Why do some say plain bearings, others hydrodynamic bearings and others fluid film bearings?

When discussing bearing technology, terms like plain bearing, hydrodynamic bearing and fluid film bearing are often used, sometimes interchangeably, sometimes with clear distinctions. This variation in terminology can lead to confusion, particularly when working across industries or international markets.

So what’s the difference? Are these three terms describing the same thing or do they refer to specific bearing types and operating principles?

This article provides clarity. We explain

what each term means,
how they relate to one another,
and how Miba uses these terms in our technical communication and product portfolio.

What is a Plain Bearing?

A plain bearing, also known as a sliding or sleeve bearing, is a type of bearing that supports relative motion between two surfaces without rolling elements. Instead of rolling, the shaft (or journal) slides against the bearing surface.

Plain bearings are among the oldest and most widely used bearing types in industrial applications.

Key Characteristics of Plain Bearings

Compact design with fewer moving parts
Excellent NVH behavior, reducing vibration and noise
Low maintenance requirements, particularly in lubricated or self-lubricating variants

Typical Applications

Plain bearings are used in a wide range of industries, including:

Internal combustion engines (e.g., main and connecting rod bearings)
Compressors and pumps
Turbomachinery (e.g., steam and gas turbines)
Marine propulsion systems
Industrial gearboxes and drives

At Miba: Plain Bearing as an Umbrella Term

Commonly plain bearing is used as an umbrella term that includes various bearing types, materials and operating principles, including hydrodynamic and hydrostatic variants. From the basic types to the highly engineered ones, what they all share is simplicity in design: a shaft, a surface and usually a lubricant.

We at Miba focus on plain bearings, which operate on hydrodynamic and hydrostatic principles, not self-lubricating.

hydrodynamic bearing on white background

What is a Hydrodynamic Bearing?

A hydrodynamic bearing is a specific type of plain bearing that operates based on the principle of fluid film lubrication. In this design, the bearing surfaces are completely separated by a thin film of lubricant, typically oil, generated by the relative motion between the shaft and the bearing.

This self-sustaining lubrication mechanism forms a pressure-driven fluid wedge that supports the applied load without any direct metal-to-metal contact during normal operation. The result: minimal wear, low friction and high reliability over extended operating periods.

Key characteristics of hydrodynamic bearings include

Load capacity increases with speed: the faster the shaft rotates, the more stable the fluid film
Virtually no contact under normal conditions, reducing wear and extending service life
Excellent suitability for high-speed and high-load applications
Requires expertise for proper start-up and shutdown

Typical Applications

Hydrodynamic bearings are commonly used in applications with high loads, high speeds or continuous operation, such as:

Large compressors and turbines
High-performance engines (main and connecting rod bearings)
Power generation equipment (e.g., generators, steam turbines)
Gearboxes

At Miba: Hydrodynamic Bearings as High-Performance Solutions

At Miba, we distinguish between two core types of hydrodynamic plain bearings, depending on application requirements and installation context:

Half shell /bushings: These are mounted directly into the housing. Their compact design allows for easy assembly and replacement
Assembled bearings: These consist of a precision-machined steel housing combined with customized bearing pads and are normally used in large rotating equipment such as turbines, compressors and generators. Assembled bearings offer maximum flexibility in terms of design cooling integration and monitoring features.

Based on the direction of the load:

Journal bearings: are designed to carry radial loads - that is, forces perpendicular to the shaft.
Thrust bearings: are used to carry axial loads, acting parallel to the shaft.

Both journal and thrust bearings in Miba’s product portfolio are typically engineered for hydrodynamic operation. This means that under proper operating conditions, a full fluid film separates the bearing surfaces, minimizing wear and friction.

What About Fluid Film Bearings?

The term fluid film bearing refers to a category within plain bearing technology that also operates with a continuous film of lubricant separating the bearing surfaces. The term is usually used for bearing assemblies like:

Key Characteristics of Fluid Film Bearings

Extremely low friction due to complete surface separation

Minimal wear and long service life when properly operated

High speed-carrying capacity, especially in large-scale or high-performance systems

Precision and stability, ideal damping properties

Application Areas of Fluid Film Bearings

As a high-performance class of plain bearings, fluid film bearings are commonly used in the following applications

Power generation (e.g., hydro, gas, and steam turbines)

Turbo gearboxes

Process industry machinery (e.g., compressors, fans, large pumps)

Test benches

These bearings are typically selected where extreme speeds or precision requirements make rolling element bearings unsuitable.

How Fluid Film Bearings Relate to Plain Bearings at Miba

At Miba, fluid film bearings are considered part of the broader hydrodynamic and even broader plain bearing family. Miba plain bearings for high-load or high-speed applications are designed to operate under hydrodynamic conditions.

This includes both:

Journal bearings (radial)
Thrust bearings (axial)

Additionally, for specific applications, hydrostatic bearing designs are also available, where the fluid film is maintained via external pressure systems, even at standstill.

Summary: Different Terms, One Core Technology

In technical communication, clarity and consistency are essential, particularly when collaborating across industries, geographies and engineering disciplines. That’s why Miba uses a clearly defined terminology in all product documentation and customer interfaces.

Plain Bearing as the Umbrella Term

At Miba, the term plain bearing is used as the umbrella term for all bearing types that operate via sliding contact,

Depending on the application, operating principle and system requirements, these bearings are further specified by their lubrication regime or geometry, but they all fall under the plain bearing category.

Use of Hydrodynamic and Fluid Film Terminology

The term hydrodynamic bearing is used when emphasizing the operating principle: i.e., when the lubricant film is generated by shaft rotation and load.
The term fluid film bearing is normally used for bearing assemblies where both hydrodynamic and hydrostatic systems are in use.
At Miba, the terms hydrodynamic and fluid film bearing are used – both fall under plain bearing.