Bolts should be stressed by design in order to avoid bending stresses, premature fatigue failure and rattling.
Preloading the bolts is the simplest and the best thing that you can do about it.
Let's dive a little more into it.
A short history
Recently there were several posts about axles in bending, like this one from Boardnamics and this one from Whooshboards.
The axles bend but without the account for preloading and the existence of the wheel. This is simpler of course, but gives you only partial picture about what's going on.
I want to fix this today by talking about axles, this is relevant to any bolt however.
Preload?
Preloading the bolt is simply making sure it's in tension, meaning it acts like a stretched spring.
That time you made sure that wheel nut is extra tightened? You stressed your axle in tension, and of course made sure the wheel is compressed against the truck.
So, yes there are stresses in your trucks even if your esk8 is just sitting at home and doing nothing.
Why is this a good idea?
Because it helps with fatigue loads and makes the whole assembly better. Let's see this with an example.
An example
We have here half of Boardnamics hanger, with a wheel.
First - only half is modeled, for making the point (and you can do anything with finite elements)
Second - yes, it's a wheel, for the calc purposes
We'll run an FEM analysis with two steps:
First: 100kg (1000N) bolt pretension (it's really not that big)
Second: 30kg load on the wheel at 50deg truck angle (the "usual" load, from here for example)
As a side note, Fusion360 doesn't support preload (at least I'm not aware) so I ran Ansys this time.
This is the adjustment of the bolt in pretension (same as preload), it's barely noticeable when you use the wrench. So you end up easily with large pretension forces.
So, how does it work?
It's called pretension, or preload, because we make tension stresses in the axles before the actual loading occurs.
Now you're using the board, and the axles want to bend, totally natural.
In order for it to experience bending, one side of the axle has to unload, to have negative stresses.
If the preload is high enough this won't happen!
We're trading re-occurring bending stresses for constant tension! Which is waay better for fatigue - no cycling loads.
Let's look at the stresses
At preload we see around 20MPa at the exposed area of the bolt. Which makes sense for 8mm axle:
S=1000/(π·4²)=19.9MPa
Disregard for a minute the higher stresses at the inside area of the bolt, these are not real (result of contacts interaction)
Now at bending:
Hardly any difference!
And that's the whole idea of preload. Compare this to the bolt stresses here.
What's the bottom line?
Don't skip the preload in bolts/axles/trucks.
You're probably doing this already and making sure the bolts are firmly tightened. The idea was to bring the inside stuff to awareness. Hope I succeeded.
Enjoy,
Dani
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