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.”

Internal Screw Thread

There is no UNR for internal threads. Use only UN Thread.
ANSI/ASME B1.1 para 1.3: “UNR applies only to external threads”
ANSI/ASME B1.1 para 2.3.2: (there is no internal UNR screw thread)”

External Screw Thread

Both UNR and UN are used for external threads.

External UN threads per ANSI/ASME B1.1

Para 1.3: UN threads have “a flat or optional rounded root contour” at the minor diameter.
Para 2.3.1: UN threads have “a flat root contour” at the minor diameter, but “a rounded root contour cleared beyond the 0.25P flat width of the basic profile is optional”…”to provide for some threading tool crest wear.”

External UNR threads per ANSI/ASME B1.1

Para 1.3: UNR have “a rounded root contour” at the minor diameter.
Para 2.3.1 (a): “The root contour…shall have a smooth, continuous, non-reversing contour with a radius of curvature not less than 0.108P at any point and shall blend tangentially into the flanks and any straight segment. At the maximum-material condition, the point of tangency shall be at a distance not less than 0.625H below the basic major diameter.”

External UNR threads per IFI, Basic Elements of Screw Thread Design

Page A-3; UNR threads have a “single difference from UN threads…a mandatory root radius with limits of 0.108 to 0.144 times the thread pitch. The minimum radius of 0.108P is the largest radius that can be fitted into the UN profile without violating the minimum material condition of the external thread. The maximum radius of 0.144P is the largest radius that can be accommodated without causing theoretical interference with an internal thread at its maximum material condition.”

Screw Thread Ring Gages

It is not necessary that the gages be marked with UNR. The UNR or UN ring gages can be used to check either the UNR or UN screw thread form. Since the amount of radius at the root of the UNR thread form is too small there is no requirement for any special adaptation of the standard UN thread for ring gages.

Screw Thread Work Plug Gages

Use the standard UN Thread Work Plug as there is no internal (female) UNR thread.

How to check the root radius

  1. For low volume applications, use an optical comparator to check the UNR thread form root radius at the minor diameter of the external screw thread.
  2. For high volume applications, it is faster and more economical to use Gaging System 22 ( Tri-Roll thread Comparator ) to monitor the minor diameter.

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

For questions or comments regarding this information, contact Thread Check Inc.

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