FTM 148: Bolted Joints

In a bolted joint should the nut be stronger than the bolt?

This edition of the Fastener Training Minute with Carmen Vertullo was originally published January 18, 2020 as “Bolted Joints” during episode 148 of Fully Threaded Radio.

The answer is that in a bolted joint the nut should be stronger than the bolt.
We can inform ourselves about that answer in a couple of different ways.
One way to think about this is sort of intuitive or maybe even obvious: if the nut is not at least as strong as the bolt then chances are if there were to be a failure, it would be the nut that would fail and not the bolt in which case we might not know there was even a failure because you can’t see it fail. It usually just strips. Also in order for us to be able to take full advantage of the tensile strength of the bolt, the nut has to be at least that strong. Otherwise the bolt won’t break. Once again, we just have stripped threads.

We can also see this play out in the SAE Fastener standards, for example in SAE J429, which is where our grades 2, 5 and 8 bolts come from, and the specifications for the tensile strength for the bolts. The companion standard for the nuts is SAE. J995. And if we were to compare a Grade 8 bolt from SAE J429 with a tensile strength of 150,000 PSI and then go and compare the nut that matches with it the Grade 8 nut from SAE J995, you can see that it’s proof load strength is also 150,000 PSI. The proof load strength of a nut is not that strength at which it would fail, it’s the strength at which it does not fail or does not suffer any permanent damage. So we see from looking at these numbers that the nut is at least as strong as the tensile strength of the bolt.

So when you go out there and you are matching nuts and bolts, always be aware of the fact that they don’t necessarily have to be Grade 8 vs Grade 8 or Grade 5 vs. Grade 5 or in the metric world of property class at 10.9 versus a class 10 nut. But as long as the nut is as strong or stronger than the bolt, you have a well-put-together, well-designed Bolted Joint.

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