Plain bearings for high speeds? What are the limits?

By Lars Butenschon

“Plain bearings are particularly suitable for high loads at low speeds.”

This statement appears in many technical references, but it can be misleading in practice. Terms such as high load and low speed are relative, and bearing performance depends on more than either factor alone.

As discussed in previous articles, load and speed are linked through the PV value, the product of pressure and velocity. The PV value provides an indication of the heat generated at the bearing interface and the system’s ability to dissipate that heat. This means the same PV value can be reached through different combinations of load and speed. In other words, a bearing suitable for high loads at low speeds may also be suitable for lower loads at higher speeds.

Heat Is the Key Limitation

When operating at higher speeds, heat becomes the primary challenge. There are two ways to address it:

• Reduce heat generation by lowering load and friction.
• Improve heat dissipation through cooling measures.

Reducing friction and pressure decreases the amount of heat generated. Improving heat dissipation is often more difficult and may require cooling fluids, like engine fuel, or operating pauses that allow components to cool.

Material selection can also influence heat management. Metals conduct heat particularly well, but they generally require lubrication to keep friction and wear at acceptable levels.

How Materials Affect High-Speed Performance

High-temperature plastics can operate continuously at temperatures between approximately 250°C and 280°C. Beyond this range, material options become more limited.

Where lubricated metal bearings are unsuitable, materials such as ceramic, carbon, graphite, and silicon carbide may be used. However, these materials can be difficult to machine and relatively brittle, making them practical only for specific applications.

Below approximately 180°C, a much wider range of engineering plastics is available, allowing designers to select materials for a variety of operating conditions.

The Advantage of Tribologically Optimized Plastics

Tribologically optimized plastics generally have lower temperature limits than metals, but they offer important advantages. They can operate with or without lubrication and are resistant to many chemicals, making them suitable for applications such as pumps and valves with direct media contact.

They can also continue operating for a limited time during emergency conditions. However, heat remains the determining factor. If temperatures exceed the material’s operating limits, performance and service life will be affected.

Conclusion

The suitability of a plain bearing for higher-speed applications depends on the balance between load, speed, friction, and heat management. Understanding the relationship between PV value, operating temperature, and material properties can help engineers identify the most appropriate bearing solution for their application.