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24-11-00
ALL ú ú 11 Page 13 ú Jan 28/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
(12)
An oil level indicator is on the bottom of the IDG. The indicator consists of a display prism with two white marks for viewing alignment. The indicator shows whether the IDG oil level is low. The indicator does not show a high level, nor does it indicate the system oil quantity numerically. A dark prism indicates the IDG oil level is adequate. A shiny bright prism indicates the IDG oil level is low.
(13)
IDG oil is drained through the case drain plug. Oil is replenished through the pressure fill elbow coupling. The overflow drain coupling is used to drain excess oil during replenishing.
(14)
ENGINES POST-RR-SB 24-9280; For the IDG to operate satisfactorily the exit oil temperature from the heat exchanger must be below 100 degrees centigrade. A thermal switch is installed in the oil exit tube of the heat exchanger. The thermal switch monitors the temperature of the exit oil and gives a signal to open or close the solenoid valve. As the solenoid valve opens (low IDG oil temperature) HP3 air is supplied to a ram. The ram extends to close two vanes in the heat exchanger duct and limit the air flow through the heat exchanger. As the oil temperature increases the thermal switch gives a signal to close the solenoid valve. The solenoid closes to stop the supply of HP3 air to the ram. The ram retracts to open the two vanes in the heat exchanger duct and allow air to flow throgh the heat exchanger and lower the IDG oil temperature.
(15)
The vent valve is a spring-actuated valve located on the IDG end housing to maintain internal pressure in the housing. The vent valve closes by use of a biasing spring acting on a piston. This action permits air pressure to build up in the IDG case. Before servicing the IDG, the valve should be pushed to vent this build-up pressure. The manual vent valve has been replaced by a case vent system on the new IDGs.
The case vent system consists on an aspirator mounted on the end housing and an aneroid valve installed in the input shaft. The purpose of the case vent system is to interchange the air inside the IDG with ambient air. Air enters the IDG through the aspirator and escapes through the aneroid valve. The continuous interchange of air extends the oil and filter change interval by removing entrained water absorbed by the ester-based oil. The entrained water forms an acid that corrodes internal metal parts, and plugs the filter with the corrosion products. The case vent system is not used with vent valve.
(16)
The case thermal relief valve allows oil to flow out of the IDG sump, if IDG oil becomes too hot. The solder plug in the relief valve opens at approximately 430°F (221°C). Oil then flows overboard through the seal drain line.
(17)
Oil exiting the IDG goes through the IDG air/oil heat exchanger. The heat exchanger provides continuous oil cooling by low pressure compressor air. After exiting the air/oil heat exchanger, oil returns to the IDG.
EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. ú
24-11-00
ALL ú ú 03 Page 14 ú Jan 28/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
(18)
A valve in the IDG limits the pressure seen across the air/oil heat exchanger. If external circuit pressure reaches 250 psi, the valve opens. Oil then flows from the output of the scavenge filter, back through the IDG. The external oil cooling circuit is bypassed.
(19)
If IDG oil-out temperature remains above 284°F (140°C) for 5 minutes, an EICAS message appears on the maintenance page: L (or R) IDG OIL TEMP. (The IDG OIL TEMP message is stored in memory by EICAS, and an ELEC AUTO EVENT occurs in the EICAS system). If IDG oil-out temperature reaches 365° | 9°F (185° | 5°C), the appropriate amber DRIVE light in the GEN DRIVE DISC switch comes on. An EICAS level C alert also appears (if the corresponding engine is on): L (or R) GEN DRIVE. (The GEN DRIVE message is stored in memory by EICAS, and an ELEC AUTO EVENT occurs in the EICAS system). To avoid further damage to the IDG, the guarded GEN DRIVE DISC switch should be pressed. This activates the input disconnect mechanism.
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