hmmmm...black oxide bearings seem to have issues

[2manyoranges]

Just really working through bearing problems and solutions.

Amongst the many bearing-related issues I have been engaged with is the conundrum of black oxide bearings moving in the bearing seats. I have always used ss bearings and have only had a few instances of VERY neglected bearings moving in the seats. I recently overhauled a very neglected Transition Scout (FS 140 travel F&R), and ALL the bearing were terribly notchy and stiff. Terrible. Some very difficult to turn with fingers. But none of them had rotated in the housings. Not so the next Transition. Black oxide, expensive bearings throughout. No notchiness in the bearings, some a tiny bit stiff due to thickened grease. But all serviceable - no need to replace apart from....the fact that they had turned in the housings...rendering the frame highly problematic. Grrrr. I inspected the bearings closely ... they turned pretty freely but the oxide coating had been entirely worn off and the surface now heavily brindled with movement-related wear. Around 0.5mm smaller diameter to the bearing than should be the case, and massive wear to the housing - argh.

I now think the sequence in the problem was...

1 bearing installed in tight housing
2 black oxide partially or fully worn off during installation since softer than stainless
3 this reduced the friction co-efficient with the housing
4 with low 'hold' in the housing any minor performance in the bearing can cause the outer to move in the housing
5 movement further wears the oxide coating, increasing the amount of movement in the housing and causing nasty wear - BAD vicious circle

There's no way that these bearings were seized or notchy - the SS ones from the Scout - same housings - WERE dreadful. But no harm to the housings. These latter oxide ones were FAR better condition but ironically really bad damage to the housings.

I am NOT using black oxide again. Even with retaining compound they will be prone to this problem since it is a coating - a thin layer of molecules. SS bearings have a surface which is integral to the structure. Better - and cheaper. Installed with medium-strength retaining compound (eg Park RC-1 or Loctite 290), they will resist water incursion and prevent housing movement.

Thoughts?

 

[Tootyred]

The coating is so thin, im struggling to see how it could effect fit or operation?.....

The coating is normally between 1 and 2 thousands of a mm thick.

The interference tolerance would be far bigger, so the coating coming off during install, should make limited to no difference.

Sure the components were running straight and true on reassembly and not twisting the bearings ?

 

[Twozaskars]

Could the original bearings in the frame have become so seized/stiff that the movement of the frame resulted in those bearings wearing the housings? Then the next bearings - you are dealing with now - are just being blamed for being in the wrong place at the wrong time.

 

[2manyoranges]

Could the original bearings in the frame have become so seized/stiff that the movement of the frame resulted in those bearings wearing the housings? Then the next bearings - you are dealing with now - are just being blamed for being in the wrong place at the wrong time.

That's a possibility - I asked about the history of the frame but it may not have been accurate of course. That's indeed there in the background.

I have been digging around on metallurgy sites and it was this that got me thinking...

from
https://www.xometry.com/resources/machining/what-is-black-oxide/

'....The physical properties of black oxide coatings are:

1. Black oxide coating enhances the hardness of the outer surface. This increased hardness has value in applications in which resistance to abrasion and wear is crucial, providing a durable surface that withstands mechanical stress.
2. The coating lends a lower coefficient of friction and greater scratch resistance to part surfaces.
3. Beyond the functional physical enhancement of the surface, black oxide coatings also add to the aesthetics of components in the form of a matte black finish.

and

black oxide coatings are less durable and offer shorter service life in wet or acidic environments...'

for sure main bearings on an FS frame are prone to getting VERY wet. It was the lower coefficient of friction which got me wondering.

Tootyred - hmm I should have thought about inference fit measurement at 30mm (the outside diameter of the bearing) in relation to the thickness of the oxide - which I have no value for...I didn't think of that and should have...

This reference says:

https://fractory.com/limits-and-fits/

'...Looking at the IT7 tolerance grade, the chart gives an allowed variance of 0.021 mm.

The letter signifies the start of the tolerance zone. For H7, the starting point is at exactly 25.000 mm. The maximum hole size is then 25.021 mm. For F7, the tolerance range is the same, but the starting point is 25.020 mm, taking the last acceptable measurement to 25.041 mm...'

So you are right 2 hundredths of a mm.

And then xometry site says:

'...Typically measured in millionths of inches, the coating thickness adds virtually no extra thickness to part surfaces, making it ideal for high-precision parts with tight tolerances...'

Ah ... so my first explanation looks suspect and the 'these were put into already-worn housings...' begins to look more likely..