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Thread classes are specified to designate the
amount of tolerance allowance and installation fit desired.
Thread classes are derived from formulas which the pitch
diameter tolerances are based on increments of the major
(nominal) diameter, the pitch, and the length of engagement of
the thread. These formulas and the class identification only
apply to Unified threads series.
There are three classes of external (1A, 2A, and
3A) and internal threads (1B, 2B, and 3B). The characteristics
and typical application of each class are as follows:
Classes 1A and 1B: These classes are the
loosest fit, in that, the largest amount play or clearance in
assembly. These are used for work where ease of assembly and
disassembly is most important, such as stove bolts and other
general rough bolts and nuts.
Classes 2A and 2B: These classes are
specified for the typical fit grade of commercial products,
such as machine screws and fasteners, where interchangeability
and reasonable quality and fit predictability is required.
These classes are the most commonly used for commercial and
aerospace applications.
Class 2AG: This application is specified
to allow for rapid assembly, and to permit the
application of a lubricant or for residual growth due to
high-temperature expansion. In these applications, the G is
specified to indicate, that when the thread is coated, the
thread tolerance and size may not be exceeded by such coating.
Classes 3A and 3B: These classes are
specified for high quality commercial, precision
applications and products, such as machine screws, where a
close or snug fit is required.
Thread specification examples:
4-40 UNC-2B or .112-40UNC - 2B
1⁄4–20 UNC-2A or 0.250–20 UNC-2A
10–32 UNF-2A or 0.190–32 UNF-2A
7⁄16–20 UNRF-2A or 0.4375–20 UNRF-2A
2–12 UN-1A or 2.000–12 UN-1A
1⁄4–20 UNC-3A-LH or 0.250–20
UNC-3A-LH (LH = Left Hand)
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