Fluid Dynamics Analysis of Bearings

In the heart of modern industry - rotating machinery, bearings play an indispensable role. Its core mission is to support the load with minimal friction and wear, ensuring smooth, efficient, and long-lasting operation of the equipment. Among various types of bearings, the performance of hydrodynamic bearings is almost entirely determined by the behavior of the internal fluid. Therefore, precise fluid dynamics (CFD) analysis of bearings has become the key to designing high-performance and high reliability rotating machinery.

1、 Core principle: Birth of fluid dynamic pressure lubrication
The working principle of fluid dynamic bearings does not rely on external pressure, but rather on the fluid effect caused by the rotation of the journal itself. Its theoretical basis can be traced back to the famous Reynolds Equation proposed by Osborne Reynolds in 1886.

When the journal rotates inside the bearing seat, it brings viscous lubricating fluid (such as oil) into a wedge-shaped gap. Due to the viscosity of the fluid and the fact that the inlet of the gap is greater than the outlet, according to the law of conservation of mass, in order to maintain continuous flow of the fluid, the fluid pressure at the narrowest point in the gap will sharply increase, thereby generating a fluid dynamic pressure sufficient to lift the journal and completely separate it from the bearing surface. This pressure oil film not only carries external loads, but also achieves an ideal full liquid friction state, transforming dry friction between solids into shear friction inside the fluid, resulting in a very low friction coefficient.

The Reynolds equation is a two-dimensional partial differential equation that describes the pressure distribution in this wedge-shaped gap, and it is the mathematical cornerstone of all fluid dynamic pressure bearing analyses.

2、 Analysis Method: From Theoretical Analysis to Digital Simulation
With the leap in computing power, the fluid dynamics analysis of bearings has undergone an evolution from simplified theoretical models to complex multidimensional simulations.

Classical analytical method:
Based on the Reynolds equation, approximate analytical solutions are obtained through a series of assumptions such as viscosity, incompressibility, laminar flow, etc. This method can quickly obtain theoretical values of key performance parameters such as bearing capacity, friction power consumption, and flow rate of bearings, which is suitable for preliminary design and trend analysis. But its accuracy is limited by simplifying assumptions, making it difficult to handle complex working conditions.

Numerical simulation method (Computational Fluid Dynamics - CFD):
This is currently the mainstream and powerful tool for analysis. With the help of CFD software such as ANSYS Fluent, OpenFOAM, etc., engineers can construct accurate 3D models of bearing clearances.

Advantage:

3D full model analysis: Complex factors such as end leakage, thermal effects, surface roughness, and cavitation effects can be considered.

Multi physics field coupling: capable of conducting fluid structure coupling (FSI) analysis, studying the elastic deformation of bearings caused by oil film pressure, as well as thermal flow coupling analysis, simulating the increase in oil temperature and viscosity changes caused by shear heating, which is crucial for predicting bearing performance.

Visualization results: It can intuitively display the pressure distribution, temperature field, flow velocity vector, etc. inside the oil film, greatly deepening the understanding of internal physical phenomena.

3、 The Value and Engineering Significance of Analysis
The value of conducting precise fluid dynamics analysis on bearings goes far beyond academic research and is directly reflected in engineering practice

Performance prediction and optimization design: Before manufacturing the physical prototype, the performance of the bearing under different speeds and loads can be predicted, thereby optimizing the geometric parameters of the bearing (such as aspect ratio, clearance, oil groove position, etc.), maximizing load-bearing capacity and stability, minimizing friction power consumption and temperature rise.

Improving reliability and lifespan: Analysis helps to avoid instability phenomena such as oil film vortex and oil film oscillation, identify areas where metal contact (wear) or local high temperature (lubrication failure) may occur, and thus ensure the long lifespan and high reliability of bearings from the design source.

Fault diagnosis and troubleshooting: When existing equipment experiences problems such as vibration and excessive temperature rise, CFD analysis can reproduce the fault conditions, help engineers locate the root cause, and propose effective improvement plans.

4、 Challenges and Future Trends
Although CFD technology has become very powerful, the fluid dynamics analysis of bearings still faces some challenges:

Two phase flow and cavitation effect: Bubbles (cavitation) may occur in the oil film due to low pressure, and accurately simulating this gas-liquid two-phase flow is a challenge.

Non Newtonian fluid characteristics: Under certain extreme operating conditions, the rheological properties of lubricating oil may deviate from the Newtonian fluid assumption.

Computational cost: High precision transient multi physics coupling simulation still requires significant computational resources.

In the future, with the development of artificial intelligence (AI) and machine learning (ML), we are expected to see surrogate models trained on CFD data that can achieve real-time prediction and reverse design of bearing performance, further accelerating the innovation cycle.

Conclusion
In summary, the fluid dynamics analysis of bearings serves as a bridge between theoretical physics and engineering practice. It starts from the microscopic fluid behavior and ultimately determines the efficiency, reliability, and lifespan of macroscopic mechanical systems. With the continuous advancement of computing technology, this analytical ability that delves into the "fluid soul" will continue to push all rotating equipment, from micro precision instruments to giant gas turbines, towards higher performance boundaries.