any new word of the Garrett BB turbo's??
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Joined: May 2005
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From: Peoria, IL
Sorry to bring up an old post, but this is something I've been pondering for a while.
If there aren't any advantages to ball bearing turbos, then where are all the claims coming from? Just manufacturers trying to sell a more expensive turbo?
If there aren't any advantages to ball bearing turbos, then where are all the claims coming from? Just manufacturers trying to sell a more expensive turbo?
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Joined: Apr 2003
Posts: 6,564
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From: Cummins Technical Center, IN
There are certain applications where a ball bearing turbo will live up to the claim of faster spoolup.
The simple reason is viscous drag. The shaft is spinning through a bath of oil. Now, imagine you are spinning yourself on a barstool. Can you do it more easily underwater than on dry land?
Of course not-- the water has more viscous drag than does the air. Hence, it is more work to spin.
In a journal bearing turbo, viscous drag and the rotational inertia of the assembly are really the only two things that stand between you and instantaneous spoolup.
Since the inertia is sort of a given with any given turbo size, a reduction in viscous friction may manifest itself as faster spoolup.
That said, the small shaft diameter of a typical turbo means that the surface velocity (rpm*PI*2R) will be small relative to RPM. Moreover, the oil viscosity of a typical diesel oil at operating temperatures is only ~13CSt-- not too thick.
What *IS* certain is that a journal bearing design is almost indisputably better for longevity. The journal bearing rides on a cushion of oil, and thus there's no metal-to-metal contact.
Compare this with a ball-bearing turbo where you have little ***** that push through the oil film much more easily, and hence there's a possibility of metal-to-metal contact. That, and the total surface area available as a bearing is MUCH lower with a ball-bearing design.
IMO, ball bearing turbos are appropriate for racing applications and have not yet earned a place in a street application when longevity is critical.
Justin (hi, it's been awhile
)
The simple reason is viscous drag. The shaft is spinning through a bath of oil. Now, imagine you are spinning yourself on a barstool. Can you do it more easily underwater than on dry land?
Of course not-- the water has more viscous drag than does the air. Hence, it is more work to spin.
In a journal bearing turbo, viscous drag and the rotational inertia of the assembly are really the only two things that stand between you and instantaneous spoolup.
Since the inertia is sort of a given with any given turbo size, a reduction in viscous friction may manifest itself as faster spoolup.
That said, the small shaft diameter of a typical turbo means that the surface velocity (rpm*PI*2R) will be small relative to RPM. Moreover, the oil viscosity of a typical diesel oil at operating temperatures is only ~13CSt-- not too thick.
What *IS* certain is that a journal bearing design is almost indisputably better for longevity. The journal bearing rides on a cushion of oil, and thus there's no metal-to-metal contact.
Compare this with a ball-bearing turbo where you have little ***** that push through the oil film much more easily, and hence there's a possibility of metal-to-metal contact. That, and the total surface area available as a bearing is MUCH lower with a ball-bearing design.
IMO, ball bearing turbos are appropriate for racing applications and have not yet earned a place in a street application when longevity is critical.
Justin (hi, it's been awhile
)
They have a "97" anniversary special that is delicious...
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