UNR designates an external Unified thread that must have a rounded root contour, the shape thread rolling naturally produces. The radius lowers the stress concentration at the root and improves fatigue strength, and UNR bolts mate with ordinary UN internal threads. UNJ goes further, with a larger, controlled root radius and an internal thread whose minor diameter is enlarged to clear it, and is common in aerospace.
Cert says 1/2-13 UNRC-2A, that just means a rolled thread with a round root, it takes the same nut.
Also heard as
- UNRC
- UNRF
Related terms
Thread rolling
Forming threads by squeezing a blank between hardened dies, displacing metal instead of cutting it away.
Fatigue
Cracking that grows a little with every load cycle until a bolt breaks far below its static strength.
UNC
Unified National Coarse: the standard coarse inch thread series, such as 1/4-20, 3/8-16 and 1/2-13.
Flank
Either sloping side face of a screw thread, the surface that actually carries load between bolt and nut.