zone of the shoe-.0% from the leading edge and 50% radially. The pos-ition of the sensor is critical in establishing the safe operating limits. As longas the probe is generally in the zone of maximum temperature， it will behighly sensitive toload， although the level of temperature may vary con-siderably as can be seen in Figure 21-10. The temperature is also dependent
the pad-backing material. At 500 psiload， the center sensor at A-II regonisters 200 0F while the sensor at B-I registers 280 0F in a steel-backedbearing. Again， these temperatures are typical and will vary withsize，type，speed， and lubrication from bearing to bearing. The difference in a copper-backed bearing can be seen to be quite significant， with A-II reading 185 0F and B-I reading 205 0F. The position of the sensor with respect to the surfaceis less significant in this bearing than in the steel-backed bearing.Again， position in the sensitive zone is important in establishing safe operating limits with respect to temperature. Axial proximity probes are another means of monitoring rotor position and the integrity of the thrust bearing. A typical installation is shown in Figure 21-11. In this case two positions are being monitored: one at thethrustrunner， and one at the end of the shaft near the centerline. This method detects thrust-collar runout and also rotor movement. In most cases this ideal positioning of the probes is not possible. Many times the probes are indexed to the rotor or other convenient locations and thus do not truly show the movement of the rotor with respect to the thrust bearing.
Figure 21-11. .ctual probes for thrust-bearing monitoring.
A critical installation should have the metal temperature sensors in the thrust pad. Axial proximity probes may be used as a backup system. If metal temperatures are high and the rate of change of those temperatures begins toalter rapidly， thrust-bearing failure should be anticipated.
.oupling Maintenance
The major inspections should also include detailed inspections of any couplings in the train. Gear couplings should be disassembled and teeth inspected for indications of problems. The most common failures encoun-tered with continuous lubrication-type gear couplings are:
1. Wear
2. Corrosive wear
3. Coupling contamination
4. Scoring and welding
Couplings with sealed lubrication systems tend to have wear problems similar to couplings withcontinuous lubrication， but they must also be checked for fretting corrosion and cold flow. These problems result from normal coupling operation. If for some reason excessive misalignmentexists， additional damage can be revealed by tooth breakage， scoring， and pitting.
Disc couplings should be checked to ensure there are no cracks in the discs or connecting shaft.If damage does notexist， which can be repaired， the coupling should be rebalanced prior to installation.
.e.uvenation of .sed Turbine 8lades
.ature of Service .amage in Turbine 8lades
Two distinct types of damage can be recognized: surface damage and internal degradation. Surface damage may be due to either mechanicalimpact or corrosion， and is generally confined to the blade airfoil. In bothcases， light damage can be removed by blending or dressing. Blades with severe surface damage or cracks are scrapped. Some of these blades can becoated with some high-temperature coatings. When properly applied， thesecoatings can increase the life of the blades considerably， in some cases even more than when they were new. Recent advances in high-temperature coat-ings for severe hot-corrosion service has resulted in the low unit cost feature of pack cementation and the economy of electroplating to yield multiple-element coatings containing precious-metal aluminides. These coatings areavailable in several combinations ofplatinum， rhodium， and aluminum for application to cobalt-and nickel-based vanes and blades.
Internal degradation is caused by microstructualchanges， which result from extended exposure at high temperature under stress. The microstruc-tural changes are responsible for the reduction in mechanical properties. Three forms of internal degradation have been verified: (1) precipitate coarsening oroveraging， (2) changes in grain boundarycarbides， and
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