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Monday, June 28, 2010

Centrifugal Pump with Journal Bearing

A journal bearing is essentially for a viscose pump, and it devices load capacity by pumping the lubricant through a small clearance region. To generate pressure, the resistance to pumping must increase in the direction of the flow. The journal moves to form a converging tapered clearance in the direction of the rotation or flow.

The eccentricity e is the total displacement of the journal from the concentric position. The attitude angle γ, is the angle between the load direction and the line of centres. Note, that because of the necessity to form a converging wedge, the displacement of the journal is not along a line that is coincident with the load vector. A positive pressure is produced in the converging region of the clearance. Downstream from the minimum film thickness which occur along the line of centres, the film become divergent. The resistance decrease in the direction of pumping, and either negative pressure occur or the air in the lubricant gasifies or cavitates and a region of atmosphere pressure occurs in the bearing area. This phenomenon is known as fluid film bearing cavitation. It should be clearly distinguished from other form of cavitation that take place in pumps, such as in the impeller, for example. The fluid is travelling at a high velocity and the inertia forces on each fluid element dominate. Implosion occur in the impeller and can cause damage.

In a bearing the viscose forces dominate and each fluid particle moves at a constant velocity in proportion to the net shearing forces on it. Thus cavitation on the bearing is more of a change of a phase of the lubricant that occurs in a region of lower pressure that permits the release of entrained gases. Generally bearing cavitation does not causes damage.