2. .uild.up of solids in rotor and.or stator passages ...plugging.. of turbine buckets.. This problem should be noticed from performance or pres-sure distribution in the machine (first-stage pressure) long before the failure occurs.
3. Off.design operation.Especially from backpressure (vacuum)， inletpressure， extraction pressure， moisture. Many failures are caused byoverload， off-design speed.
4. Compressor surging. Especially in double-flow machines.
5. .ear coupling thrust. A frequent cause offailure， especially of upstream thrust bearings. Thrust is high when alignment is perfect(friction coefficient 0.4-0.6)， decreasing to a minimum when a small misalignment is present (about 0.1 at 250 angular misalignment). Friction increases rapidly again to 0.5 or more with an increase inmisalignment. (These are rough numbersonly， to show basic relation-ships.) The thrust is caused by friction in the loaded teeth that opposesthermal expansion. Therefore， thrust can get veryhigh， since it has no relation to the normal thrust caused by pressure distribution inside the machine (for which the thrust bearing may have beendimensioned). The coupling thrust may act eitherway， adding to or subtracting from normal thrust. Much depends on tooth geometry and coupling quality. A straight-sided tooth can take misalign-ment only when the tooth fit has enough clearance to permit slantingof the male tooth inside the female teeth. For example， with vertical
misalignment， the teeth on both sides will bind when the clearance is insufficient to allow for slanting. This can causevery highthrust， sometimes one can hear a ""metallic sound"" building up until the rotors finally slip with a very noticeable ""bump."" Then the noiseand vibration aregone， at least for a while. This phenomenon， ofcourse， is torture for the thrust bearings， and it may cause failure in either direction. Dirt in the coupling can aggravate this situation or even cause it.
6. .irt in oil. A common cause of failures， especially when combined with other factors. The oil film at the end of the oil wedge is only a smallfraction of a thousandths thick. If dirt goes through， it can cause thefilm to rupture， and the bearing may burn out.Therefore， very fine filtering of the oil is required. But the best filter is no good if main-tenance personnel leave the filter or bearing case open after inspection，and the rain and sand blow in， or if they put the wet filter elements onthe sandy floor， or accidentally knock holes in the elements. It happensfar too often. Once a machine is wrecked， it is difficult to reconstruct.
.. Momentary loss of oil pressure. Sometimes encountered while switch-ing filters or coolers.
Failure protection.Fortunately， accurate and reliable instrumentation is now available to monitor thrust bearings well enough to assure safe continuous operation and to prevent catastrophic failure in the event of an upset to the system.
Temperature sensors， such as RTDs (Resistance Temperature Detectors)，thermocouples， and thermistors， can be installed directly in the thrust bear-ing to measure metal temperature. The installation shown in Figure 21-9 has the RTD embedded in the babbitted surface. It is in the most sensitive
Figure 21-.. RTD embedded in bearing surface. Figure 21-10. Temperature distribution in bearing surfaces. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：燃气涡轮工程手册 Gas Turbine Engineering Handbook 3(90)
