Comparing Thrust Bearings and Radial Bearings: Risk of Varnishing in Turbomachinery Applications

Comparing Thrust Bearings and Radial Bearings: Risk of Varnishing in Turbomachinery Applications

In turbomachinery, both thrust bearings and radial bearings play critical roles in ensuring smooth operation and reliability. However, these two types of bearings differ significantly in terms of their susceptibility to varnishing—a condition where oil degradation products form a hard, varnish-like deposit on metal surfaces. Understanding the factors contributing to varnish formation on each bearing type is essential for optimizing maintenance strategies and enhancing operational performance.

1. Overview of Bearing Functions

• Thrust Bearings: These bearings primarily handle axial loads (forces parallel to the shaft's axis). Common types include tilting-pad and tapered-land thrust bearings, which are designed to maintain separation between sliding surfaces under high loads.
• Radial Bearings: These bearings support radial loads (forces perpendicular to the shaft’s axis). Rolling-element radial bearings (such as deep groove ball bearings) and journal bearings (hydrodynamic bearings) are typical in turbomachinery.

2. Operating Conditions Affecting Varnish Formation

Varnishing risks differ between thrust and radial bearings due to varying operating conditions, such as load, temperature, and lubrication film characteristics.

2.1 Thrust Bearings

• Higher Contact Pressure: Thrust bearings experience higher localized pressure, particularly at points where oil films are thinnest.
• Oil Shear and Heat Generation: The high axial load can increase oil shear stress and lead to elevated temperatures. This accelerates oil oxidation and degradation.
• Thermal Stability Risks: If the oil's antioxidant capacity is depleted, the hot spots in thrust bearings become prime locations for varnish formation.

2.2 Radial Bearings

• Lower Contact Pressure: Radial bearings typically have lower localized pressure compared to thrust bearings, which reduces the heat generated.
• Better Lubrication Film Stability: The continuous movement of the shaft and oil flow in radial bearings often results in more consistent lubrication and heat dissipation.
• Varnish Susceptibility: Although varnish can form in radial bearings, it tends to accumulate more slowly due to the lower thermal stress.

3. Varnishing Mechanism and Contributing Factors

Both thrust and radial bearings can suffer from varnish, but the mechanisms and factors influencing varnish deposition differ.

3.1 Thrust Bearings

• Static Zones: Certain areas of thrust bearings may have limited oil flow or stagnation zones where degraded oil products settle.
• Oxidation By-Products: These products accumulate on high-contact surfaces, where fluctuating temperature cycles promote polymerization and varnish formation.
• Impact of Load Changes: Rapid changes in axial load can cause transient heating, which further stresses the lubricant.

3.2 Radial Bearings

• Dynamic Circulation: The high-speed rotation of the shaft promotes dynamic oil flow, reducing the risk of stagnant areas where varnish could build up.
• Lower Oxidation Stress: Since radial bearings generally experience less severe heat generation, oil degradation occurs at a slower rate.

4. Risk Mitigation Strategies

4.1 For Thrust Bearings

• Use of High-Performance Oils: Select turbine oils with robust antioxidant packages to delay oxidation and varnish precursor formation.
• Temperature Management: Ensure adequate cooling to minimize hot spots in the bearing.
• Filtration and Varnish Removal: Deploy advanced oil filtration systems, such as EPT Clean Oil SVR? Oil Varnish Removal System | EPT Clean Oil, to capture varnish precursors.

4.2 For Radial Bearings

• Oil Analysis Programs: Regularly monitor the oil’s oxidation levels and varnish potential using techniques like MPC (Membrane Patch Colorimetry) and RULER (Remaining Useful Life Evaluation Routine).
• Consistent Lubrication Practices: Maintain optimal oil circulation to prevent localized heating and degradation.

5. Case Studies and Field Experience

Real-world experience has shown that varnishing is more prevalent in thrust bearings than in radial bearings within the same turbomachinery systems. For example, steam turbines operating with degraded oil often exhibit varnish deposits on thrust bearing pads, leading to increased friction, misalignment, and even potential failure if not addressed.

In contrast, radial bearings tend to show less severe symptoms, although varnish may still affect bearing housings, seals, and adjacent components over time.

6. Conclusion

While both thrust and radial bearings are at risk of varnishing, thrust bearings are generally more susceptible due to higher contact pressures, localized heating, and oil shear stress. Effective varnish control measures—including proactive oil management, enhanced filtration technologies, and regular condition monitoring—can significantly reduce this risk and extend bearing life in turbomachinery applications.

By understanding the unique challenges faced by each bearing type, maintenance teams can prioritize actions to mitigate varnish formation, improve machine reliability, and optimize turbine performance.

Read more:

The Impact of Varnish on Bearings 101 | EPT Clean Oil

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