SECTION III: PRINCIPLES OF OPERATIONTM 9-2350-256-201-22 MECHANICAL TRANSMISSION AND MAIN HYDRAULIC SYSTEM PUMPNOTE1 Mechanical transmission2 Main hydraulic system pumpThe following callouts are found in Figure 1-14.The mechanical transmission and hydraulic pumpassembly is mounted in the rear of the hydraulicscompartment, under the crew compartment subfloorplates. The engine drives the mechanical transmission(1) through the PTO drive shaft coupled to the PTOcoupling at the accessory end of the engine. Themechanical transmission drives the main hydraulicsystem pump (2). The pump provides hydraulicsystem pressure.Figure 1-14. Mechanical Transmission andMain Hydraulic System Pump.1-23 MAIN HYDRAULIC SYSTEMThe main hydraulic system supplies power for the control and operation of the spade, hoisting boom, hoist winch, andmain winch, and for releasing the mechanically applied main and hoist winch brakes. System power is obtained from themain hydraulic pump driven by the mechanical transmission.1-24 APUa. GENERALThe APU consists of an overhead-valve, two-cylinder, four-cycle, air-cooled, diesel engine connected directly to a 300-A,28-V, shunt-wound, dc generator (which is also used as a starter), and an auxiliary hydraulic system pump. The APUprovides electrical power to charge the vehicle batteries, and hydraulic power to raise and lower the spade, stow andraise the boom, and retrieve the main and hoist winch cables.b. STARTING AND GENERATING SYSTEMSThe 300-A, 28-V generator mentioned above is also used as a starter motor. This is accomplished by first feeding 24 Vdc to the field coil of the generator from the preheat relay when the preheat switch is activated, and then by applying 24V dc to the generator armature through a starting relay when the start switch is activated. This motorizes the generator,which is directly coupled to the engine by a drive chain. When the engine starts, the current in the generator armaturereverses direction, and the system automatically changes from a motorizing mode to a generating mode. After theengine starts, the start and preheat switches must be released to prevent loading down the engine since the voltageregulator is bypassed during this start cycle, and full voltage is being applied to the field. After the engine has started,and the preheat and start switches are released, the generator output is controlled by a solid-state voltage regulator and acurrent-limiting device.1-31
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