Concerns regarding arms


#1

I just thought I’d voice a concern regarding the Pokitto holes. Possibly I have simply opened my Pokitto far too many times, perhaps my technic pegs are slightly different size, perhaps both. Either way, I have developed cracking around a couple of places. Anyone else had this? Or aam I just too rough with my Pokitto?


#2

Not for me, but I have not opened Pokitto too many times.


#3

Now that you mention it…

I don’t have cracking on the tabs, since I cut mine off. However, there are some on the top left and right corners. I noticed this a few days ago before opening the case for the first time in order to make my mods for measuring current. I assembled it myself as a kit but hadn’t opened it again. I’m pretty sure they weren’t there initially after it was assembled.

The photo is of the top left corner. The top right is also cracked but not as bad. As can be seen in the photo, there also appears to be a small crack at the upper back corner of the SD card slot opening. After a close examination, I don’t see cracks anywhere else yet.


#4

I’m sorry about this but there is very little that can actually be done about this.

Explanation:

the non-transparent colour plastics are ABS. The translucent colours are MABS. Introducing the transparency comes at a cost, MABS is slightly more fragile than ABS. Originally I had no plan for translucent colours, but when I saw how cool they looked I wanted to offer them. Nevertheless, MABS is still less fragile than PC (Arduboy material). What I am saying is plastic has limitations, nothing can really be done to avoid that completely.

The simple answer is thats wear and tear. I’ll put extra cases in the shop, something thats been on my to do list.

I hope this is a satisfactory response.


#5

I’d probably start stocking up on extra cases if I get the idea to pull mine apart more :wink: spares are nice to have!


#6

I figure it’d be something like that, my blue pokitto is still fine :slightly_smiling_face:


#7

I was about to say I’ve opened mine half a dozen times and never had any problem,
but I had a closer look and I do have a hairline fracture.
However it’s on my dark-green back made from whatever plastic was leftover in the machine,
so I don’t think it’s generalisable.

The transparent front has absolutely no damage.


I suspect it might be partly to do with the source of the lego ‘pegs’,
because I notice some are thicker than others.


#8

Fractures are the result of localized points of high tension, where the tension has exceeded the elasticity of the material. This property of the material is sometimes described with a values called Youngs modulus or modulus of elasticity. The Lego pegs work by a mechanism called interference fit. There are clearance fits (materials avoid each other with a gap in between) and interference fits (materials collide and need to expand to accommodate the other part)

In the case of Pokitto and technics Lego pegs, the fit is an interference fit. If you manage to keep the deformation caused by the interference fit below the treshold of the elasticity of the material, no fracturing will occur. But, if you use for example a knife to pry open the case, there may be an instance where the localized tension exceeds the elasticity, and a fracture occurs. The fracture may also be the result of a slightly tighter fit (caused by differences in molding temperature, air humidity etc) which again will increase the peak tension.

Summary: if you open very carefully, avoiding bending as much as possible, it is very likely that you will never exceed the max elasticity and you wont get any fractures. I have a Pokitto (clear, clear,clear) that I’ve used about a year for testing and it is intact. But: I open it with fingers only.

The black back you have is a prototype. The hole is ~0.3mm smaller than in later parts. Higher tension in the interference fit = max elasticity exceeded = fractures. Sorry.

(Edit: there is another mechanism.of failure called fatigue, and in the case of repeating the same tension / relaxation over and over again - think of twisting a paperclip - will also cause failures. May be a factor here also, but should be an issue after tens/hundreds of repetitions)


#9

Except I didn’t open my case at all before the fractures occurred (some time after assembly). My Pokitto has seen very little handling overall and no bumps or drops.


#10

Hmm. That is odd indeed, and not going to argue on any of that.

When you sawed off the arms, was the Pokitto in 1 piece or apart?


#11

Apart. It’s the first thing I did before assembling it.


#12

With mine, I just ordered some random pegs of ebay also, so there might be a slight size difference there, causing more stress.
Either way, I might have to amputate, I don’t think it will get infected, but you never know :mask:


#13

Out of all of the colours, are any stronger than others? Or is it simply transparent < solid?


#14

Transparent colours = transparent MABS + tiny bit of colouring
solid colours = ABS + colouring

It’s the MABS/ABS that matters


#15

So, ANY solid colour is a little stronger than ANY transparent? I’ll keep that in mind for the future :slight_smile:


#16

In a word, yes.