At AboutMechanics, we're committed to delivering accurate, trustworthy information. Our expert-authored content is rigorously fact-checked and sourced from credible authorities. Discover how we uphold the highest standards in providing you with reliable knowledge.

Learn more...

What is a Journal Bearing?

Pauline H. Gill
Pauline H. Gill

A journal bearing is a mechanical device used to reduce friction between moving parts, and it is an essential component of most engines. Engines and other related machinery often make use of many types of bearings, but journal-type models are often best for situations where there is a lot of motion. This sort of device might also be called a radial bearing or sleeve bearing, and is generally made up of four major parts that all work together to support a given load. They typically depend on a regular supply or motor oil for lubrication, too; without this they can wear down, which over time will usually lead to engine trouble. People often commit to regular oil changes for car and truck engines in part to keep these moving pieces in good working order.

Basic Movement

Man with a drill
Man with a drill

The main goal of almost any bearing is to help control movement, and those in the journal category are no different. In this context, the term “journal” describes the shaft covering, which is the part that comes into the closest contact with the bearing itself. Taken as a whole this sort of bearing operates as a moving part supporting a rapidly rotating crankshaft within an engine block. It also helps to resolve the reciprocating linear motion of pistons to the rotating motion of the crankshaft by means of crankpin journals. An in-line four cylinder engine would normally have a main bearing on each end plus one between each cylinder for a total of five, and one connecting rod bearing for each piston for a total of four.

Major Parts

The four major parts of this type of bearing are the shaft journal; the removable bearing shell halves, usually steel with a soft alloy lining; the bearing shell support halves; and the oil that actually makes up the bearing action. Since most crankshafts are either cast or forged, they tend to be all one piece, and the bearing journals are machined into the rough shape that comes from the casting or forging process. The shells and supports are split exactly in half at the bottom of the engine block to allow the crankshaft to be inserted into top half-rounds in the block. The caps that make up the bottom half rounds of each bearing are then bolted into place under the crankshaft so that each crankshaft main bearing and connecting rod journal is completely surrounded by a bearing surface that conforms tightly.

The resulting bearing clearances are ideally in the realm of ten thousandths to thousandths of an inch (thousandths to hundredths of a millimeter) and the journals are virtually perfectly round. Holes and grooves in each main bearing shell allow pressurized motor oil coming from the oil galleys in the engine block to flood each bearing with oil, which continually runs out the side of the bearings and returns to the oil pan. Besides providing a thin slippery film that prevents metal-to-metal contact, the oil performs several other functions, too. First, it hydraulically fills the bearing clearance, providing a viscous damping effect. It also cools the metal bearing surfaces as it circulates.

Importance of Regular Lubrication

That oil doesn’t just get into the bearing on its own, though; it has to be maintained. Engine owners usually have to regularly check the fluid levels to be sure that the parts have all they need to maintain proper levels of lubrication. The shaft takes oil from the reservoir and distributes it along its length.

Automotive professionals typically consider journal bearings to be sliding bearings as opposed to rolling bearings, such as ball bearings. Despite this categorization, a shaft spinning within a journal bearing is actually separated from the bearing’s metal facing by an extremely thin film of continuously supplied motor oil that prevents metal-to-metal contact. As such, it allows the crankshaft to normally be contacted only by oil, which explains the long life of engines that get regular oil changes.


The journal bearing has several advantages over other types of bearings, provided that it has a constant supply of clean high-grade motor oil. First, it handles high loads and velocities because metal-to-metal contact is minimal due to the oil film. These bearings are also remarkably durable and long lasting, and because of the damping effects of the oil film, they may also help make engines quiet and smooth running. In part because of these inherent advantages this sort of bearing is used in more than just gas and diesel-fueled piston engines. It tends to be common in many high-load, high-velocity applications, including a range of industrial machines and turbines.

Discussion Comments


Many people have begun using synthetic oils to prevent wear to the bearings in their cars. It is a bit more expensive than standard engine oil, but it provides a great deal more protection to parts since it does not break down like regular oil. This can help prevent journal bearing failure in the engine. Since synthetic oil is designed to provide that a lot more effectively and a lot longer than other oils, it actually saves money and a lot of wear and tear on vehicles in the long run.


@crunchies- Engine knocking is usually more about the kind of fuel you are running in the car rather than a bearing of any sort. There could be a problem with your main or thrust bearing that is causing this, but the first place to start is with fuel and the fuel system. The easiest thing to try is to run a tank of a higher octane fuel than you normally do. In other words, if you normally get regular gas try a tank of plus or premium. It may not fix things right away, but if you see any difference at all in the knocking that indicates that you need to be looking to the fuel or fuel system as the problem.


I have made every effort to regularly change my oil and use high quality motor oil and my engine still is loud and knocking. Does anybody have any idea why? Could it be some kind of bearing?

Post your comments
Forgot password?
    • Man with a drill
      Man with a drill