The turbine wheel and shaft assembly is commonly called the turbine wheel. It is the most critical and costly of all turbocharger parts. While it is technically an assembly, it can be thought of as a singular part, Manufacturers don’t sell turbine wheel and shaft components because the manner of assembly is not possible for the average person.
The turbine shaft is typically induction hardened in the bearing journal area only. This is for wear purposes. The shaft is ground to tolerances of around 0.0003 inch. For this reason, a tenths-reading micrometer is essential when doing turbocharger rebuilding.
On aircraft applications, the turbine wheel and shaft are all made from one piece as a fail safe dictated by the FAA. But in all other applications, the turbine shaft is welded onto the turbine wheel casting by either an electron beam weld or an inertia welding method. Most welds used in production today are the inertia weld type due to the speed of the production process it allows.
In most turbine shafts, the threaded portion of the wheel contains rolled threads instead of cut threads, which creates a stronger shaft once torqued with the compressor wheel in place. The reason is that the grain structure of the shaft metal is not cut but is compressed and therefore not weakened. A close examination of the threads on a turbine shaft will reveal smooth and shinny threads as opposed to the rougher cut threads typically seen on a machine screw.
