Threads

Thread Nomenclature

Designations Thread Series

• UN: Basic Unified National Thread Series

• UNR: Basic Unified National Thread Series with external thread controlled root radius

• UNS: Special Unified National Thread Series

• UNRS: Special Unified National Thread Series with external thread controlled root radius

• UNC: Special National Coarse Thread Series

• UNRC: Unified National Coarse Thread Series with external thread controlled root radius

• UNF: Unified National Fine Thread Series

• UNRF: Unified National Extra Fine Thread Series with external thread controlled root radius

• UNEF: Unified National Extra Fine Thread Series

• UNREF: Unified National Extra Fine Thread Series with external thread controlled root radius

What is the difference between UNR vs. UN Screw Threads?

The UNR thread form and UN thread form are identical except that the roots of the external threads are rounded with a radius of curvature between 0.108 and 0.144 times the pitch. The rounded root improves fatigue strength compared to the UN thread form.

The UNR thread designation standard was developed during the 1950s due to the request for fatigue-resistant safety- critical fasteners. Almost all externally threaded fastener manufacturers produce UNR rolled threads rather than plain unified form threads.

ANSI/ASME B1.1 – 1989 para 2.3.1: “The rounded root (UNR thread) reduces the rate of threading tool crest wear and improves fatigue strength over that of a flat root (UN) thread.”

UNR Threads

The UNR thread is modified version of a standard UN thread. The single difference is a mandatory root radius with limits of 0.108 to 0.144 times the thread pitch. When first introduced, it was necessary to specify UNR (rounded root) threads. Today, all fasteners that are roll threaded should have a UNR thread. This is because the thread rolling dies with the rounded crests are now the standard. 

UNJ Thread

The UNJ thread is a thread form having root radius limits of 0.150 to 0.180 times the thread pitch. With these enlarged radii, minor diameters of external thread increase and intrude beyond the basic profile of the UN and UNR thread forms. Consequently, to offset the possibility of interference between mating threads, the minor diameters of the UNJ internal threads had to be increased. 3A/3B thread tolerances are the standard for UNJ threads. UNJ threads are now the standard for aerospace fasteners and have some usage in highly specialized industrial applications