9. Maximum vibration levels that can be tolerated.
Bearing .esign Principles
The journal bearing is a fluid-film bearing. This description means that a full film of fluid completely separates the stationary bushing from the rotat-ing journal-the two components that make up the bearing system. This separation is achieved by pressurizing the fluid in the clearance space to the extent that the fluid forces a balance in the bearing load. This balance requires the fluid to be continuously introduced into and pressurized in the film space. Figure 13-7 shows the four modes of lubrication in a fluid-film bearing. The hydrodynamic mode bearing is the most common bearing type used and is also often called the ""self-acting'' bearing.
As can be seen in Figure13-7a， the pressure is self-induced by the relative motion between the two bearing member surfaces. The film is wedge-shaped in this type of lubrication mode. Figure 13-7b shows the hydrostatic mode oflubrication. In this type of a bearing， the lubricant is pressurized externally and then introduced in the bearing. Figure 13-7c shows the squeeze-film lubrication mode. This type of a bearing derives its load-carrying capacity and separation from the fact that a viscous fluid cannot be instantaneously
Figure 13-7.Modes of fluid-film lubrication: (a)hydrodynamic， (b)hydrostatic，
(c) squeezefilm， (d) hybrid.
squeezed out between two surfaces that are approaching each other. Figure 13-7d shows a hybrid-type bearing that combines the previous modes. The most common hybrid type combines the hydrodynamic and hydrostatic modes.
A further investigation of the hydrodynamic mode is warranted， since it is the most common type of lubrication mode employed. This type of lubrica-tion depends on the bearing member velocity as well as the existence of a wedge-shaped configuration. The journal bearing forms a natural wedge asseen in Figure13-8， which is inherent in its design. Figure 13-3 also shows the pressure distribution in the bearing. Fluid-film thickness depends on themode， lubrication， and application and varies from .0001to .01inches(.00254..254 mm.) For hydrostatic oil-lubricated bearings， the film thickness is .008 of an inch (.203 mm). In the special case of oil-squeeze film bearings where the capacity must be provided to take extremely high-revising loadswith no bearingharm， the oil-film thickness could be below .0001of an inch(.00254 mm). Since the film thickness is so very important， an understanding of the surface is very important.
Figure 13-.. .ressure distribution in a full .ournal bearing.
All surfaces， regardless of theirfinish， are made up of peaks andvalleys，and in general， the average asperity height may be 5.10 times the RMSsurface finish reading. When the surface isabraded， an oxide film will form almost immediately.
Figure 13-9a shows the relative separation of the full-film， mixed-film， andboundary. If a full-filmexists， the bearing life is almost infinite. The lim-itation in the case of full-film is due to lubricant breakdown， shockload，bearing surface erosion， and fretting of bearing components. Figures 13-9b and 13-9c are cross sections showing the various contamination types. Oil additives are contaminants that form beneficial surface films.
The bearing health can be best described by plotting a ...P versus coefficient of friction curve. Figure 13-10 shows such a curve where . isthe lubricant viscosity in centipoise， .the rpm of the journal， and P is the projected area unit loading.
As the bearing speed is increased for a given lubricant and loading， thefriction is at its lowest when full-film is achieved， after which the increase is due to the increasing lubricant shear force.
The bearing fluid film acts like a spring that is nonlinear. Figure 13-11 shows a curve of bearing load versus film thickness and eccentricity ratio. The bearing stiffness can then be obtained at any load value by drawing a line tangent to the curve at the load point. The film stiffness can then be used in determining the critical speed of the rotor.
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