UNR

The UNR thread form is identical to the UN form, except that the roots of external (male) threads are rounded with a radius of curvature between 0.108 and 0.144 times the pitch. There is no UNR standard for internal (female) threads. The required rounded root improves fatigue strength, compared to the UN form.

“The root contour...shall have a smooth, continuous, nonreversing contour with a radius of curvature of 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.”

Applying this thread form to the different Unified National thread series creates a group of thread series differing from the corresponding UN series only in the roots of the external threads. UNR bolts are used with UN nuts.

UNR, Basic Unified National Thread Series with external thread controlled root radius.

UNRS, Special Unified National Thread Series with external thread controlled root radius.

UNRC, Unified National Coarse Thread Series with external thread controlled root radius.

UNRF, Unified National Fine Thread Series with external thread controlled root radius.

UNREF, Unified National Extra Fine Thread Series with external thread controlled root radius.

ANSI/ASME B1.1-1989.

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