'The Garage' Quest TV.

On Sun, 15 Aug 2010 00:46:05 +0100, Mike G  


Well that's one of the design criteria. But generally only on startup.

That's not my understanding.
Admittedly the thin 'white metal' surface of the shell bearing will be
damaged or destroyed if unlubricated, but that isn't the primary reason for
the use of 'white metal' as a bearing material.
It's load bearing  and wear properties are the main design considerations.
Mike.

On Sun, 15 Aug 2010 03:19:12 +0100, Mike G  


Well it's quite complicated, you also need to embed any small abrasive  
particles in the surface (there's an ongoing change in philosophy about  
that) & conduct away the heat. The tin flash on the surface is mainly  
there as a dry lube for the days when there isn't any oil.

White metal bearings as far as I know, were never designed run dry as
perhaps you could view a sintered bush. As apprentices we cast shells and
machined to fit.  A test was to mount them onto a rotating shaft and run
them against a very high radial load to establish life, deformation,
temperature etc.  I remember quite clearly the outcome of the sample that
did not get an oil feed connected.
Out of interest, are there any car engines using roller bearings as those of
two stroke motorcycles and their pressed assembly crankshafts ?

Gio

Starting your engine's going to be tricky then. But almost nobody's put a  
cast babbit bearing in a new car engine since the 60s.


Yup, the bigger the clearance the faster it wears :-)

Why?. Enough oil is left in the bearing to lubricate it until the engine
starts, and oil starts to flow through the bearing.

 But almost nobody's put a

You'll find that cast, machined, Babitt bearings were discontinued long
before that.
Automotive manufacturers started using shell bearings in the 30's.

All very interesting, but it misses the point of my OP. Which is that
presumably a skilled mechanic in 'The Garage' TV prog, is telling viewers
that the 'white metal' used in shell bearings, is there to protect the
journal if there is an oil failure.

No wonder I don't trust the majority of garage mechanics when they come out
with crap like that.
Mike.   .

Mike G wrote:


I watched that one too and was a bit taken aback when they simply
changed the shells apparently without removing the crankshaft or even
checking it at all. Or perhaps the editing just gave the wrong impression.

Just changing bearing shells that have failed never works. The crank is
always damaged too.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

On Mon, 16 Aug 2010 13:15:37 +0100, Dave Plowman (News) wrote:

[...]


Been there, done that, wasted the money...

Chris

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Dave Plowman (News) wrote:

That's what I thought. Mind you, it worked as far as 'The Garage' was
concerned as the knocking stopped and didn't come back during a quick
test around the block.

You may be right, but I doubt it. They certainly didn't remove the
crankshaft, and given the state of the big-end shells due to lack of oil, I
can't see the mains surviving unscathed.

Indeed. But given the unusual circumstances, it's possible that very little
wear took place on the journals themselves. Especially if the owner took the
car to the garage as soon as he noticed the knocking. The oil itself can
cater for a small amount of wear on the journals.

If the owner is lucky the 'repair' could last for tens of thousands of
miles. OTOH the knocking could return after just a thousand or two.

I shall continue to watch the series if only to pick holes in the way they
do things. :-)

As an aside I think their prices may be quite high. There seems to be too
many staff for the amount of work being done.

Mike.

Dave Plowman (News) wrote:

Not always, I got away with it, once, on an old Ford Onion I had. It
only had to last a few months, and the bottom end had a light rattle.
New shells cured it for a few months at least....

I suspect I may have just been very lucky.

--
Pete M - OMF#9


"A one man Top Gear Challenge."

Yes. It may well last for a few hundred miles of gentle driving. Certainly
long enough for a TV prog to say it's cured. ;-)

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Willy Eckerslyke


Crap editing, then. I cannot see them not checking it, if they had half
a clue - I mean, really.

On Mon, 16 Aug 2010 01:22:32 +0100, Mike G  


There's quite often metal to metal contact at that point.


Somebody round here nearly always comes up with something you wouldn't  
want to drive that uses an odd technology :-)


Well most of what he comes out with is crap.

I doubt it. Oil is particularly tenacious, and would not be squeezed out
altogether on a large bearing surface like that of a shell bearing. Even if
there were spot contact at the side of a bearing, it would be lubricated as
soon as the crankshaft started to rotate by the oil that 'pooles' at the
bottom of the journal and crankshsft.
Mike.

On Mon, 16 Aug 2010 18:43:01 +0100, Mike G  


That's the main point of the tin flash.

If that's what you believe you are wrong..
I suggest you do a little research into why Babitt alloys are used as a
bearing material, before making statements like that.

The fact remains that just because the 'white metal' bearing surface will
melt and be destroyed if the bearing overheats through lack of oil was never
an engine design requirement. A bearing that could last for many thousands
of miles (200k is not uncommon) with little deterioration was.
Mike.

 

On Tue, 17 Aug 2010 04:14:24 -0000, Mike G


The tin goes on top of the babbit. You can look it up yourself.

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