Flaking occurs as a result of normal fatigue, i.e. the bearing has reached the end of its normal life span. However, this is not the commonest cause of bearing failure. The flaking detected in bearings can generally be attributed to other factors. If the flaking is discovered at an early stage, when the damage is not too extensive, it is frequently possible to diagnose its cause and take the requisite action to prevent a recurrence of the trouble. The path pattern of the bearing may prove to be useful, see section Path patterns.
When flaking has proceeded to a certain stage, it makes its presence known in the form of noise and vibrations, which serve as a warning that it is time to change the bearing.
The causes of premature flaking may be heavier external loading than had been anticipated, preloading on account of incorrect fits or excessive drive-up on a tapered seat, oval distortion owing to shaft or housing seat out-of-roundness, axial compression, for instance as a result of thermal expansion. Flaking may also be caused by other types of damage, such as indentations, deep seated rust, electric current damage or smearing.
Flaking caused by preloading
|Heavily marked loading pattern in the raceways.||Preloading on account of fits being too tight.||Alter the fits or select bearing with larger internal clearance.||Flaked cone and rollers of taper roller bearing. Heavy loading and inadequate lubrication are the causes of this damage (fig 1).|
|Flaking usually in the most heavily loaded zone||Exessive drive-up on a tapered seat||Do not drive the bearing so far up its tapered seat. Follow carefully the instructions given by SKF.||Outer ring of self-aligning ball bearing that has been driven up too far up its tapered seat (fig 2).|
|Single row angular contact ball bearings or taper roller bearings adjusted to give excessive preload.||Re-adjust the bearings to obtain lighter preload.|
|Temperature differential between inner and outer rings too great.||Select bearing with larger internal clearance.|
Flaking caused by oval compression
|Heavily marked path pattern at two diametrically opposed sections of either bearing ring.
Flaking in these sections.
|Oval shaft or oval housing seat. The latter is a common defect in split housings and machine frames.||Usually it is necessary to manufacture a new shaft or a new housing to remedy this defect. One expedient is to spray metal on the components and then regrind. If it is a matter of an oval shaft with the bearing mounted on a sleeve, it is possible to adjust the shaft by grinding, in certain cases.||Flaking in the outer ring of spherical roller bearing that has been mounted in a housing with oval bore (fig 3).|
|The bore of plummer blocks mounted on an uneven base becomes oval when the base bolts are tightened.||Adjust the base.|
Flaking caused by axial compression
|Deep groove ball bearings: heavily marked path patterns displaced to one side of both rings.
Self-aligning ball bearings and spherical roller bearings: severely marked raceway path patterns from one row of rolling elements.
Flaking in these areas.
Single row angular contact ball bearings and taper roller bearings: same appearance as damage resulting from preloading see.
|Incorrect mounting which results in axial loading, e.g. excessive preloading of angular contact ball bearings and taper roller bearings.||Check adjustments when mounting the bearings.||Outer ring of self-aligning ball bearing subjected to excessive axial loading. Flaking in the load zone (fig 4).
Flaked inner ring of a spherical roller bearing. The extent of the flaking around one entire raceway indicates that the axial load has been very heavy in relation to the radial load (fig 5).
|The non-locating bearing has jammed.||Check the fit and lubricate the surfaces.|
|Axial freedom of movement has not been sufficient to accommodate the thermal expansion.||If temperature differential between shaft and housing cannot be reduced, provide greater freedom of movement.|
Flaking caused by misalignment
|Deep groove ball bearings: diagonal path pattern, severely marked at two diametrically opposed sections.
Cylindrical roller bearings: flaking at the edge of the raceway.
|Bearing seats out of alignment.||Rectify the seats.||Deep groove ball bearing outer ring that has been out of alignment with the shaft. The ball path has an oval configuration on account of the misalignment. The result is the same as with oval compression (fig 6).
Cylindrical roller bearing inner ring with flaking at one side of the raceway, as a result of overloading due to misalignment (fig 7).
|Bearing mounted on the skew.||Use mounting sleeve with parallel faces.|
Flaking caused by indentations
|Flaking in conjunction with indentations coinciding with the rolling element spacing.||Indentations resulting from faulty mounting practice or overloading of the non-rotating bearing, see "Flaking caused by preloading".||Various stages of flaking in the inner ring of a deep groove ball bearing. The ring has been mounted with interference fit on the shaft and blows have been directed against the outer ring, causing the mounting force to pass through the balls, which have produced the indentations (fig 8).
Flaking (the dark areas) initiated by the adjacent indentations - 100× magnification (fig 9).
|Flaking in conjunction with small indentations.||Indentations made by foreign particles, see section Indentation.|
Flaking caused by smearing
|Flaking at the start of the load zone in raceways of roller bearings.||Skid smearing, see section Smearing.|
|Flaking , coinciding with the roller spacing, in raceways of roller bearings.||Transverse smearing resulting from faulty mounting practice, see section Smearing.||Inner ring of a cylindrical roller bearing with extensive flaking caused by smearing of the kind depicted in next fig (fig 10).
Inner ring of a cylindrical roller bearing with smearing, at intervals corresponding to the roller spacing, caused by incorrect mounting (fig 11).
Flaking caused by deep seated rust
|Flaking originating from rust damage.||Deep seated rust, see section Corrosion.||Flaking originating from deep seated rust on the roller of a spherical roller bearing (fig 12).
Cross section through the roller depicted in previous figure, showing the way in which the fatigue crack extends beneath the surface (fig 13).
Magnification of the damage shown in previous figs (fig 14).
Flaking caused by fretting corrosion
|Flaking in the raceway of either the inner or outer ring. Corroded area at corresponding part of the inner bore of outside surface.||Fretting corrosion, see section Corrosion.||Flaking in the raceways of the outer ring of a spherical roller bearing. Corresponding area of advanced fretting corrosion on the outside surface (for this photo the ring has been placed in front of a mirror). Development of the corrosion has been accompanied by an increase in volume that has led to deformation of the bearing ring and localised overloading. The results have been premature fatigue and flaking (fig 15).|
Flaking caused by fluting or craters
|Flaking in conjunction with bright or corroded fluting or craters.||Wear resulting from vibrations while the bearing was not running, see section Wear.||Flaking in both raceways of the inner ring of a spherical roller bearing. The flaking has originated from the vibration markings (fig 16).|
|Flaking in conjunction with dark-coloured or burnt fluting or craters.||Electric current damage, see section Passage of electric current.||The outer ring of a self-aligning ball bearing with flaking caused by craters that have formed in conjunction with the passage of electric current (fig 17).|