Figure 13-11. Journal bearing load capacity versus minimum film thickness and eccentricity ratio.
With higher speeds and unusual fluid lubricants， turbulence in the fluidfilm is no longer rare. Normally， thin film is thought of as beinglaminar，but with highspeeds， low viscosity， and sometimes high-densityfluids， the lubricant can be turbulent in the film space. This turbulence manifests itself as an abnormal increase in power loss. As compared to laminar-flow con-ditions， a Reynoldsnumber， even in the transitionregion， can double thepowerand， deep in the turbulentregion， can increase the power tenfold.Although this phenomenon， because of its random nature， is difficult toanalyze， there is an unusual amount of theoretical work that has been done and some experimental work that is available. Just as aguide， one can assume that the transition point will occur at a Reynolds number of about
800. As to film thickness， there is evidence indicating that under turbulent conditions it is actually greaterthan calculated， based on laminar-flow theory.
.ilting-Pad Journal Bearings
Normally， the tilting-pad journal bearing is considered when shaft loadsare light because of its inherent ability to resist oil whirl vibration.However，this bearing， when properly designed， has a very high load-carrying capacity. It has the ability to tilt to accommodate the forces being developed in the hydrodynamicoilfilm， and therefore operates with an optimum oil-film thickness for the given load and speed. This ability to operate over a large range of load is especially useful in high-speed gear reductions with various combinations of input and output shafts.
Another important advantage of the tilting-pad journal bearing is its ability to accommodate shaft misalignment. Because of its relatively shortlength-to-diameter ratio， it can accommodate minor misalignment quite easily.
As shownearlier， bearing stiffness varies with the oil-film thickness so that the critical speed is directly influenced to a certain degree by oil-film thick-ness. Again， in the area of critical speeds， the tilting-pad journal bearing has the greatest degree of design flexibility. There are sophisticated computer programs that show the influence of various load and design factors on the stiffness of tilting-pad journal bearings. The following variations are pos-sible in the design of tilting-pad bearings:
1. The number of pads can be varied from three to any practical number.
2. The load can be placed either directly on a pad or to occur between pads.
3. The unit loading on the pad can be varied by either adjusting the arc length or the axial length of the bearing pad.
4. A parasitic pre-load can be designed into the bearing by varying the circular curvature of the pad with respect to the curvature of the shaft.
5. An optimum support point can be selected to obtain a maximum oil-film thickness.
On a high-speed rotorsystem， it is necessary to use tilting-pad bearings because of the dynamic stability of these bearings. A high-speed rotor system operates at speeds above the first critical speed of the system. It should beunderstood that a rotor system includes therotor， the bearings， the bearingsupportsystem， seals，couplings， and other items attached to the rotor. The system's natural frequency is therefore dependent on the stiffness and damping effect of these components.
.ommercial multipurpose tilting-pad bearings are usually designed for multidirectional rotation so that the pivot point is at pad midpoint. How-ever， the design criteria generally applied for producing maximum stability and load-carrying capacity locates the pivot at two-thirds of the pad arc in the direction of rotation.
Bearing pre-load is another important design criterion for tilting-pad bearings. Bearing pre-load is bearing assembly clearance divided by machined clearance
.oncentric pivot film thickness
Pre-load ratio二 C.jC二 Machined clearance
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