FTM 173: How do I install Jam Nuts?

FTM 173: How do I install jam Nuts

This edition of the Fastener Training Minute with Carmen Vertullo was originally published February 16th, 2022 as “How do I install jam Nuts?” during episode 173 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 here in the beautiful El Cajon California.

As is often the case, today’s Fastener Training Minute topic comes from a question that I got from someone who came to one of our Fastener Training Institute classes. This question comes up frequently, and sadly there is no good answer to this question. However I’m going to provide a partial answer and a solution going forward. And the question is this: when using a jam nut, (you all know what a jam nut is, it looks like a regular hex nut except it is thin, or I should say thinner, say about half-height), when using a jam nut, should the jam nut go on top of the full-sized nut or underneath the full-sized nut?

Now most common sense folks would think that you would put the jam nut on after, but in our world, when we looked this up, from a technical perspective, we find out that most of the folks who think they know what they are talking about say to put the jam nut on first and then the full-height nut. This sounds very confusing, and it is very confusing. And when we come back, I will try to clear up the confusion and then ultimately propose a method by which we can have a final answer.

Well, welcome back everybody to the Fastener Training Minute, today’s topic is Jam Nuts. Now if you’ve ever looked up any technical book or manual about the proper use of jam nuts, what you probably discovered is: oh my goodness, I’ve been doing it wrong all these years. The jam nut goes on first and then the thick or the regular nut goes on top of it. Well, to me there’s a little bit of a problem with that, and here are some of the questions that we have to ask and answer to accept that strategy.

The first question is: what should the torque of the Jam Nut be when we install it? Well, theoretically it is a half-height nut, so maybe it should be half the torque of the full-height nut. If we only torque it to half the torque of the full height nut, we are obviously not going to get the full tension on the bolt. So now we are going to put the full-height nut on top of that Jam Nut, and theoretically we are going to torque it to it’s proper torque, and somehow magically its going to pull through that Jam Nut and add tension to the bolt.

I don’t think so. I don’t think that would work. And should we hold the Jam Nut from turning while we are tightening the full-height nut or let it grab the Jam Nut and turn it more? Those are questions that we have to answer.

So, once we have that all figured out, it’s obvious that we still don’t know how much tension we are putting on the bolt. So, there are two answers to this.

One answer is, well first off, Jam Nuts are not the best solution for vibration-resistant fastening. they are very popular but there are better solutions than that. We will talk about those better solutions in another Fastener Training Minute. But if that is the solution that has been chosen, we then have to come up with a strategy for the installation: one of which I just described which is the most common one. Here’s the thing – it’s probably not suitable for high-strength bolting, or any bolting that is intended to be used in it’s full tension application. That all works fine if the bolt is in shear, and we are not really concerned with tension, we are not going to fully tension the bolt anyway. So that might be the answer to the whole Jam Nut problem.

The reason this also comes up for me is that I just spent the middle of the night, literally 2 o’clock ’till 5 o’clock in the morning on web meetings with the ISO fastener standards TC-2 committee, sub-committee 12 on nuts, upon which I sit. And these are the world’s greatest experts. They are unbelievably smart people and they are all in other parts of the world, and they all think that the Jam Nut should go on second. We really didn’t solve the problem in that meeting, but we did identify it as something that needs to have some resolution.

So here is what I propose, and it would be a relatively simple resolution. We have some test equipment here because we happen to be a fastener testing laboratory and we can simply do some experiments. Let’s find out what happens to the bolt tension when we put on our Jam Nut first and our full-height nut second. Lets do some vibration testing and find out whether it is more or less effective to put the Jam Nut on under the full-height nut, or on top of the full-height nut. And I intend to do these experiments, and if anyone is interested in joining me in doing them, maybe you can provide the product and we write the nice magazine article about what we find out, and we will give you some credit for that.

Well, I know I didn’t probably solve the problem that many of you may have with the use of Jam Nuts, but at least we now know what the problem is, and it has been well defined, and we are on our way to a solution that we will be describing in a future Fastener Training Minute.

This has been Carmen Vertullo with the Fastener Training Minute, thank you for listening.

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