Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGdrive shaft, which carries the double helicalforward pinion at the after end of the gear box.The forward pinion is in constant mesh with thedouble helical main gear, which is keyed on thepropeller shaft. By following through the geartrain, you can see that, for ahead motion, thepropeller rotates in a direction opposite to theengine's rotation.The parts of the airflex clutch which givethe propeller astern rotation are illustrated inthe lower view of figure 22-32. The reverseclutch is inflated to engage the reverse drum,which is then driven by the engine. The reversedrum is keyed to the short reverse shaft, whichsurrounds the forward drive shaft. A largereverse step-up pinion transmits the motion tothe large reverse step-up gear on the uppershaft. The upper shaft rotation is opposite tothe engine's rotation. The main reverse pinionon the upper shaft is in constant mesh with themain gear. By tracing through the gear train,it may be seen that, for reverse rotation, thepropeller rotates in the same direction as theengine.The diameter of the main gear of the airflexclutch is approximately 2 1/2 times as greatas that of the forward and reverse pinions. Thus,there is a speed reduction of 2 1/2 to 1 fromeither pinion to the propeller shaft.Since the forward and main reverse pinionsare in constant mesh with the main gear, theset that is not clutched in will rotate as idlersdriven from the main gear. The idling gearsrotate in a direction opposite to their rotationwhen carrying the load. For example, with theforward clutch engaged, the main reverse pinionrotates in a direction opposite to its rotation forastern motion (note the dotted arrow in the upperview of figure 22-32. Since the drums rotate inopposite directions, a control mechanism is installedto prevent the engagement of both clutchessimultaneously.The airflex clutch is controlled by an operatinglever which works the air control housing,located at the after end of the forward pinionshaft. The control mechanism, shown with theairflex clutches in figure 22-39, directs the highpressure air into the proper paths to inflatethe clutch glands (tires). The air shaft, whichconnects the control mechanism to the clutches,passes through the forward drive shaft.The supply air enters the control housingthrough the air check valve and must pass throughthe small air orifice. The purpose of the restrictedorifice is to delay the inflation of theclutch to be engaged, when shifting from onedirection of rotation to the other. The delay isnecessary to allow the other clutch to be fullydeflated and out of contact with its drum beforethe inflating clutch can make contact with itsdrum.The supply air goes to the rotary air jointin which a hollow carbon cylinder is held tothe valve shaft by spring tension. This preventsleakage between the stationary carbonseal and the rotating air valve shaft. The airgoes from the rotary joint to the four-way airvalve. The sliding-sleeve assembly of the fourwayvalve can be shifted endwise along the valveshaft by operating the control lever.When the shifter arm on the control leverslides the valve assembly away from the engine,air is directed to the forward clutch. The fourwayvalve makes the connection between the airsupply and the forward clutch, as follows: thereare eight neutral ports which connect the centralair supply passage in the valve shaft withthe sealed air chamber in the sliding member.In the neutral position of the four-way valve, asshown in figure 22-39, the air chamber is adead end for the supply air. In the forwardposition of the valve, the sliding member uncoverseight forward ports, which connect withthe forward passages conducting the air to theforward clutch. The air now flows through theneutral ports, air chamber, forward ports, andforward passages to inflate the forward clutchgland. As long as the valve is in the forwardposition, the forward clutch will remain inflatedand the entire forward air system will remainat a pressure of100 psi.LUBRICATION.— On most large gear units,a separate lubrication system is used. Onelubrication system is shown in figure 22-40.Oil is picked up from the gear box by an electricdrivengear-type lubricating oil pump and issent through a strainer and cooler. After beingcleaned and cooled, the oil is returned to thegear box to cool and lubricate the gears. Intwin installations, such as shown in figure 22-40 a separate pump is used for each unit and astandby pump is interconnected for emergencyuse.Hydraulic Clutches or CouplingsThe fluid clutch (coupling) is widely usedon Navy ships. The use of hydraulic coupling588