Chapter 22. -DIESEL AND GASOLINE ENGINES 



FORWARD DRUM 



REVERSE STEP-UP PINION 



75.247 

 Figure 22-32.— Clutch and reverse reduction 

 gear assembly. 



contains two drive pinions and the main drive 

 gear. Each pinion is driven by the clutch or 

 coupling shafts of two engines, through splines 

 in the pinion hubs. The pinions drive the single 

 main gear, which is connected to the propeller 

 shaft. 



Electric drives are used in the propulsion 

 plants of some diesel-driven ships. With elec- 

 tric drive, there is no mechanical connection 

 between the engine(s) and the propeller(s). In 

 such plants, the diesel engines are connected 

 directly to generators. The electricity produced 

 by such an engine-driven generator is trans- 

 mitted, through cables, to a motor. The motor 

 is connected to the propeller shaft directly, or 

 indirectly through a reduction gear. When a 

 reduction gear is included in a diesel-electric 

 drive, the gear is located between the motor 

 and the propeller. 



The generator and the motor of a diesel- 

 electric drive 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. Since 

 the speed of a d-c motor varies directly with 

 the voltage furnished by the generator, the con- 

 trol system of an electric drive is so arranged 

 that the generator voltage can be changed at 



any time. An increase or decrease in genera- 

 tor voltage is used as a means of controlling 

 the speed of the propeller. Changes in genera- 

 tor voltage may be brought about by electrical 

 means, by changes in engine speed, and by a 

 combination of these methods. The controls of 

 an electric drive may be in a location remote 

 from the engine, such as the pilot house. 



In an electric drive, reversing the direction 

 of rotation of the propeller is not accomplished 

 by the use of a reverse gear. The electrical 

 system is arranged so that the flow of current 

 through the motor can be reversed. This re- 

 versal of current flow causes the motor to 

 revolve in the opposite direction. Thus, the 

 direction of rotation of the motor and of the 

 propeller can be controlled by manipulating the 

 electrical controls. 



DIRECT DRIVES.— In some marine engine 

 installations, power from the engine is trans- 

 mitted to the driven unit without a change in 

 shaft speed; that is, by a direct drive. In a 

 direct drive, the connection between the engine 

 and the driven unit may consist of a "solid" 

 coupling, a flexible coupling, or a combination 

 of both. A clutch may or may not be included 

 in a direct drive, depending upon the type of 

 installation. In some installations, a reverse 

 gear is included. 



Solid couplings vary considerably in design. 

 Some solid couplings consist of two flanges 

 bolted solidly together. In other direct drives, 

 the driven unit is attached directly to the en- 

 gine crankshaft by a nut. 



Solid couplings offer a positive means of 

 transmitting torque from the crankshaft of an 

 engine; however, a solid connection does not 

 allow for any misalignment nor does it absorb 

 any of the torsional vibrations transmitted from 

 the engine crankshaft or shaft vibrations. 



Since solid coupling will not absorb vibra- 

 tion and will not permit any misalignment, most 

 direct drives consist of a flange-type coupling 

 which is used in connection with a flexible 

 coupling. Connections of the flexible type are 

 common to the drives of many auxiliaries, such 

 as engine-generator sets. Flexible couplings 

 are also used in indirect drives to connect the 

 engine to the drive mechanism. 



The two solid halves of a flexible coupling 

 are joined by a flexible element. The flexible 

 element is made of rubber, neoprene, or steel 

 springs. Two views of one type of flexible 

 coupling are shown in figure 22-35. 



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