Working Principle and Tribological Fundamentals of Plain Bearings (Sliding Bearings)

Plain bearings, which are often called EPEN sliding bearings, are important parts of many mechanical systems. The way these bearings work is that two surfaces slide against each other with very little friction. The basic idea is to put a small layer of lubrication between the moving parts. This keeps them from wearing down and lets them work smoothly. Understanding how plain bearings work requires a good understanding of tribological fundamentals, which include things like friction, lubrication, and wear mechanisms. Engineers may make bearings that are more durable, efficient, and reliable for a wide range of uses, from car engines to industrial machines, by improving these parts.

1. Core Principle: Fluid Film Creation

The primary working principle is to separate the sliding surfaces (shaft and bearing) with a lubricant to prevent direct metal-to-metal contact. This drastically reduces friction and wear.

The process can be broken down into three key operational regimes:

Boundary Lubrication (Startup/Shutdown):

At very low speeds, during startup, shutdown, or under high load, the shaft and bearing are partially in contact.

A very thin layer of lubricant (only a few molecules thick) adheres to the metal surfaces, providing just enough protection to prevent severe wear and seizure. Friction is relatively high.

Mixed Lubrication:

As the shaft speed increases, it draws more lubricant into the converging gap between the shaft and bearing.

The load is partially carried by the fluid pressure and partially by the contacting surface asperities (high points). Friction begins to decrease.

Hydrodynamic Lubrication (Full-Film - Normal Operation):

This is the ideal operating condition. At sufficient speed, the rotating shaft acts like a pump, dragging viscous lubricant into a wedge-shaped space between the shaft and the bearing.

This action generates enough pressure within the fluid film to fully lift the shaft and center it within the bearing.

The surfaces are completely separated by a thin film of lubricant (can be microns thick). There is no physical contact, resulting in very low friction and virtually no wear.

Key Design Feature: The Clearance and the Wedge

The bearing is designed with a slightly larger inner diameter than the outer diameter of the shaft. This creates a radial clearance. Furthermore, the bearing is often slightly offset or the shaft deflects under load, forming a converging wedge—a space that narrows in the direction of motion. This wedge is essential for generating the hydrodynamic pressure that supports the load.

2. Tribological Fundamentals

Tribology is the science of interacting surfaces in relative motion, encompassing friction, wear, and lubrication. The performance of a sliding bearing is entirely governed by tribological principles.

A) Friction

Friction in a sliding bearing transitions through the three lubrication regimes:

Boundary & Mixed: Friction is higher and is determined by the shear strength of the boundary lubricant layers and the contact between surface asperities.

Hydrodynamic: Friction is purely due to the viscous shear of the fluid film. It is calculated using the petroffs equation and is a function of:

Lubricant viscosity (μ)

Relative speed (N)

Bearing dimensions (D, L, c)

It is independent of the load and the surface materials, as long as the film is maintained.

B) Wear

Wear is the progressive loss of material. In a properly operated hydrodynamic bearing, wear is negligible because there is no contact. However, wear becomes a critical factor in:

Startup/Shutdown cycles (boundary lubrication).

Overloading or insufficient speed, which collapses the fluid film.

Contamination by abrasive particles (e.g., dirt, metal shavings).

Lubricant starvation or failure.

To mitigate wear, bearing materials are chosen for their compatibility, embeddability (ability to trap contaminants), and corrosion resistance.

C) Lubrication

The lubricant is the lifeblood of the bearing. Its key properties are:

Viscosity (μ): The most critical property. It is the resistance of a fluid to flow. Higher viscosity generates a thicker, more robust fluid film but also increases viscous friction and heat generation. The correct viscosity is a careful balance for the operating conditions (speed, load, temperature).

Viscosity Index (VI): A measure of how much viscosity changes with temperature. A high VI means the viscosity remains relatively stable, which is desirable.

Additives: Modern lubricants contain additives for:

Anti-wear (AW): Form protective layers on surfaces during boundary lubrication.

Extreme Pressure (EP): Form protective chemical films under very high loads and temperatures.

Oxidation inhibition: Prevent the lubricant from breaking down at high temperatures.

Rust and corrosion inhibition.

3. Bearing Materials

The choice of material is a tribological compromise. No single material has all ideal properties. Common materials include:

Babbitt (White Metal): A tin or lead-based alloy. Excellent conformability (ability to adapt to misalignment) and embedability. Excellent compatibility to prevent shaft wear. Low strength, so it's usually bonded to a stronger steel shell.

Bronze: A copper-based alloy. Good strength, fatigue resistance, and thermal conductivity. Less conformable than Babbitt. Often used with a lead-tin overlay for better surface properties.

Aluminum Alloys: Good corrosion resistance and fatigue strength. Moderate cost.

Multilayer Materials: Modern bearings are complex layered structures (e.g., steel backing for strength, a bronze layer for load capacity, and a Babbitt overlay for surface properties).

Summary Table: Key Tribological Aspects

4. Load Distribution and Bearing Surfaces

Plain bearings excel in distributing loads across their surfaces. The design of these bearings allows for even pressure distribution, which is crucial for maintaining stability and reducing wear. The bearing surface, typically made from materials like bronze, bimetal composites, or engineered polymers, plays a vital role in this process. These materials offer unique properties that enhance load-bearing capacity and reduce friction.

For instance, bronze bushings, whether oil-impregnated sintered bronze or cast bronze, provide excellent load distribution properties. Their porous structure allows for oil retention, ensuring consistent lubrication during operation. Bimetal bushings, with their steel backing and softer lining materials, offer a combination of strength and low friction, making them ideal for high-load applications.

