Aluminum and steel cable joint 1

[Deefer]

Hi All, hoping you experts can advise on best course to maximize longevity of a public art piece.

Details:
I have a 1/8" thick x 14" diameter aluminum disc suspended between a concrete floor and metal post above, using 1/8" metal cable. The disc has a 3/8" hole at top and at bottom, to thread the cable through. The disc's entire surface including the holes is sealed by powder coated paint.

This is part of an outdoor public art piece in Texas with extreme humidity, rain, sun, all weather. Eventually, the powder coating will wear away where the cable rubs against it. When that happens, I'm concerned about corrosion at those points of contact.

An engineer told me under non-marine outdoor conditions, it would take decades for noticeable corrosion to happen between aluminum and stainless. But after researching online, I'm not so sure. An important point may be that most of the aluminum disc's surface will be covered with powder-coated paint.

Here's a photo of a typical use case for wire-rope rigging: the cable nestles inside a thimble. It is the thimble that would come into contact with my aluminum disc.

My goal is to protect the aluminum disc from as much corrosion as possible; it needs to last at least 10 years without looking discolored with rust. I'm far less concerned about the thimble and wire rope, which can be replaced once each year if needed.

I originally planned to use a stainless steel thimble with stainless steel uncoated wire rope. But, am now wondering if it's better to use an aluminum thimble. That way, the stainless steel would touch the "sacrificial" aluminum thimble which could be replaced once yearly. But I'm unclear on how quickly and to what extent the corrosion would move through the aluminum thimble and into the aluminum disc.

Here's a quick sketch of what I have in mind. PLEASE let me know of improved suggestions. I read elsewhere in this forum it might be smarter to use standard steel instead of stainless, to force the corrosion into the steel instead of the aluminum. I simply don't know enough about metals to make the judgement.

I'm also wondering if there's a suitable synthetic rope to use instead of steel, but it'd need to be tough enough so vandals couldn't cut it with a knife.

 

[DBronson]

Deefer:

You might consider using carbon fiber cord to eliminate corrosion issues with the cable, then I'd look at using a nylon thimble to get away from dissimilar metals in contact with each other. If the edges of the aluminum holes could be radiused the thimble could, possibly, be omitted.

Regards,

DB

 

[MintJulep]

None of the materials you mention "rust".

The powder coat on the inside edge of the hole will wear away very soon.

Consider adding a grommet to the holes.

https://www.amazon.com/Hilitchi-Alu...3071222011&s=industrial&sr=1-1&ts_id=16409771

Plain old dirt will make this look bad long before corrosion.

Consider a stainless rod with formed loops at the ends instead of cables. Far less bulky looking than a cable loop with thimble and crimped ferrule.

 

[EdStainless]

Carbon fiber on Al is worse than SS on Al.
First question, what Al alloy is it? Can you use a 5052 or 5083? That would help.
I don't see the SS rusting, but you should use coated cable. Even outdoors the coating should have enough UV resistance to last over a year. Look for cable used in sailboat rigging.
Honestly using a brass or fiberglass thimble (if they exist) might eliminate the issue all together.

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P.E. Metallurgy, consulting work welcomed

 

[Tmoose]

What gets hung on the lower cable? Description and weight.
Is this wind powered mobile?

I'm picturing a short grommet made of clear vinyl tubing super-glued in the holes after power coating, as long as Sglue won't craze the PCoating.

 

[Deefer]

Thanks for replying everyone. Some information for you:

The aluminum disc is 5052 grade by necessity, it's the only grade used by the vendor preparing the discs.

Answering Tmoose's questions about what gets hung on the cables, let me offer more details. There are actually two cables suspending each disc (I originally only described one being used, just to make the discussion easier). Each line of cable holds at maximum two 18" wide aluminum discs. The cable lines are stretched taught vertically between a cement anchor and a metal rail 8 feet above. These are not mobiles, it is more of an image "curtain" as pictured in the included photo.

It's sounding like the corrosion is not something I need to worry about. Rather, the main concern would be structural integrity where the cable loops through the disc's holes. I'm inclined to fit a stainless steel eyelet/grommet into the disc holes, so there is no constant motion wearing against the aluminum, rather the motion would then be stainless thimble on stainless grommet.

So some questions moving forward:

1) Is it necessary to use some kind of gasket or fixative to lock the grommet into place on the holes, to prevent movement? Or will snapping/crimping them into position be sufficient?

2) Assuming I need to engineer this to withstand a 250 pound adult grabbing a disc and throwing their weight onto the two cables, what working load should I be targeting? I'm guessing that's somewhere around 1000 pounds without going overboard, but please correct me if I'm wrong.

3) Given your answer to #2, what's the max distance from disc edge for drilling the holes into the aluminum discs? I'd like to know the minimum amount of aluminum I need to leave between the hole and the disc edge. The holes' diameter is 3/8" - 1/2" (1/2" max), the aluminum is 5052 grade 1/8" thick. Again, this needs to survive an adult male messing around with it. As easily accessible outdoor public art, I need to account for unpredictable human interaction.

4) Edstainless recommended using coated cable. If I'm using a thimble and eyelet, what is the long-term gain from using a coated stainless cable? Seems it would be need to be replaced more frequently than bare cable, as the coating deteriorates. I don't want to overly burden the City with replacement because new cable ferrules would need to be re-swaged each time and turnbuckles loosened then properly re-tensioned. Seems like it would be easier to require periodic cleaning of the bare stainless cables, unless the value add of using a coated cable is significant. Please help me understand the benefits better.

please note the bottom photo shows only one cable, but in reality two cables will attach to each disc.

 

[EdStainless]

You could look at the rules for cable railings (stairs and balconies) for an idea of loads.
You will need the cables tight enough to meet the 'ball insertion' requirements so that a child couldn't get there head stuck in between them. In fact the hardware for these railings might be very useful.
Well, on the cable I figured that if the coating held up for a few seasons on a sailboat it would likely last 5 yr in your application, but not strictly needed.
This is going to require periodic re-tensioning of cables (at first maybe every week for a while) and cleaning (hosing off with fresh water).
You do need to make sure that you don't get rust running down from your overhead steel, unless it is SS also.
If it is SS make sure that it gets acid passivated after fabrication and prior to installation (and then forbid touching it with and steel) or you will have rust spots everywhere.
If they go with powder coated then your SS eyes mounted in it will need to be sealed so that rusty water from the inside can't run down the cables.

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P.E. Metallurgy, consulting work welcomed

 

[Deefer]

EdStainless The overhead steel will most likely be stainless, though I may end up going with powder-coated steel instead. Help me understand the rust running down the cables. If I'm using stainless steel (cable and rigging, and overhead steel) and aluminum discs, where does the rust come from? In my imagination, neither aluminum nor stainless steel would produce rust.

 

[EdStainless]

If you have a powder coated steel tube the inside will collect moisture (thermal cycling and condensation), then the inside will rust, and then that rusty water will run out of the holes that have your mounting eyes, down the cables, and onto you Al discs.
Things like this are common in 'artistic' installations.
As are issues with rust spots on SS because the SS touched or was handled using steel (chains, pry bars, truck beds) and got iron on the surface. Often architectural items are fabricated, brushed, acid passivated, and then covered with a plastic film to protect them during transit and installation.

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P.E. Metallurgy, consulting work welcomed