Causes of rusting in NSK bearings in electric motors

Causes of rusting in NSK bearings in electric motors

The causes of rusting in NSK bearings in electric motors are complex and can be attributed to various factors. It could be due to poor rust prevention treatment of imported NSK bearings, moisture-induced corrosion, or corrosion caused by paint mist.

Most electric motor manufacturers use insulation varnishes that release acidic components, mainly consisting of four types:

1. Unreacted acid components from low molecular weight varnish materials (anhydrous isophthalic and terephthalic acid, anhydrous trans-butenedioic acid).
2. Acidic components generated during varnish curing (when using phenolic-modified varnish, formaldehyde in water converts to formic acid).
3. Low-level fatty acids generated by heating or oxidative decomposition (formic acid, acetic acid).
4. Acidic components generated after the hydrolysis of varnish solidified by moisture. The main acid component produced by insulation varnish is formic acid, which is closely related to the occurrence of corrosion.

Lubricating grease absorbs the volatile acidic components in the varnish, leading to grease deterioration. Moreover, this acidic component promotes grease hydrolysis, resulting in an increase in the concentration of acidic components and leading to corrosion of NSK bearings.

The working principle of lubricating grease is to use an oil film to separate the steel balls from the inner and outer raceways. By utilizing the viscosity properties of the grease, it adheres to the surfaces of the steel balls and raceways, separating the metal interfaces and reducing wear on the metal surfaces. It also protects the metal from air and moisture corrosion, reduces vibration and noise, enabling smooth rotation of NSK bearings. However, when the lubricating grease deteriorates, the viscosity changes, and the oil film breaks, causing the loss of the grease's separation and adhesion characteristics. This leads to the following consequences:

1. The acidic components generated by the insulation varnish directly oxidize the metal surface, resulting in bearing corrosion.
2. Direct contact between the steel balls and the inner and outer raceways causes wear during rotation, leading to increased noise.