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A fan disk is the part of a turbofan jet engine that secures the bypass fan assembly and connects it to the low pressure compressor and turbine via the low pressure shaft. The fan disk is one of the critical parts of any turbofan engine and serves several vital functions. The most important of these is securing the blades of the primary or bypass fan. The disk also serves to connect the bypass fan to the low pressure shaft which is, in turn, connected to the rest of the low pressure components of the engine. All peripheral fan components such as the nose cone or spinner are also supported by the fan disk.
Modern turbofan jet engines consist of two basic sections: low and high pressure. These both consist of compressor and turbine components. The low pressure section has one additional part that is possibly the most important part of all. This is the bypass fan which is the large, multi-bladed fan so prominently visible when a jet engine is viewed straight on. This fan is one of the most critical parts of the engine and a major contributor to the engine's thrust capacity.
As large and impressive as this fan may be, its most important single component is fairly small and rarely, if ever, seen. This is a rather unassuming looking hub at the center of the fan blade assembly. Known as the fan disk, this part of the bypass fan serves several vital functions in the overall operation of the engine.
The first is the critical task of supporting the huge fan blades.These blades are subject to intense mechanical stresses during operation; as the most exposed part of the engine, they are often subjected to impact from foreign objects.These unforeseen stresses can be caused by ingestion of loose objects on the runway, ice, or even bird strikes. The disk has to support the fan blades in such a way that they can absorb these impacts without breaking off and destroying the engine.
The second important function of the fan disk is the transfer of rotation from the fan to the low pressure compressor and turbine. This is facilitated by a shaft which passes from the fan disk through to the rear of the engine core. The low pressure compressor is mounted on this shaft right behind the fan and the low pressure turbine at the rear of the engine.
There are several other peripheral components mounted on the disk including the spinner or nose cone. This is the conical center piece on the fan which ensures an aerodynamic flow of air into the fan. All of these reliant elements mean that a fan disk failure constitutes one of the most serious engine problems pilots can encounter. For this reason, most fan disks are made from one piece titanium alloy forgings specially profiled to spread loads and stress effectively.