Friction and Wear Characteristics

Understanding friction and wear is crucial for optimizing plain bearing performance. Friction in plain bearings is influenced by factors such as surface roughness, material properties, and lubrication conditions. Wear, on the other hand, is the gradual removal or deformation of material from the bearing surfaces.

Different bearing materials exhibit unique friction and wear characteristics. For instance, bronze-based composites often provide a good balance between wear resistance and low friction. Special alloy bushings, designed for extreme conditions, may incorporate materials that offer exceptional wear resistance in high-temperature or corrosive environments.

Engineered polymer bushings, such as those made from nylon or other high-performance plastics, can offer extremely low friction coefficients and good wear resistance, particularly in applications where traditional metallic bearings might struggle.

5. Material Selection and Optimization

Choosing the right material for a plain bearing is a nuanced process that considers multiple factors. The operating environment, load conditions, speed, and lubrication availability all play crucial roles in material selection. Advanced plain bearing designs often involve composite materials or layered structures to optimize performance.

For instance, bimetal bushings combine the strength of a steel backing with the tribological properties of a softer lining material. This construction allows for high load-bearing capacity while maintaining excellent friction and wear characteristics. The lining material can be tailored to specific applications - lead-based linings for traditional high-load scenarios, or lead-free alternatives for environmentally sensitive applications.

Polymer bushings represent another area of material innovation. Advanced engineering plastics like PTFE, POM, and high-performance nylons offer unique combinations of low friction, chemical resistance, and self-lubricating properties. These materials are particularly valuable in applications where traditional lubrication is impractical or where weight reduction is critical.

Surface Engineering and Coatings

Surface engineering has emerged as a powerful tool in enhancing plain bearing performance. By modifying the surface properties of bearings, engineers can dramatically improve wear resistance, reduce friction, and extend operational life.

Techniques such as nitriding, carburizing, or the application of thin-film coatings can significantly alter the surface characteristics of metallic bearings. For example, a hard chrome coating on a steel-backed bearing can provide exceptional wear resistance and corrosion protection.

In the realm of polymer bearings, surface treatments can enhance bonding with backing materials or improve load-bearing capacity. Some advanced polymer bearings incorporate solid lubricants or nanoparticles in their surface layers, providing enhanced tribological properties.

Computational Modeling and Simulation

The advent of powerful computational tools has revolutionized plain bearing design. Finite element analysis (FEA) and computational fluid dynamics (CFD) allow engineers to simulate complex bearing behaviors under various operating conditions.

These simulations can predict factors such as pressure distribution, film thickness, and thermal gradients within the bearing. By analyzing these parameters, designers can optimize bearing geometry, material selection, and lubrication strategies before physical prototyping.

Advanced modeling techniques also enable the study of edge effects, misalignment sensitivities, and transient behaviors that are difficult to observe in physical tests. This capability is particularly valuable when designing bearings for critical applications or when pushing the boundaries of traditional design limits.

For instance, in developing special alloy bushings for extreme environments, computational modeling can help predict material behavior under conditions that would be challenging or expensive to replicate in physical tests. This approach accelerates the development of innovative bearing solutions for emerging technologies and demanding applications.

Conclusion

Plain bearings, which look simple, are actually a complicated mix of tribological concepts. The field of plain bearing technology is always changing, from the basic ideas of load distribution and lubrication to more advanced ideas in material science and computer modeling. By learning about and improving these basic tribological principles, engineers can make bearings that work better, last longer, and use less energy in a wide range of situations. As industries push the limits of how well machines can work, well-designed plain bearings become more and more important for making sure that everything runs smoothly and reliably.

FAQ

What are the main types of EPEN Sliding bearings?

The main types include bronze bushings (oil-impregnated sintered bronze, cast bronze), bimetal bushings (steel-backed with various linings), polymer bushings (PTFE, POM, Nylon), and special alloy bushings for extreme conditions.

How do plain bearings(sliding bearings) reduce friction?

Plain bearings reduce friction through lubrication mechanisms like hydrodynamic and boundary lubrication, as well as material properties that promote low-friction sliding.

What factors should be considered when selecting a plain bearing ?

Key factors include load requirements, operating speed, temperature, environmental conditions, lubrication availability, and specific application needs.

Choose the Right Plain Bearing(Sliding Bearing) for Your Application | About EPEN

Jiashan Epen Bearing Co.Ltd. is a professional manufacturer of plain bearings and wear plates, and has grown rapidly to a point where now all types of plain bearings can be supplied. Standard catalogue sizes, special sizes and designs can be produced at competitive prices and to a high quality standard. Jiashan Epen Bearing Co.Ltd. Serves both the domestic and international markets. The Jiashan Epen Bearing Company intend to stay at the forfront of this market.

EPEN's existing main products are metal plastic composite series sliding bearings, bimetal series bearings, single metal series sliding bearings, etc.

Products are widely used in more than 30 fields such as automobile industry, metallurgy, engineering machinery, construction machinery, plastic machinery, machine tool industry, water conservancy and hydropower.

Contact us at epen@cnepen.cn to discuss your plain bearing needs and discover how our solutions can enhance your machinery's performance and longevity.

References

Johnson, K.L. (1985). Contact Mechanics. Cambridge University Press.

Stachowiak, G.W. and Batchelor, A.W. (2013). Engineering Tribology. Butterworth-Heinemann.

Khonsari, M.M. and Booser, E.R. (2008). Applied Tribology: Bearing Design and Lubrication. John Wiley & Sons.

Bowden, F.P. and Tabor, D. (2001). The Friction and Lubrication of Solids. Oxford University Press.

Neale, M.J. (1995). The Tribology Handbook. Butterworth-Heinemann.

Harnoy, A. (2002). Bearing Design in Machinery: Engineering Tribology and Lubrication. CRC Press.