Comparative Analysis of Fixed and Tilting Pad Bearings in Rotordynamic Applications

Abstract

Bearings constitute one of the most critical components in turbo and rotating machinery today. Bearings are fundamental elements in rotating machinery such as turbines, compressors, motors, and pumps, as they support the rotor, control vibration, and influence system stability. Their stiffness and damping properties critically affect rotordynamic behaviour, including critical speeds and overall stability. Among the various types, fluid film bearings are widely used due to their ability to separate surfaces with a thin lubricant film, providing near-infinite life when properly maintained. Operating regimes range from dry friction and mixed lubrication to full hydrodynamic lubrication, with the latter playing the most significant role in rotordynamic performance. Fluid film bearings exist in fixed-geometry (sleeve) and variable-geometry (tilting pad) configurations. Fixed-geometry bearings are simpler and provide higher damping but are more susceptible to instability, while tilting pad bearings offer superior stability by eliminating cross-coupling effects, albeit at higher cost and complexity. The orientation (load-on-pivot vs. load-between-pivots), preload, and design parameters of tilting pad bearings strongly influence stiffness, damping, and thermal behaviour. This paper highlights the rotordynamic implications of bearing design, configuration, and operating regimes, emphasizing their role in vibration control, stability enhancement, and overall machine reliability.

Introduction

Bearings are essential components in rotating machinery such as turbines, motors, compressors, and pumps. They support the rotor, influence vibration, and are critical to the system's dynamic behavior and stability. The type and properties of a bearing—especially its stiffness and damping—greatly affect how the rotor responds to imbalances or disturbances. Improper bearing selection or wear can lead to excessive vibrations, performance issues, or system failure.

A. Fluid Film Bearings

• These bearings use a thin lubricant film (oil, air, water, etc.) to separate rotating and stationary surfaces, preventing direct contact.

• Hydrodynamic bearings create film pressure via the motion of the rotor, enabling non-contact support during operation and infinite life if properly maintained.

• Lubrication operates under three regimes:

  1. Dry Friction (start/stop),

  2. Mixed Lubrication (partial contact),

  3. Full Hydrodynamic Lubrication (complete separation).

• Damping is a key advantage, aiding in vibration control and dynamic stability.

• Hydrostatic bearings, while stiffer and useful in certain subcritical applications, are limited in use.

B. Fixed-Geometry Sleeve Bearings

• Simpler and more damped than variable-geometry bearings.

• Support is generated through a hydrodynamic pressure wedge formed between the shaft and the bearing.

• Suffer from cross-coupling effects, which can reduce stability, especially at higher speeds.

• Sub-synchronous vibrations (oil whirl) are influenced by the L/D ratio (Length/Diameter) of the bearing.

• Stability can be enhanced by modifying geometry, e.g., adding axial grooves to reduce internal fluid rotation and improve performance.

C. Variable-Geometry Tilting Pad Bearings

• Favored in high-speed, supercritical machinery due to high inherent stability and low cross-coupling.

• The bearing consists of multiple pads that tilt or pivot to align with the load.

• They produce reaction forces directly in line with shaft centers, improving dynamic response.

• Although more complex and expensive than fixed-geometry bearings, they offer greater stability for demanding applications.

Conclusion

The reliability issues at Hindalco’s Mahan plant clearly show that soft foot—a condition where one or more motor feet fail to Bearings are fundamental to the performance, reliability, and stability of rotating machinery. Their stiffness and damping properties strongly influence rotor dynamics, critical speeds, and vibration behaviour. Fluid film bearings, particularly hydrodynamic types, provide long life due to non-contact operation and inherent damping, though stability concerns such as oil whirl must be addressed. Fixed-geometry sleeve bearings offer simplicity and higher damping but can be prone to instabilities unless modified with features like grooves. On the other hand, variable-geometry tilting pad bearings deliver superior stability by minimizing cross-coupling effects, with load orientation (LOP vs. LBP) and preload significantly affecting their dynamic response. While more complex and costlier, tilting pad designs allow greater flexibility to tailor performance for specific applications. Overall, proper bearing selection and configuration—balancing stability, damping, load capacity, and cost—are critical to ensuring safe, efficient, and reliable operation of high-speed rotating machinery.

References

[1] Smith, D. M. The motion of a rotor carried by a flexible shaft in flexible bearings [2] Robertson, D. Whirling of a journal in a sleeve bearing [3] Poritsky, H., “Contribution to the Theory of Oil Whip”, Transactions of the ASME. [4] McHugh, J. D., “Principles of Turbomachinery Bearings”, [5] Machinery vibration and rotordynamics / John Vance, Brian Murphy, Fouad Zeidan.

Copyright

Copyright © 2025 Sushil Sharma. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.