Can nylon gall aluminum

[cobra46]

I am working an a design that involves a linear sliding interface. One of the surfaces is 6061-T6 and the other is MDS filled nylon.

I chose MDS filled nylon because I heard it was good for bearing surfaces. It turns out that the aluminum surface, which initially was smooth (~32), now looks like what can best be described as a galled surface.

Is it possible for nylon to gall aluminum? What materials would be better than MDS filled nylon? I can not use any lubrication nor can I use Teflon.

Thanks,

Kevin

 

[KENAT]

Is the aluminum anodized or anything? If not I wonder if oxidation could be a contributing factor?

UHMWPE or maybe Delrin or some other acetyls like Pomalux might be worth a look depending on application.


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[MintJulep]

Is your MDS filled Nylon perhaps graphite filled Nylon by mistake?

 

[cobra46]

Sorry....the aluminum is not plated or coated.

I was going to try both Delrin and UHMW. I think we have some laying around here. I'm not familiar with Pomalux.

 

[cobra46]

MintJulep,

I suppose it's possible. I bought it from Mcmaster. They have made mistakes before.

 

[byrdj]

need to look at your loading density, ie too little surface area

 

[DesignerGuy16]

Delrin would tear itself up, you'd be better off with a HDPE, UHMW or Nylon for wear.

Look at what Igus is doing, they've got great information on their bearing materials and wear characteristics with different shafting materials.

James Spisich
Design Engineer, CSWP

 

[MintJulep]

Nylon should not gall aluminum. However if it were graphite instead of MDS then you would very likely have anodic corrosion of the aluminum, which might look like galling.

 

[KENAT]

Another thought, depending on application, could some other contamination be getting in and causing the issue?

KENAT,

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[cobra46]

MintJulep - How long do you think the anodic corrosion would take to cause an issue? The damage occurs in less than two hours of operation.

Kenet - We have not ruled out contamination. After the first test there were aluminum particles as well as a few very small machining type chips laying around. The orientation is such that any particles shed fall away from the test surface. I know this may be hard to imagine without a picture. The test surface is near the upper most portion of the machine.

Thanks for the ideas so far.

 

[IRstuff]

How many cycles in the 2 hours of operation?

TTFN

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[cobra46]

The actual numbers are 11300" at 70"/min. This works out to 161 min ( A bit longer than my initial guess). It runs along a path that is about 27" and then returns over and over.

 

[IRstuff]

How much pressure is applied? It's less than 500 cycles.

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[patprimmer]

I have never actually seen MoS2 filled nylon. It has always been a mixture of MoS2 and graphite. as MoS2 does not work as a lubricant when embedded in nylon but graphite works very well in that regard. The MoS2 actually helps by acting as a very effective nucleating agent and creates a much higher level of crystallization and a much finer crystal structure in the nylon. This crystal structure (however it is achieved) gives a harder smoother bearing surface.

The best plastic to use as a bearing is very dependent on a number of factors, including the nature of the opposite surface, lubrication, cooling, speed, pressure and cycle rate.

Hard anodizing the aluminium would help a lot. Even a bit of water as a lube can help. unmodified nylon might do better. Are you sure your nylon is not glass fibre reinforced. The presence of glass fibres certainly gauls aluminuim. I have never seen molly and graphite filled nylon gaul aluminium in the past, but I have only ever used it in contact with aluminium in piston buttons to hold gudgeon pins in place where the circlips are prone to fail. This is hot, bathed in oil and has little motion against the aluminium surface. I suspect glass fibre is present in the nylon.



Regards
Pat
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[MintJulep]

Ohhh..

MoS2+ graphite + aluminum = battery.

The MoS2 acts as an electrolyte and the aluminum corrodes.

 

[plasgears]

Neat nylon has been under-rated as a structural, wear resistant material. Two experiences with major projects were solved with neat nylon.

 

[patprimmer]

I agree with plasgears and note that a highly nucleated grade makes a better bearing for the same reasons as MoS2 filled.

Bar stock is unlikely to be nucleated. Fast cycle injection moulding grades are nucleated, but may be difficult to get in the size and numbers you require but it's no big deal if you can't get it unless the nylon part fails.

Nylon 4.6 or Stanyl is a better bearing than type 6 or 6.6 and is inherently crystalline enough to not need extra neucleation, but is a lot more expensive and not justified unless you have a failure with the others.

Regards
Pat
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[GregLocock]

Just a gentle thought, why are we concentrating on the nylon, rather than wondering why a sliding joint uses aluminium anyway? I know if you anodize it you can get some hardness and wear resistance into it, but, frankly, like the Cornish villager said to the tourist when asked for directions, If Oi wanted to get there Oi wouldn't be starting from here.


Cheers

Greg Locock

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[HDS]

As has been suggested I would look at commercial solution from someone like IGUS or trstar.

Is there any reason you don't want a bearing grade of teflon?

UHMW as many have said is a good solution. It can be found in tape form for a quick test. It could also test the galvanic corrosion theory.

Hard anodized AL parts can also be treated to reduce friction.

 

[NomLaser]

There are some companies that can apply Teflon to your aluminum surfaces. We used this for potting tooling, where a plug was need to create a recess in potting. It was a nice coat and it was pretty flexible too.