What Is a Journal Bearing?

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  • Written By: Pauline H. Gill
  • Edited By: C. Wilborn
  • Last Modified Date: 16 March 2014
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Journal bearings are widely used in gasoline and diesel-fueled piston engines in motor vehicles, and allow parts to move together smoothly. Main bearings support a rapidly rotating crankshaft within an engine block, while connecting rod bearings help resolve the reciprocating linear motion of pistons to the rotating motion of the crankshaft by means of crankpin journals on the crankshaft. 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.

Automotive professionals 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 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 comprises the bearing action. Since most crankshafts are either cast or forged, they are 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. First, it hydraulically fills the bearing clearance, providing a viscous damping effect. It also cools the metal bearing surfaces as it circulates.

The journal bearing has several advantages over other types of bearings, providing 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 help make engines quiet and smooth running. With their inherent advantages, these bearings are also used in other high-load, high-velocity applications, such as machines and turbines.


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Post 3

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.

Post 2

@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.

Post 1

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?

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