Journal bearings

Excellent antifriction properties and good hardness (qv) make lead—antimony—tin alloys suitable for journal bearings. The alloys contain 9—15 wt % antimony and 1—20 wt % tin and may also contain copper and arsenic, which improve compression, fatigue, and creep strength important in bearings. Lead—antimony—tin bearing alloys are Hsted in ASTM B23-92 (7). 

Babbitt ahoys ate suitable for hundreds of types of iastahations involving the movement of machinery, eg, the main, crankshaft, connecting tod big end, camshaft, and journal bearings associated with marine propulsion, tailtoad and automotive transportation, compressors, motors, generators, blowers, fans, rolling-mill equipment, etc. 

Types of Bearings Figure 10-119 shows a number of different types of journal bearings. A description of a few of the pertinent types of journal bearings is given here ... 

Usually, a journal bearing and a thrust bearing are combined in one assembly, and oil is injected so as to feed both of them. The rate of flow usually is adjusted so as to carry the heat away with a temperature rise of the order of II to 17° C (20 to 30° F). 

The diffu-ser equalizes radial loading on the shaft, impeller and journal bearings. 

The journal bearing diameter (based on the bearing manufacturer s experience)... 

Journal bearing length (based on an assumed load capacity)... 

Monitoring should include the usual parameters worthy of surveillance in high-speed turbo machinery the temperature of journal bearings, vibration and axial position of the pinions, inlet and discharge temperatures combined with discharge pressure from the individual compressor stages, and various lube oil system devices. Competent manufacturers make sure all measurement locations are completely prewired on the machine and made available at predefined interfaces or in terminal boxes for connection at the plant site. 

Throw-off temperatures from journal bearings and thrust bearing... 

If supported by journal bearings, the shaft may describe an orbit so that the axis of rotation itself rotates about the geometric center of the bearings. 

Plain journal. Bearing is bored with equal amounts of elearanee (on the order of 1to two thousands of an ineh per ineh of journal diameter) between the journal and bearing. 

Pressure or pressure dam. Used in many plaees where bearing stability is required, this bearing is a plain journal bearing with a pressure poeket eut in the unloaded half. This poeket is approximately 1 /32 of an ineh (.8 mm) deep with a width 50% of the bearing length. 

Figure 13-11. Journal bearing load capacity versus minimum film thickness and eeeentrieity ratio.

Another important advantage of the tilting-pad journal bearing is its ability to aeeommodate shaft misalignment. Beeause of its relatively short length-to-diameter ratio, it ean aeeommodate minor misalignment quite easily. 

One of the most serious forms of instability encountered in journal bearing 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. 

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