Tightening forces on a bolt

Fastener strength

How strong are bolts?

Bolts are usually made out of different types of steel, but they can also be made from materials other then steel. The strength of every material differs and the choice of the bolt strength can be really important in your design.

To be honest in my designs I only use two bolt strengths or bolt grades. In the metric system they are 10.9 – for mating steel parts and 8.8 for mating aluminum parts, and here is why.

Never the less it is important to distinguish the bolt grades and to understand what they mean. This is a subject that has many resources. Rather than repeating something that you can read elsewhere I have collected some good resources for you here.

First to help you visualize the forces in the bolt watch this video:

In this video you can see how the forces that are acting on the bolt are spread out throughout the fastener.  Also at 1:25 mark you can see how the bolt is deforming in assembled state. This compression state, that is on the part of the bolt between the materials that are mated, is producing a spring-like effect. So you can imagine that a bolt operates like a very tight spring. This spring is holding the material(s) in its compressed section. The friction between the bolt head and the material and the friction in between the tread of the bolt and the thread of the nut (threaded hole) are what keeps the bolt from un-tightening. There are a lot of more or less successful methods of securing the fastener from undoing. Those will be covered in another post.

Let’s understand the tensile bolt strength a bit better

So in these tests you can see how the elongation of the test rod changes with respect to the force acting on set rod.  This curve that is drawn here in this test is different for every type of material. This is a standard test that determines what are the strengths of materials and there by the bolts.

So now that you are a bit more familiar with the strength of material and what forces are acting on the bolt I think that you will have a greater understanding of what are these numbers associated with the bolt strength

So how to know how strong is a bolt? (or what do does number on the bolt head mean)

Here is a great page where you can check out what are the theoretical strengths of different types of bolt


Some other references that I used:


http://www.smartbolts.com/fundamental s/



For you that got to the end of a post I have prepared a little present. You can download a free, printable, one page document, that has the most important proof strengths of metric 8.8 10.9 and 12.9 bolt classes. So you can use it as a reference 🙂

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