Malfunctions of friction clutches will vary, depending upon the type of clutch. The troubles discussed in this section- slippage and wear, freezing, and noise-are common to twin-disk clutches.

SLIPPAGE AND WEAR.-Slippage and wear of mechanical clutches must be considered together, since each can be the cause of the other and each intensifies the others effect. Slippage generally occurs at a high engine speed when the engine is delivering the greatest torque. Slippage causes lower efficiency, loss of power, and rapid wear of the clutch friction surfaces.

There are several possible causes of clutch slippage: wear, insufficient pressure, overload, and fouling. Over a period of operation, extended engaging and disengaging of the surfaces will cause a normal amount of wear. If the surfaces are rough, wear will be excessive. Do not engage the clutch while the engine is racing. It may cause excessive wear, and it will strain the entire drive system.

When an engine is overloaded, torque may be increased to such an extent that slippage will occur. Obviously, you can prevent this trouble by keeping the load within specified limits. Whenever an engine is fully loaded, watch for symptoms that indicate slippage.

The clutch may slip when the lining surfaces become fouled with oil, grease, or water. Oil or grease on the clutch surfaces is usually the result of careless maintenance practices, such as forcing too much grease into the bearings or over-filling the gear case with oil. When oil in a gear case foams, there will probably be leakage from the shaft bearings. Foaming may result from overfilling. When foaming occurs, check for the proper oil level.

When filling a reduction gear case, add only enough oil to bring the level up to the FULL mark. Do not add or measure oil when the unit is in operation, because you will not get an accurate oil reading.

In a twin-disk clutch installation, an oil leak at the rear main bearing of the engine may cause oil to appear on the clutch surfaces. The leakage may be caused by excessive bearing clearance, overfilling of the engine crankcase, a plugged crankcase breather cap, or excessive crankcase pressure due to piston blowby. The crankcase breather cap must be cleaned periodically so that it will not become clogged.

Another source of fouling is grease that may get on the linings of a dry-type clutch during overhaul. Do not handle the parts with greasy hands, and remove any grease deposits with an approved cleaner. For pneumatically operated friction clutches where rubber parts are used, use only a clean, dry cloth to wipe off clutch faces and linings.

When there is clutch slippage, immediately take steps to correct the trouble. The clutch surfaces are probably worn, so measure the thickness of the clutch linings. When a lining is worn excessively, replace it; tightening the adjusting device (installed on some units) will not correct for excessive wear of the linings. Instead, such adjustments may lead to scoring of the mating clutch surfaces, particularly when the linings are fastened to the clutch faces with rivets.

The spring-loaded clutch-operating mechanism of the twin disk is pressure set at the factory. It should not be necessary to adjust the mechanism, as it is designed to follow up and compensate for wear of the friction surfaces on the clutch plates. The simplest way to determine when the disks need to be replaced is to check the position of the plungers of the spring-loaded mechanisms in the engaged position. The plungers are permitted to travel a specified amount according to the specifications listed in the NAVSEA technical manual.

FROZEN CLUTCH.-When a clutch fails to disengage, it is frozen. Failure of the clutch to disengage may be caused by a defective clutch mechanism or by water absorbed in the material that lines the clutch plates.

When a clutch becomes frozen, inspect the operating mechanism. Check the control rods for obstructions or loose connections, and check for excessive clearances in the throw-out bearing pressure plate, the pivots, and the toggles. In a twin-disk clutch, warped disks will cause the clutch to freeze. (Warped disks are caused by extended running in neutral position.)

If a clutch has molded clutch linings, moisture will cause the linings to swell and become soft. When this occurs, many linings tend to stick to the mating surfaces. Every effort should be made to prevent moisture from getting to the clutch linings. If a molded lining becomes wet, let it dry in the disengaged position. Allowing the linings to dry in the engaged position increases the possibility of sticking.

CLUTCH NOISE.-Dry-type clutches may produce a chattering noise when the clutch is being engaged. Excessive clutch chatter may cause damage to the reverse and reduction gears and may cause the clutch linings to break loose, resulting in complete clutch failure.

The principal cause of clutch chatter is oil, grease, or water on the linings. You should take every possible precaution to keep oil, grease, or water out of the unit, because replacement of the linings is the only satisfactory means of repair. All metal parts of the clutch may be cleaned according to instructions in the appropriate NAVSEA technical manuals.