(2)
Attach the caps to the drip pan with the bolts.
R. Remove placard from BATTERY SWITCH.
S. Do a test for the fire and overheat detection system (AMM 26-51-0/501).
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APU LUBRICATION SYSTEM - DESCRIPTION AND OPERATION
1. General
A. Lubrication of the APU turbine engine is by a self-contained, positive pressure, dry-sump system, which cools and lubricates all gears and bearings in the engine by providing a pressurized supply of oil. The components of the system are the oil pump, oil cooler, and oil tank (Fig. 1).
B. When the unit is started, the pressure pump draws oil from the oil tank and delivers it under pressure to the accessory drive section and to the bearing between the second stage compressor and the turbine. The oil drains into the accessory drive housing sump and into the second stage compressor and turbine bearing cavity. The scavenge pump draws the oil from the sump and bearing cavity and pumps it through the oil cooler back in to the tank.
2. Oil Pump
A. The oil pump consists of separate pressure and scavenge pumps, pressure regulating valve, oil filter, and a filter bypass valve. The pump is installed on the accessory drive gearbox (Fig. 1).
B. The pressure and scavenge pumps are driven by a common shaft. The pressure pump is a two gear, positive displacement pump which supplies oil underpressure to the bearings, gears, and shafts of the turbine engine. The scavenge pump is a three gear positive displacement pump which scavenges oil from the accessory drive oil sump and from the bearing cavity between the second stage compressor impeller and the turbine wheel. It returns the oil through the oil cooler to the oil tank. External outlet and inlet lines are connected to threaded ports in the body, but scavenging from the accessory drive gearbox is through an internal opening between the oil pump and the gearbox.
C. The maximum delivery pressure is regulated by a spring-loaded relief valve contained within the pump body. The function of this valve is to open when the pressure pump delivery exceeds 90 psig system pressure, thus allowing oil to bypass back to the inlet side of the pressure pump to maintain nearly constant system pressure.
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D. The oil filter is located on the oil pump. It filters all the oil flowing from the pressure pump. The filter base is integral with the pump body. The removable case screwed into the base encloses a replaceable filter element.
E. The filter bypass valve is a spring-loaded relief valve contained within the pump body. The function of this valve is to yield to pressure slightly in excess of normal filter resistance, so that if the filter should become blocked, oil is permitted to bypass the filter and maintain a flow through the system.
3. Oil Cooler
A. The oil cooler is a cylindrical unit constructed of mechanically bonded aluminum tubes housed in a shell. It is installed on the lower front side of the APU (Fig. 1). Oil delivered by the scavenge pump passes around the tubes in the cooler before returning to the tank, while air supplied by the cooling fan flows through the tubes and cools the oil.
4. Oil Tank
A. The oil tank contains the supply of oil for the system and is mounted on the left side of the APU (Fig. 1). Oil is drawn from the tank by the pressure pump and returned, through the oil cooler, by the scavenge pump. The tank is vented to the turbine exhaust. An air-oil separator in the tank separates the oil from the air going through the tank to the vent. An oil level switch, contained within the tank, is used to transmit a low oil quantity signal.
5. Operation
A. When the unit is started, the oil pressure is supplied by the oil pressure pump gears through the oil pump filter (Fig. 2). If the filter is clogged, the bypass valve opens to permit continuing oil flow. From the oil pump, the oil is routed to the accessory drive gearbox and to the bearing between the second stage compressor impeller and the turbine wheel to provide pressure and spray lubrication for the engine gears and bearings. An oil pressure regulator valve in the oil pump operates to maintain the output oil pressure at 90 psig at 100% service speed.
B. Lubricating oil in the unit drains into the accessory drive gearbox oil sump and 2nd-stage compressor and turbine bearing cavity. It is pumped from the sump and cavity by the oil scavenge pump gears (Fig. 2). Scavenge oil is directed through the oil cooler back to the oil tank. Any air trapped in the 2nd-stage compressor and turbine bearing cavity is released through the oil tank vent line into the turbine exhaust.
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