Bearing SMEARING and its Causes.

Smearing

When two inadequately lubricated surfaces slide against each other under load, material is transferred from one surface to the other. This is known as smearing and the surfaces concerned become scored, with a "torn" appearance. When smearing occurs, the material is generally heated to such temperatures that rehardening takes place. This produces localised stress concentrations that may cause cracking or flaking.
In rolling bearings, sliding primarily occurs at the roller end-guide flange interfaces. Smearing may also arise when the rollers are subjected to severe acceleration on their entry into the load zone. If the bearing rings rotate relative to the shaft or housing, this may also cause smearing in the bore and on the outside surface and ring faces. In thrust ball bearings, smearing may occur if the load is too light in relation to the speed of rotation.

Smearing of roller ends and guide flanges

In cylindrical and taper roller bearings, and in spherical roller bearings with guide flanges, smearing may occur on the guiding faces of the flanges and the ends of the rollers. This smearing is attributable to insufficient lubricant between flanges and rollers. It occurs when a heavy axial load acts in one direction over a long period, for instance when taper roller bearings are subject to excessive preloading. In cases where the axial load changes direction, smearing is much less common as the opportunity is provided for the ingress of lubricant when the roller end is temporarily relieved of load. Such smearing can be avoided to a considerable extent by selecting a suitable lubricant.

Figure 1 - Smearing on the surface of a roller from a spherical roller bearing - 100 x magnification.	Figure 2 - A cylindrical roller with end smearing caused by heavy axial loading and improper lubrication.	Figure 3 - Guide flange smearing attributable to the same causes as the smearing in figure 2.

Smearing of rollers and raceways

In certain circumstances, smearing may occur on the surface of rollers and in raceways of spherical and cylidrical roller bearings. This is caused by roller rotation being retarded in the unloaded zone, where the rollers are not driven by the rings. Consequently their speed of rotation is lower than when they are in the loaded zone. The rollers are therefore subjected to rapid acceleration and the resultant sliding is so severe that it may produce smearing.

Figure 4 - Skid smearing in both raceways of a spherical roller bearing outer ring.

Raceway smearing at intervals corresponding to the roller spacing.

Far too often, when cylindrical roller bearings are being mounted, the ring with the roller and cage assembly is entered askew, without being rotated. The rollers then scratch the raceway of the other ring, causing smearing in the form of long, transverse streaks. The rollers may be smeared too. This type of damage can be avoided if the bearing is well lubricated and one of the rings is rotated. When large numbers of bearings are to be mounted it is expedient to employ a mounting ring, see fig 5. Similar damage may arise if the bearing rings are mounted with fits that are too tight in relation to the internal clearance, so that preloading occurs. Smear streaks may also be found in the raceways of spherical and taper roller bearings. These streaks are the result of careless handling or incorrect mounting practice. Blows or heavy pressure applied to the wrong ring, without rotating the bearing, cause the rollers to produce narrow, transverse streaks of smearing in the raceways, see fig 7.

Figure 6 - A cylindrical roller bearing with smear streaks in the inner ring raceway and on the rollers. The smearing has been caused by the roller assembly being entered askew without being rotated.

Figure 7 - Outer ring raceway of a spherical roller bearing with smear streaks caused by a blow against the inner ring.	Figure 8 - One of the smear streaks shown in fig 7 – 50 x magnification

Smearing of external surfaces.

Smearing may occur on the external surfaces of heavily loaded bearings. Here, the smearing is the result of movement of the bearing ring relative to its shaft or housing. Smearing of the inner ring bore, outer ring outside surface and ring faces can only be avoided if the fits are tight enough to prevent movement of the ring concerned in relation to its seating. Increasing the axial compression does not result in any improvement.

Figure 9 – Smeared face of a cylindrical roller bearing inner ring.	Figure 10 – Smeared outside surface of a spherical roller bearing outer ring. Material transfer has occurred from the housing bore to bearing ring.

Smearing in thrust ball bearings.

Smearing may occur in the raceways of thrust ball bearings if the rotational speed is too high in relation to the loading. The centrifugal force then impels the balls to the outer part of the shallow raceways. There the balls do not roll satisfactorily and a great deal of sliding occurs at the ball-to-raceway contacts. This leads to the formation of diagonal smear streaks in the outer part of the raceway. In the case of thrust ball bearings operating under light loads and at high speeds, such damage can be prevented by subjecting the bearings to extra loading, for instance by applying springs, see fig 12.

Figure 11 – Thrust ball bearing raceway with smear streaks on account of the rotational speed having been too high in relation to the load.	Figure 12 – Preloading thrust ball bearing by means of springs.

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