FTM 163: What is a reduced load ability head

Fastener Training Minute: Episode 163

This edition of the Fastener Training Minute with Carmen Vertullo was originally published April 15, 2021 as “What is a reduced load ability head” during episode 163 of Fully Threaded Radio.

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

I am immutable, however, from time to time, Eric does mute me. So you don’t really get to hear the best part because he always edits that out. In any case today, we are talking about a topic comes through an email question, which is often the case. And this question had to do with a very small socket screw, about an M5 that had a special head on it. This head was not a low-head, it was actually what’s called a very low or extra low head. And as a result, this property class 12.9 socket screw was not capable of bearing very much load because of the head configuration. Now, some standards do a pretty good job of accommodating this situation. One of those standards is ISO 898 part 1, the standard from which the property class 12.9 comes from. It gives this case a special term, and that special term is reduced load ability, which kind of sounds like it should come out of the state of California, Firearms regulation. But this has nothing to do with Firearms, it has to do with screws. And when we return, I will tell you about the intersection of reduced load ability and hydrogen embrittlement.

Well, welcome back everybody. This is Carmen Vertullo finishing up with the Fastener Training Minute. Today, we’re talking about reduced load ability. This concept is very important for those who use fasteners where we have a head configuration that’s non-standard, that is, it’s not a standard socket head cap screw, or a hex head cap screw, but there’s something about the head configuration or the internal drive, typically that removes so much material from the head that the head does not have enough strength to take the full advantage of the strength of the fastener material. The standards call these types of fasteners, reduce load ability fasteners.

Since we can’t really take full advantage of the strength of the fastener, we have to know how to accommodate this situation. One of the more common ones, for example, are standard alloy steel socket button head,  and flat countersunk head screws from ASTM F835.

Even though the screw has the same material that the socket head cap screw would have in the smaller sizes, (180 KSI), we only can load it to 150 KSI because of the reduced load ability brought about by the head configuration. We run into some other issues with this situation as well.

In the metric world, in ISO 898-1, they accommodate this very well in the testing standards, the testing part of ISO 898-1 tells us which products have reduced load ability and how we are to go about accommodating that during testing. But when we run into a part that has extreme reduced load abilities, such as the extra low socket head cap screw, we can barely tighten the fastener down without breaking it. Sometimes we can’t tighten it down at all anywhere near the desired load because the drive is so short, we can’t get enough to tighten it to its reduced load ability capability.

So one of the ways we can fix this problem is use a lubricant. In other words, a little bit of grease on the screw or a coating that has a very low K-factor reducing the friction will give us some extra load for the same amount of torque. In the application situation, it’s very common to see an engineer say “oh I don’t have room for a hex head” or “I don’t have room for a socket head”, “so I’m going to go to a button head or a flathead”. While not realizing that those screws even though they’re made out of the same material and have the same characteristics in terms of strength as the standard screw, they can’t bear the same load.

We had a situation here recently with that very low socket screw and we discovered the problem through the hydrogen embrittlement test. We were doing the stress durability test on the screws and they failed. In the process of watching them fail, we saw that when the head came off, there was a hole right through the head where the socket was and there was very little material at the head body junction.

So one of the things we have to be aware of with these reduced load ability screws when they’re from ASTM F835 or ISO 898-1 property class 12.9, we have an intersection of reduced load ability and hydrogen embrittlement which makes these parts particularly susceptible to hydrogen embrittlement.

So the reality is there is really no good reason to ever make a flat head or a button head screw or a low head socket screw out of this property class 12.9, or F835 material because they’re not capable of taking advantage of the load. So if I could turn things around a little bit here, I would go back and say, hey, let’s fix the standards up, and let’s not make ourselves vulnerable to hydrogen embrittlement.

The reduced load ability screws screws are particularly vulnerable, because they have a very small cross section at the head body Junction and thus when we try to load them we push them even further into the hydrogen embrittlement susceptibility Zone. And in fact, this is borne out in reality because we see a lot of failures for hydrogen embrittlement with butting heads and flat head screws of these high hardness values.

Well, now, you know, everything there is to know about reduced load ability and the risk intersection of reduced load ability and hydrogen embrittlement.

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

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