Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 31

The turbine and compressor are machines supported by a common bearing system. The bearing system has many considerations that go into its design. The bearing system must allow for rotational speeds in excess of 100,000 rpm; it must also withstand shaft motion gyrations imparted onto the turbine shaft by the engine’s pulses at or near peak torque, handle radial and axial or thrust loads, withstand high temperatures, and contain enough imbedability of contaminates to keep the turbo from failing prematurely. Despite this fact, contaminated lube oil has remained the number one cause of premature turbo failure.

The most common type of bearing system is the three-piece bronze bearing system that includes two journal bearings and one thrust bearing. The journal bearings are typically full floating and have specific clearances between the turbine shaft and bearing inside diameter, as well as the bearing outside diameter and the bearing housing bore. The two journal bearings typically rotate in relation to the bearing housing while the turbine shaft rotates in relation to the bearings. The bearings will typically rotate about one-third of turbine shaft speed, which divides the speed differential between the relative bearing surfaces to help minimize internal wear.

The third piece of the three-piece bronze bearing system is the thrust bearing. The thrust bearing has two bearing lands, one on each side. The oil enters from the center via a small oil galley and flows outwardly between the bearing surface and the thrust rings made from hardened steel held to very close tolerances.

The typical three-piece bronze bearing system design dominates the volume of production turbochargers. It consists of two journal bearings and a flat thrust bearing. Note the holes around the two journal bearings. These bearings are full floating, meaning that while the turbine shaft rotates inside the bearings, the bearings themselves rotate relative to the bearing housing. The thrust bearing is installed in a fixed positioned in a pocket inside the bearing housing. (Courtesy Diesel Injection Service Company, Inc.)

Boost pressure acts upon the back face of the compressor wheel just as turbine pressure acts upon the back wall of the turbine wheel. This causes thrust load on the rotor assembly. When the pressure is greater in the compressor than it is in the turbine, it’s called a positive pressure differential. When the turbine pressure is greater than compressor pressure it’s called a negative differential. An engine will run best in a positive differential and a good turbo match will produce a positive differential at or near the designed match point. However, during hard acceleration, like in a drag race, the turbo will almost always be in a negative differential condition because the compressor boost can never keep up with the turbine pressure that is constantly rising. Once the vehicle is in a higher gear, the rate of change slows and the boost pressure should overcome turbine pressure. A positive pressure differential is desired for both power and economy.

Since the pressure on each respective side of the turbo acts on the back face of each wheel, the pressures tend to pull each wheel away from the other. The thrust surface for the compressor end is the bearing surface closest to the turbine and vice versa. This is a commonly confused issue, but important to understand. Most turbos will accommodate a maximum pressure differential of about 20 lbs of boost. If your turbo fails due to a pressure differential issue, it’s important to understand which surface is which.

This illustrates how compressor and turbine pressures act upon the back face of each wheel in a typical turbocharger.

Some turbo models will feature thrust-bearing upgrades because there may have been a larger compressor wheel developed later in that model’s life cycle. A higher load capacity thrust-bearing will tend to have a slightly larger bearing land and/or a ramp on the feed side of the bearing land that uses an inclined plane as a form of pump to improve oiling under high-stress conditions. That part would be called a ramped thrust bearing.

The oiling system can be separate fed or end fed in a two piece full floating bearing system. Following are the different types of bearing system used in turbos today.

This close-up of a turbocharger bearing system shows what is called a separate fed, full-floating bearing system. Note the yellow colored oil passages that branch out to separately feed each of two journal bearings and the thrust bearing. In an end-fed bearing system, the main oil inlet will be positioned more centrally between the two journal bearings and will pass all the way through to the shaft. Oil will fill the center area and flow outwardly and lubricate the bearings from their ends. Note the snap rings that hold the journal bearings in place. (Courtesy Diesel Injection Service Company, Inc.)