TM 1-1520-238-10 2-56 Change 9 Section VII. POWER TRAIN SYSTEM 2.40 POWER TRAIN. The power train (fig 2-32) transmits engine power to the rotors and transmission-mounted accessories. The power train includes two engine nose gearboxes, two shafts to the main transmission, the main transmission, main rotor drive shaft, tail rotor drive shafts, intermediate gearbox, tail rotor gearbox, and APU drive shaft, and couplings. 2.40.1 Engine Nose Gearboxes. The engine nose gearboxes are mounted on the nose of each engine. They reduce nominal engine speed and change the angle of drive. Both nose gearboxes have self-contained pressur- ized oil systems with provisions to ensure limited opera- tion if a total loss of pressurized lubrication occurs. These self-contained systems feature a pressure pump driven by the gearbox output shaft, filter with impending bypass fil- ter indicator and filter bypass capability, high-pressure re- lief valve, and sump. Mounted on the nose gearbox output drive is an axial-type fan that draws air through the gear- box fairing and past cooling fins on the gearbox. The input drive shafts have flexible couplings that require no lubrica- tion. Externally accessible accessories include a filler, breather, oil level sight gage, chip detector/temperature sensor, pressure transducer, and temperature probe. a. Engine Nose Gearbox Caution Light Provi- sions. High oil temperature, low oil pressure, and the presence of chips are detected in each gearbox. The pilot is alerted to the condition by six caution segments: OIL HOT NOSE GRBX 1, OIL HOT NOSE GRBX 2, OIL PSI NOSE GRBX 1, OIL PSI NOSE GRBX 2, CHIPS NOSE GRBX 1, and CHIPS NOSE GRBX 2. Either the ENG 1 or ENG 2 segment will simultaneously illuminate on the CPG caution/warning panel (fig 2-44). 2.40.2 Main Transmission. The main transmission combines the two engine nose gearbox inputs and pro- vides drive to the main rotor, tail rotor, accessories, and rotor brake disc. Two overrunning clutches at the main transmission permit either engine to be disengaged from the transmission during autorotation. The main transmis- sion reduces the rpm input to the main rotor, tail rotor, and the rotor brake disc. The main transmission is mounted below the main rotor static mast base which allows its re- moval without removing the upper controls, mast, hub, or blades. The main rotor drive shaft is designed to carry torque loads only. The rotor hub is on a static mast which carries vertical and bending loads. The drive shaft rotates inside the static mast. The main transmission has primary and accessory drive trains. The primary drive train, through three stages, changes the angle and speed of the power drives to the main rotor, tail rotor, and rotor brake. Overrunning clutches allow the APU to drive the transmis- sion accessory gearbox when the engines are not operat- ing. The transmission accessories consist of two alternat- ing-current generators, two hydraulic pumps, and a shaft driven compressor. A magnetic pickup measures main ro- tor rpm. a. Main Transmission Lubrication. The main transmission has two independent oil systems. Each sys- tem has its own sump, pump, filter and heat exchanger. Oil level sight gauges are located in the transmission housing at each oil sump. These systems are not totally independent in the usual sense because during normal operation, the oils will mix. If oil loss occurs in either sump or in either heat exchanger, the diverter (float) valve will seal off that sump to prevent a total loss of oil. There are provisions throughout the transmission so that even with a total loss of oil there will be limited lubrication. Each oil fil- ter has a bypass capability, an impending bypass capabili- ty and an impending bypass filter indicator. If a filter be- comes partially clogged, differential pressure causes the impending bypass indicator to pop. Further clogging causes the filter bypass valve to close, routing oil directly to the pump. The pilot/CPG has no cockpit indication of either occurrence. Each sump has a chip detector/tem- perature sensor and temperature transducer. Pressure is measured downstream of each heat exchanger. Oil pass- ing through the heat exchanger mounted inboard on each engine firewall, is air cooled. The heat exchangers have thermal bypass provisions for cold starts. A third oil pump, driven by the accessory drive, lubricates the accessory gears during APU operation. This pump draws oil from the right oil system sump.