Chapter 22. -DIESEL AND GASOLINE ENGINES 



Types of Drive Mechanisms 



The term indirect drive describes a drive 

 mechanism which changes speed and torque. 

 Drives of this type are common to many marine 

 engine installations. Where the speed and the 

 torque of an engine need not be changed in order 

 to drive a machine satisfactorily, the mecha- 

 nism used is a direct drive . Drives of this 

 type are commonly used where the engine 

 furnishes power for the operation of auxiliaries 

 such as generators and pumps. 



INDIRECT DRIVES.— The drive mechanism 

 of most engine-powered ships and many boats 

 are of the indirect type. With indirect drive, 

 the power developed by the engine(s) is trans- 

 mitted to the propeller(s) indirectly, through an 

 intermediate mechanism which reduces the shaft 

 speed. Speed reduction may be accomplished 

 mechanically (by a combination of gears) or by 

 electrical means. 



Mechanical drives include devices which 

 reduce the shaft speed of the driven unit, pro- 

 vide a means for reversing the direction of shaft 

 rotation in the driven unit, and permit quick- 

 disconnect of the driving unit from the driven 

 unit. 



The combination of gears which effects the 

 speed reduction is called a reduction gear. In 

 most diesel engine installations, the reduction 

 ratio does not exceed 3 to 1; there are some 

 units, however, which have reductions as high 

 as 6 to 1. 



The propelling equipment of a boat or a ship 

 must be capable of providing backing-down power 

 as well as forward motive power. There are a 

 few ships and boats in \yhich backing down is ac- 

 complished by reversing the pitch of the pro- 

 peller; in most ships, however, backing down is 

 accomplished by reversing the direction of rota- 

 tion of the propeller shaft. In mechanical drives, 

 reversing the direction of rotation of the pro- 

 peller shaft may be accomplished in one of two 

 ways: by reversing the direction of engine rota- 

 tion, or by the use of reverse gears. Of these 

 two methods, the use of reverse gears is more 

 commonly employed in modern installations. 



More than reducing speed and reversing the 

 direction of shaft rotation is required of the 

 drive mechanism of a ship or a boat. It is 

 frequently necessary to allow an engine to oper- 

 ate without power being transmitted to the 

 propeller. For this reason, the drive mecha- 

 nism of a ship or boat must include a means of 



disconnecting the engine from the propeller 

 shaft. Devices used for this purpose are called 

 clutches and couplings . 



The arrangement of the components in an 

 indirect drive varies, depending upon the type 

 and size of the installation. In some small in- 

 stallations, the clutch or coupling, the reverse 

 gear, and the reduction gear may be combined 

 in a single unit; in other installations, the clutch 

 or coupling and the reverse gear may be in one 

 housmg and the reduction gear in a separate 

 housing attached to the reverse-gear housing. 

 Drive mechanisms arranged in either manner 

 are usually called transmissions. The arrange- 

 ment of the components in two different types of 

 transmissions are shown in figures 22-30 and 

 22-31. 



In the transmission shown in figure 22-30 

 the housing is divided into two sections by the 

 bearing carrier. The clutch or coupling assem- 

 bly is in the forward section, and the gear as- 

 sembly is in the after section of the housing. 

 In the transmission shown in figure 22-31, note 

 that the clutch assembly and the reverse gear 

 assembly are in one housing, while the reduction 

 gear unit is in a separate housing (attached to 

 the clutch and the reverse gear housing). 



In large engine installations, the clutch or 

 coupling and the reverse gear may be combined; 

 or they may be separate units, located between 

 the engine and a separate reduction gear; or 

 the clutch or coupling may be separate and the 

 reverse gear andthe reduction gear may be com- 

 bined. An assembly of the last type is shown in 

 figure 22-32. 



In most geared-drive, multiple-propeller 

 ships, the propulsion units are independent of 

 each other. An example of this type of arrange- 

 ment is illustrated in figure 22-33. 



In some installations, the drive mechanism 

 is arranged so that two or more engines drive 

 a single propeller. This is accomplished by 

 having the driving gear which is on, or con- 

 nected to, the crankshaft of each engine trans- 

 mit power to the driven gear on the propeller 

 shaft. In one type of installation, each of two 

 propellers is driven by four diesel engines. 

 The arrangement of the engines, the location of 

 the reduction gear, and the direction of rotation 

 of the crankshaft and the propeller shaft in one 

 type of "quad" power unit are illustrated in 

 figure 22-34. 



The drive mechanism illustrated includes 

 four clutch assemblies (one mounted to each 

 engine flywheel) and one gear box. The box 



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