Why do cones pit in hub bearings? So called cones are the the inner races in angular contact bearings in hubs. The outer races, usually simply called races, are usually integral to the hub and cannot be replaced.
Cones however are easy to replace, often one would simply get a new axle with cones and counter nuts included.
Apparently, it is quite common for cones to become pitted. Where the surface of a race is covered by a number of small depressions and in worse cases becomes rough and This may happen in a process like Brinelling where the hardened steel balls leave a small concave impression on the race or spalling where after an initial crack flakes from the surface are shed.*
Pitting is often encountered only on the cones while the outer races remain in good shape. Evidence for the frequency of this failure mode is that replacement cones are readily available, which also indicates demand.
Why is that so?
Is this perhaps coincidental due to load geometry? (Concave vs convex ball race interfaces.)
Or is this by design, for example by different hardness outer races &gt; balls &gt; cones?
*In case of insufficient lubrication and/or corrosion fretting may also lead to pitting.
According to my research, which consists of a couple of web searches and looking at a few photos, the failure mechanism that causes pits in bearing cones is spalling. It is the result of metal fatigue from balls repeatedly rolling over the same part of the cone, and it can be recognized by the irregular shape of the pits. This is different from mechanical wear, which produces a smooth groove and brinelling, which produces smooth indentations at even intervals.
The reason why cone is affected first is that the load is concentrated on smaller surface. The inner diameter of the bearing is smaller than the outer bearing, so the surface is smaller to begin with, and in bicycle wheels the cone does not rotate so the load is concentrated on a small part at the lower side of the cone. Because the outer race rotates, the load is spread to the entire bearing track. The quality of materials has also some effect here: I have seen this damage only on two Alivio hubs. The cheap no-brand cup and cone hubs that I have seen had a worn groove at the bottom side of the cone, and I have not had cone failures in higher tier hubs.