2.
Circum仲erential grooved bearing . Normally has the oil groove at halfthe bearing length. This configuration provides better cooling， but reduces load capacity by dividing the bearing into two parts.
3. Cylindrical bore bearings. Another common bearing type used in turbines. It has a split construction with two axial oil-feed grooves at the split.
4. Pressure or pressure dam. Used in many places where bearing stability isrequired， this bearing is a plain journal bearing with a pres-sure pocket cut in the unloaded half. This pocket is approximately 1/32 of an inch (.8 mm) deep with a width 50% of the bearing length.
This groove or channel covers an arc of 135o and terminates abruptly in a sharp-edge dam. The direction of rotation is such that the oil is pumped down the channel toward the sharp edge. Pressure dam bearings are for one direction of rotation. They can be used in conjunction with cylindrical bore bearings as shown in Figure 13-6.
5. .emon bore or elliptical. This bearing is bored with shims at the splitline， which are removed before installation. The resulting bore shape approximates an ellipse with the major axis clearance approximately twice the minor axis clearance. Elliptical bearings are for both direc-tions of rotation.
6. ..ree.lobe bearing. The three-lobe bearing is not commonly used in turbomachines. It has a moderate load-carrying capacity and can be operated in both directions.
7. .仲仲set
.alves. In principle， this bearing acts very similar to a pressure dam bearing. Its load-carrying capacity is good. It is restricted to one direction of rotation.
8. .ilting.pad bearings. This bearing is the most common bearing type in today's machines. It consists of several bearing pads posed around the circumference of the shaft. Each pad is able to tilt to assume the most effective working position. Its most important feature is self-alignment when spherical pivots are used. This bearing also offers the greatest increase in fatigue life because of the following advantages:
a. Self-aligning for optimum alignment and minimum limit.
b. Thermal conductivity backing material to dissipate heat developed in oil film.
c. A thin babbitt layer can be centrifugally cast with a uniform thickness of about 0.005 of an inch (0.127 mm). Thick babbitts greatly reduce bearing life. Babbitt thickness in the neighbor-hood of .01in. (.254 mm) reduce the bearing life by more than half.
d. Oil film thickness is critical in bearing stiffness calculations. In atilting-pad bearing， one can change this thickness in a number of ways: (1) changing the number of pads; (2) directing the load on or in-between the pads; (3) and changing axial length of pad.
The previous list contains some of the most common types of journal bearings. They are listed in the order of growing stability. All of the bearings designed for increased stability are obtained at higher manufacturing costs and reduced efficiency. The antiwhirl bearings all impose a parasitic load onthe journal， which causes higher power losses to the bearings， and inturn， requires higher oil flow to cool the bearing. Many factors enter into the selection of the proper design for bearings. Some of the factors that affect bearing design follow:
1. Shaft speed range.
2. Maximum shaft misalignment that can be tolerated.
3. .ritical speed analysis and the influence of bearing stiffness on this analysis.
4. Loading of the compressor impellers.
5. Oil temperatures and viscosity.
6. Foundation stiffness.
7. Axial movement that can be tolerated.
8. Type of lubrication system and its contamination.
中國航空網(wǎng) www.k6050.com 航空翻譯 www.aviation.cn 本文鏈接地址：燃氣渦輪工程手冊 Gas Turbine Engineering Handbook 2(82)
