Motor Bearing Corrosion Analysis and Disposal Methods

Several large-scale electric fan manufacturers in Guangdong have encountered an issue where motor assemblies passed all factory tests but, after being shipped to other regions or even exported to the Middle East, were found to have abnormal noise upon inspection. Upon disassembly, severe bearing corrosion was discovered. The bearings showed signs of rust on the steel balls and raceways, which significantly affected the motor's performance. This led to returns from customers, damaging the manufacturer’s reputation and causing operational issues.

The motor manufacturer suspected that the problem stemmed from the bearing quality and anti-rust treatment provided by the bearing supplier. They approached our bearing product quality inspection station for assistance in analyzing the issue and finding a solution to prevent future occurrences.

Corrosion Characteristics of Motor Bearings

Through analysis of samples from various manufacturers, it was observed that the corrosion incidents had several common features:

1. These issues primarily occurred in household appliances and office equipment such as fans and cold-air motors, which typically use small, fully enclosed motors with power ranging from a few watts to hundreds of watts.
2. The motor units showed no abnormalities during initial testing, but after storage for several months, increased noise was detected, indicating internal bearing corrosion.
3. The corrosion mostly occurred during the production process or while in storage, particularly during high humidity periods in March and April.
4. Bearings were usually purchased directly from the bearing factory, then greased by the motor manufacturer before installation. Some grease formulations contained additives that could lead to corrosion when combined with moisture.
5. Internal corrosion was often more severe than external, especially in environments with medium humidity (around 70% RH). Certain types of grease, such as ester-based oils, were more prone to corrosion.
6. Some smooth grease had a distinct paint-like odor, suggesting chemical interactions between the grease and the paint used in the motor.
7. Corrosion was directional, often occurring more on the side of the bearing facing the rotor. The raceway and retainer on this side were most affected.

Analysis of Motor Bearing Corrosion

The primary cause of bearing corrosion is related to the insulating varnish inside the motor. Acidic components from the varnish are absorbed by the grease and, when combined with moisture, lead to grease breakdown. This results in the formation of acidic compounds that corrode the bearing, especially at the contact points between the balls and raceways.

Four main types of acid components are commonly associated with this issue:

1. Low-molecular lacquer acid components (e.g., anhydrous phthalic acid, maleic acid).
2. Acid components released during the curing of the paint (e.g., formic acid from formaldehyde).
3. Primary fatty acids (e.g., formic acid, acetic acid) formed through thermal or oxidative degradation.
4. Acid components generated after hydrolysis of the cured varnish.

Formic acid is the most common acidic component and has a direct correlation with the severity of the corrosion. When the grease absorbs these acidic components, it loses its protective properties, leading to metal oxidation and increased wear, resulting in higher noise levels and reduced bearing life.

Key Factors Contributing to Corrosion

Several factors contribute to the corrosion of motor bearings:

1. The use of 1032 melamine acid impregnated insulation varnish, which contains organic solvents that can promote rust if not properly applied.
2. Failure to dry the painted coils adequately allows acidic gases to gradually escape into the bearing, especially under high humidity conditions.
3. Improper curing temperatures (below 130°C) or insufficient curing times (less than 180 minutes) may leave acidic residues unremoved.

Disposal Measures and Preventive Actions

To prevent paint-induced corrosion, the following steps should be taken:

1. Use solvent-free paints whenever possible.
2. Choose impregnating varnishes without oxidizing components, such as epoxy-urethane or epoxy-based ones.
3. If using melamine acid varnish, increase the curing temperature slightly (to around 135°C) and extend the curing time beyond 180 minutes.
4. Select paints that do not contain volatile acids or have poor hydrolysis resistance.
5. Use mineral oil-based grease for bearings, which is less likely to react with acidic components.
6. Ensure that the grease container is sealed to prevent moisture and dust from entering during application.
7. Use plastic packaging with micro-perforations to allow ventilation before use, preventing trapped moisture and paint vapors.
8. Ensure proper grease filling according to specifications, such as 0.60–0.80g for a 6202 bearing.
9. Collaborate with bearing suppliers to incorporate more effective rust inhibitors during production.
10. Involve all relevant parties—motor manufacturers, bearing producers, and paint suppliers—in a joint effort to address the root causes of corrosion.

Conclusion

We have shared our findings with the motor manufacturers, who have confirmed the analysis and implemented improvements in their production processes. These changes have yielded positive results, reducing the occurrence of bearing corrosion incidents.

Related Bearing Knowledge

• Bearing Common Sense: Understanding Bearing Block Cracking Failures
• Factors Affecting the Service Life of INA Bearings
• Rolling Bearing Terminology (Part 2)
• Sliding Bearing Technical Standards (Part 1)

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