FTM 170: How tight is a “snug-tite” joint

Fastener Training Minute 170: How tight is a "snug-tite" joint

This edition of the Fastener Training Minute with Carmen Vertullo was originally published November 18th, 2021, as “How tight is a ‘snug-tite’ joint” during episode 170 of Fully Threaded Radio.

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

Today’s Fastener Training Minute is about a particular word in the fastener installation process, and this word is “snug” as in snug-tite. It normally applies to structural bolting applications, however, understanding the need for properly snug, tightened joints in the structural bolt application, greatly informs, our understanding of all bolted joints, proper tightening of bolted joints, and some of the options that we have for tightening bolted joints.

So, when I come back, I’m going to explain more to you about “snug-tite” and why knowing about snug is a good thing for all bolted joints.

Well, welcome back everybody, this is Carmen Vertullo with the Fastener Training Minute talking to you about snug-tite in the structural bolt world.

The first step in installing these bolts or tensioning them is called snug tightening. And what snug tightening means is that we’re going to install the bolts and the nuts, and we’re going to tighten the nuts until all the plies in the bolted joint are in what’s called firm contact. That means that there is no more wiggle room in the joint, all of the bending, or non-flatness of the joint that can be squeezed out, has been squeezed out. And the way we do that is that we start from the center of the joint or the most rigid part, and we tighten these bolts even if it’s just two bolts, (but normally there’s going to be two, to many, to dozens), in an orderly fashion, starting from the center part of the joint and working our way outward.

Now, one of the things that may happen as we tighten up a joint or a bolt in the joint, is that the bolt next to it, that has already been tightened, may come loose, because we pushed the plies of the joint closer together.

So there’s a little bit of a back-and-forth double-checking operation. But once all of those bolts have been snug-tightened as defined by two things: one is that all the plies are in firm contact, and the other is the force on the nut or the torque on the nut is that which is applied by an ironworker with a normal spud wrench. So a small bolt, like a half inch bolt, will have a relatively small wrench, a larger boat, like a one and a half inch bolt, will have a larger wrench. So pretty much as tight as you can make it by wrench, by hand is what snug-tight is. And in many structural bolting applications, once we’ve done that, we’re done. That’s called a snug-tightened joint. We don’t need to tighten the joint any further.

These are called bearing joints. And essentially, we’re depending upon the shear strength of the bolt to hold the steel together.

One step beyond that, we have more sophisticated joints, which are called fully tensioned joints or slip critical joints. In these cases, we now need to tighten the bolt much tighter, but what this snug-tighten operation has done for us in preparation for that final tightening, which is actually called pre-tensioning (it’s not a word I’m fond of, but that’s the word we use: pre-tensioning the joint), is that we have a zero point. We now have a place to start, and we mark the bolt, we mark the nut, we mark the socket and we mark the steel. And now we can turn the nut, a certain number of degrees and that will tell us that we have achieved the tension required to make those plies in the joint squeeze together. Tight enough to provide a fully tensioned joint or in a more sophisticated requirement, what’s called a slip-critical joint, where we’re no longer depending on the tension of the bolts to hold everything together, but the tension of the bolts providing friction between the plies as they are tightened together; so tightly that they can’t move or slip. And so we have a slip critical joint application there.

So what does that have to do with other bolted joints? Well, it turns out that this method called angle or torque and angle, is a better method for tightening any joint, not just for structural joints, but the key is finding zero.

So once we learn how to find where that zero is, that is the snug-tightened position in any bolted joint, whether it’s automotive, a piece of equipment or structural joint, we can use angle as our method correctly tighten, and angle is more precise. It’s easier to train people to do, it’s less likely to cause a problem because we don’t need calibrated torque wrenches and so on. We even have a new product in the structural bolt world called the torque and angle joint, where we have a special tool that gives us our snug-tite with a low torque and then gives us our final tensioning with an angle. Those are very new on the market, and if you’re in the structural bolt world, you probably need to learn about the TNA product, and that’ll be a topic of another Fastener Training Minute. As well as how do we actually know when we’re in the steel that this method that we’ve chosen to use provides us this accurate amount of tension.

So snug is the word of the day. I don’t like the word because when I think of snug, I think of, you know, a hug, snug as a bug in a rug, just kind of, not very tight, but snug-tite when it comes to bolts, is actually a lot more than snug, it’s pretty dang tight.

So I hope you can use this information going forward whether you’re in the structural bolt world or in just any kind of assembly situation, where you might want to consider using angle as your method for tightening the bolt, instead of torque.

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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