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Plain bearings.

Penis Rotor

GT Godfather!
Joined
Oct 28, 2008
Messages
2,833
Location
Bungendore, NSW Australia
Their sudden failure and how they work.

Recently one of the new contributors here from NZ experienced a sudden and catastrophic failure of his engine in his almost new 1200 Sport. On examining his paperwork from the first service he found that the incorrect oil had been used and was ready to sheet all the problems home to that. Now while it *may* of been a small factor in the failure, (But I doubt it.) I felt it might be usefull if I gave a bit of an explanation of WHY I thought that oil was not likely to be the culprit and to do that I thought I'd better do a bit of explaining as to WHY oil choice for the 8V motor is critical and also, if my guess is correct that the big ends and mains have failed, give an explanation of how and why plain bearings work or don't.

What I have tried to do is keep ity simple without 'Dumbing it down' and thought that it might also be of use to some here as a way of understanding 'Why Shit Works'. So I'll C&P my email below.

************************************************************************************************

Chris. Your owner's handbook has the suggested oil specifications in it but
before you make a decision as to this being the reason why your motor has
blown up can I please ask you to read and study my synopsis as to why I
think it is unlikely that this is the cause. It certainly may of been a
contributory factor but on your motor, at its age and mileage, it is
unlikely to be 'The Cause' of the failure.

I'm going to be trying to explain, in laymans terms, some fairly complicated
engineering, so please. bear with me. It may also come to you in a couple of
emails as I'll probably have to take a break and come back to it. OK?

Firstly, lets look at oil and what its function is. Forget any ideas you may
have about oil being 'Slippery'. While it's 'Slipperyness' is a minor factor
in its performance there are two main reasons why you need to have oil in a
motor aith a third linked to the second. Oil has to be used to;

a.) Lubricate. This essentially means keeping solid parts separate from each
other.

and

b.) To cool. Linked to this second item is oil's ability to CLEAN and this
is very important when looking at the choice of oil for a motor like the
Guzzi 8V.

Despite some outward similarities to the earlier OHV motors the 'Cam in
Head' motor is fundamentally an 'All New' engine. Unlike the OHV motors
which were principally air cooled the 8V is liquid cooled with 'air assist'.
It is just that rather than having the heat removed and transported away
from the hot areas by glycol in a 'Water Jacket' the Guzzi motor uses its
oil for both lubrication AND cooling.

There are two completely different oil circuits with two oil pumps. The
first one is a 'Conventional' pump delivering oil in low volume but high
pressure, through a filter and past an oil pressure relief valve to supply
the bearings in the engine. The second pump delivers much larger quantities
of oil but at lower pressure. this pump feeds the cooling circuit after
passing through the oil cooler and is un-filtered apart from a strainer on
its pick-up.

The reason a very high-spec, full ester synthetic oil is specified is NOT so
much because it is needed for lubrication. it is because it is needed for
cooling and CLEANING. The cylinder heads of the new motor are riddled with
oil galleries. Around the exhaust valve seats there are a series of
drillings that allow oil to circulate around the seats removing heat and
taking it to the sump where it will then shed that heat as it passes through
the oil cooler on its next trip. Likewise beneath the bores there are oil
jets also fed from the cooling circuit that spray cooling oil onto the
underside of the piston crowns allowing the pistons to be both lighter and
less succeptible to heat damage and distortion. one of the major advantages
of a top of the range, full ester, synthetic oil is its abillity to
withstand heat without breaking down. the areas around the exhaust valve
seats are some of the very hottest in the engine. The only way the exhaust
valves can dump heat, (And when the exhaust gasses are flowing past them
they are in the region of 1000*C, very close to the temperature of an oxy
flame.) is when they are closed, through the seats, and through the
interface betwixt valvestem and valve guide. The use of the oil cooling
galleries around the seats is neccessary to allow the fitment of the very
light valves with very thin stems used in the 8V motor. Without that cooling
a much heavier valve would be needed.

The problem with using a mineral oil is that in the extreme conditions it
will meet within those galleries, (And beneath the piston crowns.) it will
be more likely to form deposits of carbon and varnish as it oxidises and
these can EVENTUALLY block the galleries sufficiently to hinder the flow of
cooling oil. The result you would expect to see in these circumstances would
be the head dropping off an exhaust valve BUT these deposits would take a
considerable period of time to build up. I certainly wouldn't expect, even
with the cheapest and nastiest oil available, that it would become a problem
within the first few thousand Km.

