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Journal bearings reduce friction between moving parts in engines and machinery. Trunnion designs are best for high motion situations. They consist of four main parts and require regular lubrication. They handle high loads and speeds and are used in various applications.
A journal bearing is a mechanical device used to reduce friction between moving parts and is an essential component of most engines. Engines and other related machinery often use many types of bearings, but trunnion designs are often best for situations where there is a lot of motion. This type of device could also be called a radial bearing or sleeve bearing and is generally made up of four main parts that all work together to support a given load. They also usually depend on a regular supply or engine oil for lubrication; without this they can wear out, which over time usually leads to engine problems. People often make the effort to regularly change the oil in their car and truck engines in part to keep these moving parts in good working order.
Basic movement
The primary goal of nearly all pads is to help control motion, and the ones in the journal category are no different. In this context, the term “journal” describes the shaft lining, which is the part that comes into closest contact with the bearing itself. Taken together, this type of bearing functions like a moving part that supports a rapidly rotating crankshaft within an engine block. It also helps resolve the reciprocating linear motion of the pistons to the rotary motion of the crankshaft by means of crankpins. A straight-four engine would normally have one main bearing at 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.
Main parts
The four main parts of this type of bearing are the shaft journal; the removable half-shells of the bearings, generally in steel with a soft alloy coating; the bearing shell support halves; and the oil that actually makes up the bearing action. Since most crankshafts are cast or forged, they tend to be all in one piece and the bearing journals are machined into the rough shape that results from the casting or forging process. The shells and mounts are split exactly in half at the bottom of the engine block to allow the crankshaft to fit into the upper half-turns of the block. The caps that make up the lower half bearings of each bearing are then bolted into place under the crankshaft so that each crankshaft main bearing and crankpin is completely surrounded by a seating surface that conforms tightly.
The resulting bearing clearances are ideally in the range 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 engine oil from the oil vents in the engine block to flood each bearing with oil, which continuously flows out the side of the bearings and back into the oil pan. In addition to providing a thin, slippery film that prevents metal-to-metal contact, oil also performs many other functions. First, it fills the bearing clearance hydraulically, providing a viscous damping effect. It also cools the metal surfaces of the bearings during circulation.
Importance of regular lubrication
However, that oil doesn’t get into the bearing by itself; must be maintained. Engine owners usually need to check fluid levels regularly to be sure the parts have what they need to maintain proper lubrication levels. The shaft draws oil from the tank and distributes it along its length.
Automotive professionals typically view journal bearings as sliding bearings as opposed to rolling bearings, such as ball bearings. Despite this categorization, a rotating shaft within a journal bearing is actually separated from the bearing’s metal lining by an extremely thin film of continuously fed motor oil that prevents metal-to-metal contact. As such, it allows the crankshaft to be normally contacted by oil only, which accounts for the long life of engines that undergo regular oil changes.
Advantages
The journal bearing has several advantages over other types of bearings, provided it has a constant supply of clean, high-quality engine oil. First, it handles high loads and speeds because metal-to-metal contact is minimal due to the oil film. These bearings are also remarkably strong and long lasting, and due to the damping effects of the oil film, they can also help make engines quiet and smooth running. Partly because of these inherent benefits, this type of bearing is used in more than just gas- and diesel-powered piston engines. It tends to be common in many high speed, high load applications including a range of industrial machines and turbines.
