Principles of naval engineering - Historic Naval Ships Association

Chapter 22. -DIESEL AND GASOLINE ENGINESFORWARD DRUMany time. An increase or decrease in generatorvoltage is used as a means of controllingthe speed of the propeller. Changes in generatorvoltage may be brought about by electricalmeans, by changes in engine speed, and by acombination of these methods. The controls ofan electric drive may be in a location remotefrom the engine, such as the pilot house.In an electric drive, reversing the directionof rotation of the propeller is not accomplishedby the use of a reverse gear. The electricalsystem is arranged so that the flow of currentthrough the motor can be reversed. This reversalof current flow causes the motor torevolve in the opposite direction. Thus, thedirection of rotation of the motor and of thepropeller can be controlled by manipulating theelectrical controls.REVERSE STEP-UP PINION75.247Figure 22-32.— Clutch and reverse reductiongear assembly.contains two drive pinions and the main drivegear. Each pinion is driven by the clutch orcoupling shafts of two engines, through splinesin the pinion hubs. The pinions drive the singlemain gear, which is connected to the propellershaft.Electric drives are used in the propulsionplants of some diesel-driven ships. With electricdrive, there is no mechanical connectionbetween the engine(s) and the propeller(s). Insuch plants, the diesel engines are connecteddirectly to generators. The electricity producedby such an engine-driven generator is transmitted,through cables, to a motor. The motoris connected to the propeller shaft directly, orindirectly through a reduction gear. When areduction gear is included in a diesel-electricdrive, the gear is located between the motorand the propeller.The generator and the motor of a dieselelectricdrive may be of the alternating current(a-c) type or of the direct current (d-c) type;almost all diesel-electric drives in the Navy,however, are of the direct current type. Sincethe speed of a d-c motor varies directly withthe voltage furnished by the generator, the controlsystem of an electric drive is so arrangedthat the generator voltage can be changed atDIRECT DRIVES.— In some marine engineinstallations, power from the engine is transmittedto the driven unit without a change inshaft speed; that is, by a direct drive. In adirect drive, the connection between the engineand the driven unit may consist of a "solid"coupling, a flexible coupling, or a combinationof both. A clutch may or may not be includedin a direct drive, depending upon the type ofinstallation. In some installations, a reversegear is included.Solid couplings vary considerably in design.Some solid couplings consist of two flangesbolted solidly together. In other direct drives,the driven unit is attached directly to the enginecrankshaft by a nut.Solid couplings offer a positive means oftransmitting torque from the crankshaft of anengine; however, a solid connection does notallow for any misalignment nor does it absorbany of the torsional vibrations transmitted fromthe engine crankshaft or shaft vibrations.Since solid coupling will not absorb vibrationand will not permit any misalignment, mostdirect drives consist of a flange-type couplingwhich is used in connection with a flexiblecoupling. Connections of the flexible type arecommon to the drives of many auxiliaries, suchas engine-generator sets. Flexible couplingsare also used in indirect drives to connect theengine to the drive mechanism.The two solid halves of a flexible couplingare joined by a flexible element. The flexibleelement is made of rubber, neoprene, or steelsprings. Two views of one type of flexiblecoupling are shown in figure 22-35.581