Accessory drive
Gearbox driving essential engine and aircraft accessories.
The accessory drive is a gearbox found in gas turbine engines. Though it sits outside the engine’s core, it powers components essential for the engine or aircraft to function, including generators, fuel and oil pumps, air compressors, hydraulic pumps, and the engine starter. On large engines, these drives handle between 400 and 500 horsepower.
Power for the drive comes from the central shaft connecting the turbine and compressor. An internal gearbox couples this shaft to a radial driveshaft—sometimes called a towershaft—which then drives an external gearbox. The internal gearbox is challenging to design due to high heat and tight space; it is usually placed between the compressor outlet and the combustor, though in turboprops or engines with centrifugal compressors it may sit ahead of the compressor.
In two-shaft engines, the accessory drive is typically taken from the high-pressure shaft—the outer, shorter of the two concentric shafts—because it spools up faster during engine start. The drive and accessory gearboxes can be split, with one driven by each shaft, to distribute loads. Engine-critical systems, like the starter, are placed on the high-pressure shaft, while aircraft systems run off the low-pressure shaft. The high-pressure shaft also rotates faster, which can affect how accessories are arranged.
To accommodate thermal expansion, the drive from the main shaft uses one of three methods: a direct bevel gear on the main shaft; a tubular stub shaft fitted over the main shaft via splines, carrying the bevel gear; or an idler shaft running parallel to the main shaft, driven by spur gears and turning the bevel gear to the main gearbox.
The radial driveshaft runs at high speed to keep its diameter small, minimizing airflow disruption and the size of its enclosing fairing. If a straight path is impossible, the shaft may be split into two sections linked by an intermediate gearbox—common in high-bypass turbofans with large fans.
The external gearbox is often a curved casing, allowing accessories to mount close to the engine. It consists of two light alloy castings with separate machined pads for each accessory. Inside, a train of spur gears provides the drive, with accessories arranged on both sides of the driveshaft entry in descending order of speed.
- Power range
- 400–500 hp
- Drive source
- central shaft linking turbine and compressor sections
- Internal gearbox location
- usually between compressor outlet and combustor; ahead of compressor in turboprops or designs with centrifugal compressors
- Shaft types
- direct drive, stub shaft drive, idler shaft drive
- External gearbox casing material
- light alloy castings
- Gear types
- plain spur gears, idler gears, helical gears for high-torque drives
Lore & Background
Power for the accessory drive is taken from the central shaft linking the turbine and compressor sections of the engine. This requires an internal gearbox that couples the drive to a radial driveshaft or towershaft that drives an external gearbox. The design of the internal gearbox is complicated by the heat and small space available in which to connect the driveshaft. It is usually placed between the compressor outlet and the combustor. In turboprops or designs with centrifugal compressors, it may be placed ahead of the compressor. For two-shaft designs, an accessory drive will be taken from the high-pressure shaft, i.e. the outer and shorter of the two concentric shafts. This shaft comes up to speed more quickly when the engine is started. The drive and accessory gearboxes may also be split in two, one driven from each engine shaft, so as to distribute their loads. The engine-critical systems, including the starter drive, are arranged on the high-pressure shaft, with aircraft systems on the low-pressure shaft. The high-pressure shaft also rotates faster than the low-pressure shaft, which may influence the distribution of accessories. To allow for thermal expansion, the drive from the main shaft may be taken by one of three means: direct drive by a bevel gear on the main shaft; stub shaft drive, where a tubular stub shaft is fitted over the main shaft and driven from it by splines; or idler shaft drive, where a narrow shaft runs parallel to the main shaft, driven by spur gears. The radial driveshaft runs at high speed to allow small diameter, reducing disruption to airflow and the size of the hollow fairing. If a single straight path is not possible, the driveshaft may be arranged in two sections linked by an intermediate gearbox, most commonly required for high-bypass turbofans with large diameter fans. The external gearbox is usually arranged as a curved casing, with separate machined mounting pads for each accessory. The drive within the casing is provided by a train of spur gears. Accessories are arranged on both sides of the driveshaft entry, in reducing order of their speed. Idler gears are commonly used to increase spacing between accessories. Helical gears are sometimes used for high-torque drives, typically the starter, as these give smoother running, but they generate end-thrust requiring more complicated thrust bearings. The complexity of an accessory drive and its gears was used as a theme by the anthropomorphic illustrator Boris Artzybasheff in advertising for the Avco Lycoming company, who were making drive gearboxes for the Westinghouse J40 engine.
Reader's Guide
The accessory drive is a critical subsystem of a gas turbine engine, enabling the operation of essential engine and aircraft accessories. Its significance lies in its role as a mechanical power take-off from the engine's main shaft, converting rotational energy to drive fuel pumps, generators, hydraulic pumps, air compressors, and the engine starter. The article notes that accessory drives on large engines handle between 400–500 hp, indicating the substantial power required. The design is constrained by heat, space, and the need to accommodate thermal expansion, leading to three distinct drive methods: direct, stub shaft, and idler shaft. The internal gearbox is typically placed between the compressor outlet and combustor, but may be ahead of the compressor in turboprops. For two-shaft engines, the high-pressure shaft is used for critical systems due to its faster spin-up and higher speed. The external gearbox is a curved light alloy casing with machined mounting pads, using spur gears and sometimes helical gears for high-torque applications. The complexity of these gearboxes was notable enough to be featured in advertising by Boris Artzybasheff for Avco Lycoming, who made drive gearboxes for the Westinghouse J40 engine. Bleed air may also be tapped for accessories, particularly for cabin pressurization or cross-starting other engines. The accessory drive's legacy is as an indispensable component that balances mechanical power distribution, thermal management, and packaging within the nacelle.
Did You Know?
- For two-shaft designs, the high-pressure shaft is used for engine-critical systems including the starter drive.
- The complexity of accessory drive gears was used as a theme by illustrator Boris Artzybasheff in advertising for Avco Lycoming.
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