Jet Engine Gearbox

[BA 77]

For quite a while now I've been wondering what the gearbox is actually used for in a Jet Engine. Can someone tell me which parts use it (parts like the HP/IP/LP compressors/turbines or front fan etc). Also, does anyone know an approximate weight of one?

Thanks, BA 77.

[tristar 500]

The gearbox on jet engines is normally used to drive the ancillary items, such as IDG/generator, hydrualic pump(S) HP fuel pump & if memory serves it is driven by the HP section of the engine.

It also is driven intially by the starter motor to rotate the engine on start.

As for weight "HEAVY!!"

tristar 500

[Rigga]

Its usually known as the Accessory Gearbox and most often it's used for driving the Electrical Generator, Oil Pump and Hydraulic Pump.

What drives it depends on the specific engine type and design.

[celtic mech]

The 2 replies above hit the nail on the Head. What it drives etc can change from engine to engine. If you take an example of the CFM56, The High Pressure Compressor (N2), via gears and a tower shaft, drive the Accessory Gearbox. In turn, items such as the Hydraulic Pump, IDG, HMU, Oil Pump, Fadec Alternator etc are connected directly on the Accessory Gearbox.
In order to start the engine, we have a Starter Motor which is Pneumatically operated. This motor is mounted to the Accessory Gearbox also. Therefore, provided we have sufficient air supply, by opening the Start Air Valve we turn the starter motor...via the tower shaft this then turns the N2 (Hp compressor and Hp turbine). Off hand on a 320 series a/c, the N2 is rotated and at 16% N2 we get ignition, 22% is Fuel flow then engine accelerates to Idle.

[BA 77]

Thanks a lot for the replies, that's been a great help! Can I just quickly ask what IDG and HMU stand for? And also, is Fadec just the make of Alternator or does Fadec mean something?

Sorry for the ignorance in these areas, thanks for the replies, BA 77.

[One Outsider]

You should get yourself a copy of this one.
*
http://www.rolls-royce.com/Images/rolls_royce_jet_engine_200_tcm92-11118.jpg
*
The Jet engine - Rolls-Royce (http://www.rolls-royce.com/about/publications/jet_engine_book/)

[mrmagooo]

IDG= Intergrated Drive Generator, a generator with a built in CSD to give a constant output at varying engine speeds.

HMU= Hydromechanical Unit......

FADEC= Full Authority Digital Engine Control. It basically controls the engine without the old fashioned cables. It is normally powered by a 28vdc PMA which is also mounted on the gearbox.

[Dodo56]

Also note many FADEC units are not mounted on the engine. As electronic boxes, they don't need to be engine mounted and can be located on the airframe in an area away from heat and vibration and more accessible for troubleshooting/change.

The only fuel system components that do need to be engine mounted are the pump (driven via gearbox) and hydromechanical metering/shutoff unit (these days normally integrated into a single LRU).

LRU= Line Replaceable Unit by the way ;)

BA77 if you do want to find out more the recommendation of the RR book is very good. If you don't want to buy it check the ISBN number and see if your local public library can lend it to you.

[mitzy69]

gear box full of gears to drive things at different speeds as required.
weight of gear box on B747-400 RB211 is 135 kilogrammes.
with all the pumps, drives, generators, and pipes on the gearbox weighs a lot more.

[N1 Vibes]

BA 77,

Just to list what basic components most g'boxes have fitted to them and why:

- Fuel pump - often low and high pressure combined, draws fuel out of wing tanks

- Fuel controller - varies/controls the amount of high pressure fuel, based on throttle input/autothrottle inout, going to the fuel spray nozzles in the combustion chamber

- Oil pump assembly - providing pressure to supply all mainline bearings, and the g'box itself and scavenge/suction to recover oil afterwards

- Dedicated Generator/Permanent Magnet Alternator - provides electrical power to the engine systems - on a modern engine it's the FADEC (Full Authority Dedicated Engine Control system that needs this power - which includes the EEC/ECU - the 'brain' of the engine)

- IDG/CSD+Gen - Integrated Drive Generator or Constant Speed Drive and Generator assembly - providing main electrical power to the a/c systems

- Hydraulic Pump - supplying the main hydraulic pressure to the a/c systems for flight surface control, rudder, elevator, ailerons, landing gear retract etc

There are a number of other things on various g'boxes - oil pressure switches, speed indiactors, temp probes etc. But the above list is basically common to most large civilian a/c engines.

Brgd's

N1 Vibes

[dixi188]

On the ALF 502 & 507 fitted to the BAe 146 & RJ there is reduction gearing between the LP turbine and the fan. These gears are in the front of the engine just behind the fan. As a result of this the fan rotates in the opposite direction to the turbine.
I think this arrangement will probably appear in the next generation of bigger engines as the major manufacturers are talking about Geared Fans.

Hope this helps.

[TURIN]

Such as this one for example....

PW1000/GTF (http://www.flug-revue.rotor.com/FRHeft/FRHeft07/FRH0702/FR0702c.htm)

[JamesA]

BA77,
I don't know what you do but, I would certainly recommend this book, as has already been done.
One point though - BUY your own, but DO NOT lend it out.(Likely never to be seen again). It is a mine of information and gets better with every new edition.
Worth every penny, even cheap at twice the price.

[dixi188]

Rolls Royce book "The Jet Engine".

Excellent.

On ebay, 15 pounds + postage.

But as JamesA says, don't lend it out. I had my fathers copy with the Spey on the front and lost that to someone, then I bought the one with the RB 211on and that disappeared too at a later date.