In recent years, the demand of motors for insulating bearings has increased. Higher motor speeds and the wider use of variable frequency drives mean that adequate insulation is needed if current damage is to be avoided. This insulation must remain stable regardless of the environment. This is a specific problem faced by bearings during storage and handling in a humid environment. Electric corrosion can damage bearings in the following three ways: 1. High-current corrosion. When current flows from one bearing ring to another bearing ring through the rolling body and passes through the bearing, it will produce an effect similar to that of electric arc welding, forming a higher current density on the smaller contact surface. This heats the material to a tempering or even melting temperature, creates a fading zone (varying in size) at the tempering, secondary quenching or melting point of the material, and creates a pit at the melting point of the material.
When leakage current corrosion current continues to flow through the working bearing in the form of an arc, the raceway surface will be affected by high temperature and corroded even if it is a low density current, because thousands of micro-pits are formed on the surface (mainly distributed on the rolling contact surface). These pits are very close to each other and are smaller in diameter than corrosion caused by large currents. Over time, there will be grooves (corrugation) in the raceway of the rings and rollers, known as secondary effects. The degree of damage depends on several factors: bearing type, bearing size, electrical mechanism, bearing load, speed and lubricant. In addition to the damage to the bearing steel surface, lubricant properties near the damage may degrade, resulting in poor lubrication and surface damage and spalling.
The local high temperatures caused by the current can cause the additives in the lubricant to be burnt or burnt, causing the additives to be consumed more quickly. If lubricated with grease, the grease will become black and hard. This rapid destruction greatly shortens the service life of the grease and the bearing. Why pay attention to humidity? Wet working conditions pose another challenge for insulated bearings in countries such as India and China. When the bearing is exposed to a humid environment (such as during storage), moisture can permeate the insulating material, thereby reducing the electrical insulation effect and shortening the service life of the bearing itself. Grooves in the raceway are usually secondary damage caused by destructive currents passing through the bearings. Miniature pit caused by high frequency current leakage corrosion. Comparison of ball bearings with (left) and without (right) miniature pits with cylindrical roller bearings outer rings with retainers, rollers and grease: current leakage causes the grease on the retainer beam to scorch (blacken).
