Abstract: Aiming at the premature failure of a certain type of tapered roller bearing, the operating condition and damage morphology of the failed bearing are analyzed in detail. By investigating bearing failure mechanisms and reconstructing the failure process via simulation, root causes of premature failure are determined, targeted solutions are proposed to improve the service life of the bearing.

Key words: tapered roller bearing; bearing failure; service life

1 Introduction

Rolling bearings are critical fundamental components for mechanical equipment, whose operating status directly determines the overall running performance of machinery and equipment. Even high-performance bearings will fail to reach designed service performance if improperly installed or applied. A bearing is defined as failed once it cannot fulfill specified functional requirements during service, which would trigger equipment shutdown and abnormal malfunction. Therefore, rapid root-cause identification and corrective measures are required after unexpected bearing failure occurs.

Compared with general mechanical parts, rolling bearing failure features diversified failure modes and complicated inducing factors. Bearing failure is classified into normal fatigue failure (end-of-life damage under designed load) and premature early failure. Most failure analysis projects focus on early failed bearings; identifying failure causes and proposing optimization improvements can effectively extend bearing service life and reliability, as well as upgrade product quality of bearings.

Tapered roller bearings can withstand combined radial and axial loads, as well as pure axial loads; its axial load capacity rises with the increase of contact angle.

Figure 1: Schematic drawing of tapered roller bearing

2 Bearing Failure Analysis

2.1 Description of Failed Component

The tested bearings are mounted on both sides of the drive pinion of a vehicle axle (one set per side). According to feedback from the equipment manufacturer, the bearing suffers frequent premature damages including overheating, cage fracture, severe damage of inner ring and rollers. Macro damage features are summarized as below: bright spalling marks on inner ring raceway, flared and worn-off large rib of inner ring, flared small rib of inner ring, and deep spline indentations on one side of inner bore.

Figure 2: Severely damaged inner ring with flared large rib

Figure 3: Inner ring with flared small rib

Figure 4: Deep penetrating indentations on inner bore surface

2.2 Analysis of Failure Mode

Wear damage is defined as continuous material loss from two contacting surfaces under rolling/sliding motion due to interaction of surface micro-asperities, divided into abrasive wear and adhesive wear:

· Abrasive wear originates from insufficient lubrication or intrusion of external particulate contaminants; surface darkening degree depends on particle size and property. Detached wear debris accelerates abrasive wear and eventually induces complete bearing failure.

· Adhesive wear refers to material transfer between contact surfaces accompanied by frictional heating, local tempering or secondary quenching, resulting in concentrated contact stress, surface cracking and spalling.

Based on macroscopic damage features (bright worn raceway, scuffing between raceway and rib), the bearing is confirmed to fail due to adhesive wear.

2.3 Failure Verification Test

Metallographic structure and hardness inspection are carried out on rings and rollers of failed bearing.

Figure 5: Quenched & tempered metallographic microstructure of bearing inner ring

Table 1 Hardness test results of failed bearing rollers / HRC

2.4 Root Cause Analysis

Metallographic and hardness test results prove that the inner ring has experienced high-temperature tempering above 350℃ during operation. Bright polished wear on raceway is caused by improper lubricant selection or insufficient grease/oil supply; adhesive scuffing between inner ring rib and raceway results from excessive axial load on rollers plus inadequate lubrication, leading to seizure at roller end faces.

Romax bearing simulation software is adopted for life calculation with actual operating parameters: the gear shaft is equipped with helical gear and paired with two sets of 30215 tapered roller bearings on both sides. The rated allowable axial load of 30215 bearing is 41.5kN, while actual axial force generated by helical gear transmission reaches 54.5kN (exceeding the recommended limit: axial load ≤50% of basic rated load). Excessive axial load squeezes rollers against inner ring large rib and triggers intensive sliding friction, sharp temperature rise, material temper softening, surface scuffing & spalling, irreversible rib flaring under roller extrusion and final premature bearing failure.

Figure 6: Structure drawing of gear shaft

Table 2 Calculated bearing life (Long-shaft end)

Table3 Calculated bearing life (Short-shaft end)

3 Optimization Solutions

Original vehicle axle uses spur/helical bevel gear with low axial load and stable bearing service performance, yet high gear noise leads to customer complaints. To reduce transmission noise, the manufacturer replaced original gear with high-helix-angle helical gear; noise is effectively reduced but axial load rises sharply beyond bearing allowable limit and induces premature failure.

Optimization 1: Adjust bearing contact angle per GB/T297 standard for tapered roller bearings; max allowable contact angle for 75mm bore bearing:28°48′39″. After contact angle optimization, service life is calculated as Table4 & Table5, which still fails to satisfy full-vehicle design life requirement.

Final recommendation: ① Increase bearing overall dimension to upgrade rated axial load capacity; ② Optimize gear design or transmission layout to reduce actual axial load on bearings, to eliminate repeated premature failure.

Table4 Calculated life after contact angle modification (Long-shaft end)

Table5 Calculated life after contact angle modification (Short-shaft end)

4 Conclusion

Proper bearing selection matching practical working condition is the core measure to prevent premature bearing failure. Bearing type selection is a detailed and complicated technical work including structural selection, performance matching and service life calculation, with key considerations listed below:

1. Mechanical structural constraint, equipment performance index and application requirements: available installation space, disassembly accessibility (structure); precision, noise, friction torque and rated life (performance); lubricant ageing property, operating temperature and service medium (application condition).

2. Precision grade: select bearing type, internal clearance and dimensional tolerance according to host equipment technical specifications.

3. Service life verification: calculate theoretical bearing life based on equipment warranty requirement and actual working load to guarantee practical service requirement.

In summary, comprehensive evaluation should be completed during bearing type selection to avoid early-stage bearing failure and ensure stable equipment operation.

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