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System Description
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WHITE MESSAGE Require immediate crew attention. These messages are accompanied by flashing master warning, triple chime attenson with or without voice message or dedicated tone. Require immediate crew awareness and future crew action may be required in the form of alternate system selection or performance limitation. These message are accompanied by flashing master caution and a single chime. Indicate safe or normal system operation which require crew awareness, over and above the dark cockpit philosophy. Indicate minor failures or reduction in systems capability, which require no crew action.

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Overview

The accessory drives supply mechanical power necessary to operate the hydraulic pump, aircraft generator, air-oil separator, permanent magnet alternator (PMA), hydromechanical unit (HMU), and the cartridge oil pump. The accessory gearbox (AGB) provides a drive pad for the air turbine starter (ATS).

Accessory drives are part of the AGB, attached by two mounting points to the front frame at the bottom forward end of the engine. An oil tank in the AGB supplies oil to the accessory drives through oil jets or by splash lubrication. The oil drains to a drain mast in case of failure. The tower drive shaft supplies the mechanical connection between the bevel gears of the compressor shaft and the AGB input shaft.

Gearbox

The engine’s mechanical energy from the high-pressure (HP) spool (N2) is transmitted through the towershaft drive system to the accessory gearbox (AGB). The towershaft drive system turns the internal gears inside the AGB. There are aircraft and engine accessories on the forward and aft side pads of the AGB. The internal gears of the AGB transmit the mechanical power to operate these different aircraft and engine accessories.

During an engine start, the AGB is driven by the air turbine starter (ATS). When the engine gets to a constant idle speed, the AGB is driven by the HP N2. The HP spool is connected to a tower drive shaft, and the tower drive shaft connects to and drives the AGB.

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Accessory Gearbox (AGB)

The AGB is a unit with a set of internal gears. It is at the bottom forward end of the engine and it has two mounts which are attached to the engine front frame.

The AGB internal gears are made from steel, and its housing is made from cast aluminum. The AGB has attachment connections to different aircraft and engine accessories. It has an oil tank for oil storage and oil distribution during engine operation. Primarily, the AGB supplies necessary mechanical power to operate aircraft and engine accessories.

The AGB has five drive pads, three on the forward side and two on the aft side. The forward side has the drive pads for the aircraft system accessories. These aircraft system accessories include the aircraft DC generator, air turbine starter (ATS), and hydraulic pump. The aft side has drive pads for the engine system accessories, except for the fuel heater/oil cooler (FHOC). The engine system accessories include the permanent magnet alternator (PMA), hydromechanical unit (HMU), and FHOC.

The drive pads turn at different speeds and directions, as shown by a mark on the pad. The speed of the drive pads is calculated on 100% N2 speed (28,100 rpm). Each drive pad has a seal drain that drains to a drain mast when the seal becomes defective. The AGB has internal cored openings and carbon face seals in the accessory pad locations.

The drive pads for the generator, ATS and hydraulic pump on the AGB have a quick attach/detach (QAD) adapter at the pad. The QAD adapter for the ATS has a break-away feature that lets the QAD breakage occur at the AGB interface. This prevents damage to the AGB housing if a fan blade-out occurs.

The AGB also has attachments for other components. These components are the N2 speed sensor, AGB 1 debris monitor, forward sump, aft sump and AGB 2 debris monitors. Other components include the oil pump (internal), sight gauge, and an oil filter assembly with its oil pressure adjustment valve. The AGB also has an air pressure test port, AGB oil pressure test port, and oil fill port for the lubrication system.

The integral oil tank inside the AGB distributes oil to cool and lubricate the engine bearings, gears, splines, and seals under all operation conditions. The oil pump, oil filter assembly, and FHOC are the main components for oil distribution. The integral oil tank, sight gauge, and oil pressure valve are the main components for oil storage. The electronic chip detector supplies signals to the maintenance diagnostic system through the electronic control unit (ECU), if particles are found in the oil supply.

There are ball and roller bearings in the AGB. Some bearings and gears in the AGB get oil jet spray lubrication, the other bearings get oil splash lubrication. Sealing is done with the noncontact and liftoff carbon face seals. No gaskets are necessary between the AGB and the accessories, but there is a gasket used for the hydraulic pump. The ATS and DC generator drive splines are oil lubricated.

