Bearing Types

Cylindrical Bearing The most common type of journal bearing is the plain cylindrical bushing shown schematically in Figure 1. It can be split and have lubricating feed grooves at the parting line. A ramification is to incorporate axial grooves to enable better cooling and to improve whirl stability (described in more detail below in the discussion of cylindrical bearings with axial grooves). The principle advantages of cylindrical bearings are (1) simple construction and (2) a high-load capacity relative to other bearing configurations.

This type of bearing also has several disadvantages:

• Whirl Instability: This is prone to subsynchronous whirling at high speeds and also at low loads. Whirling is an orbiting of the journal (shaft) center in the bearing, 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.

• Viscous Heat Generation: Because of the generally large and uninterrupted surface area of this bearing, it generates more viscous power loss than some other types.

• Contamination: The cylindrical bearing is more susceptible to contamination problems than other types 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 advantages 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 applications (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 grooves extending most of the way through the bearing. Usually, a slight land area exists at either end of the groove to force the inlet flow to each groove into the bearing clearance region (see Figure 7), rather than out the groove ends. This configuration is a little less simple than the plain cylindrical 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 contaminants in the lubricant, and thus the grooved bearing can tolerate more contamination than the plain cylindrical bearing.

In general, this bearing can be considered as an alternate to a plain bearing if the former can correct a whirl or overheating problem.

Elliptical and Lobe Bearings Elliptical and lobe bearings have noncircular geometries. Figure 8 shows two types of three-lobe bearings with the clearance distribution exaggerated so that the lobe geometry is easily discernible. An elliptical bearing is simply a two-lobe bearing with the major axis along the horizontal axis.

The lobe bearing shown in Figure 8a is a symmetric lobe bearing where the minimum concentric clearance occurs in the center of each lobed region. 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 cavitation, pressures.

The canted lobe in Figure 8b, on the other hand, 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 reasonably 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 bearings are often used because they provide better resistance to whirls than cylindrical configurations. They do so because they have multiple load-producing pads that assist in preventing large-attitude angles and cross-coupling (see the section on bearing dynamics). Elliptical and lobe bearings are generally used for high-speed, low-load applications where whirls might be a problem.

FIGURE 7 Cylindrical bearing with axial grooving.

FIGURE 8 (a) symmetric lobe bearing and (b) canted lobe bearing.

FIGURE 8 (a) symmetric lobe bearing and (b) canted lobe bearing.

FIGURE 9 Five-pad tilting pad bearing.

Elliptical, or two-lobe, bearings generally have poor horizontal stiffness because of the large clearances along the major diameter of the ellipse. The split elliptical configuration, however, is easier to manufacture than the other types because it is two cylindrical bearing halves with material removed along the parting line. Lobe bearings are usually clearance- and tolerance-sensitive. The other types of lobe bearings are complicated to manufacture.

Tilting-Pad Bearings Tilting-pad bearings are used extensively, especially in highspeed applications, because of their whirl-free characteristics. They are the most whirl-free of all bearing configurations.

An important geometric variable for tilting-pad bearings is the preload ratio, defined as shown in Figure 9.

cc where c = machined clearance

- = concentric pivot film thickness

The variable - is an installed clearance and is dependent upon the radial position of the pivot. Figure 10 displays two pads. Pad 1 has been installed such that the preload ratio

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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