What Is a Journal Bearing and Why Are They Important?

What Is a Journal Bearing

A journal bearing is one of the simplest types of bearing. A journal bearing is a bearing that allows a shaft to rotate freely within a supporting sleeve or shell. The part of the shaft which fits in the bearing is the journal, giving rise to the name.

Unlike roller and ball bearings, journal bearings do not have any rolling elements. As a result, journal bearings are only capable of supporting radial loads, which stems from the weight of the shaft.

Journal bearings are important as they improve the service life of engineering systems and have only a small cost. Due to their simplistic design and manufacturing requirements, they are commonly used in many engineering industries and are one of the most popular types of bearings.

Journal bearings run in the hydrodynamic lubrication regime after startup, which operates in the boundary lubrication. The rotational speed of the journal in the bearing forces oil between it and the bearing surface, creating an oil wedge – a wedge must be present for hydrodynamic lubrication to allow sufficient pressure to be generated in the fluid. If you want to learn more about lubrication and the different regimes, learn more here.

Types of Journal Bearings

Integral

An integral plain bearing has been integrated into the object as a hole to act as a bearing surface. The most common type of integral journal bearing is a pillow block bearing. Pillow block bearings tend to have the housing made of a cast iron, and the bearing surface is made of material chromium steel alloy. They tend to have grease nipples on the housing to help maintain good lubrication throughout their service life.

Bushing

A bush is a mechanical component fitted into housings, commonly by interference fit, to provide a bearing surface. The size of the housing and shaft will determine the necessary internal and external diameter of the bushing.

The most common kind of bushing is a sleeve, which can be plain or flanged.

A flanged sleeve is a common type of busing - a type of journal bearing — Credit: Silberwolf License: CC BY-SA 2.5

A sleeve is a common type of busing - a type of journal bearing — Credit: Silberwolf License: CC BY-SA 2.5

Two-piece

Two-piece journal bearings are, as the name suggests, a bearing split into two pieces. The halves are referred to as shells and are used on large diameters, for example, a crankshaft. To prevent the bearing to move during operation, it is common for a dowel to be used.

Like roller and ball bearings the two halves of bearings are manufactured slightly larger than their housing. During installation, the shells are compressed, producing a radial force preventing the bearing from spinning.

Materials In a Journal Bearing

A common material used in journal bearings is ‘white metal’. A type of white meal is Babbit metal which is used for the bearing surface. Babbitt metal is soft with hard crystals within the structure. The softer part of the metal wears, generating a lubricating film between the journal and the harder particles in the Babbitt metal. Additionally, the hard particles wear the surrounding surface and generate a path, aiding rotation.

Tin is commonly used in babbitt metal, and from the friction generated, it melts and acts as a lubricant.

Bearings also consist of a backing material which is a harder material. For two-piece journal bearings, it is common for the material to be copper-alloy or bronze. A general backing bronze material is CC493K, SAE 660 and LG4 (BS 1400). These bronzes have slightly different compositions of elements, but all serve the same purpose.

The composition of the bronze can change slightly for the environment. For example, CC493K has a higher zinc content, approximately 3.0% – 5.0%. The higher zinc content provides greater corrosion resistance to seawater, making it attractive for marine applications.