David Stevens IEng - Bearing failures

Bearing CRACKS and Their Causes.

Cracks

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 seating 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.

Fractured outer ring.

Figure 1 - 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.

Cracks caused by rough treatment

Appearance	Cracks or pieces broken off, generally at one face of the bearing.
Cause	Blows, with a hammer or hardened chisel, have been directed against the ring when the bearing was being mounted.
Action	Always use a soft drift or mounting sleeve. Never subject the bearing to direct hits.

Cracked inner rings.

Figure 2

Left hand image : Cracked inner ring of a spherical roller bearing. One roller has been removed to allow the raceway of the leff-hand of the photograph 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.
Right hand image : Inner ring of a spherical roller bearing with outer flange fracture produced by direct hammering.

Cracks caused by smearing

Appearance	Crack or cracks in conjunction with smearing of the bearing ring. The ring may have cracked right across. smearing cracks generally form across a smearing.
Cause	See smearing
Action

Cracked inner ring

Figure 3

Left hand image : Spherical roller bearing inner ring that has cracked right across following smearing of one face. The ring has been mounted to a but spacer that has not been a sufficiently tight fit on the shaft. Consequently the spacer has rotated relative to the shaft and the bearing ring.
Right hand image : Smearing damage on the face of a bearing ring. Note the incipient transverse cracks.

Cracks caused by fretting corrosion

Appearance	Cracks, transverse in inner rings and generally longitudinal in outer rings, in conjunction with fretting corrosion.
Cause	see Fretting corrosion
Action

cracks4.jpg (43536 bytes)

Figure 4
left hand image : Spherical roler bearing inner ring with fretting corrosion and a transverse crack right through the ring. The fretting corrosion has caused the cracking.
Right hand image : Longitudinal crack in a deep groove ball bearing outer ring with freting corrosion.

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