Nuts and bolts - what is so interesting about them

Maybe you really know everything about nuts and bolts. That would be great.

I once believed I know everything about nuts and bolts and then I learned I know nothing. So, are you really sure you know how nuts and bolts are actually working? Test it here…

On the picture you see a typical nut-and-bolt combination at work. Two contacted pieces of bulk material (In this example I call them the base piece and the extension piece) are joined together by a single nut-and-bolt combination.

Now we will add external force to the extension piece that will try to separate it from the base. The function of our nut-and-bolt duo is to transfer this applied external force from extension to base.

It is obvious that added external force is going to be directed only through our nut-and-bolt duo stressing it heavily and that the duo will have to handle all the added force by itself…. Well that is WRONG, WRONG, WRONG! I better start this article once again but concluding more carefully…

One more try...

Again, on the picture you see a typical nut-and-bolt combination at work. Two contacted pieces of bulk material are joined together by a single nut-and-bolt combination.

A simple fact about above picture is that the nut is screwed firmly generating preload and making both pieces of material powerfully squeezed together. On the other word, although nothing is happening, there are quite powerful forces developed inside this assembly. This is shown on the picture below. On this picture, forces are illustrated using red arrows (separatly for every member of the assembly).

You can see that both, the base piece and the extension piece are getting compressed while the nut-and-bolt is stretched. All forces being the same by their absolute value and all members of the assembly are force-balanced.

Now we will see what happens when we add the external force. Check carefully what happens and compare it with our previous picture.

As you can see, the force that is “felt” by our nut-and-bolt duo is in fact not changed. The majority of stress is “felt” by bulk material pieces (oddly, the force that squeezes them becomes reduced). And still the force applied to our extension peace is transferred to the base piece – you can see that the base piece is the one that “feels” the force imbalance.

Charming. This is how nuts and bolts do. I was very surprised when first time learned this, but now it is quite obvious to me. Of course, my example above is idealized. In real life a bolt would “feel” some small change when external force changes, but not nearly all of it.

An engineer can do things right or can do things wrong. If he/she chooses wrong bolts for his/hers application then such bolts will “feel” majority of the stress and a bridge or a building may collapse due to fatigue developed in bolts. Note the fact that a fatigue is mostly generated by variations of force, and not so much by the steady force even if it is quite excessive. This is why a bridge or building will stand much, much longer if you use right bolts – ones that will not “feel” all those ever-changeable external forces.

It all depends on how elasticity coefficients of a bolt and contacted pieces (joint) are related. If bolt’s elasticity coefficient is a lot smaller than elasticity coefficient of contacted pieces then the bolt will mostly not “feel” external forces. Luckily, in the real life this often is the case even if a bolt is made of stiffer steel than contacted pieces because the cross-section of a bolt is usually much smaller than a contact section between contacted pieces.

Oddly enough, sometimes bolts have intentionally reduced diameters to diminish the possibility of fatigue (reduced diameter reduces its elasticity coefficient – but of course, it also reduces the maximal allowed force applicable to the bolt).

And finally, note that it is only possible to utilize the described effect if the joint is preloaded with force that is greater than the maximum expected external force. If ever the external force grows larger than the pre-squeeze force – our little nut-and-bolt will be forced to do all the work by itself.

Danijel Gorupec, 2006