Journal Bearing

Journal Bearing

A reliable compressor depends heavily on the quality of its internal components, and bearings are central to smooth, efficient operation. High-performance bearings and bushings help minimize friction, reduce wear, and maintain consistent performance under varying loads and speeds.

Journal bearings are among the simplest types of bearings used in rotating machinery. They support the shaft inside a smooth, cylindrical sleeve, allowing it to rotate freely with minimal friction. The section of the shaft that rests within the bearing is known as the journal—hence the name.

Unlike rolling-element bearings, journal bearings do not use balls or rollers to carry the load. Instead, they rely on a thin film of lubrication between the journal and the bearing surface to support radial loads and allow smooth operation. Despite their simple design, journal bearings are capable of handling substantial weight due to the distribution of load across the contact surface and the inherent strength of the shaft itself.

Journal bearings operate under both boundary and hydrodynamic lubrication regimes, depending on the stage of operation. During startup and shutdown, there is direct metal-to-metal contact between the shaft and the bearing surface, resulting in boundary lubrication. This condition involves higher friction and wear due to limited oil film thickness.

Once the shaft begins to rotate and oil is supplied to the contact area, pressure builds up and creates a hydrodynamic film that lifts the shaft away from the bearing surface. In this regime, the shaft is fully supported by a fluid film, significantly reducing friction and wear. Hydrodynamic lubrication allows journal bearings to support higher loads and operate efficiently at elevated rotational speeds, making them ideal for heavy-duty, high-speed compressor applications.

Journal bearings are generally classified into two main types based on their assembly: plain bearings and two-piece bearings. Each type serves specific applications depending on load, speed, and maintenance requirements.

• plain bearings are built directly into the housing structure, where a machined hole acts as the bearing surface. One of the most common examples is the pillow block bearing. In this design, the housing is typically made of cast iron for structural strength, while the bearing surface itself is lined with harder materials such as chromium steel alloys to resist wear. Lubrication in integral bearings is usually maintained through grease nipples that deliver lubricant directly into the bearing surface. This ensures consistent lubrication, reduces friction, and prolongs service life, especially in medium-speed, moderate-load applications.

Two-piece bearings consist of an upper and a lower section, often referred to as bearing shells. The lower part is fixed to the application, while the upper part is removable for easy inspection and maintenance. This configuration is especially useful in large-diameter applications, such as crankshafts in high-power reciprocating compressors. These bearings are typically manufactured slightly larger than the housing to ensure a secure fit. During installation, they are compressed into place, creating a tight and reliable seating that maintains alignment under load. The split design allows for straightforward replacement without dismantling the entire assembly, reducing downtime and simplifying maintenance in heavy-duty equipment.