Dynamic Method Like the static method, the dynamic method requires a dial indicator and instrument placement to measure the shaft deflection. Dynamic measurements are done while the propulsion shaft is in operation. Communications must be established from the area of the thrust bearing to the station with throttle control. The ship must be operated in the ahead direction at a 1/ 3 bell for 10 minutes. Allow the shaft to coast to a stop and then position the dial indicator against the shaft flange. After the indicator is in place, operate the ship in the astern direction at a 1/ 3 bell for 5 minutes. After allowing the ship to coast to a stop, record your deflection reading. This procedure is repeated two more times (three times total). You must use the average of these three readings to obtain the main thrust bearing clearance. Designed thrust bearing clearance is 0.030" to 0.045", with a maximum of 0.080".

For specific detailed information on these procedures, consult the applicable PMS maintenance requirement card (MRC) or manufacturer's technical manual for the speed decreaser gear installed on your ship.

CLUTCH AND BRAKE ASSEMBLIES Depending on the type of ship to which you will be assigned, you will encounter either one or both of the two types of current clutch assemblies used on gas turbine-powered ships. The first and most widely used clutch 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 a pneumatically operated, forced-synchronization type of clutch assembly. The forced-synchronization clutch assembly is installed on DD-963 and DDG-993 class ships.

Along with the two types of clutch assemblies, there are two types of power turbine (PT) brake assemblies installed on gas turbine-powered ships. The type of brake 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 normal operation and maintenance related to both types of clutch assemblies and all the brake assemblies. Because of the complexity, the elaborate control system, and the large 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 he forced-synchronization type of clutch. Because of this phase-out, we will focus our discussion on the maintenance practices associated with the SSS type of clutch.

NORMAL OPERATIONS Both types of clutch assemblies perform the same function. They connect a GTM or the GTMs to the MRG to drive the propulsion shaft. It is not the function, but the method of clutch engagement that varies drastically between the SSS and forced-synchronization clutches.

Forced-Synchronization Clutch The forced-synchronization type of clutch requires ship's service air and MRG LO availability before normal engagement can occur. The clutch is made up of a friction pack and dental clutch assembly. The friction pack is needed to bring the GTM input shaft speed to within 11 rpm of the first reduction pinion. Once the speed permissive are met, air pressure is applied to the dental clutch to complete engagement. When the dental clutch is engaged, all torque is transmitted from the GTM input shaft to the MRG's first reduction pinion. This clutch assembly also houses a friction-type PT brake that serves two purposes. The primary purpose of the PT brake is to stop and hold the PT stationary. If the PT brake is used with an engaged clutch, the PT brake also acts as a shaft brake. The forced-synchronization clutch is being phased out of Navy service in favor of the SSS clutch.

Synchro Self-Shifting Clutch Like the forced-synchronization clutch, the SSS clutch performs the same functions by transmitting engine torque through the input shaft to the MRG first reduction pinion. It does not, however, require any external controls to perform the engagement sequence. For clutch engagement to occur, the SSS clutch requires only that the input shaft speed be greater than the speed of the first reduction pinion. The SSS clutch is fully automatic. By design, centrifugal force causes the main sliding member to move and engage with the output assembly.

Depending on the ship class, the SSS clutch system uses two different types of PT brake assemblies. The CG-47 and DD-963 class ships have a similar PT brake assembly. The PT brake assembly is an internally housed friction clutch design that is mounted to, but operates independently of, the SSS clutch assembly. There is one main difference between the CG-47 class ships brake and the one installed on the DD-963 class ships. The CG-47 brake cannot be used as a shaft brake. Because of the SSS clutch design, even if the brake is applied with the clutch engaged, the clutch will disengage once the PT input speed drops below the speed of the first pinion.

The other type of PT brake assembly is the one installed on the DDG-51 and FFG-7 class ships. This is a single-disc caliper brake assembly that is externally mounted to each PT input shaft. These brakes are used with the SSS type of clutch in which their only purpose is to stop and hold the PT stationary when required.

The last type of brake assembly we will discuss is the shaft brake assembly. Shaft brake assemblies are installed only on FFG-7 class ships. The shaft brake assembly is also a single-disc caliper brake assembly, such as the PT brake assembly we just described Consisting of two complete units, a shaft brake assembly is mounted on each starboard first reduction quill shaft. Once all permissive are met, the single purpose of thiS brake is to stop MRG rotation.

MAINTENANCE OF EQUIPMENT AND COMPONENTS The maintenance of the clutch and brake assemblies and associated equipment and components is normally done according to the PMS. General cleaning, tests, and inspections will be your primary concern. Because of the good operational track record associated with the SSS clutches, troubleshooting and repairs should be minimal. This good operational record is the main r e a s on the Navy is p h a s i ng o u t t he forced-synchronization type of clutch in favor of the SSS clutch.

In the following paragraphs, we will discuss some general maintenance and repair practices associated with clutch and brake assemblies. You, the GS supervisor, must be familiar with these practices so you can properly supervise maintenance and repairs.

Cleaning The cleaning of the clutch and brake assemblies is done primarily when the MRG is cleaned Cleaning the clutch assembly is limited to external cleaning. Pay attention to the areas around the inspection and access cover plates and the clutch position indicating ports.

The cleaning of externally mounted brake assemblies requires a little more effort. To properly and thoroughly clean any of the external brake assemblies, you must remove the guard screen. Once you remove the screen, pay particular attention to removing any dust accumulation on the brake and to the cleanliness of the disc. It is important that you keep dust accumulation to a minimum. Once dust mixes with oil, it can be deposited on the brake disc or absorbed into the pads. Excessive dust and/ or oil accumulation can seriously degrade the brake's operation.

Tests and Inspections

Unless a casualty occurs to either the clutch or brake assemblies, all tests and inspections are performedaccording to the PMS. There are no tests or inspections related to SSS clutch assemblies, unless you are assigned to a CG-47 class ship. Remember, that particular type of SSS clutch has an internal PT brake assembly. The ship's maintenance action plan periodically requires that an inspection of the disc assembly be made and the clearances between the discs measured.

Additionally, you must check the externally mounted PT and shaft brake assemblies on a regular basis according to the PMS. These inspections normally include checking the brake pad thickness measurements, rotor condition, proper operation of air or hydraulic actuators, and proper lubrication of vital moving parts.

TROUBLESHOOTING Because SSS clutches are reliable, problems that require troubleshooting are usually minimal. Like all other gear-driven assemblies, SSS clutches have a tendency to wear and produce noise with age. Normal failures are usually limited to faulty position indicator switches and failures related to the PT brake assemblies. We will not dwell on the clutch assemblies, but move on to some of the problems related to the installed brake assemblies and the ways in which you, the GS supervisor, can better identify them.

The basic operation of both the PT and shaft brakes is the same as the disc brake system installed in most automobiles. All brake systems require some type of medium (air, oil, or air and oil) to force the caliper piston against the brake pad which, in turn, is pushed against the disc. This action slows the rotation of the disc until the disc stops. Next are some common malfunctions that may occur in this system and ways that you can isolate the cause.