FTM 120: Class 2A and Class 3A threads

This edition of the Fastener Training Minute with Carmen Vertullo was originally published September 28, 2017 as “the difference in strength between Class 2A and Class 3A threads” during episode 12o of Fully Threaded Radio.

This is Carmen Vertullo with the Fastener Training Minute coming to you from the Fastener Training Institute and the Carver FACT Center here in beautiful. El Cajon, California. I recently wrote a magazine article for Fastener Technology International magazine within which I discuss the issue of thread stripping and a lot went into that in regards of understanding thread strength.

I got a very interesting reply back from a university professor who said “I read your article and it was very interesting, but you did not address the difference in strength between Class 2A threads and Class 3A threads, or in the case of a nut Class 2B and Class 3B. He’s correct, I did not address it and there’s a reason why. I probably should have addressed it but I didn’t, so what is the strength difference between Class 2A and class 3A threads? We know that Class 3A is used in aerospace, and we know that Class 2A is used for commercial fasteners, and that there’s only one commercial product that has Class 3A threads.

Well, welcome back everybody. Here we are with again with the Fastener Training Minute talking about Class 2A threads and class 3A threads in regards to thread strength. Well Class 2A threads and Class 3A threads are the most common thread classes that we see on Commercial Fasteners. In fact almost all commercial Fasteners have class 2A threads. Class 3A threads are reserved only for ASME B18.3 socket screws in the smaller sizes. So what’s the difference between these two classes of threads? By the way, when we have the letter “A” that means external threads, we’re talking about screws and bolts.

Well, the fact of the matter is, those two tolerances overlap each other significantly. So you could actually have a completely compliant Class 3A thread and a completely compliant class 2A thread, and the Class 3A thread could have less material than the Class 2A thread. So we really cannot make any strength determination based on Class 2A and Class 3A threads. For the purposes of strength, there is no significant difference. This has actually been borne out in some experiments. Now in the higher-end threads in aerospace, (those strong socket head cap screws), they specified 3A for the tighter tolerances and for a more Precision Fit. The more important thing though, is that for high-strength fasteners, we have a root radius where the thread flanks come together in the thread root, we want to have a nice rounded contour there that gives us better fatigue resistance and a higher strength thread. So the bottom line is, for all commercial applications, specify Class 2A threads with rounded root radius. There’s no reason to use a Class 3A thread in any commercial fastener.

I think if we had it to do over again, we probably would not use Class 3A threads, even in the socket screws. So this is for inch threads, when it comes to metric it’s a much simpler system. We only have one standard class 6G and it works fine. It’s been well worked out.

So now you know that there’s really no difference in strength between Class 2A and Class 3A threads. This is Carmen Vertullo with the Fastener Training Minute, thanks for listening.

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