Chapter 22. -DIESEL AND GASOLINE ENGINES 



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Figure 22-34.— Four engines (quad unit) arranged to drive one propeller. 



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direct-propulsion engines, the engines are re- 

 versed. 



Reduction gears are used to obtain low 

 propeller-shaft speed with a high engine speed. 

 When accomplishing this, the gears correlate 

 two conflicting requirements of a marine engine 

 installation. These opposing requirements are: 

 (1) for minimum weight and size for a given 

 power output, engines must have a relatively 

 high rotative speed; and (2) for maximum ef- 

 ficiency, propellers must rotate at a relatively 

 low speed, particularly where high thrust capa- 

 city is desired. 



Friction Clutches and Gear Assemblies 



Friction clutches are commonly used with 

 smaller, high-speed engines, up to 500 hp. 

 However, certain friction clutches, in com- 

 bination with a jaw-type clutch, are used with 

 engines up to 1400 hp; and pneumatic clutches, 

 with a cylindrical friction surface, with engines 

 up to 2000 hp. 



Friction clutches are of two general styles; 

 the disk and the band styles. In addition, fric- 

 tion clutches can be classified into dry and wet 



types, depending upon whether the friction sur- 

 faces operate with or without a lubricant. The 

 designs of both types are similar, except that 

 the wet clutches require a large friction area 

 because of the reduced friction coefficient be- 

 tween the lubricated surfaces. The advantages 

 of wet clutches are smoother operation and less 

 wear of the friction surfaces. Wear results 

 from slippage between the surfaces not only 

 during engagement and disengagement, but also, 

 to a certain extent, during the operation of the 

 mechanism. Some wet-type clutches are filled 

 with oil periodically; in other clutches the oil, 

 being a part of the engine-lubricating system, 

 is circulated continuously. Such afriction clutch 

 incorporates provisions which will prevent 

 worn-off particles from being carried by the 

 circulating lubricating oil to the bearings, gears, 

 etc. 



The friction surfaces are generally construc- 

 ted of different materials, one being of cast iron 

 or steel; the other is lined with some asbestos- 

 base composition, or sintered iron or bronze 

 for dry clutches, and bronze, cast iron, or steel 

 for wet clutches. Cast-iron surfaces are pre- 

 ferred because of their better bearing qualities 

 and greater resistance to scoring or scuffing. 



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