the rotating journal. To generate pressure, the resistance to pumping must increase in the direction of the flow. In the figure, 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 its concentric position. The attitude angle g in Figure 1 is the angle between the load direction and the line of centers. 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 occurs along the line of centers, the film becomes divergent. The resistance decreases in the direction of pumping, and either negative pressures occur or the air in the lubricant gasifies or cavitates and a region of atmospheric pressure occurs in the bearing area. This phenomenon is known as fluid film bearing cavitation. It should be clearly distinguished from other forms of cavitation that take place in pumps, such as in the impeller, for example. Here the fluid is traveling at a high velocity and the inertia forces on each fluid element dominate. Implosions occur in the impeller and can cause damage.

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

Regimes of Lubrication Whether or not a fluid film can be formed, journal rotation is dependent on several factors including the surface speed, viscosity, and load capacity. A parameter1 is often used to determine a particular regime of lubrication, ZN/P, where

N = rotating speed, rpm

P, = average pressure of the bearing, lb/in2 (bar)*

A plot of the coefficient of friction versus ZN/P generally has the form shown in Figure 2. At low values of ZN/P, a combination of viscosity, speed, and load places a bearing in a boundary lubricated regime where typical coefficients of friction are 0.08 to 0.14. Boundary lubrication implies intimate contact between the opposed surfaces. As the value of the parameter increases as a result of the increased speed, increased viscosity, or lowered load, there is a dramatic reduction in the coefficient of friction. In this region, there is a mixed film lubrication and the coefficient of friction varies between 0.02 and 0.08. By mixed film lubrication, it is meant that the journal is partly surrounded by a fluid film and is partly supported by rubbing contact between the opposed members. As ZN/P increases boundary ffmction boundary ffmction

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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