Bearing Maintenance

[porthole]

FAA-H-8083-30, Aviation Maintenance Technician Handbook-General, FAA-8083-31, Aviation Maintenance Technician Hanbook-Airframe Volume 1 and AC 43-4A. The dark lines running perpendicular to the race or parallel with the rollers, (water marks) is usually caused by water between the bearing roller and the race of a stationary bearing. The water acts as an electrolyte or medium allowing for a small electric current to start between the two surfaces causing dissimilar metal corrosion and eventually pitting of one or both surfaces. In the early stages the marks can probably be lightly buffed off because the corrosion is on the surface. Inter granular corrosion is caused by the same electrolyte action but between different alloys within the metal itself usually caused by lack of uniformity during the manufacturing process i.e. improper heat treating. Aluminum structures like wing spars are more susceptible to this type of corrosion but it can and does occur in steel alloys. Inter granular corrosion can show up as a discoloration of the material or not at all but either way since it is corrosion that starts below the surface it can't be buffed out. The more technical name for these types of corrosion is electrochemical corrosion. Heat damage will cause a blueish color in the race or rollers and more advanced heat damage will cause galling. Galling is caused when the surface of the roller and surface of the race get hot enough to weld together and as the bearing moves or turns a layer of welded surface material is pulled up from the race or off of the roller.

I would be interested in the text that refers to Inter granular corrosion caused by the two dissimilar metals in an electrolyte and how it relates to automotive wheel bearings.
Some typical corrosion issues are with different metals immersed in an electrolyte. The further apart on the nobility scale the greater the corrosion potential.
Boat trailers can be susceptible to this because of of trailer dunking when the bearings are warm or hot.

Anything I have seen on Inter granular corrosion points to it being more of an issue with the various stainless steels and aluminum.

The three main parts of a wheel bearing are similar metals, and a couple drops of water is not enough of an electrolyte, especially fresh water.
It has been my understanding that the corrosion / lines you find on a race, that are parallel with the rollers, is caused by "fretting"

Galling is not welding. Galling is caused by metals in sliding contact with improper lubrication.

 

[whp4262]

I would be interested in the text that refers to Inter granular corrosion caused by the two dissimilar metals in an electrolyte and how it relates to automotive wheel bearings.
Some typical corrosion issues are with different metals immersed in an electrolyte. The further apart on the nobility scale the greater the corrosion potential.
Boat trailers can be susceptible to this because of of trailer dunking when the bearings are warm or hot.

Anything I have seen on Inter granular corrosion points to it being more of an issue with the various stainless steels and aluminum.

The three main parts of a wheel bearing are similar metals, and a couple drops of water is not enough of an electrolyte, especially fresh water.
It has been my understanding that the corrosion / lines you find on a race, that are parallel with the rollers, is caused by "fretting"

Galling is not welding. Galling is caused by metals in sliding contact with improper lubrication.

Ok now you are going to make me work.
Galling is caused by metals sliding together without lubrication which generates enough heat to weld the surfaces. Picture of the input bearing on my dump truck the night I lost all the gear oil and some text.

Fretting is caused by two normally stationary surfaces moving slightly.

Inter granular corrosion is an attack along the grain boundaries of a material. Each grain has a clearly defined boundary which, from a chemical point of view, differs from the metal within the grain center. The grain boundary and grain center can react with each other as anode and cathode when in contact with an electrolyte. (Copied from AC 43-4A). You are correct that it is most common in aluminum alloys and stainless.

Heat damage.

Water marks, those little dark lines.

The other perpendicular lines or marks on the race that can occur which are not water marks is False Brinelling caused by vibration of the bearing while in a static state. Hope you can read the text in the pictures.


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[porthole]

I have seen a bearing or two like that in my former career, and happy that I no longer have to give the customer the bad news on the costs of repairs

Ok now you are going to make me work.
Galling is caused by metals sliding together without lubrication which generates enough heat to weld the surfaces. Picture of the input bearing on my dump truck the night I lost all the gear oil and some text.

I get that this is out of an aircraft manual, and aircraft types are far smarter then me - but this goes against what a weld truly is, which is why I brought it up before.

