Bolts are tiny, compared to the bigger structure, and still the loads need to go through.

Bolted joints are capable of transferring moments and forces, but the bolts are better off in tension and shear only.


We use bolts to connect parts one to another. (Here's an excellent post by Mike Maner about different types of bolts).

It’s clean, convenient, and it’s not permanent (like glue) and therefore serviceable and replaceable. Trucks to a deck, motor mount to a truck, motor to the motor mount…

We call these bolted joints.

Moent is transferred to axial forces in bolts
Moent is transferred to axial forces in bolts

Simple Example

Look at your truck and the way it connects to the deck. The wheels are not centered between the bolts, so it will transfer moments (M) to the deck. And vise versa.

The bolts won't see this moment, but will see the pair of forces - one tension and one compression (F).

We don't care about compression here (it will go through the truck base to the deck, but the tension force will be taken by the bolt(s).


It's simple to calculate:  F=M/d


Now imagine there is no second bolt.

The moment will be taken by the single bolt.


Why isn't this a good idea? Let's explore.

Bolts are Easy

Bolts are simple to analyze, these are cylinders.

The manufacturers provide the tensile strength of the bolt which depends on the material. This is the failure stress in tension. Here's an example.


For 18-8 Stainless Steel (SS), the Tensile stress is 70000psi.

For Alloy-Steel bolts it's even higher at 170000psi.


What can we expect from 1/4" 18-8 SS bolt?

F(max)=(Tensile Stress)·(cross-section area)=70000·π/4·(1/4)²=3436 lb (1561 kg)

For Alloy-Steel it's even higher at 8344 lb (3792 kg)


Impressive isn't it? Over 1.5 ton in tension, from a tiny 1/4" bolt.


For shear loading we'll use 60% of the Tensile values.
Our 1/4" SS bolt will fail at ~937 kg in shear. Still impressive!


Now, what happens in bending?

Let's use the same 70000 psi of allowable stress. And the formula which we used before:


"Stress equals bending moment (M) times offset (y) and divided by the moment of inertia (I)".


For a bolt y will equal the radius, and I=π·(r^4)/4.

Putting everything in, using diameter, we'll get the following for the allowable moment:


For our tiny SS 1/4" bolt, we'll get: M(max)=107 lb·in (1.2 kg·m)



Bolt that can hold 1.5 tons in tension will break when 1.2 kg is applied at 1 meter distance!

Is it really that low?


Here is me breaking it with not much effort.

(and we can all agree I won't be pulling 1.5 ton with pliers anytime soon)


In Conclusion

Not bending bolts is easy, loose bolts tend to be the obvious victims and are prone to bending:

    1. Make sure the head and the nut are not too small (use washers if necessary).
    2. Use moderate force to tighten the bolts - don't leave them loose and don't overdo.
    3. Check your joints once in a while.




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