Influence of Electrocorrosion on Insulated Bearings

Whenever electric current flows through insulated rolling bearings for motors, it can threaten the reliability of your equipment. Galvanic corrosion can damage and degrade bearings in traction motors, electric motors and generators, resulting in costly downtime and unscheduled maintenance. With its latest generation of insulated bearings, SKF has raised the bar of performance. INSOCOAT bearings improve equipment reliability and increase equipment uptime in electrical applications, even in the most challenging environments. Effects of galvanic corrosion The demand for SKF insulated bearings in electric motors has increased in recent years. Higher motor speeds and the wider use of variable frequency drives mean that adequate insulation is required if damage from current flow is to be avoided. This insulating property must remain stable regardless of the environment; this is a specific issue faced by bearings when stored and handled in wet environments. Electrocorrosion damages bearings in the following three ways: 1. High current corrosion When current flows from one bearing ring through the rolling element to the other bearing ring and passes through the bearing, it will produce an effect similar to arc welding. A higher current density is formed on the surface. This heats the material to tempering or even melting temperatures, creating discolored zones (of varying sizes) where the material is tempered, requenched, or melted, and also creates pits where the material melts. Current leakage Corrosion current flows continuously in the form of an arc through a working bearing, even with low-density currents, the raceway surface is affected by high temperatures and corrodes due to the formation of thousands of micro-dimples on the surface (mainly distributed in the rolling contact surface). The pits are very close to each other and have a small diameter compared to corrosion caused by high currents. Over time, this will cause grooves (shrinkage) in the raceways of the rings and rollers, a secondary effect. The extent of damage depends on several factors: bearing type, bearing size, electrical mechanism, bearing load, rotational speed and lubricant. In addition to damage to the bearing steel surface, lubricant performance near the damage can degrade, eventually leading to poor lubrication and surface breakage and spalling. Localized high temperatures caused by electrical current can cause additives in the lubricant to scorch or burn, causing additives to be consumed more quickly. If lubricated with grease, the grease will turn black and hard. This rapid destruction greatly reduces grease and bearing life. Why care about humidity? In countries such as India and China, wet working environments present another challenge for insulated bearings. When a bearing is exposed to moisture, such as during storage, moisture can penetrate the insulating material, thereby reducing the electrical insulating effect and shortening the service life of the bearing itself. Grooves in the raceways are usually secondary damage due to destructive current passing through the bearing. Micro pits caused by high frequency current leakage corrosion. Comparison of balls with (left) and without (right) micro pockets Cylindrical roller bearing outer ring with cage, rollers and grease: current leakage causes grease to scorch (blacken) on cage beams .