Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 29

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.

The areas of the turbine shaft and wheel assembly include: (1) the exducer diameter, (2) the turbine wheel contour, (3) the turbine inducer or tip, (4) the seal ring, or piston ring groove, (5) the bearing journal or shaft bearing surface, (6) the stub shaft, (7) the turbine wheel, (8) the turbine shaft, (9) the shoulder, and (10) the rolled threads used to clamp the compressor wheel onto the shaft. (Courtesy Diesel Injection Service Company, Inc.)

A close-up of the turbine wheel and shaft assembly reveals the difference between the seal ring groove and the oil flinger, which is designed to throw oil away from the seal ring area during turbine rotation. (Courtesy Diesel Injection Service Company, Inc.)

Turbine shaft threads are rolled instead of cut to maintain material grain structure, which makes the threads stronger. When threads are cut, the grain is interrupted and the shaft is weakened.

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.

There are knock-off turbochargers made in this big world that some will call counterfeit. Design aspects such as these escape the average consumer’s eye. Materials selection, manufacturing methods, and subtle design differences can make all the difference in a piece of machinery whose internal rotating parts can, in some applications, exceed the speed of sound! Buyers beware. I have to express my opinion here that some Pacific Rim turbo knock-offs are not a good way to spend your money. There are legitimate companies in this region that use genuine quality designed turbochargers from sources such as Garrett or Borg-Warner in their complete turbo kits. I am primarily referencing those that manufacture complete turbochargers, not kits, and sell them on eBay at low prices to uninformed turbo owners.

The turbine wheel is made from an investment casting process and is made from high nickel content materials such as GMR235 Super alloy or Inconnel 713C. “Inco” is typically desired due to its strength and higher heat resistance over the GMR235. However, you cannot select a turbo model and specify whether you get Inco or GMR235, each model is either made with one or the other. The GMR235 alloy is typically fine for most diesel applications, and even for many automotive uses. But high-performance gasoline applications typically use Inco for its higher heat resistance. If you’re building a high-performance gas turbo car, I recommend using turbos that have Inco turbine wheels. Those turbos marketed in automotive catalogs will tend to contain the correct material. However, in many high-performance applications, a large-frame turbo originally designed for a production diesel may have the necessary flow range you need, but its material may not be correct. Be sure to determine this often overlooked fact. Both diesel and gasoline high-performance applications will need Inco turbine wheels.

Schwitzer brand turbos have always used Inco as their standard turbine wheel material. If you’re contemplating a diesel turbo because of its flow range on your project, you would be wise to contact your nearest distributor and research the material used on the model you’re considering. As a general rule, if your turbine inlet is expected to rise near or above 1,400 degrees F, go for the Inco wheel.