those Unified
Thread Series agreed upon by the United Kingdom, Canada, and the United States to
obtain screw thread interchangeability among these three nations. These Unified threads
are now the basic American standard for fastening types of screw threads. In relation to
previous American practice, Unified threads have substantially the same thread form and
are mechanically interchangeable with the former American National threads of the same
diameter and pitch.
The principal differences between the two systems lie in: 1) application of allowances;
2) variation of tolerances with size; 3) difference in amount of pitch diameter tolerance on
external and internal threads; and 4) differences in thread designation.
In the Unified system an allowance is provided on both the Classes 1A and 2A external
threads whereas in the American National system only the Class I external thread has an
allowance. Also, in the Unified system, the pitch diameter tolerance of an internal thread is
30 per cent greater than that of the external thread, whereas they are equal in the American
National system.
Revised Standard.—The revised screw thread standard ANSI/ASME B1.1-1989
(R2001) is much the same as that of ANSI B1.1-1982. The latest symbols in accordance
with ANSI/ASME B1.7M-1984 (R2001) Nomenclature, are used. Acceptability criteria
are described in ANSI/ASME B1.3M-1992 (R2001), Screw Thread Gaging Systems for
Dimensional Acceptability, Inch or Metric Screw Threads (UN, UNR, UNJ, M, and MJ).
Where the letters U, A or B do not appear in the thread designations, the threads conform
to the outdated American National screw threads.
Advantages of Unified Threads.—The Unified standard is designed to correct certain
production difficulties resulting from the former standard. Often, under the old system, the
tolerances of the product were practically absorbed by the combined tool and gage tolerances,
leaving little for a working tolerance in manufacture. Somewhat greater tolerances
are now provided for nut threads. As contrasted with the old “classes of fit” 1, 2, and 3, for
each of which the pitch diameter tolerance on the external and internal threads were equal,
the Classes 1B, 2B, and 3B (internal) threads in the new standard have, respectively, a 30
per cent larger pitch diameter tolerance than the 1A, 2A, and 3A (external) threads. Relatively
more tolerance is provided for fine threads than for coarse threads of the same pitch.
Where previous tolerances were more liberal than required, they were reduced.
UN External Screw Threads: A flat root contour is specified, but it is necessary to provide
for some threading tool crest wear, hence a rounded root contour cleared beyond the
0.25P flat width of the Basic Profile is optional.
UNR External Screw Threads: To reduce the rate of threading tool crest wear and to
improve fatigue strength of a flat root thread, the Design Profile of the UNR thread has a
smooth, continuous, non-reversing contour with a radius of curvature not less than 0.108P
at any point and blends tangentially into the flanks and any straight segment. At the maximum
material condition, the point of tangency is specified to be at a distance not less than
0.625H (where H is the height of a sharp V-thread) below the basic major diameter.
UN and UNR External Screw Threads: The Design Profiles of both UN and UNR external
screw threads have flat crests. However, in practice, product threads are produced with
partially or completely rounded crests.
UN Internal Screw Thread: In practice it is necessary to provide for some threading tool
crest wear, therefore the root of the Design Profile is rounded and cleared beyond the
0.125P flat width of the Basic Profile.There is no internal UNR screw thread.
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