This edition of the Fastener Training Minute with Carmen Vertullo was originally published June 20, 2018 as “Screw Locking Devices” during episode 129 of Fully Threaded Radio.
Welcome to another edition of the Fastener Training Minute. This is Carmen Vertullo coming to you from the Carver Facts Center here in beautiful San Diego and from the Fastener Training Institute where we solve all kinds of fastener problems and ask all kinds of fastener questions and answer most of them.
One of the questions that came to me early last week was from someone who wanted to know about putting locking elements on screws. We know locking elements as various types of nylons or adhesives that we would put on screws. In this case it was attached to the screw as either a pellet or a strip, and they wanted to know which one works best, and if they were interchangeable. This is a very very good question. It has both functional and financial implications.
So when we come back, we’ll tell you if you’re trying to choose between a patch, a pellet, or a strip, which one should you choose?
If you’ve been in the fastener industry for any length of time, one of the things you know is that one of the good solutions for keeping fasteners from coming loose, especially small short ones, is that we will apply some nylon to the threads of the fastener. We have a couple of different ways of applying the nylon. We call it patch, pellet, or strip. We are going to distinguish these methods from adhesives, which we are not going to talk about right now.
So when we apply the patch to a screw, what we have to do is is literally take that screw and somehow get that nylon to stick to it. We heat the screw up and we spray some microscopic droplets of powdered nylon onto and it melts on to the screw threads and makes what looks like a little scab on there. And so when we install a screw into the hole or on the nut that it goes on, that nylon creates an interference which causes friction on the opposite side of the screw, keeping it from coming loose.
In the case of a pellet or a strip the difference is we actually machine a place on the screw. If it’s a pellet, we drill a little hole and we insert a small plug of nylon into it if it’s a strip, we machine a little groove and we put a little strip of nylon. They are all very effective and they actually are all about exactly the same in effectiveness and by specification. They all meet the same requirements for anti-vibration or what would be called the prevailing torque, the torque that we have to overcome When we install the screw.
Now as you can imagine, the biggest difference by far between those three options is the cost. By far, the patch is a lower cost, then the pellet and then the strip, and it has to do with the amount of Labor that it takes to install whichever feature that we use. If this is the case, why do we even use a pellet or a strip, when a patch is so much less expensive.
Well, the primary reason is that if we are not restricted, the patch is obviously our first choice. But often times, especially in the cases of military specifications, the drawing will say use a pellet or use a strip, in which case we have to do that. So that’s where that comes from. If the government and the military would catch up with technology a little bit, my guess is we would see the pellet and the strip go away because they do have one small issue with reliability.
And that is even though they do an outstanding job of installing these pellets and strips, occasionally, they will fall out. So if you have a giant box with a 10,000 parts in it every now and then you may see a piece of strip or a piece of pellet left in the bottom of the box, and that means that one of the screws lost its locking element.
So that’s the story on patch, pellet, or strip. And if you have a customer or client who’s using a pellet or a strip and they’re looking to save some money and they’re not otherwise restricted, you might want to encourage them towards using a patch. If so, talk to your patch, pellet, or strip service provider. There are a couple of them in the industry. I’m not going to endorse any of them at this time. They’re all very good. They all provide great service, and all their products work about the same.
Well, that’s it for the Fastener Training Minute. I hope, you know a few things about patch pellet and strip that maybe you didn’t know before. Thanks for listening.