As far as we so far know your bike has not dropped a valve. While until it
is stripped we won't really have a clue as to what has happened it appears
to have run its big ends and mains and locked up the bottom end, (And
incidentally, any engine that has had a seizure of any part will have
shockingly dirty oil, no matter what quality it was to start off with so the
state of your oil is unsurprising.) To look at why it might of done that,
and why it too is unlikely to be related to the TYPE of oil used we need to
look at hydrodynamic, ('Plain') bearings and how they work and why they fail
and that will be the subject of the next e-mail.

I do though strongly caution you against going in to the shop with all guns
blazing to make an issue of the oil choice at this point. While it may well
help you in any claim you may have to make regarding the failure alienating
the shop owner and his staff will be counter-productive and ultimately
fruitless. If we all try to work together on this there will be a MUCH
greater chance of a happy resolution to the problem than if it turns into a
shitfight!

***********************************************************************************

OK, to continue on the subject of plain bearings.

The way a 'Plain' or Hydrodynamic bearing works is both simple but at the
same time complex. You essentially have a shaft, the 'Journal' that spins
within the bearing. In the case of the main bearings the shaft spins within
the bearing but in the case of the big ends of the connecting rods it is
more as if the barings are rotating around the jurnal. It makes no odds, the
principle is the same. The reason why the bearing matrial is usually made of
something 'Softer' than the journal is simply to make one of the two parts
sacrificial. If you are lucky, if the bearing fails, the journal will be
recoverable. There is nothing 'Magical' about the material out of which they
are made.

The 'Magic' comes from how they work!

Any plain bearing needs a constant feed of oil under pressure from an
external source. Usually this oil is fed through galleries in either the
bearing, (In the case of the main bearings or cam bearings.) or through a
gallery in the centre of the journal and drillings to the bearing faces, (In
the case of the big ends.). This ensures that the bearing surface and the
journal are kept apart. In service they should NEVER touch, being kept
separated by a thin film of oil. BUT... The pressure of the oil delivered to
the bearings by the oil pump is a.) limited, (In the case of the Guzzi motor
to about 60PSI) and b.) of insufficient pressure to actually keep the
journal and bearing separated due to the uneven nature of the forces exerted
on the bearing. In the case of the connecting rods not only is there the
pressure of the force being exerted onto the crankpin when the power stroke
is taking place but every 180 degrees of crank rotation the whole rod
assembley has to stop dead and then start accellerating in the other
direction. At 7,500 RPM that is happening 15,000 times a minute! The crank
also has significant forces imposed upon it from various different
directions as it spins, as do camshafts.

The clever thing is that as the journal spins within the bearing it produces
a rolling pressure wave front within the thin film of oil that due to the
laws of physics will produce a large increase in the oil pressure and
therefore the strength of the protective film RIGHT AT THE POINT IN THE
BEARING WHERE THE MAXIMUM PRESSURE IS BEING APPLIED TO IT! Brilliant eh?!!!

The problem is that for this wonderful protective wavefront which is called
a 'Hydro-dynamic wedge' to form there are several very important 'rules'
that have to be observed.

Firstly both the journal and the bearing have to be perfectly round. Any
ovality in either will hinder the formation of the wedge weakening the
bearing's ability to cope with load..

Secondly the clearance between the journal and the bearing is absolutely
critical. As a 'Rule of Thumb' you can say that a conventional 'Plain'
bearing will need about one thousandth of an inch clearance for every inch
of journal diameter BUT, the tighter you can make it the GREATER the
strength of the wedge. But this is where you run into the 'other' problem.

While liquids are, to all intents and purposes, in-compressible, (This is
why hydraulic systems like brakes work.) when you get down to the very small
thicknesses of the oil film in a hydrodynamic bearing with the forces being
applied to it stuff starts to happen at a mollecular level and the oil
mollecules are pushed more tightly together and they, quite simply, rub
together and the internal friction of the oil causes it to heat up. Not only
does it heat up quickly it will also heat up a lot! In fact if it isn't
moved on quick-smart it will start to burn and melt the bearing.

So as well as needing to be small enough to allow the formation of a good,
strong, hydrodynamic wedge the clearance in the bearing between journal and
bearing surface has to be large enough to allow sufficient through-flow of
oil to keep the bearing COOL!

