||The Unified national Coarse Thread
was introduced in the mid nineteenth century by Whitmore and
is still used today. In fact, within the past twenty years there
has been a popularity shift to UNC, which analysts say is due
to the need to simplify the diameter/pitch group combinations.
||The Unified National Fine thread is commonly
used for high strength applications, having less thread depth
and larger minor diameters than UNC threads. Less wall thickness
is needed for internal threads. External threads of the UNF
series are stronger in tension and have higher torsional and
transverse shear strengths. One disadvantage is that UNF threads
have a tendency to loosen due to their smaller lead angle.
||Unified National threads require that the
root of the threads have a radius. This call-out was used more
in the past since today all fasteners classified as Unified
National threads are manufactured with a root radius if they
are smaller than one inch in diameter. Use UNR callout only
on diameters larger than one inch.
||The Unified National Extra Fine series is
used for threaded parts and equipment requiring fine adjustments
and for thin wall nuts and tubes.
||Unified National "J" formed threads
are characteristically known by the controlled root radius and
increased minor diameter. They offer superior fatigue strength
without sacrificing static strength. Thread "fit"
is 3A and 3B used for critical applications such as aerospace.