Nuts and bolts are not just a snack mix – this combination is used all over, from engines to building structures, precision machines and even toys. The challenge that engineers have in designing these items is to… well, make sure they stay together, forever. What’s the use in an engine that loses an essential part due to bolt failure? And who wants to be in a building designed with the wrong combination of nuts and bolts? Luckily today’s Technology helps with the production of Fasteners, Nuts and Bolts like Hex Bolts.

None and nobody.

So what are the factors that go into determining the nut and bolt compatibility? And why is this important to the strength of the fastener?

Factors for Nuts and Bolts to Get Along

Similar to finding the right fastener material for your application, finding the nut material that will work well with your bolt is vital. The IFI standards claim it is “relatively straightforward,” although from personal experience (being married to an engineer for 13 years) I’ve found that nothing in engineering is anywhere near straightforward.

That being said (sorry, hun), the IFI guidelines are summed up fairly easily.

For average needs when using carbon steel fasteners you should take a look at the strength of both mating parts: bolt / screw and nut. Make sure that the nut has the same level of strength or higher than the bolt or screw. Nut proof stress (aka strength) is also measured in pounds per square inch (or psi) – so if you have a fastener made of medium carbon that has been heat treated (usually somewhere around 120,000 psi), you need to look for a nut with at least 120,000 psi strength as well.

Easy, right?

If you are concerned about safety and want to provide a high level of assurance, IFI recommends that you choose a nut with a proof stress that is 20 percent higher than the strength of the fastener.

Stainless steel and non-ferrous fasteners require a nut with equal strength – so match the psi of your bolt with the proof stress of the nut (also in psi). Full thickness nuts generally don’t fall any lower than 90,000 psi, which is good for most fasteners unless you move into the stronger stainless steel or Nickel-based alloy products.

Matching Alloys

It’s also recommended that, if at all possible in your application, the same alloy is used for both the fastener and the nut. You could be introducing a whole other set of issues by mixing and matching alloys.

Overtapped Means Less Strength

You may hear of a nut being overtapped, which occurs when plating and coatings are applied. This process actually lowers the proof stress of the nut (although it improves resistance to corrosion) and can reduce the lowest grade of full thickness nuts to as low as 60,000 psi.

When hardware is hot dipped galvanized the plating process may clog up the threads, making it even more important that the mating parts both be hot dipped galvanized.  Coordination is about more than matching your shoes and socks.

Balance is Vital

Another element to remember is that you need balance between the nuts and bolts. Don’t choose a nut that is near the minimum accepted proof stress level and mate it with a fastener that sits at the maximum end of the range of accepted strength. In stress situations your bolt will hold up nicely, but the tension will be too much for the weaker nut and you’ll end up stripping the nut thread entirely.

To be sure this doesn’t happen (in case your application demands complete assurance), choose a nut that meets or exceeds the maximum strength of the bolt.

Also, opting for a larger diameter nut (with a thicker wall) is a cheaper option than going with a heat treated nut – which will save on the cost. In certain situations space may be tight and the heat treatment may be your only option.

There are a lot of factors that go into matching the right bolt with the proper nut. Count on the strength and durability of a well paired combo. That’s the kind of thing those engineers are looking for.