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TABLE 19.18 Long Gas Journal Bearing Performance Parameters, Limiting Cases, Long-Bearing Theory
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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JOURNAL BEARINGS 19.45
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19.7.2 Stability Considerations The journal bearing of a rotating machine cannot be treated separately. It is part of a complex dynamic system, and its design can influence the dynamic behavior of the entire system. Bearing damping controls vibration amplitude and tolerance to any imbalance and, to a large extent, determines whether the rotor will be dynamically stable. A bearing running in an unstable mode can lose its ability to support load, and rubbing contact can occur between the bearing and the shaft surfaces. Rotor bearing instabilities are particularly troublesome in gas-lubricated bearings; the film is considerably thinner in gas bearings than in liquid-lubricated bearings and so does not possess the damping capacity. We can differentiate between two forms of dynamic instability: synchronous whirl and half-frequency whirl (also called fractional-frequency whirl and film whirl). Synchronous Whirl. A rotating shaft experiences periodic deflection (forced vibration) because of the distribution of load, the method of shaft support, the degree of flexibility of the shaft, and any imbalance within the rotating mass. This deflection causes the journal to orbit within its bearings at the rotational speed. When the frequency of this vibration occurs at the natural frequency (or critical speed) of the system, a resonance condition exists. In this condition, the amplitude of vibration (size of the journal orbit) increases and can cause bearing failure. Because the shaft rotational speed and the critical speed coincide, this form of instability is termed synchronous whirl. Since stable operation occurs on either side of the critical speed, the bearing system must be designed so that critical speeds do not exist in the operating speed range. This can be accomplished by making the critical speeds either very large or very small. Large critical speeds can be established by increasing the bearing stiffness by reducing the bearing clearance. Lower critical speeds can be established by increasing the shaft flexibility. The instability can also be suppressed by mounting the bearing in a flexible housing which introduces additional system damping. Half-Frequency Whirl. A deflected rotating shaft has its center whirl, or orbit, around the center of the bearing. When the whirl speed is equal to or somewhat less than half the rotational speed of the shaft, , the hydrodynamic capacity of the bearing to support a load is diminished and, in fact, may fall to zero. With any subsequent rotational speed increase, the whirl amplitude will increase, and the bearing and journal may come into violent contact. This form of self-activated vibrational instability is called half-frequency or half-speed whirl. Because the whirl speed for gas bearings is generally less than the rotational speed, the instability is also termed fractional-frequency whirl. Figure 19.20 is a map for assessing the stability of a full journal bearing. Half-speed whirl may be suppressed by altering the circumferential symmetry of the bearing. This can be accomplished by providing axial grooves in the bearing surface, by dividing the circumference into a number of sections, or lobes, or by using tilting pads. Also, introducing more damping into the system through the use of flexible housing mounts can be helpful.
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19.7.3 Cylindrical Ungrooved Gas Journal Bearings Complete plain cylindrical journal bearings are generally used in applications that require support of a fixed, unidirectional load. Self-excited whirl instability is a com-
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Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
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