Correct use of NPT (National Pipe Thread) Thread Gages

thenson

Starting to get Involved
#1
I've recently been thinking about this same questions and it does surprise me that I cannot at least find some quantifiable values that some companies use...

We have a new threaded part that we will be shipping to Mexico and at this point do not know how they will gage them or how picky they will be and we want to be proactive.

What I found out, is that our quality engineering and inspection people all agree that with this 3/4 -14 plastic NPT threaded insert, that when gaging, using about 2-3 inch pounds seems reasonable and we can agree on that amount of force. Even though this may not be an industry agreed upon value, we will at least have an internal benchmark and know how we are testing them and how we are establishing our accept/reject criteria.

But, as I have been reading some of the comments here, it occurred to me that this could also be evaluated by not using a specified amount of force, but by doing a G R&R study. If we had a set of parts that us and our customer could agree on (some good parts, some parts too small/tight, some too big/loose) and then we do a GR&R study, then we would know that our system does have the ability to descriminate between a good part and a bad part, which is the goal. Then if we would do a correlation between us and our customer, there would not need to be an actual amount of force specified.

For us geeks, that do like data, we could then take the readings for each part and measure them with a torque gage or meter.

Meaning that if the group agrees that part #1 checks at 3.75 turns, then record the actual force it takes to turn the gage in to 3.75... this way it is actually scientifically documented.

Another reason I like this suggestion is that the actual/reasonable force you want on a plastic to plastic fitting would very much depend on the size of the thread and be much different if you were talking about a cast pipe to a brass thread-curshing NPTF thread. You may end up with less than 1 inch pound on the plastic thread and look for 10 inch pounds on the cast pipe to brass thread-crushing NPTF thread...

Just my humble opinion... but if anyone has any actual scientific studies or publications related to this, I would love to see a copy. You can e-mail me at "[email protected]".

thanks and let me know what you think.
thenson
 
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Wayne

Gage Crib Worldwide
#2
Re: Torque specification for setting thread rings vs. "Finger Force" and "Snug"

Mark;
This requires its own thread.
Title: Correct use of NPT thread gages.
 

Wayne

Gage Crib Worldwide
#3
Torque specification for setting thread rings vs. "Finger Force" and "Snug"

I've recently been thinking about this same questions and it does surprise me that I cannot at least find some quantifiable values that some companies use.... thanks and let me know what you think.
thenson;

This is not the same question, but a whole different topic. The preceding posts are related to straight thread gages and the adjustment of the thread ring gages using the set plug gages.

Here you are addressing useage of NPT, which is a taper thread gage. The NPT ring gages do not get adjusted, so torque between the ring gage and the set plug gage is not an issue.

Do you have a copy of ANSI/ASME B1.20.1? Check out paragraph 8.3 Gaging Practice. It outlines the use proceedures for NPT gages. Once you read that I think you will come back with a whole new set of questions and comments.
 

thenson

Starting to get Involved
#4
Re: Torque specification for setting thread rings vs. "Finger Force" and "Snug"

Mark, thanks for you comments. I made a few changes but did start a new thread under "Inspection and Testing"... hope this was a good choice of threads.

Thread Gaging: Hand Tight Definition (post434102)

Mark, I also did read the section of the B1.20.1 but it didn't seem to help me much in regards to establishing a benchmark or standard amount of force that would distinguish between 90 Granny's 3 finger snug and Gorilla Bob's 5 finger crank method...

thenson
 

Wayne

Gage Crib Worldwide
#5
Re: Torque specification for setting thread rings vs. "Finger Force" and "Snug"

My comments will be on the new thread.
 

Wayne

Gage Crib Worldwide
#6
Re: Torque specification for setting thread rings vs. "Finger Force" and "Snug"

Do you have a copy of ANSI/ASME B1.20.1? Check out paragraph 8.3 Gaging Practice. It outlines the use procedures for NPT gages.
I ... did read the section of the B1.20.1 but it didn't seem to help me much in regards to establishing a benchmark or standard amount of force that would distinguish between 90 Granny's 3 finger snug and Gorilla Bob's 5 finger crank method...
thenson;
I appreciate your search for precision, but this question may be compared to asking how much tension to put on a tape measure when taking a length measurement.

Let me explain.

NPT per ANSI/ASME B1.20.1 is designed to be a sloppy thread; lose and forgiving by design. Designed for a pipe fitter to hand make a thread on the jobsite. NPT is designed to be sealed with joint compound, Teflon tape or some other kind of goop. The guy in the field will likely have never even seen a thread gage for this thread, yet the thread is designed to work.

A dirty little secret about the NPT thread and its associated gages is that they have huge tolerances. The reason is related first to the forgiving nature of the thread and second to the difficulty of holding a consistent tolerance in an economical fashion. If you take a NPT product thread made at one end of the product tolerance and two NPT thread gages, one at each end of the gage tolerance; one gage will pass the part, and one gage will fail the part. At this point the question of how much torque becomes irrelevant.

