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.”
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:
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