Many 737 engines have an accessory gearbox on the 9'o clock position, which is part of why the engine has a flat-bottomed shape. What exactly is an accessory gearbox?
The accessory gearbox is generally driven by the N2 shaft (connecting the high-pressure turbine to the high-pressure compressor) and in turn drives the engine accessories such as generators, hydraulic pumps, oil pumps, etc. The air turbine starter is also part of this gearbox so during starting the accessory gearbox drives the high-pressure compressor.
This analogous to the serpentine belt on your car, which connects your crankshaft to your cars engine accessories (alternator, water pump, power steering, air conditioning, etc).
From this answer, an accessory gearbox for the Rolls Royce Trent. The accessories connected to the engine spool(s) are:
- The Engine Driven hydraulic Pump
- Fuel pump
- Oil pump
- Pneumatic starter, requiring the highest loads and therefore dimensioning the gear strength.
- The IDG: Integrated Drive Generator which provides the AC power.
Only the pneumatic starter must be connected to the HP spool. The other accessories may be connected to the LP spool, which turns at lower RPM.
The Accessory Gearbox can be placed relatively freely along the engine outer radius: on top, underneath, or to the side as in the B737.