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Inexplicable Staining on Conveyor Belt

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Dear Engineering Colleagues

I'm stuck with an eerie problem, with no solution whatsoever: As a start: I'm in front of a conveyor with a white belt (glossy white, Polyester fiber core with Polyurethane coating). The belt is moved with a drum motor and redirected several times around return shafts (1.4301 stainless steel) that are held with sealed ball bearings. The belt runs atop a sheet metal on the upper side of the system. That's it. The problem: After only days of use, the under side of the white belt turns black. The upper side that has seemingly no contact with anything but the drum motor: It's black. The return shafts have large areas that are covered with a substance that seems like molten plastic/resin, however, inspection with a thermal camera yields no higher temperatures than room temperature anywhere. Additionally it looks almost tempered black. The sheet metal: It's black and has those stains looking like molten plastic/resin as well. Everything is accompanied with a powdery pitch black substance in inexplicable quantities (mass conservation does not seem to exist here haha). This thing is proven to to run over years (motor is sealed and has no leakage) , but becomes dirty in almost sterile environments very fast. Suppliers of the belt and motor have confirmed a correct construction of the set-up, forces are well below what would be allowed for all parts. Everyone is baffled. I'm suspecting the belt to have a problem, but I have no idea how a white PU belt would become stained pitch black under contact with only steel parts?!
I hope anyone that one of you has encountered a similar problem over the years, as I'm really perplexed from what I'm witnessing here.

Kind regards, harry

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Hi Baumi,

Have you got pictures that we can have a look at ? It would be pretty useful for me to see a few pictures.

How quickly is the belt moving ? It could be it may be a friction issue and its creating heat when it is under contact to the steel components ?

Hope this helps.


Kind regards,

Ricardo Santos



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Hi Baumi

Seems like a classic thing many of us have experienced . Due to the not obvious way that stainless steel protects itself.  Stainless steel actually protects itself with a very thin layer of chromium oxide that prevents further oxidation. Not by not forming an oxide as you may think.  The oxide is very thin, a few microns , It Is not really visible, forms very quickly BUT is not very wear resistant. You are supposed to AVOID wearing on it. It will form again instantly. What you experienced will happen. Seemingly wear product emerging from nothing. The colour of the wear product will depend on what wears it of reasonably and present dirt etc. Black seems reasonable. What you are doing is very very slowly wearing on the stainless pulley. To prevent this you could use a material with hard  surface oxide instead. Anodized aluminium should work well for that.  



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