[Diy_efi] Ball Bearing vs. Plain Bearing

[Bernd Felsche]

On Wed, Jan 01, 2003 at 06:46:16PM -0800, efi student wrote:

[please TRIM your responses and quote consistently
 http://www.netmeister.org/news/learn2quote.html ]

>> They do; as do other transient consitions where there is
>> insufficient oil flow or shaft speed to sustain a hydrodynamic
>> film.

>> Ball-bearings are much less susceptible under those circumstances
>> and have low turning resistance from static.

> And this would have what net effect on our crank, rods, pistons,
> cams, etc.?  I'm sure lots of people on this list can recognize
> the symptoms of lubrication failure on a rod bearing or a piston
> skirt or in cam journal.  We all know that modern engines running
> modern lubricants have far more service life than anything built
> in the 30's.  Start up wear is a really minor consideration in
> bearing selection.

There are situations other than startup where a hydrodynamic film
will not prevent metal-to-metal contact; and somewhat before that,
the turning resistance of the bearing increases dramatically,
perhaps reducing shaft speed and further tending to tear the
lubricating film.

> Again, I find it impossible to believe that any ball bearing has
> less drag than a properly sized and  pressurized plain bearing in
> service, and since this discussion is about turbochargers in
> service, it seems pointless to describe situations under which all
> engine components suffer.

Why is it "pointless" to try to make a turbo more reliable than a
big-end bearing?

You assume that the hydrodynamic bearing is properly sized and
pressurised - there's also the requirement for an oil flow through
the bearing to make it work.

> So, I'm back to my original request.  Does anyone (outside of
> Garrett's marketing department) have hard data to support the
> claim that a ball bearing turbo spools faster than a plain bearing
> turbo?

IMHO turbo spooling speed is much more defined by the rate of change
of gas flow into the turbine than the bearing resistance - given
reasonable lubrication of some sort. I think Mike may share that
opinion as well, given his thoughts on injecting additional gas flow
on throttle changes.

There are design details that can make a ball-bearing slower to spin
up than a hydrodynamic bearing - including the issue of rotating
inertia. It depends a great deal on all design decisions made in a
specific turbo as to which will spool up faster.

-- 
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