Oldfeller--FSO
Serious Thumper ModSquad
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Hobby is now "concentrated neuropany"
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What is life is like inside a turbocharger ..... http://www.epi-eng.com/piston_engine_technology/turbocharger_technology.htmBEARING SYSTEMS
The bearing system which supports the rotor assembly (turbine, shaft and compressor) resides in the turbocharger center housing. That bearing system must reliably position and support the rotor from zero up to speeds that can approach 150,000 RPM. In addition to the rotating loads on the bearings, there can be substantial thrust loads in either direction, depending on operating conditions. The bearing system also has an influence on critical rotor speeds, vibration and shaft instability.
The temperature of the turbo environment also presents a challenge to the bearing system. If the engine is shut down immediately following a run at high power output, the turbine and turbine housing temperatures are toward their upper limits, and suddenly all gas flow through the turbine stops and all oil flow through the center housing stops. All that heat must go somewhere, and an easy path is into the center housing. The resulting temperatures can easily cook the oil to a solid with potentially disastrous results on the next run.
The bearing system has evolved from the early days, when most were hydrodynamic sleeve and face bearings which required uninterrupted oil supply to avoid damage from loss of fluid film and from overheating.
Today's turbos feature dual ball bearing systems with very high bi-directional thrust capacities and reduced frictional drag, allowing faster spool-up times. To combat flat-spotting of bearings during heat-soak, an upgrade in bearing material from 52100 to M2 tool steel is also available.
The centrifugal force at very high speeds can cause steel balls to lift off the inner race, and to skid on the inner race during acceleration. To combat that issue, some manufacturers have switched over to bearings having ceramic balls, and others are moving in that direction. The ceramic ball bearings are also reported to be more resistant to damage from high temperatures.
Garrett Bearing Cartridge
Figure 7
Garrett Bearing Cartridge
Garrett uses an integrated dual ball bearing cartridge (Figure 7) which contains an angular-contact ball bearing at each end, providing a huge bi-directional thrust capacity, and which adds bending stiffness to the shaft system, helping to prevent critical speed issues.
Borg-Warner is developing a two-ball-bearing system which is expected to be fully ceramic.
A turbocharger lives in a terribly hostile environment. The turbine is driven by exhaust gasses that can exceed 1875°F (1025°C) and which are very corrosive. Exhaust valves experience those same corrosive, high-temperature gasses, but exhaust valves do not approach the peak temperature of the exhaust gas. An exhaust valve in a competition engine spends at least half of the time on the valve seat (production engines more like two-thirds of the time). Valves continuously transfer heat through the stem to the guide, and when they are seated, they rapidly transfer heat into the cylinder head through the valve seats. Those cooling paths keep exhaust valve temperatures well below EGTs.
The turbine wheel, however, lives in a continuous, high-velocity jet of those gasses. Although there is expansion across the turbine nozzle, therefore some cooling of the gasses, the temperature at the tips of the turbine rotor can approach exhaust gas temperatures. Further, the rotor system on many turbochargers operates well in excess of 100,000 RPM, and some approach 150,000 RPM. That imposes huge tensile loads from the centrifugal forces, as well as bending and vibratory loads. That environment requires the use of nickel-based superalloys for the turbine wheels. Those alloys can retain high strength values at these high temperatures.Do you think your bike's engine heat stresses your oil more than a dull red hot on the inside big rig turbocharger? I don't think so ..... 
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