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Journal bearing

A journal bearing is a simple bearing in which a shaft, or "journal", or crankshaft rotates in the bearing with a layer of oil or grease separating the two parts through fluid dynamic effects. The shaft and bearing are generally both simple polished cylinders with lubricant filling the gap. Rather than the lubricant just "reducing friction" between the surfaces, letting one slide more easily against the other, the lubricant is thick enough that, once rotating, the surfaces do not come in contact at all. If oil is used, it is generally fed into a hole in the bearing under pressure, as is done for the most heavily-loaded bearings (main, connecting rod big-end and camshaft) in an automobile engine. A simple oil "slinger" in the sump and an appropriate feed hole in the bearing shell are considered adequate for small single-cylinder engines, such as those used in lawnmowers.

The casing that houses the journal bearing is called the journal box.

Types of Journal Bearing

Liquid journal bearings can be hydrodynamically lubricated or hydrostatically lubricated. The difference between hydrostatic and hydrodynamic forces is in the way the pressure that supports the bearing is initially and subsequently maintained.

In a hydrostatic bearing, the pressure is always present at a value that is desirable and is achieved by an external pump which forces lubricant into the system. The pump provides a magnitude of pressure that aims to supplement the pressure which is created by the bearings rotation, if any. In a hydrodynamic bearing the pressure in the oil film is maintained by the rotation of the shaft itself.

Hydrodynamic Bearings

Hydrodynamic bearings require much greater care in design and operation than hydrostatic bearings. They are also much more prone to initial wear because lubrication does not occur until there is rotation of the shaft. At low rotational speeds the lubrication may not attain complete separation between shaft and bush. As a result, hydrodynamic bearings are often aided by secondary bearings which support the shaft during start and stop periods, protecting the fine tolerance machined surfaces of the journal bearing.widely use in automobile industry.

General Theory

A journal bearing works on the principle that, over an infintesimally small length of the shaft circumference, the theory of a lubricated pair can be applied. The convergence as well as the viscosity and velocity of fluid generate a pressure film. As one surface moves, it drags oil into the gap that is made between it and the other. As the oil moves forward, the space decreases. The oil can be considered to be incompressible enough to generate pressure. This pressure prevents oil from entering the gap created. The oil within the gap reaches a pressure limit after which it pushes oil through the smaller space.