The bearing I cleaned and killed
Removing the lubricant along with the contamination.
A bearing can be cleaned to death because the solvent wash or compressed-air blast that strips away contaminating grit can just as easily strip away the thin film of lubricant the rolling surfaces depend on. What remains is a bearing that looks spotless and has nothing left protecting it from the wear the cleaning was meant to prevent.
The interval after cleaning
A bearing's grease or oil does two jobs at once: it keeps the rolling contact between ball and race running smoothly, and it forms a continuous film that stops hard, precisely finished metal surfaces touching each other directly. A solvent strong enough to dissolve and carry away the grit described in the previous article will also dissolve the grease that grit was embedded in, and a jet of compressed air displaces a residual oil film as readily as it displaces dust. The bearing is at its most vulnerable in the short interval after cleaning and before fresh lubricant has been worked back into the contact, when its hardened surfaces are at once cleanest and most exposed. Left like that, bare steel starts picking up moisture from ordinary humid air, and corrosion can begin on a rolling surface within the time it takes to reassemble the machine around it.
Washing a cast-iron pan
A well-seasoned cast-iron pan carries a thin, baked-on layer of oil that keeps the bare iron from meeting air and moisture. Washing it with soap and hot water strips that seasoning along with the food residue, leaving bare iron that looks perfectly clean and starts rusting within days unless it is dried and re-oiled. The soap has done exactly what it was asked, removing everything on the surface without telling residue from protection. A cook who dries the pan straight away and works a thin coat of oil in while it is still warm rarely sees rust, while a pan left to air dry overnight turns up spotted by morning. A degreased bearing is in the washed pan's position, spotless and exposed, and it stays protected only if someone deliberately restores the film.
Why the spotless bearing is the dangerous one
A visibly dirty bearing announces its condition, and nobody mistakes it for ready to run. A freshly cleaned one gives no such warning, because an eye or a fingertip can tell that grit is gone but cannot tell whether a protective film remains. The trap tends to catch conscientious people, since taking a bearing apart to clean it thoroughly is the more responsible choice, which is why the failure feels so unfair when it arrives.
A procedure worth trusting treats relubrication as part of the cleaning step itself, working fresh grease or oil into the contact as soon as the solvent or air blast has finished, before the bearing is set aside or reassembled. Labelling a cleaned bearing as still needing lubrication, instead of trusting memory to carry that step across an interruption or a shift change, closes the gap where this failure lives.
My key error with this
The bearing had picked up contamination and I wanted it properly clean, so I used a general purpose degreaser, which removed the contamination extremely effectively and also removed rather more than that. The solvent attacked the elastomer seal inside the bearing, and a seal that has been softened stops keeping the grease in and the dirt out, so a component that had been running quietly for a long time began making noise within days and then failed properly not long after. It ran fast enough that the loss of retained lubricant showed up quickly rather than gradually. What replaced the belief is that cleaning is a chemical operation performed on every material present rather than only on the dirt, and that a sealed bearing is an assembly containing rubber and grease with defined chemistry, so the correct question before applying any solvent is what else in there is soluble.