FTM 197 – do you torque the bolt or the nut and does it even matter?

Fastener Training Minute 197 - do you torque the bolt or the nut and does it even matter?

This edition of the Fastener Training Minute with Carmen Vertullo was originally published February, 20th 2024 as “do you torque the bolt or the nut and does it even matter?” during episode 197 of Fully Threaded Radio.

Well hello everyone, this is Carmen Vertullo with your Fastener Training Minute,  coming to you from Carver Labs in beautiful El Cajon California, and the Fastener Training Institute.

Today’s Fastener Training Minute comes from the other side of the world, at least the other side of the world from where I’m at  in San Diego, and this Fastener Training Minute comes all the way from Cape Town in South Africa. And by way of that exchange, I just wanted to give you a reminder about some really good Fastener Technical Resources, because for this particular inquiry, we had some back and forth, and I’m going to read you the last line of the email from this gentleman, because one of the things I said to him was: “how did you know to call me about this from South Africa?” And he said this: “last month I found the article on the FCH Sourcing Network that you wrote titled do you torque the bolt or nut and does it even matter, and decided to reach out to you. I’m sure glad I did, my motto is follow where your curiosity leads. So I just wanted to remind you that the FCH Sourcing Network, fastenersclearinghouse.com is a great technical resource, and there are over 100 Fastener Training Minutes that you can listen to and also read. So just wanted to throw that out there so you’ll notice from the sign off, my new friend from South Africa asked the question do you torque the bolt or the nut or does it even matter?  This is not the first time, it may not even be the second time that we have addressed this on the Fastener Training Minute, but in this context it’s a little bit different. So when I come back I’m going to share a few things about this particular application and something that kind of dovetails into it which of course is the overall tightening strategy which we have talked about many times in many contexts on the Fastener Training Minute so stick around.

Welcome back everyone, this is Carmen Vertullo with the Fastener Training Minute. Today’s topic is do you torque the bolt or the nut and does it even matter? and I got this question from a gentleman in South Africa. He is assembling large carports with solar panels on top of them, they come as a kit, and if they use like M16 size Hardware with nylon insert lock nuts. It’s all stainless steel. I’m not going to address that right now but his question had to do with should we torque the nut or the bolt when we’re putting these assemblies together? Now fortunately for this particular situation, and with these questions we don’t always have this one resource which I like to call an authoritative citation. The IFI the industrial Fasteners Institute has a document written many years ago by Joe Greenslade that addresses this very issue. if you want that document you can go to the IFI and buy it, or just call me and I’ll give you my version of it. Now there are obviously a variables in the assembly process. We’re going to do this thing. This guy’s going to tighten the bolt. This guy is going to tighten the nut. And  by that we mean, which one is going to be the held component, and which one is going to be turned component? And that matters, because the turned component has a bearing surface which has friction, which greatly impacts the torque tension relationship. Does it matter that much? Well I’m going to read you exactly what I wrote to this gentleman.

I said: “All things being equal, a rigid joint with parallel surfaces, hard washers, same bearing surface area, same coating or lubricant, (and by the way adding lubricant or friction control coating is an excellent way to control the K-factor variable), all that being equal I say I would standardize on tightening the nut. And you say short socket if possible. OK, the reason I say the short socket is because I saw a photograph of his situation. The bolt was much too long, it had a couple of diameters of thread sticking out, and that’s a waste of a bolt, but he says, we use the same bolt lengths so we don’t have to worry about people using the wrong bolt somewhere. So some are too long but you have to use a deep socket. And that deep socket can be a variable on the torque wrench as well. So we use the word variable which is the very important word because in the manufacturing world, as a manufacturing engineer, and in a subset of manufacturing engineering called process engineering, our job is to identify all the variables in a process. Whether it’s a cold heading process, a machining process, a plating process, a heat-treating process, even the testing process. Identify all the variables and eliminate anything that you can eliminate, and control what you can’t eliminate. That way, if you ever do have a problem, you don’t have so many different things to ask questions about, and it’s much less likely that you are going to have a problem. Because there’s not a lot that can go wrong, because you’ve eliminated all that stuff (or as much of it as you can) so sometimes the best reason, sometimes the only reason (but it could be the best reason) to do something the same way all the time, is to eliminate the variables that can occur when you do it a different way. And in that case you have to wonder: did that different way cause a different outcome? when I’m trying to troubleshoot a problem

So back to this tightening strategy, they used torque wrenches. In going further, I advised him (and now we’re going to segue into a topic that we have discussed before on the Fastener Training Minute) another method of achieving consistent tension is the torque angle method. In this method we don’t use the torque wrench as our final tightening strategy, we use it as our initial tightening strategy to get a snug tight fit. You probably could do that without a torque wrench in this application because it’s highly controlled and pretty well engineered, and then we turn the fastener a certain number of degrees, (or wrench flats, or wrench corners) to achieve the desired tension. So how do we know how many degrees or wrench flats to turn it? Well we know that we are

going to do an experiment in our Skidmore Wilhelm bolt tension calibrator, to determine how many degrees were going to turn it. And this is a very common procedure used on structural job sites where we have structural steel held together with structural bolts. And we have to do that down on the ground in the Skidmore and that informs the installer how to install the fastening up in the steel. It’s a very very good method, I highly recommend it, and it’s easy to learn, and it’s easy to train for. As far as having a torque wrench, or using a torque wrench, there are so many variables that make torque not the most reliable method for achieving a given amount of tension.

So two things today I hope you learned, (well three actually)

One.     All things being equal, it really doesn’t matter whether you tighten the head of the bolt or the nut.

Two.     Eliminate all the variables that you can in any given manufacturing or assembly operation, the ones you can’t eliminate you should control. So sometimes the best reason to do something the same way all the time is it eliminates a variable.

Three.  Finally and maybe most importantly, remember that you have a great technical resource at the  FCH Sourcing Network and use it.

Well I hope you enjoyed today’s Fastener training minute this has been Carmen vertullo thanks for listening.

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