Cracks may form in bearing rings for various reasons. The most common cause is rough treatment when the bearings are being mounted or dismounted. Hammer blows, applied direct against the ring or via a hardened chisel, may cause fine cracks to form, with the result that pieces of the ring break off when the bearing is put into service. Excessive drive up on a tapered seat or sleeve is another cause of ring cracking. The tensile stresses, arising in the rings as a result of the excessive drive-up, produce cracks when the bearing is put into operation. The same result may be obtained when bearings are heated and then mounted on shafts manufactured to the wrong tolerances.
The smearing described in an earlier section may also produce cracks at right angles to the direction of slide. Cracks of this kind produce fractures right across the rings.
Flaking, that has occurred for some reason or other, acts as a fracture notch and may lead to cracking of the bearing ring. The same applies to fretting corrosion.
Cracks caused by rough treatment
|Cracks or pieces broken off, generally at one face of the bearing ring.||Blows, with hammer or hardened chisel, have been directed against the ring when the bearing was being mounted.||Always use soft drift or mounting sleeve. Never subject the bearing to direct hits.||Fractured outer ring of a self-aligning ball bearing. The indentations visible at the bottom edge of the ring were caused by rough treatment and the crack originated at one of these indentations (fig 1).
Cracked inner ring of a spherical roller bearing. One roller has been removed to allow the raceway of the left-hand of the photo to be examined. The roller has then been hammered back in place, causing part of the centre flange to break away. The impacts have been transmitted via a roller in the other row and part of the outer flange has broken off too. At the same time the ring has cracked right through (fig 2).
Inner ring of a spherical roller bearing with outer flange fracture produced by direct hammering (fig 3).
Cracks caused by excessive drive-up
|The bearing ring has cracked right through and has lost its grip on the shaft.||Excessive drive-up on a tapered seat or sleeve.||Follow carefully the SKF mounting instructions concerning bearings on tapered seat.||Section of the inner ring of a spherical roller bearing - 3,5× magnification. The ring has cracked because of excessive drive-up. The fracture originated at the dark area by the bore chamfer (fig 4).
Fractured surface of the inner ring in previous figure (fig 5).
|Interference fit on cylindrical seat too heavy.||Alter the fit|
Cracks caused by smearing
|Crack or cracks in conjunction with smearing of the bearing ring. The ring may have cracked right across. Smearing cracks generally form across the smearing.||Smearing. See section Smearing.||Spherical roller bearing inner ring that has cracked right across following smearing of one face. The ring has been mounted to abut a spacer that has not had a sufficiently tight fit on the shaft. Consequently the spacer has rotated relative to the shaft and the bearing ring (fig 6).
Smearing damage on the face of a bearing ring. Note the incipient transverse cracks (fig 7).
Cracks caused by fretting corrosion
|Cracks, transverse in the inner rings and generally longitudinal cracks in outer rings, in conjunction with fretting corrosion.||Fretting corrosion. See section Corrosion.||Spherical roller bearing inner ring with fretting corrosion and a transverse crack right through the ring. The fretting corrosion has caused the cracking (fig 8).
Longitudinal crack in a deep groove ball bearing outer ring with fretting corrosion (fig 9).