I recently noticed a strange "sand paper feeling and noise" when turning the wheel of my bike, and decided that it was time to learn to service the bearings of the hubs (cups and cones bearings, not sealed ones).
I discovered that the cone looks like this:
The cup on the hand seems to be fine.
I have few questions related to this:
Is it normal wear, or can it be avoided by more systematic maintenance? - if the info is relevant to answer, it's the side with the brake rotor, the other side has a discolored mark, but is smooth.
What would be the cause, ingress of dust/sand in the bearings? (I'm riding between 200km and 400km per month, mix of road, gravel (dusty) and light forest trails, the bike is almost 2 years old), almost only dry. There was still a fair amount of grease in the bearing.
I couldn't find a matching parts on large online bike shops: is this part part of a wider standard and can I find only the cone, or I should I replace full bearing? (cone/cup/bearings). In the second case, what would be the "key measurements" to consider when looking for bearings? (I didn't find the info on Formula's website - the hub is a formula DC20/22 according to the spec sheet of the bike).
If finding parts is difficult, are there consequences of continuing to use the wheels/hub? (except accelerated wear?). Before this discovery, I was actually considering buying a second pair of wheels and keep the current ones for winter/spares.
[ADDED] I also noted clear bearing marks on the cone — I lock the suspension when riding on roads, would it be possible that a shock has cause such markings (not speaking about the fine marks that can be caused by sand)?
That is NOT normal wear and proper cleaning, lubrication, and adjustment would have prevented that damage.
Riding in the rain, through puddles, washing the bike with water, or just dirt getting carried in by the air. From your description of your riding, there is plenty of dirt and grit to get in there. It just happens.
Any LBS with a service department should have a supply of axle cones. The one you are showing is pretty common. You probably also need to replace the cup which is in the hub. The ball bearings are held between the cone and the cup. If one is damaged the other is almost always in similar condition.
This will not run smoothly again and the wear will just accelerate. Eventually you will cause damage to non-replaceable parts if that has not happened already. Then you're looking at new wheels or at least new hubs.
UPDATE: You posted a photo of the cup and at least from what I see in the image, there does not appear to be damage there. So you need only to replace the cone. Check the bearings themselves and see if they need to be replaced also. They should be shiny and smooth with no pits or discoloration.
If the bearing hasn't been contaminated, there's a good chance that this is a manufacturing defect or a cheap part that wasn't intended to survive in continuous use. Both would be good for warranty replacement if you notice them early enough.