AmTech manufactures this Generator Shaft Assembly for use in the starter generator of an aircraft.
The purpose of a starter generator is to crank the aircraft engine or APU system up to a specified RPM to start the aircraft.
Basically, the starter generator rotates the turbine engine fast enough so the fuel can be ignited during start up. It then takes over as a generator providing DC power to the electrical system after start. This Generator Shaft Assembly has strict FAA requirements.