Bending stresses or direct forces – what kills you faster?

I’ve mentioned this in the previous post, also there's this *safety factor* thing.

Let’s see what it’s all about.

This is the fifth post about esk8 Structures.

What do you think? Do you like it? Hate it?

Did you learn something new?

Are you ready to design your own parts to hold real world loads?

Are there things you’re still interested in?

Can I cover this for you?

What can I improve?

Do let me know at: dani@dosimplecarbon.com

# Direct Forces

We'll use the binding example. These are gaining popularity.

Doing this more interesting and increasing the loads by 10.

Using the direct force of 100kg and bending moment of 5000kg.mm.

As before the bending stress will be:

**S=M·y/I**

Which is around 1 kg/mm².

Stresses from direct forces are even easier to calculate:

**S=F/A**

"Stress equals the force divided by the cross-section area"

Which will be ~0.07 kg/mm²...

Amazing isn't it?

The stresses from the force are ~7% of those from the bending moment.

So, can you just neglect it entirely?

Usually yes, and focus on the moments only.

Just make sure to get the bending stresses right.

Then again, you might practice calculating this, just to get a feeling for the numbers.

# Safety Factor (SF)

SF is that extra commission which ensures your part or structure will remain in one piece.

Time for an active visualization to clarify:

Imagine you’re designing a deck.

You made all the calculations, estimated all the loads and you are certain that this deck will hold 200kg load just fine.

Now you put it to use and make a big red sign above it which says: 100kg max.

(This specific deck can hold much more, a blast from the past which is no longer in use)

What did you just do?

You’ve created a safety factor of 2.

# Why Would You Do That?

"SF is the ratio between failure load/stress and the actual load/stress"

SF is that extra insurance for everything that you don’t know.

Just think about it:

What are the chances to calculate all the loads exactly right?

Or to get the material properties exactly right?

Maybe an occasional abuse of your part/structure during it's life cycle?

Using safety factor make sure you still have a structure in the end of the day.

In our example, instead of using failure stress of 3 kg/mm², you can use 1.5 kg/mm² for SF of 2.

# In Practice?

I believe a safety factor of 2-4 should be enough for any practical use.

What are you going to design next?

Enjoy,

Dani

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PS

I’m doing something new here.

A short series of posts dedicated to designing your own, as strong as you want, structures.

I’ll cover the basics, so you can do the rest.

Let me know what you think: dani@dosimplecarbon.com