A quill shaft is a type of shaft that is used to couple parts of the engine that are subject to torsional loads, such as a generator to an engine.
It consists of a long, thin, hardened shaft with splines on each end. One end splines into the drive shaft and the other into the generator. When an electrical load is suddenly applied to the generator, the quill shaft absorbs the load by twisting. This prevents the load being applied to the gear train of the engine as a sudden shock.
AmTech supplies quill shafts for a leading Aviation product manufacturer