Straight pipe thread

[EngJW]

I found some old drawings that call out a straight pipe thread where a pipe plug goes into the end of an oil gallery. The pipe plug is a standard part. In 30 years of calling out NPT or NPTF threads, I have never seen this kind of thing. There is a brief mention of them in Machinery's Handbook, but not much about what they are used for. Why would anyone want to use a straight pipe thread, especially when liquid under pressure has to be sealed?

 

[dgallup]

They are easier to make since being straight you don't have to control the depth of tapping.

"These joints are recommended for comparatively low pressures only."

 

[11echo]

I've used N.P.S. threads a number of times! This is what we used in a large engineering firm for adjustable field supports. We would weld a dummy leg on a pipeline elbow with the dummy leg end threaded with N.P.S. for about 6" (this support is in the vertical just to be clear ...hopefully). Then you'd take an electrical conduit coupling of the same size (it is straight threaded as well – this is the normal conduit thread) weld a steel base plate with the coupling centered on the plate, now you thread on a conduit lock nut and then this base plate assembly. Now you can adjust the base plate assembly to come in contact with floor or poured foundation and lock it in place with the conduit lock nut.
"IF" you used tapered threads you won't be able to adjust the height of this support because it would tighten as you decreased the length. Clear as mud? *G*
FYI ...British Standard Parallel threads are the same, the sealing is done at the end of the pipe as apposed to the sides as with the tapered thread.

 

[EngJW]

Mr. Echo, I think you hit on the key to this thing when you said you can adjust the straight thread but not the tapered. Thanks!

 

[dgallup]

Straight on straight does not make a seal. Straight on straight is just like any other threaded fastener. Not what you would use in an oil gallery.

 

[EngJW]

dgallup, that's the thing that puzzles me. They use NPTF all over this part except for two places that are NPSF. Of course, the people who made the original drawing are long gone. And if someone wants a straight thread, why chose pipe instead of UNC or UNF? I even wonder if NPTF is over-kill, since standard procedure here is to put sealant on the threads.