Investigation into the Strength of Lego Technic Liftarms and Brick Beams

[nerdsforprez]

1 hour ago, Milan said:

May not be him per se, but few producers of metal parts compatible with Lego. Look for them on Ali/Ebay.

Okay, I will look.  Thanks @Milan.

 

Btw... this begs the question.  Does anyone know the strongest orientation of a liftarm?  For really short ones it does not matter, but lets say a 15L liftarm.  Which orientation is it strongest when used horizontally?  Holes vertical or horizontal ? 

Wondering if this has been tested before.  Would be a great experiment for that brick experiment channel on YT.  

[1gor]

8 minutes ago, nerdsforprez said:

Okay, I will look.  Thanks @Milan.

 

Btw... this begs the question.  Does anyone know the strongest orientation of a liftarm?  For really short ones it does not matter, but lets say a 15L liftarm.  Which orientation is it strongest when used horizontally?  Holes vertical or horizontal ? 

Wondering if this has been tested before.  Would be a great experiment for that brick experiment channel on YT.  

You should face holes in direction of impact. Example: if it is heavy model than turn holes to be visible from top (and bottom). At least my mocs (that are bigger) were more sturdy with beams placed to make holes oriented upwards

[nerdsforprez]

Hmm..... I would have thought just the opposite.  My rational?  The arch is one of the strongest shape configurations out there.  If holes are oriented so you can see them from top and bottom you don't use the arch.  At least from a vertical plane which for most MOCs that is what you are concerned about (not bending the chassis up and down, no one really cares about side to side).   

[TeamThrifty]

If you physically apply a load to a beam it flexes less with holes not in the direction of impact, so horizontal holes for vertical load.

I think this more to do with the amount of plastic available to resist the bend. With holes in the direction of bend, there's less material. Not sure if the arch contributes, but the amount of plastic certainly does.

[Lipko]

Theoretically it would be stronger if the holes are perpendicular to the force (more material on the relevant upper and lower faces - arces are not doing much here), but in reality the thin material is more prone to buckling on the compressed side in this setup.

I think...

Edited April 8, 2021 by Lipko

[Milan]

Since @nerdsforprez asked about strength of liftarms, we can discuss it here (These posts have been split from the Zetros topic).

Also, here is a great example of tests, the very same he was looking for, as PDF:

DOWNLOAD:

https://easyupload.io/z0ef6y

[1gor]

1 hour ago, nerdsforprez said:

Hmm..... I would have thought just the opposite.  My rational?  The arch is one of the strongest shape configurations out there.  If holes are oriented so you can see them from top and bottom you don't use the arch.  At least from a vertical plane which for most MOCs that is what you are concerned about (not bending the chassis up and down, no one really cares about side to side).   

Perhaps you could look them not as arches but shirt tubes...i just mentioned my experience, but it is not 100 certain, as @Milan pointed only test is relevant.

[Erik Leppen]

20 hours ago, nerdsforprez said:

Does anyone know the strongest orientation of a liftarm?  For really short ones it does not matter, but lets say a 15L liftarm.  Which orientation is it strongest when used horizontally?  Holes vertical or horizontal ? 

Give the Wikipedia page I-beam - Wikipedia a read.

What matters is the amount ofmaterial at the parts that are in tension (bottom) and compession (top). When holes are horizontal, the beam acts as an I-beam with a continuous strip of material at top and bottom, so this is stronger.

I always found it funny how a nearby train station uses I-beams with holes in the construction, proving that the amount of material in the middle doesn't matter much for the strength. All it does is keep the top and bottom strips at the required distance. maxresdefault.jpg (1280×720) (ytimg.com) 

Edited April 9, 2021 by Erik Leppen

[nerdsforprez]

oh wow.... very useful information everyone... Especially the link to the study that was done @Milan.  I thought that horizontal orientation would be the strongest...

[Lipko]

The strength depends on the load too. For a mostly bending load,or evenly distributed load (like in the case @Erik Leppen showed), the horizontal holes doesn't make the I-beam much less weaker.
But it there is significant shear load (for example one end is fixed and a downwards force is applied on the other end), this setup is weaker, because the upper and lower face both can bend "parallelly". The round holes will reduce this weaknes, rectangular holes would clearly be much weaker.

In case of tower cranes, you'll see huge holes, but the holes are not "round", the small beams between the upper and lower beam are always triangular and straight, which prevents this parallelogram-like deformation of the holes. The round holes in Lego is more like having only vertical beams between the upper and lower beam, which you'll never see in cranes.

Imagine a ladder with sideways force on one end, it will not hold. But if you apply even load, or you apply the load in the middle, it will hold much better.

I hope it's clear, I didn't find any resources to back the post up, I don't have time to dig deep into it.

Edited April 9, 2021 by Lipko

[Lipko]

I couldn't resist.
Though my "science" behind it is very rusty, it was some 10 years ago when I learnt it in university and I haven't used my structular mechanics "knowledge" ever since. This diagram is far from deep or accurate, but you'll get the idea.

No holes is clearly stronger that having holes if there's shear load and if you neglect the dry-weight/load of the beam itself (and it's pretty rare to have only bending on a structural beam). Maybe in some cases by applying holes, you decrease the dry-weight (thus dry-load) of the beam more than the strenght decreases. Adding holes have some other advantages, for example leading pipes and stuff through them, but at the same time, it makes the beam more expensive to manufacture. That's the reason you don't see too much of the stuff @Erik Leppenposted.

And yup, typo in the graph....

Edited April 9, 2021 by Lipko

[Lipko]

Sorry, last reply, but I need to correct some things.

Practically beams with horizontal holes will allways be weaker than no holes if there's shear force, no matter if it's evenly distributed or not (again if we neglect the dry weight of the beam). And in practical situations (excluding purely longitudinal loads) there will always be significant sheer forces.

And that holes doesn't matter in case of purely bending loads is not fully true either. Because holes make the I-beam much more prone to local buckling at the holes.

With all that crap said, maybe the Technic studless beams are still stronger with horizontal holes, I couldn't open the test report because of the company firewall.
I can imagine that in case of horizontal holes, the amount of deformation is smaller with the same load, but at the same time the maximum load it can bare without failure is smaller than with vertical holes.

Edited April 9, 2021 by Lipko

[Captainowie]

You can test this with your fingers. Hold a beam with your thumbs in the middle, and forefingers at the ends and see how much force it takes to deflect. Then rotate the beam around it's long axis and try again - there's a noticeable difference in the amount of deflection you get for a given amount of finger pull.

Orient your beams with the holes parallel to the load!

Edited April 13, 2021 by Captainowie

[nerdsforprez]

Obviously I tried this.  However, this is not precise enough for me. As  a practicing clinical neuropsychologist, I am more than aware of the evaluative inconsistencies that can accompany sensory perception.  It is anything but a straightforward process.  

But, as laid out by others, I think the question has been satisfactorily answered by others.....