NPT Thread Depth

[HR ShoveNstuff]

I've had several issues with the depth of internal NPT threads. The main issue I have seen is not having enough internal threads, and the external threaded member "running out" of threads causing galling. Primarily an issue if the threads are "single pointed" on a lathe or thread milled. I've never had issues with tapped threads, as the taps are long enough that if you gage correctly the tap construction takes care of the depth.

I've seen some replies to threads stating to not specify, and referencing NPT or ASME B1.20.1 is enough to properly specify internal thread depth. I agree, however in my designs I prefer not to leave that up to chance. Also, I'm usually the person called when there is an issue, so prefer to avoid the issue altogether and specify depth.

Also, plug gauges do not work to make sure there are enough internal threads. Plug gauges only go to the L1 plane.

I use this forum fairly frequently to look for answered questions when I'm stumped, so thought I would contribute (sorta, ha). I frequently have arguments with machinists/machine shops so am looking for community reinforcement of my interpretation of the NPT (ASME B1.20.1) standard.

I've seen several threads on this forum point to L1 (depth of hand tight plane) + L3 (wrench make-up) as the appropriate depth for internal threads. I agree, however this is the nominal depth of the internal threads. You have to also account for the gaging tolerance (plus or minus one turn for both thread elements) or L1 + L3 + 2 turns. Just so happens that this is only slightly less than the L4 dimension on all that I have checked, so I now just use L4 an call the difference the tolerance.

Tried to respond to the threads I've found, but all I could find were closed, so making a new one. example: thread1103-219294, thread404-138915, or thread1103-170383. Also, most didn't bottom-out the issue to my satisfaction, so here we are.

Any thoughts?

 

[CheckerHater]

The simple "practical" recommendation not going deep into B1.20 looks like this:

https://files.engineering.com/getfi...-77e0-4867-916a-9f9cb640cef9&file=Capture.PNG

But you probably need more then that.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Wuzhee]

What are the unquestionable benefits of female tapered threads? I can't think of one...

Never had an issue with male tapered and female straight. Is it a code/customer requirement?

 

[CheckerHater]

Wuzhee,

If you stick in enough Teflon tape you can get away even with strait to strait. BUT for proper "dryseal" you may need tapered to tapered.

I don't know neither OP's or yours thread application, so you may be both right.

Everyone's application is different. National Pipe standard even includes little known thread series for building guarding and handrails.


"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[HR ShoveNstuff]

CheckerHater,
Thank you for that response & reference. I compared for the sizes I normally use (1/4 & 1/2), it looks like that table uses the L1+L3. I used to do that until I had issues with a batch of parts, internally threaded part gauged a little big, and external threaded part gauged a little small (pressure transducers)(both within tolerance), had all kinds of galling issues, needless to say boss wasn't too happy about the couple of scrapped transducers and having to rework the entire batch of internally threaded parts. My assessment at the time was that the L1+L3 depth was nominal and you need to consider the gauging tolerance stack potential.

Wuzhee,
Thank you for your response. Interesting, I've never heard of doing that. I would presume the leak tight pressure of such a joint must be lower than a taper/taper connection. That prompted me to read into B1.20.1 a little more and that appears to be a practice considered in the standard. I typically work with high pressures (3,000 - 10,000psi) so am reluctant to try. Sounds like you have done that, curious what pressures were your successful applications?

 

[CheckerHater]

HR,
I've seen different recommendations, going like this:
[tt]Thread: 1/4; 3/8; 1/2
Hole depth, mm: 20; 21; 26.5
Thread depth
including run-off, mm: 15; 16 ; 21[/tt]

W,
From experience, high-pressure applications, pneumatic and, especially, hydraulic require tapered-to-tapered

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Wuzhee]

what pressures were your successful applications?

10 bar and lower. Pneumatic cylinders and stuff in assembly lines. We don't have problems with R and G combined.