Copyright Goodheart-Willcox Co., Inc. Heavy Equipment Power Trains and Systems 596 Spalling Spalling is a surface contact stress fatigue in which the mating surfaces of a component or components (such as gear teeth) chip, break, or flake off in pieces. See Figure 17-44. Common causes of spalling are lack of proper lubri- cation, extended periods of excessive loading, or load misalignment. Spalling can also occur as a result of initial surface pitting or bearing brinelling. To prevent future occurrences, ensure that the component receives proper lubri- cation, is properly aligned, and is not excessively loaded. Brinelling Brinelling is the result of a hardened metal surface exhibiting one or more indentations. Bearings are the most common component with brinelling type failures. The needle, roller, or ball bearing create a valley in the component, such as an indentation in the bearing’s race or in the yoke of a universal joint. The precision grinding of bearing races leaves fine lines in the race that are difficult to see. If the fine lines (precision grinding marks) are visible in the valley of the brinelling indentations, the condition is known as true brinelling. True brinelling results in permanent indentations in the bearing race without the loss of metal. It occurs due to an impact or shock overload that causes the bearing to indent the race. If brinelling is due to a load, inves- tigate whether it is a static overload or a shock load. When the overload source has been determined, take steps to correct the cause of the overload. Pro Tip Hammering a bearing into position can cause true brinelling. This cause of brinelling can be prevented by using a hydraulic press to properly push the bearing into place. If the brinelling indentations show no evidence of precision grinding marks, the condition is known as false brinelling. False brinelling occurs over time and is often caused by excessive vibration that occurs when the bearing is stationary. The indentations are worn into the race and the mate- rial has been removed. False brinelling indentations can also be caused by electrical current flow through the bearing, large particle contamination, and corrosive etching. Preventative action includes eliminating bearing con- tamination and ensuring that components have good electrical grounds to prevent current flow through the bearing. See Figure 17-45. Fretting Corrosion Fretting corrosion is a surface wear failure that occurs on the surfaces of two close-tolerance stationary components that have moved. Examples are the outside race of a bearing pressed into a housing, or the inside race of a bear- ing pressed onto a shaft. Vibration or movement causes the two normally sta- tionary components to make minor oscillations in the absence of lubrication. The slight back-and-forth movement of the two components causes wear on microscopic asperities (tiny high spots on the component’s surface). Fretting corrosion often results in brownish-red oxide on the component’s surface, Figure 17-46. The color depends on the type of lubricant and the metal oxide formed by the adhesive wear. Corrective action is needed to restore proper fit so the components remain stationary. A B C Figure 17-44. Spalling is the flaking or breaking away of the outside surface of a component. A—Spalling on the outside edge of planetary pinion gear teeth. The gear was misaligned, causing the outside tooth contact surface to overload and spall. B— Spalling on tapered roller bearings. C— This bearing’s inside race was installed on the rear axle of an agricultural tractor. The race is fixed stationary in the axle housing. Spalling occurred on one side of this bearing and on one side of the bearing on the opposite side of the drive axle. The identical spalling failures indicate that the tractor’s axle was overloaded. Goodheart-Willcox Publisher Goodheart-Willcox Publisher Goodheart-Willcox Publisher