0.015
0.01
0.005
0
Figure 13-14. Babbitt fatigue characteristics.
creep under the softening influence of the rising temperature. .reep can occur with generous film thickness and can be observed as ripples on thebearing surface where flow took place. With tin babbitts， observation has shown that creep temperature ranges from 375 oF (190 o.) for bearing loads below 200 psi (13.79 Bar) to about 260.270 oF (127.132 o.) for steady loads of 1000 psi (69 Bar). This range may be improved by using very thin layers of babbitt such as in automotive bearings.
Bearing and Shaft Instabilities
One of the most serious forms of instability encountered in journal bear-ing operation is known as ""half-frequency whirl.'' It is caused by self-excited vibration and characterized by the shaft center orbiting around the bearing center at a frequency of approximately half of the shaft rotational speed as shown in Figure 13-15.
As the speed is increased， the shaft system may be stable until the ""whirl'' threshold is reached. Whenthe threshold speed is reached， the bearing becomesunstable， and further increase in speed produces more violentinstability until eventual seizure results. Unlike an ordinary criticalspeed，the shaft cannot ""passthrough，'' and the instability frequency will increase and follow that half ratio as the shaft speed is increased. This type ofinstability is associated primarily with high-speed， lightly loaded bearings.At present， this form of instability is well understood， can be theoreticallypredicted with accuracy， and avoided by altering the bearing design.
It should be noted that the tilting-pad journal bearing is almost com-pletely free from this form of instability. However， under certain conditions，
0 500 1000 1500 2000 2500 Bearing life (hrs)
the tilting pads themselves can become unstable in the form of shoe (pad)flutter， as mentioned previously.
All rotating machines vibrate whenoperating， but the failure of the bearings is mainly caused by their inability to resist cyclic stresses. The level of vibration a unit can tolerate is shown in the severity charts in Figure 13-16. These charts are modified by many users to reflect the critical machines in which they would like to maintain much lower levels. .are mustalways be exercised when using these charts， since differentmachines have different size housings and rotors. Thus， the transmissibility of the signal will vary.
.hrust Bearings
The most important function of a thrust bearing is to resist the unba-lanced force in a machine's working fluid and to maintain the rotor in its position (within prescribed limits). A complete analysis of the thrust loadmust be conducted. As mentioned earlier， compressors with back-to-back rotors reduce this load greatly on thrust bearings. Figure 13-17 shows anumber of thrust-bearing types.Plain， grooved thrust washers are rarelyused with any continuousload， and their use tends to be confined to cases where the thrust load is of very short duration or possibly occurs at astandstill or low speed only. Occasionally， this type of bearing is used for light loads (less that 50 lb/in2.3.5 bar.)， and in these circumstances the operation is probably hydrodynamic due to small distortions present in the nominally flat bearing surface.
When significant continuous loads have to be taken on a thrustwasher， it is necessary to machine into the bearing surface a profile to generate a fluid film. This profile can be either a tapered wedge or occasionally a small step.
The tapered-land thrust bearing， when properly designed， can take andsupport a load equal to a tilting-pad thrust bearing. With perfect alignment， it can match the load of even a self-equalizing tilting-pad thrust bearing that pivots on the back of the pad along a radial line. For variable-speed oper-ation， tilting-pad thrust bearings as shown in Figure 13-18 are advantageous when compared to conventional taper-land bearings. The pads are free to pivot to form a proper angle for lubrication over a wide speed range. The self-leveling feature equalizes individual pad loadings and reduces the sen-sitivity to shaft misalignments that may occur during service. The major drawback of this bearing type is that standard designs require more axial space than a nonequalizing thrust bearing.
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