In construction machinery repair, replacing rolling bearings is a frequent task. However, given today’s uneven quality in the spare parts market, the reliability of construction machinery is often questionable, and after repair, the quality can be even worse. This involves both part quality issues and workmanship issues, but problems related to rolling bearings account for a significant share.

In the past, large state-owned enterprises had dedicated procurement departments. If a part was defective, it was simply returned and replaced, so quality problems were rare. Today, especially for privately owned machines and small contractors, poor-quality bearings often cause serious headaches. Small repair shops, in particular, worry about counterfeit parts leading to disputes with customers.

For example, just recently a colleague reported that a loader’s front axle reduction gear set failed prematurely due to bearing quality problems.

I cannot change the market, but I can share the experience-based methods I’ve developed for evaluating bearing quality. These can help improve repair quality and extend machine life.

Here, I focus on small and medium-sized rolling bearings (outer diameter < 400 mm). Large bearings and plain bearings are excluded because they are too heavy to handle with simple on-site checks.

Background

Rolling bearings are assembled products, which means there are many human factors affecting quality. This has always been the case—even back in the 1950s, when wooden axles on carts were replaced with iron axles and bearings. In fact, some early bearings were literally hammered into shape by blacksmiths!

Today, bearings are of course manufactured by machines, but their quality varies widely. Even imported bearings differ greatly in precision.

Take an example: you buy a bearing for a loader, such as an overrunning clutch bearing (similar in principle to a bicycle freewheel). The quality can vary wildly—some fail quickly even if assembly is correct. Or, after carefully adjusting the gears and clearances of a differential, the gearset fails early due to poor bearings. Even transmissions carefully rebuilt with correct procedures sometimes fail within months because of bearing outer race slippage or shaft seat wear.

I personally recall in the 1990s, after painstakingly rebuilding a Czech-made Tatra dump truck chassis, the entire axle housing failed within half a year—partly due to bearing failures.

This experience pushed me to explore field methods for checking bearing quality without precision instruments. Over years of practice, I’ve identified practical techniques, which I now share.

Experience-Based Bearing Quality Identification Methods

General Principles

1. Hardness Test

   • Use a hacksaw or file to scratch the inner/outer race of the new bearing and compare with the old one. If the new one is softer, its wear resistance is worse.

2. Reuse of Original Bearings

   • For tapered roller bearings, if no wear or corrosion is observed, simply adjust the clearance rather than replacing them.

   • For cylindrical or spherical roller bearings, avoid replacing unless defects are found—the original OEM bearings often outlast many new domestic ones.

3. Avoid Random Substitutions

   • Do not casually replace imperial-size bearings with metric substitutes, or adjust thickness with shims. Incorrect substitutions lead to rapid failure.

4. Do Not Mix New and Old Bearings

   • Never mount an old bearing and a new bearing on the same shaft, especially under heavy loads.

5. Machines Exposed to Electricity, Fire, or Flooding

   • After electrical discharge, rollers may develop micro-cracks (arcing scars). Always inspect carefully—common on travel reducers, swing bearings, or track rollers.

Practical On-Site Inspection Methods

1. Cleaning

   • Thoroughly wash and dry the new bearing before inspection.

2. Fit and Shake Test

   • Hold the inner and outer rings together, press firmly, and shake the bearing with maximum force.

   • If any knocking sound is heard, the bearing is defective.

   • This applies to all types: roller, ball, thrust, overrunning clutch, etc.

   The principle: A qualified bearing set must maintain consistent dimensional precision across all components. If internal clearances are inconsistent, knocking will occur.

3. Hardness Verification

   • Perform the scratch test (file/hacksaw) on the races to compare hardness.

4. Rotation Test

   • Rotate the outer ring while holding the inner ring. A good bearing rotates smoothly without noise.

   • If possible, compare rotation speed and smoothness with the original bearing.

Precision Classes

• For construction machinery, P5 grade bearings (ISO standard, equivalent to former Chinese E-class) are sufficient.

• Bearings without a marked precision grade are typically P6 (lower precision), regardless of whether they’re made in Germany, Japan, the USA, or elsewhere.

Conclusion

These methods allow mechanics to quickly identify poor-quality bearings without instruments. I’ve taught these to many colleagues, and they’ve been consistently effective. This kind of practical knowledge is rarely found in manuals—yet it directly impacts machine reliability and repair quality.