UNR Screw Thread Form

The UNR thread was developed for fatigue resistant safety critical fasteners during the 1950s.

The UNR thread form applies only to external threads. Its design form is the same as that of the external UN thread except that the root is required to be rounded. UNR threads are applied most often to high volume commercial fastener threads produced by rolling

Per ANSI/ASME B1.12003 – paragraph 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.”

Thread Designations:

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 = Unified 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 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

External UNR threads per ANSI/ASME B1.1

Paragraph 1.3: UNR threads have "a rounded root contour" at the minor diameter.

 

Paragraph 2.3.1(a): "The root contour ... shall have a smooth, continuous, non-reversing contour with a radius of curvature not less than 0.10825318P 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.”

 

For internal threads there is no UNR requirement. Use a standard UN thread plug gage.

THREAD RING GAGES:

The difference between a UNR and UN thread ring gage is marking only. The thread ring gages are identical. Although Thread Check Inc. can mark thread ring gages with the R designation, it is not a standard requirement. Use the same ring gages to check either the UNR or UN screw thread form. The amount of radius at the root of the UNR thread form is too small to require a modification of the standard UN thread form ring gages.

 

UNR Inspection Method for External Root Radius:

  • Optical Comparator or Vision System
  • Tri Roll thread comparator with minor diameter type 7 rolls ( 55° included angle ). The Tri Roll thread comparator may be more practical in a high volume production environment

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