used for all other gas turbine-powered ships. Take abrief look at these two methods to measure main thrustbearing clearances.Static MethodIn the static method, a dial indicator is mounted ontop of the aft end of the thrust housing with the zeropointer against the forward face of the first shaft flangeaft of the thrust bearing. While the controllable andreversible pitch (CRP) system is operating, the propellerpitch is advanced to 100 percent ahead with the localcontrols at the oil distribution (OD) box. The electricCRP pump is then secured, and the hub servobottomed-out by use of the emergency pitch hand pump.The reduction gear is then rocked with the turning gearmotor by use of the ratchet wrench to turn the motor.This movement assures that the shaft is bottomed-outon the ahead thrust collar.If the clearance is within normal limits, and noabnormal conditions exist, log the readings and put thesystem back in service. If the clearance is not withinnormal limits, or a noticeable increase or decrease ismeasured, inspect the thrust bearing. Check for anyabnormal rendition (scored or wiped shoes) and takecorrective action as necessary.NOTEApproximately six to eight turns of thewrench in either direction will remove thebacklash; turn an additional two to fourturns. Repeat once or twice in each directionwith 2500 to 3000 psi on the emergency handpump.Dynamic MethodLike the static method, the dynamic methodrequires a dial indicator and instrument placement tomeasure the shaft deflection. Dynamic measurementsare done while the propulsion shaft is in operation.Communications must be established from the area ofthe thrust bearing to the station with throttle control.The ship must be operated in the ahead direction at a 1/3bell for 10 minutes. Allow the shaft to coast to a stopand then position the dial indicator against the shaftflange. After the indicator is in place, operate the shipin the astern direction at a 1/3 bell for 5 minutes. Afterallowing the ship to coast to a stop, record yourdeflection reading. This procedure is repeated two moretimes (three times total). You must use the average ofthese three readings to obtain the main thrust bearingclearance. Designed thrust bearing clearance is 0.030"to 0.045", with a maximum of 0.080".For specific detailed information on theseprocedures, consult the applicable PMS maintenancerequirement card (MRC) or manufacturer’s technicalmanual for the speed decreaser gear installed on yourship.CLUTCH AND BRAKE ASSEMBLIESDepending on the type of ship to which you will beassigned, you will encounter either one or both of thetwo types of current clutch assemblies used on gasturbine-powered ships. The first and most widely usedclutch assembly is the synchro self-shifting (SSS) type.This type of clutch assembly is installed on all CG-47,DDG-51, and FFG-7 class ships. The other type is apneumatically operated, forced-synchronization type ofclutch assembly. The forced-synchronization clutchassembly is installed on DD-963 and DDG-993 classships.Along with the two types of clutch assemblies, thereare two types of power turbine (PT) brake assembliesinstalled on gas turbine-powered ships. The type ofbrake assembly used depends not only on the ship class,but also on the type of clutch assembly installed.In this section, we will briefly discuss the normaloperation and maintenance related to both types ofclutch assemblies and all the brake assemblies. Becauseof the complexity, the elaborate control system, and thelarge number of labor hours required for maintenance,t h e N a v y i s g r a d u a l l y p h a s i n g o u t t h eforced-synchronization type of clutch. Because of thisphase-out, we will focus our discussion on themaintenance practices associated with the SSS type ofclutch.NORMAL OPERATIONSBoth types of clutch assemblies perform the samefunction. They connect a GTM or the GTMs to theMRG to drive the propulsion shaft. It is not the function,but the method of clutch engagement that variesdrastically between the SSS and forced-synchronizationclutches.3-8
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