Other than the bearings used for the DC generator and the air-oil separator breather, all other bearings are the same. The HMU and the hydraulic pump have two ball bearings. The bearings are flange-installed and made from bearing steel. The ball and roller bearings are moved by the radial loads supplied by the spur gears and give axial location control for the gearshaft. The ball bearings have zero thrust loads during operation.

The machined spline inserts lubricate automatically and are installed in the DC generator, HMU and hydraulic pump pads. This makes spline oil lubrication unnecessary. The self-lubrication decreases the spline wear and prevents damage to the AGB gear splines.

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Rigid Coupling Shaft

There are three rigid coupling shafts which have 23 external and 16 internal teeth. The diameter of the rigid coupling shafts are 1.172 to 1.182 in. (29.76 to 30.02 mm). One is used for the generator gear spline insert, the second is used for the hydraulic pump gear spline insert; and the third is used for the HMU/oil pump spur gear.

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Operation

During an engine start, the ATS supplies the initial mechanical power to the AGB. The ATS turns the HP spool (N2) until the HP spool has sufficient speed to operate the engine. When the engine is in operation, the HP spool (N2) transmits mechanical energy to the towershaft drive system. The towershaft drive system has an internal power takeoff, towershaft and set of drive gears. The power takeoff module is near the forward sump and has an upper bevel gear and pinion gear connected to the HP spool (N2). The towershaft is in the 6 o’clock strut in the engine front frame. It supplies the mechanical connection between the upper bevel gear in the engine and the lower bevel gear in the AGB.

The upper bevel gear supplies torque to move the towershaft during the engine operation. The lower bevel gear supplies torque to move the AGB internal gears. The upper and lower bevel gears as well as bearings support and align the towershaft.

The pinion gear is held by the annular ball bearing to the HP spool (N2). The pinion gear transmits the torque to the upper bevel gear and towershaft. And the towershaft transmits the mechanical power from the HP spool (N2) to turn the internal gears of the AGB. The AGB internal gears together make a reduction gear train. The reduction gear train decreases the speed and increases the torque. The increased torque from the internal gears supplies more power to operate the aircraft and engine accessories connected to the AGB.

The ATS spur gear is held at its forward end by a ball bearing, and at its aft end by a roller bearing. The ball bearing is flange-installed directly to the AGB housing. The bearing moves with the radial loads supplied by the spur gears and supplies axial location control for the gear shaft. The bearing has zero thrust loads during operation. The roller bearing moves with the radial loads supplied by the spur gear mesh. The spur gear supplies the mechanical connection to turn the towershaft and N2 spool during engine start.

The PMA is attached on the back side of the ATS shaft and controlled by the ATS bevel spur gear. The PMA provides access for manual turns of the gearbox and HP spool for maintenance of the engine, such as borescope inspection.

The HMU/oil pump spur gear supplies a mechanical connection to move the HMU and oil pump. The HMU and oil pump use the same spur gear. The oil pump coupling connects directly into the HMU gear. The HMU/oil pump spur gear is held at its aft end by a duplex ball bearing. The duplex ball bearing moves with the radial loads and more momentum is supplied by the spur gear forces. The duplex ball bearing also supplies axial location control for the gear shaft. It has zero external thrust loads during operation. The outer ring of the duplex bearing is flange-installed on the AGB accessory cover. The duplex bearing cannot be separated.

The hydraulic pump spur gear supplies a mechanical connection to operate the hydraulic pump. The ATS spur gear controls the hydraulic pump spur gear through the HMU/oil pump and stepdown gearbox spur gear. A duplex ball bearing gives support to the hydraulic pump spur gear. The ball bearing gives axial position control for the spur gear. The ball bearing moves with the radial loads supplied by the spur gear mesh.

The AGB lubrication system is a self-contained oil system. It cools and lubricates all engine bearings, seals, and gears, and all splines where lubrication is necessary. It also provides fuel heating (through the FHOC) and shaft vibration damping (through the use of the hydraulic mounts). The towershaft splines are lubricated at both ends.

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System Interface

The gearbox interfaces with the following systems/components:

  • Primary DC Power System
  • Left and Right Hydraulic System
  • Maintenance Diagnostic System
  • Electronic Control Unit (ECU)
  • Speed Sensing
  • Oil Storage System
  • Oil Distribution System
  • Engine Starting System

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