Inter granular corrosion is an attack along the grain boundaries of a material. Each grain has a clearly defined boundary which, from a chemical point of view, differs from the metal within the grain center. The grain boundary and grain center can react with each other as anode and cathode when in contact with an electrolyte. (Copied from AC 43-4A).

You are correct that it is most common in aluminum alloys and stainless.

This is what piqued my curiosity.
Years ago I had some issues with stainless boat shafts and the experts couldn't agree on whether it was galvanic, inter granular, pitting, crevice, lack of oxygen in the water, not austenitic enough or what.

Either way, cost me a lot of $$$.

You are correct that it is most common in aluminum alloys and stainless.

The only reason I brought it up as it relates to our wheel bearings.

 

[porthole]

There are some bearing companies that over simplify this by just calling it rust.

Water marks, those little dark lines.

The other perpendicular lines or marks on the race that can occur which are not water marks is False Brinelling caused by vibration of the bearing while in a static state.

 

[whp4262]

There are some bearing companies that over simplify this by just calling it rust.

Ouch! Just a guess but I'm thinking you can't get one of those shafts at Walmart for a $100.00. I would guess inter granular corrosion. If my memory serves me correctly galvanic corrosion occurs when two dissimilar metals are attached i.e a steel fitting bolted to an aluminum structure etc.

I got my first paying mechanic job 40 years ago. Worked on cars, trucks and did plant maintenance the first 12 years before going into aircraft. I've been a licensed aircraft mechanic (A&P) for the last 28 years and held an Inspection Authorization (one step above an A&P) for the last 25 years. I think we both understand a little something about corrosion but we know it by different names, different environments and explain it differently. Good discussion though.


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[jbeletti]

I think we both understand a little something about corrosion but we know it by different names, different environments and explain it differently. Good discussion though.

I think that's a good characterization. You both seem super-knowledgable in this area but the different environments may be the reason why you see things a little differently or know things by another name.

Great conversations in any event and thank you both for a lively and civil debate

 

[Jim.Allison]

I wonder is purging with new grease, via the EZ lube system, before and after each season, would help with such problems?

 

[whp4262]

I think that's a good characterization. You both seem super-knowledgable in this area but the different environments may be the reason why you see things a little differently or know things by another name.

Great conversations in any event and thank you both for a lively and civil debate

Thanks jebetti. Guess the way I see it is it defeats the purpose of a fun, friendly and educational forum if you are going to get all twisted up because someone disagrees with or questions your opinion on a subject. Friendly debates make you think and see things in a different way.


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[porthole]

Ouch! Just a guess but I'm thinking you can't get one of those shafts at Walmart for a $100.00. I would guess inter granular corrosion.

You are correct on that guess. 12' by 1 3/4" of Aquamet 22 shafting, at the time was $2400 (included new coupling machined flat to existing trans coupling).

My guess was inter-granular as well, but there is the argument for crevice corrosion as well. Although, the area where the boat was docked was very clean and well oxygenated water.

If my memory serves me correctly galvanic corrosion occurs when two dissimilar metals are attached i.e a steel fitting bolted to an aluminum structure etc.

Two dissimilar metals, for those following along that are on a "nobility scale" in relation to each other, typically immersed in an electrolyte.
See http://en.wikipedia.org/wiki/Galvanic_series

The shafts were just one form of expensive corrosion we experienced that year. We also had props ruined ($4200) from galvanic corrosion, rudders fall apart and almost lost most of the underwater metals, including all the paint (copper based) around any underwater fitting.

All thanks to crappy wiring in the marina and one nearby boater with electrical issues.


On a previous boat - go out diving one day and notice all the zincs are gone, odd, they are only a few weeks old.
Come back in and start changing out the zincs - and some of them start to bubble!

Find out my new neighbor as an automotive battery charger plugged in (cheap sailboter, won't buy a marine grade charger).
Automotive charges are not isolated properly and it was causing stray current in the surrounding waters.
I unplug it and the problem stops.

 

[whp4262]

Interesting. Don't think I would have guessed the battery charger or wiring issue but that makes sense.


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