FTM 159: Thread Run-Out

Fastener Training Minute 159: Thread run-out

This edition of the Fastener Training Minute with Carmen Vertullo was originally published December 16, 2020 as “how do we measure thread pitch run-out during episode 159 of Fully Threaded Radio.

Well, hi everyone, this is Carmen Vertullo with the Fastener Training Minute, coming to you from the Fastener Training Institute and the AIM Testing Laboratory in beautiful El Cajon, California.

I’ve had an issue come up about three times this past year with some clients, and this last time it made me think that perhaps it’s something worth talking about on the Fastener training Minute.

And the issue has to do with the dimensional inspection of nuts, hex nuts in particular and even more specifically hex flange nuts. Now, we know that there are relatively common things we have to do to inspect a nut. We have to measure across the flats, across the corners, and the height. And of course we also have to measure the threads with a Go / No-Go gauge, and we have to use a plain pin gauge to inspect the minor diameter, but there is one criteria in particular, which is a little bit more difficult to inspect.

Even manufacturers don’t have the right capacity to do this measurement, and that is: inspecting the bearing surface of the nut for its perpendicularity to the threads, which is sometimes called “run out”. And so we had some issues with run-out on a particular flange lock nut. And when we come back. I’ll tell you what those issues were and how you can possibly avoid them.

And today we’re talking about measuring the perpendicularity of the bearing surface of a flange nut to the threads, in particular to the thread pitch diameter, which is the common call out in most inch and metric standards. We want to know how perpendicular the surface of the thread pitch diameter is to the bearing surface to the nut.

That’s important because imagine if the hole in the nut was not tapped straight, but crooked. Let’s say you begin to spin the nut down and instead of the bearing surface sitting flat, one side of the nut hits first. That would not work. You would have all kinds of torque tension issues. You might bend the stud that the nut is going on. It would damage the surface that you are tightening against and so on. So we want that surface to land all at the same time, and so to do that, we control the perpendicularity of the surface to the pitch diameter. The way that we measure that sometimes is with an inspection called run out where we install the nut on to some kind of a thing that can grab it on the pitch diameter.

Normally its going to be a thread gauge, but it could be just a threaded stud or something that we can tighten it against. We put that in a fixture where we can rotate it, and we put a gauge up against the bearing surface. It’s called a run-out gauge actually, and we rotate the assembly to see how much change there is in the position of the gauge from 0 both directions plus and minus. We call that the total indicator reading, or total run-out, and the tolerances on that measurement are not particularly tight. There are relatively manageable and also the upside is that it’s relatively easy to make a nut perfect. When nuts are made, they are tapped in a fixture that holds them very tightly. The tap goes in straight, the bearing surface of the nut is located against the surface, and it’s pretty hard to mess that up.

So most nuts that I’ve inspected are pretty much perfect with very little if any detectable run out. But every now and then we have an issue and in this particular case that I’ve seen three times, the issue had to do with all metal lock nuts. In these lock nuts the nut was pinched or crimped and in the process of crimping the nut, the bearing surface got deformed ever-so-slightly. In the case of one relatively large large metric nut, M22, the crimp was significant and the bearing surface was cupped. We could tell it was cupped because when we put it on a piece of sandpaper and sanded the bearing surface of the nut down ever so slightly, we could see the high points. These high points were exactly opposite where the crimp was. So that told us what had happened there.

Now the fact of the matter is, the chances are that in an application when when we tighten the nut down, those two high spots are going to hit first and as we develop tension or stress on the nut, they’re probably going to flatten out and work perfectly fine. However, it doesn’t inspect perfectly fine. As a matter fact, it has significant amount of run out, and the typical amount for a nut might be a few thousandths to 8/1,000 or so in the inch world (that translates into about a tenth of a millimeter maybe in the metric world). In this case we had way more than that, and we had a print that we had to follow and there was just no way around it. It was very difficult to convince the end-user that this nut was probably usable and we just needed to maybe change the criteria a little bit or do some testing to prove that this particular dimensional non-conformance did not affect the function of the part.

So just keep in mind that the nut was probably absolutely stunningly perfect up until the point where they crimped it to put the Locking feature in. So we have to be aware when we are buying or manufacturing nuts that have bearing surface perpendicularity or run out requirements, when we crimp the nut you might lose the feature’s dimensional integrity.

Well now, you know something about nut bearing surface run out. Not all standards have requirements for bearing surface run out, most of them do, but not all.
That might be a good “Get out of Jail Free” card for you, depending on what standard to use. Thanks for listening to the Fastener Training Minute.

This is Carmen Vertullo coming to you from AIM Testing Laboratory.

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