Fractures, impact load and the rare real manufacturing defect
Broken rings, split rollers and broken cages usually follow heavy overload or misalignment. A true bearing material defect is rare and has a recognisable signature.
Fractures
Fractures in races or rolling elements are normally due to heavy overload or misalignment. In the example set below, a winch gear shaft with a mating gear showing severe misalignment: the outer race at one end fractured from the prying action of the misaligned shaft, the rib on the other end fractured in a brittle way from shock, several rollers split in half (ductile in the soft core, brittle in the hard case), and the housing at one end fractured first, which caused the misalignment.
Impact load damage
Impact and vibration also break cages and chip rings. Look at where the indentations are: if dents on a ball track repeat at ball pitch and later flake, an impact load was the start.
When it really is the bearing
A bearing failed after only a few hundred hours: the cage was broken into many pieces; the outer race and balls looked almost normal with only light scuffing from cage pieces; the inner race had heavy plastic deformation and a notch that could be cut with a file, hardness below 20 HRC. There was no heat discoloration on the parts, so the hardness could not be explained by overheating. The inner race had most probably not been heat treated correctly.
- Check hardness on the failed part and on a spare from the same lot before deciding.
- No heat colour and a soft ring is the signature of a heat treatment problem.
- Such cases are rare. Fit, alignment, lubrication and handling first.