Bearing INDENTATIONS and Their Causes.

Indentations

Raceways and rolling elements may become dented if the mounting pressure is applied to the wrong ring, so that it passes through the rolling elements, or if the bearing is subjected to abnormal loading while not running. Foreign particles in the bearing also cause indentations.

Indentations caused by faulty mounting or overloading

The distance between the dents is the same as the rolling element spacing. Ball bearings are prone to indentations if the pressure is applied in such a way that it passes through the balls during the mounting or dismounting operations. Self-aligning ball bearings are particularly susceptible to damage in such circumstances. In spherical roller bearings the damage originates as smearing and subsequently, if the pressure increases, develops into a dent. The same conditions apply in taper roller bearings that are unduly preloaded without being rotated.
Bearings that are mounted with excessively heavy interference fits, and bearings with tapered bore that are driven too far up the shaft seating or sleeve, also become dented.

Appearance	Indentations in the raceways of both rings with spacing equal to the distance between the rolling elements.
Cause	Mounting pressure applied to the wrong ring. Excessively hard drive-up on tapered seating. Overloading while not running
Action	Apply the mounting pressure to the ring with the interference fit. Follow carefully the manufacturers instructions concerning mounting bearings on tapered seating. Avoid overloading or use bearings with higher basic static load ratings.

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Figure 1 - Washer of a thrust ball bearing subjected to overloading while not running. The indentations are narrow and radially aligned, not sphered as in radial ball bearings.


Figure 2	Figure 3	Figure 4

Figures 2-4 are examples of the results of improper handling. A roller in a double row cylindrical roller bearing has suffered impact (fig 2). A periphery camera view of the roller shows two diametrically opposed indentations (fig 3). The roller has, in turn, dented the inner ring raceway (fig 4).

Indentations caused by foreign particles

Foreign particles, such as swarf and burrs, which have gained entry into the bearing cause indentations when rolled into the raceways by the rolling elements. The particles producing the indentations need not even be hard. Thin pieces of paper and thread from cotton waste and cloth used for drying may be mentioned as instances of this. Indentations caused by these particles are in most cases small and distributed all over the raceways.

Appearance	Small indentations distributed around the raceways of both rings and in the rolling elements.
Cause	Ingress of foreign particles into the bearing.
Action	Cleanliness to be observed during the mounting operation. Uncontaminated lubricant. Improved seals.

Smearing damage and cracks.

Figure 5 - Indentations, caused by dirt, in one of the raceways of a roller bearing - 50 x magnification.

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