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Friday, June 18, 2010

Bearing Types

The most common type of journal bearing in the plain cylindrical bushing. It can be split and have lubricating freed groove at the parting line. A ramification is to incorporate axial grooves to enable better cooling and to improve whirl stability. The principle advantages of cylindrical bearing are (1) simple construction ad (2) a high load capacity relative to other bearing configuration.
 
This type of bearing also has several disadvantages:
  • Whirl instability: this is prone to sub synchronous whirling at high speed and also at low loads. Whirling is an orbiting of the journal (shaft) centre in the bearings, a motion that is superimposed upon the normal journal rotation. The orbital frequency is approximately half the rotating speed of the shaft. The expression half-frequency whirl is commonly used. The reason for the occurrence of this whirl and more details concerning bearing dynamics are presented in the section on bearing dynamics.
  •  Viscose Heat Generation: because of the generally larger and uninterrupted surface area of this bearing, it generates more viscose power loss than some other types.
  • Contamination: The cylindrical bearing is more susceptible to contamination problems than other because contaminants that are dragged in at the leading edge of the bearing cannot easily dislodge because of the absence of grooves or other escape paths.
 The advantage of simplicity and load capacity make the plain journal a leading candidate for most applications, but performance should be carefully investigated for whirl instability and potential thermal problems. Cylindrical bearings are generally used for medium speed (500 in/s (200 mm/sec) surface speed) and medium to heavy load application (250 to 400 lb/in2 (17 to 28 bar) on a projected area.
 
Cylindrical Bearing with Axial Grooves
 
A typical configuration of this type of bearing is a plain cylindrical bearing with four equally spaced longitudinal groove extending most of the way through the bearing. Usually, a slight land area exist at either end of the groove to force the inlet flow to each groove into the bearing clearance region, rather than out the groove ends. This configuration is a little less simple than the plain cylinder bearing, and because the grooves consume some land area, this configuration has less load capacity than the plain bushing. Since oil is fed into each of the axial grooves, this bearing requires more inlet flow but also will run cooler than the plain bushing. The grooves act as convenient outlets for any contaminant in the lubricant, and thus the grooved bearing can tolerate more contamination than the plain cylindrical bearing.
 
Empirical and Lobe Bearings
 
Elliptical and lobe bearing have noncircular geometries. As elliptical bearing is simply a two lobe bearing with the major axis along the horizontal axis. Thus at the leading edge region, a converging clearance produces positive pressure, but downstream from the minimum film thickness, a divergent film thickness distribution can be found with resulting negative, or cavitations, pressure.
 
The canted lobe, generally develop positive pressure throughout the lobe because the bearing is constructed with a completely converging film thickness in each lobed region. This design has excellent whirl resistance (superior to that of the symmetric lobe bearing) and a reasonable good load capability. A 2:1 ratio between leading and trailing edge concentric clearance is generally a reasonable compromise with respect to performance.
 
Elliptical and lobe bearing are often used because they provide better resistance to whirl than cylindrical configurations. They do so because they have multiple load producing pads that assist in preventing large attitude angles and cross coupling. Elliptical and lobe bearing are generally used for high speed, low load applications where whirls might be a problem.

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