FTM 171: fasteners and their reuseability

Once a fastener has been installed and tightened, everything is correct and we expect it to work work as advertised. If we then take it apart, when we put it back together, do we expect that fastener to behave in the same way?

This edition of the Fastener Training Minute with Carmen Vertullo was originally published December 16th, 2021 as “fasteners and their reuseability”” during episode 171 of Fully Threaded Radio.

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

I’m here fresh back from my two days teaching the Fastener Training Institute‘s Certified Fastener Specialist program in La Mirada, California.

The first two days were my part. One of the things we did there was that we talked and talked about the torque tension relationship. As you know, that’s one of my favorite topics, the great equation, T = K x D x P. We taught and demonstrated where torque is equal to a K-Factor (the secret sauce. number) times the DIAMETER of the fastener times the PRELOAD of the Fastener. And we teach that and we demonstrated that with some instrumentation.

Then later in the day, we break up into groups and each group gets to do some actual experimenting with torque wrenches and the infamous Skidmore, Wilhelm bolt action, calibrator. One of the things that happened was that there was a little bit of an error that led our experiment in a certain direction and we discovered something very interesting about Fastener reuse ability. And when I come back, I’m going to talk generally about fasteners and their reuseability, and tell you what happened with our experiment and the kind of data that we generated.

Well, welcome back everyone, this is Carmen Vertullo returning with the Fastener Training Minute and today we’re talking about fastener reuseability, and an experiment that we conducted at the Certified Fastener Specialist training program this week.

One of the things that end users and OEMs often wonder, is once a fastener has been installed and tightened (presumably correctly), everything is correct and we expect it to work work as advertised. If we then take it apart, when we put it back together, do we expect that fastener to behave in the same way? In other words, if we tightened it initially to a certain torque and achieved a certain clamp load, would be expect to be able to receive or recreate that same clamp load with the same torque? OEM’s advertise in their maintenance and repair bulletins and instructions, torque values when parts are taken apart and put back together, theoretically based on the torque values that they originally used.

The experiment that we did at the Fastener Training Institute program involved using zinc-plated nuts bolts and washers 3/8-16 diameter. And what we were expecting to do in our experiment was we were simply going to tighten them down with the zinc plating to find out what the K-Factor would be. Normally it would be .22 for zinc. the we were going to add some lubrication and see how that lubrication would affect the clamp load. Now, I have done this many many times over the years in the classroom, and in the laboratory and for customers and clients. And so I have the opinion that zinc as a K-Factor control coating is notoriously unreliable. Even though the advertised K-Factor is .22, I’ve seen it anywhere as low as .16 or as high as .25. And the reason for that generally, under controlled conditions, stuff goes sideways within the Skidmore with the torque wrench in the laboratory.

However, it’s not just the zinc that determines the K-Factor. To an even larger extent, it’s the top coating that goes on the zinc, sometimes called the passivation coating. Normally, it’s some kind of chromate; hexavalent chromium, trivalent, chromate, or perhaps some other kind of a non hexavalent chromium that’s not trivalent. Whatever the plating applicator chooses to use that meets the requirements for the salt spray test that they would achieve the correct corrosion resistance. Zinc generally does not come with a stated or guaranteed K-Factor. So whatever they use is what you get and you may or may not know.

Now fortunately for folks who use zinc as a fastener coating probably are not using that fastener in a critical application, where the K-Factor being off by a few points is going to make the wheels come off the wagon. However, many other applications for fasteners do require an accurate and consistent K-Factor and they tend to either shy away from zinc, or if they want to use zinc, they will specify a lubrication type top coat that will produce if not a low K-Factor, at least a very tightly controlled K-Factor. What happened in our experiment, after we put the first bolt in, we got to K-Factor was kind of low. I think it was around .19, and then we were supposed to remove that bolt & nut and put a fresh one in with some lubrication. But we got sidetracked in a conversation and we failed to remove it, and we ended up retightening that same bolt and nut, and we got a K-Factor of .26, drove it through the roof.

Once we figured out what happened, we decided we were going to change our experiment from what happens with the lubrication, to what happens on the second tightening. Now, we had four groups of six people, each group got to do three experiments with a first and second tightening. So that’s quite a number of experiments. And what we discovered first off was that the K-Factor on zinc on the first tightening, ranged between .19 on the low side up to .24 on the high side, about what you would expect for zinc. It really kind of met the advertised expectations. I was surprised at that. However, on the second tightening, every one of them added at least five points to the K-Factor. They went as high as .36, and most of them were over .30.

So what we discovered was that on the second tightening of the bolt, something deteriorates in the top coat that makes it so that we no longer have the same torque tension relationship. Or when we maintain that item, we should not be able to expect to get a good consistent, reliable, clamp load, compared to the original assembly. So that leads us into faster reusability. If we’re going to reuse fasteners, that’s something we need to be aware of. I did have a case once where we did the same experiment for a client. They had cadmium plated fasteners. we had to determine whether or not they should use a different maintenance torque than the original assembly torque. It turned out in that case, that cadmium did not deteriorate. It worked very well. Cadmium is a great torque tension control coating. So, we didn’t have to change anything.

But it might be that if you are producing product with zinc plated hardware, and you expect it to be maintained in the field, you may need to increase the torque for the maintenance operation, compared to the original assembly operation, to produce the same torque-tension relationship.

It was a fun experiment, and the students enjoyed it. I enjoyed it. And I will probably make a more formal version of it sometime and maybe a turn it into a magazine article.

Well, this has been Carmen Vertullo with the Fastener Training Minute, coming to you again from the Fastener Training Institute and AIM Testing Laboratory. Thanks for listening.

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