Principles of naval engineering - Historic Naval Ships Association

Chapter 22. -DIESEL AND GASOLINE ENGINESTypes of Drive MechanismsThe term indirect drive describes a drivemechanism which changes speed and torque.Drives of this type are common to many marineengine installations. Where the speed and thetorque of an engine need not be changed in orderto drive a machine satisfactorily, the mechanismused is a direct drive . Drives of thistype are commonly used where the enginefurnishes power for the operation of auxiliariessuch as generators and pumps.INDIRECT DRIVES.— The drive mechanismof most engine-powered ships and many boatsare of the indirect type. With indirect drive,the power developed by the engine(s) is transmittedto the propeller(s) indirectly, through anintermediate mechanism which reduces the shaftspeed. Speed reduction may be accomplishedmechanically (by a combination of gears) or byelectrical means.Mechanical drives include devices whichreduce the shaft speed of the driven unit, providea means for reversing the direction of shaftrotation in the driven unit, and permit quickdisconnectof the driving unit from the drivenunit.The combination ofgears which effects thespeed reduction is called a reduction gear. Inmost diesel engine installations, the reductionratio does not exceed 3 to 1; there are someunits, however, which have reductions as highas 6 to 1.The propelling equipment of a boat or a shipmust be capable of providing backing-down poweras well as forward motive power. There are afew ships and boats in \yhich backing down is accomplishedby reversing the pitch of the propeller;in most ships, however, backing down isaccomplished by reversing the direction of rotationof the propeller shaft. In mechanical drives,reversing the direction of rotation of the propellershaft may be accomplished in one of twoways: by reversing the direction of engine rotation,or by the use of reverse gears. Of thesetwo methods, the use of reverse gears is morecommonly employed in modern installations.More than reducing speed and reversing thedirection of shaft rotation is required of thedrive mechanism of a ship or a boat. It isfrequently necessary to allow an engine to operatewithout power being transmitted to thepropeller. For this reason, the drive mechanismof a ship or boat must include a means ofdisconnecting the engine from the propellershaft. Devices used for this purpose are calledclutches and couplings .The arrangement of the components in anindirect drive varies, depending upon the typeand size of the installation. In some small installations,the clutch or coupling, the reversegear, and the reduction gear may be combinedin a single unit; in other installations, the clutchor coupling and the reverse gear may be in onehousmg and the reduction gear in a separatehousing attached to the reverse-gear housing.Drive mechanisms arranged in either mannerare usually called transmissions. The arrangementof the components in two different types oftransmissions are shown in figures 22-30 and22-31.In the transmission shown in figure 22-30the housing is divided into two sections by thebearing carrier. The clutch or coupling assemblyis in the forward section, and the gear assemblyis in the after section of the housing.In the transmission shown in figure 22-31, notethat the clutch assembly and the reverse gearassembly are in one housing, while the reductiongear unit is in a separate housing (attached tothe clutch and the reverse gear housing).In large engine installations, the clutch orcoupling and the reverse gear may be combined;or they may be separate units, located betweenthe engine and a separate reduction gear; orthe clutch or coupling may be separate and thereverse gear andthe reduction gear may be combined.An assembly of the last type is shown infigure 22-32.In most geared-drive, multiple-propellerships, the propulsion units are independent ofeach other. An example of this type of arrangementis illustrated in figure 22-33.In some installations, the drive mechanismis arranged so that two or more engines drivea single propeller. This is accomplished byhaving the driving gear which is on, or connectedto, the crankshaft of each engine transmitpower to the driven gear on the propellershaft. In one type of installation, each of twopropellers is driven by four diesel engines.The arrangement of the engines, the location ofthe reduction gear, and the direction of rotationof the crankshaft and the propeller shaft in onetype of "quad" power unit are illustrated infigure 22-34.The drive mechanism illustrated includesfour clutch assemblies (one mounted to eachengine flywheel) and one gear box. The box579