FTM 174: How tight is too tight for a Nylon Insert Locknut?

FTM 174: How tight is too tight for a Nylon Insert Locknut?

This edition of the Fastener Training Minute with Carmen Vertullo was originally published March 17th, 2022 as “How tight is too tight for a Nylon Insert Locknut?” during episode 174 of Fully Threaded Radio.

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

As is often the case, today’s Fastener Training Minute comes out of a question I received from one of our students or clients. And the question was: “how do I know how tight I need to make the nylon insert lock nut?” Or what is the torque value that I should apply on a nylon insert locknut, not just any nylon insert lock nut but a stainless steel nylon insert locknut. This is a very difficult question to answer, and it’s a very important question to know the right answer to and when we return I’ll tell you what the answer is.

Welcome back everybody. Today, we’re talking about the proper torque for stainless steel, nylon insert, lock nuts. So let’s start with the basic reason why we use torque as a tightening strategy in the first place.

The reason is because it’s easy to measure compared to a lot of other things that we could use and it can in most cases directly affects tension. That is, there is a direct relationship between torque and tension. The more torque we apply, the more tension we put into the fastener joint. But we’re talking about nylon insert lock nuts here, stainless steel nylon insert lock nuts, which theoretically would be installed on to some other stainless steel Fastener. A screw, a bolt, or a stud that theoretically would also be stainless steel.

So we run into a couple of issues here, the first one is: it’s not the nut that we need to be concerned about in terms of the tension, it’s the screw. The screw is the thing that is going to stretch. So assuming that we know how much tension we want to put on that screw or that bolt or that stud, then based on the torque tension relationship, we can decide how much torque we should put on the nut. Now stainless steel fasteners, and stainless steel nylon insert lock nuts in a particular, lead to a very difficult problem to solve. That is the problem of galling.

Galling is where stainless steel in conjunction with another stainless steel part fitted closely together, causes this material to rub one component onto and they weld together into a cold weld. And they get stuck, and it leads to lots of frustration, especially with stainless steel nylon insert lock  nuts. One of the ways that we combat this problem is with a lubricant. Another way, is that typically stainless steel nylon insert lock nuts are sold with wax already on them, to help combat that galling problem. Now the wax has an effect on the torque-tension relationship which lowers the friction in the nut, on the surface, and in the threads, thereby requiring less torque to achieve tension and thereby helping us to prevent the galling issue.

But what it all comes down to though, is that at some level we have to decide how much tension we want to put on the stainless steel fastener. We have to know two things: how strong the fastener is, and how much torque it can withstand, or how much tension it can withstand, and what is the torque tension relationship, based on the lubricant or the coating that is going to be on the nut. And this varies greatly obviously from size, very small size screws will take a very small amount of torque and very large size screws will take a larger amount of torque. And it’s surprising how low the torque value should be for small stainless nuts. The reason is that stainless steel is not really very strong compared to steel.

You can probably estimate its strength to be somewhat better than a Grade 2 bolt which would be about 70 KSI, nowhere near as good as a Grade 5 bolt at 120 KSI. So about 90 KSI is a good place to start. But it is not only the tensile strength that we really need to be concerned about, it is the yield strength of the stainless which will be lower than that. So typically stainless steel yield strength is going to be around 65 KSI. We have to be careful though because there are some stainless steel fasteners where the specification torque is only thirty thousand PSI or 30 KSI. So once we know all of that we have a very handy dandy formula you’ve all heard it: (T = KDP), that we can use to determine what the torque for a stainless steel nylon insert locknut, with some amount of lubricant should be.

So, because a client asked me to do it, I’ve developed a table that we can share with you. If you want a copy of this, just email me at CarmenV@aimtestlab.com and I’ll share it with you. It’s an Excel spreadsheet that allows us to input the lubricant and the strength of the stainless steel, and it will give us a torque value for the Fasteners. Typically in a very average situation, with off-the-shelf stainless steel hardware, off-the-shelf stainless steel nylon insert lock nuts with wax.

On the very small side of #4-40, the torque would be 5 inch pounds, very low. #10-32 would only be 28 inch pounds. 1/4-20 would be 58 inch-pounds I could go up and down the scale and I will in a minute. But then we get to 1/4-28 for example which is fine thread, it is 67 inch pounds. It’s a good idea to avoid fine threads with stainless steel nylon insert lock nuts because the galling problem is exacerbated. But let’s keep going up the scale a 3/8-16 nylon insert locknut should be torqued at 213 inch-pounds. I hope you’re all writing this down we go all the way up to a 1/2-13, the torque is 519 inch-pounds. Now I just want to warn you, if you use these values and they don’t work for you at your own fault, don’t blame it on Fully Threaded Radio, or Fasteners Clearing House, or me, or anybody else.

If you really would like to have an authoritative citation on this, you can use the IFI Technology Connection to get some of this same information. So again stainless steel nylon insert lock nuts: very difficult item to determine the proper torque, because it depends on the screw, and it depends on the lubricant. But you can do it and I can help show you how. If you need any more help just email me at CarmenV@aimtestlab.com.

Well this has been your Fastener Training Minute, thank you for listening

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