NPFC - FED-STD-H28/2
SCREW-THREAD STANDARDS FOR FEDERAL SERVICES SECTION 2 UNIFIED INCH SCREW THREADS- UN AND UNR THREAD FORMS
|Publication Date:||20 August 1991|
This section provides the standard for unified inch screw threads to be used by the Federal Services.
Only UN and UNR screw threads are covered in this section. For UNJ threads (controlled external thread rounded root with increased basic minor diameter) see FED-STD-H28/4 (MIL-S-8879). For UNM threads (miniature threads) see FED-STD-H28/5.
The UN thread is intended for general purpose fastening applications. Its external thread root may be either flat or rounded.
The UNR form applies only to external threads. It 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.
NOTE: The mandatory rounded root of the UNR thread greatly reduces the concentration of stress, hence increases the fatigue life of threaded parts.