NPT Jam Nuts

[emichaels]

So, NPT jam nuts exist, though they're uncommon. I've found them in a few places, McMaster Carr, for one: High-Pressure Brass Pipe Fitting, Locknut, 1/8 NPT Female | McMaster-Carr

My question is: what are they for? Wouldn't a jam nut completely negate the purpose of an NPT thread? NPT threads are made with a taper so that the threads crush together when tightened down, creating a partial seal. Wouldn't a jam nut simply go on to the male fitting, lock down, and prevent the fitting from actually tightening into the female port?

The only way these would make sense to me is if the jam nut threads are loose enough to allow the male fitting to thread completely in and get torqued down, then the nut is installed to prevent loosening. Or if the nut was installed on the far side of the sealing surface. But the latter seems unlikely to be possible for most pressure sealing applications, and if it isn't pressure sealing, why not just use a straight thread?

Has anyone used these before?

Thanks in advance!

[Kelly_Bramble]

At the bottom of the webpage you referenced was written:

"Locknuts thread onto the male end of a fitting to provide extra holding power so the fitting won’t loosen from vibration."

Which it the answer I would have given without the reference.