Tue Mar 8 2011

AN Threads

One of the important facets of the mil-spec I mentioned yesterday is that the threads are UNJF, Class 3. The J in the designation is arguably the most important factor that differentiates true AN fittings from 37 degree JIC fittings. It includes a modified (larger) root radius to reduce stress the stress concentration. It has been said that the UNJF threads are also rolled (rather than cut), although I was unable to find a reference to that or any manufacturing method in MIL-S-8879. Maybe it's in the the superceding document AS8879...which I couldn't find on the net.

Here you can see the thread profile from the ACS check valve. While I don't have any way of measuring it, I don't believe these are UNJF threads. This fitting is an AN-6 which uses 9/16-18 threads, so they should have a root radius of .0083 to .0100. For the AN-4 threaded connection I used on the end of the check valve, the root radius should be between .0075 and .009 inch. That might be possible to do on a lathe, but I doubt it's feasible to have the threads rolled, however.

Compare the previous threads to these on a real AN fitting. Notice the nice blended radius at the root of the threads? This will greatly reduce the stress concentration at the root of these threads. One other notable difference between the AN and JIC fittings is the thread class. JIC fittings use a class 2A thread, whereas AN fittings use a class 3A thread. If memory serves, that basically means the AN fittings are held to tighter tolerances and will have less clearance (slop) between the male and female threaded parts. The last point about the UNJF threads is that since the thread root has a larger radius, that means the thread isn't cut as deeply, thus leaving more material. i.e. the minor diameter is larger. Another factor which increases the strength of these fittings.