Spalled Bearings

Situation

On a EUCLID EH 1600 haul truck, a rigid-frame truck used in topsoil and stripping activities, the differential pinion bearing failed, the cage broke and the inner race spalled. In this application, the inner race is the rotating race while the outer race is stationary.

The strange thing about this failure is that the inner race pitted for half of the circumference while the remaining half does not appear to be damaged at all. The outer race is not damaged and appears to be in relatively good order.

The crown wheel and pinion gear teeth and supporting bearings are in good condition and were not replaced. It is not clear why the failure affected only half of the inner race circumference.

Question

Has anyone seen something similar? What might have caused this strange occurrence?

Answer No. 1
This case history describes a macropitting failure (sometimes called spalling) that occurred on a rear axle pinion bearing.

Macropitting over a sector of the inner ring (IR) of a tapered-roller bearing is not unusual, even with a rotating IR.

This type of damage can be caused by a load that rotates with the IR such as a force due to unbalance. With a rear axle, the drive shaft could be unbalanced and hence impose a load that rotates with the pinion-bearing IR. Additionally, we frequently find macropitting on only a sector of a rotating IR in applications where the load is known to be unidirectional.

Once macropitting starts, it creates a geometric stress concentration at the edge where damaged and undamaged surfaces meet. As the rollers roll over the edge of the macropitted boundary, geometric stress concentration creates high Hertzian stresses that cause the macropitting to advance opposite to the direction of rotation of the IR.

Furthermore, we are firm believers in the “hydraulic propagation theory” that hypothesizes macropitting propagates because oil is trapped in cracks associated with macropitting and pressurized by the rollers. Pressurized oil at tips of cracks causes tensile stresses that advance the cracks. With time, macropitting works its way around the IR. If the bearing is removed from service soon after the macropitting started, surface damage will be evident on only a sector of the IR.

The case history also describes a broken cage. This usually occurs in tapered-roller bearings when they have excessive clearance, when axial loads reverse quickly, or when debris from a failure lodges between a roller and the cage. In this case history, the cage probably failed because of excessive clearance created by the macropitting or possibly from roller jamming due to debris from the macropitting. The cage failure is probably a secondary failure that occurred as a consequence of the primary failure mode of macropitting.

Bob Errichello, Consultant
GearTech

Answer No. 2
It appears there are axial marks on the unspalled portion of the inner ring raceway. These axial patterns may be caused by corrosion or vibration. It is likely the spalling initiated at the location of one of these axial marks. The spalling then progressed in the direction of rolling. With the degree of surface spalling that has occurred, it is inevitable that particle damage exists throughout the remainder of the inner ring and the outer ring raceway surface and the outer ring raceway surface. The inner ring raceway is subjected to a higher stress than the outer ring raceway.

Tom Jendzurski, Senior Applications Engineer
SKF USA Inc.