Unified Screw Threads

Unified (UN) screw threads are a standard form of thread used in the United States and other countries around the world. They are widely used in a variety of applications, including fasteners, machinery, and other equipment that requires secure and reliable threaded connections. The thread standard is defined in ANSI/ASME B1.1(2019).

The UN thread system was developed to harmonize standard threads used by the United States of America, Britain, and Canada (the ABC council). The goal was to create a standard thread form that would be compatible with existing threading systems while also improving thread quality, strength, and consistency.

Internal UN Thread Form (Shown at Maximum Material Condition)

The UN thread system (like the ANSI/ASME M Profile metric thread system) is based on a 60-degree angle between the threads, which creates a triangular-shaped thread form. The threads are spaced at a constant distance, known as the pitch, which is measured in threads per inch (TPI). For example, a ΒΌ inch UNF bolt has a diameter of 0.25 inches and a thread pitch of 28 TPI. The UN profile is identical to the metric profile except for the values used for pitch and diameters.

The UN thread system is divided into several categories, including coarse thread (UNC), fine thread (UNF), and extra fine thread (UNEF) and others. Coarse thread is used for general-purpose applications, while fine and extra fine threads are used for more precise applications where a strong and reliable threaded connection is required.