I have had discussions with many companies who own many copies of one size NPT gage and are finding measurement discrepancies from gage to gage. There is a solution here, but it requires a Master be purchased and all gages are calibrated to that single master.

Another issue is the statement in ANSI/ASME B1.20.1:1983; page 18; pp 8.3.1 Precautions where it states: "In gaging pipe threads it is common practice to tap or rap the part to assure proper seating of the gage in or on the product thread." Is this manditory? How hard is a tap or rap? In which direction is the tap or rap force applied? How many taps or raps are exessive?

Assuming that a torque is set, the next question is how do you quantify the visual measurement of +/- one turn from the start of the thread? Will you be using a height gage to make the measurement? You will also have to apply the requirements of pp 8.4.

Basically IMHO; when it comes to NPT thread measurement, worrying over the applied torque should be way down the list of concerns.
 
Last edited:

Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#7
Re: Correct use of NPT Thread Gages

Posts split off to new thread per request.
 
Y

YankeePapa

#8
Wayne,
I'm a little new to the finer points of NPT gages. I have a new Van Keuren plug gage and a Vermont Gage ring gage, both 2" NPT. I also have carefully read B1.20.1.

Table 2 shows L1 to be 5.01 turns to hand tight. After mating my carefully cleaned and oiled plug into the ring, my offset is 0.019" (0.22 turns) short of flush, which is within the 0.047" tolerance listed in column 15 of Table 9 (maximum offset between any working plug and any working ring). So even though it's not a great fit, the two are certainly within spec.

OK, now the following has me mystified: When counting turns from hand tight to disengagement, I only have 2.75 turns. If add in the 0.22 turns we're up to just over two full turns short of spec. Certainly some of the discrepancy is due to the partial end threads being removed from both plug and ring, per paragraph 7.1.1. But can we get a full turn of engagement from a partial thread? In examining the gages under magnification, it appears that more than just partial threads are removed.

So the what about paragraph 8.4.1, "Turns-Engagement Method of Gaging," which refers back to Table 2, col. 7 which lists the 5.01 turns? The 5.01 figure will obviously not work directly with plug and ring gages, and suitable methods would need to be developed to implement this.

The turns-engagement method is of particular interest because that's how the dispute I'm working on is currently framed.

Did I go wrong somewhere? Or do I have it right?

Thanks!
 

Wayne

Gage Crib Worldwide
#9
I'm a little new to the finer points of NPT gages. I also have carefully read B1.20.1.
I will say that you are now more educated than the average gage user, and many gage distributors (including me as I do not even own a copy of ANSI B1.20.1 :notme: ).

I have a new Van Keuren plug gage and a Vermont Gage ring gage, both 2" NPT.
The brand of gage should make no difference as they are both made to B1.20.1. :frust:

?some of the discrepancy is due to the partial end threads being removed from both plug and ring, per paragraph 7.1.1. But can we get a full turn of engagement from a partial thread? In examining the gages under magnification, it appears that more than just partial threads are removed.
The process of convolution is one which the gage makers do not tightly control. It is something which is not certified even though there is a stated requirement with tolerance. Convolution is a hand process and considered ?good enough? if all of the incomplete thread is removed. This may result in more than one full turn of thread being removed. :caution:

So what about paragraph 8.4.1, "Turns-Engagement Method of Gaging,"
If this method is employed, then the amount of convolution should be certified (good luck with that :rolleyes: ). Check with the person who is presenting this problem :argue: (I assume a customer) for a copy of their gage certification and look for the convolution certification. :whip: If they cannot prove their gage is in tolerance...

Table 2 shows L1 to be 5.01 turns to hand tight.
IMHO: The data discussing the 5.01 turns is for mathematical calculation of the L1 position only and the start point is usually physically nonexistent. :cool:

I hope this begins to help in your situation. ;) I am open to further discussion, which might require consultation with some experts I know, or the purchase of the standard and some reading on my part. :read: :nope:
 
Y

YankeePapa

#10
Thank you, Wayne. I'm not familiar with the term convolution. I've googled it and a convolution seems to mean one revolution of thread. But what does it mean in the thread gage community?

And a bigger question: Where can I find a stated requirement on convolution with tolerance? That word doesn't appear in B1.20.1.

Regarding the 5.01 turns in column 7, I think you're totally correct. There's no way that this number is the rotation from hand tight to disengagement. Since L1 is measured from the ends of the two parts, when you're starting the thread, you have something like one turn of engagement length even when no threads are engaged. So the "turns engagement" method needs a big caveat in the standard to say that its particulars need to be devised on a case-by-case basis.
 
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