So in fact the clearance is a juggling act! In any engine you will find that
there is not A specific clearance given but a range of sizes between a
minimum and a maximum. this is known as the 'Tollerance' of the bearing and
it is the range of size that will allow the formation of a strong enough
wedge while at the same time allowing sufficient throughput of oil to keep
the bearing cool.

The problem is that if for any reason oil pressure is lost, even for an
instant, damage starts immediately and very, very rapidly. A plain bearing
CANNOT run without oil in sufficient quantity or under sufficient pressure.
If delivery fails for whatever reason? So does the bearing. By the time your
oil pressure warning light comes on? At anything other than idle and you
catch it immediately? It's all-over-red-rover. the bearings will be deader
than the dinosaurs!

As long as pressure and volume of clean oil is kept up to a plain bearing it
will NEVER wear out because the journal and the bearing surface in a
correctly tolleranced and assembled one will NEVER TOUCH, (A condition known
as 'Boundary Lubrication'.) 95% of ALL bearing wear will occur in the first
couple of minutes after start up before the oil has reached operating
temperature and 90% of that wear occurs in the critical moments at start up
before the oil pump has a chance to pressurise the oilways and galleries in
the engine on the way to the bearings.

OK, so your motor isn't apart yet. But you DO know that the oil was very
black, dirty and smelly. The heat created by the melting bearings will of
done that if, as I suspect, it has run its mains and big ends. It is almost
always the big ends that will go first as they are the most heavily loaded.
What we DO know is that the motor has had a catastrophic failure. The
challenge is to find out WHY that failure has occured.

Once one of the bearings has lost its tollerance, (By melting.) it no longer
acts as an 'Inhibitor' to oil flow from its feed gallery. Like any fluid
under pressure the oil will seek the path of least resistance and gush out
of the feed gallery of the damaged bearing and substantially lowering the
delivery pressure to ALL the remaining bearings fed by the low volume/high
pressure circuit. This will start a viscious chain reaction in the other
bearings until something seizes up. As you found out, this only takes
seconds and once the damage is done it cannot be 'Un-Done'.

My 'Guess', and that is all it is. I haven't got the motor. Is that
something has caused a sudden, catastrophic loss of oil pressure. The most
likely culprits are going to be the oil pressure reliefvalve jamming open or
falling out, the thermostat for the oil cooler, (If there is one? I can't
remember?) failing for some reason or some of the internal plumbing in the
sump coming loose and dumping pressure. OR it could be an oil pump failure.
Either the gear has come loose/off or one of the pump rotors or vanes has
failed.

the fact that the engine ran fine for several thousand Km would indicate
that prior to the failure the bearing clearances and tollerances were good.
As I sated before, plain bearings either work or they don't. If it was going
to blow up because of an issue with a bearing it would of happened within
seconds of the engine being first started so we can rule that out.

While it has been running with a mineral oil, as I have expalined this is
something that is most likely to cause long term problems. NOT a
catastrophic failure of this type. While analysing totally fucked bearings
is always fruitless understanding what causes them to fail and then looking
for a cause is the way to set about chasing down the problem and skewering
it.

Regardless of anything else though the motor has to come out and be taken to
bits to the last nut and bolt. I'll wager that there will be sufficient
damage to enough major components for the cheapest and simplest sollution to
be a full engine replacement. Providing the warranty hasn't been invalidated
by fitment of non-approved accessories I would exspect that as soon as pics
of the damage have been submitted and a cause found warranty approval should
be fairly straightforward BUT I'm not the warranty manager and know nothing
of how the system works in NZ.

I hope that this has been helpfull and informative for you.

Pete
 
Pete, good to see this post. Until very recently I was in the lubes industry and you've pretty well nailed it. There's lots of mistruth/untruth on the internet so it's good to see this.
 
I'm a mechanic. It's my JOB to know this shit. Problem is there are many people who claim to be mechanics, (Or better yet, 'Technicians', whatever the fuck they are.) who don't.

pete
 
One fellow living not far from here had engine failure happening to his Guzzi caused by loss of oil pressure due to an uncsrewing oil filter. Pete's Engine Strip series shows how the same had happened to an engine due to an unscrewing pressure relief valve.

I've already started securing the oil filter with a hose clamp, racing style even though the filter sits inside the sump (ol'tonti). Is the next measure to uncrew and secure the relief valve with the toughest loctite in the shop as an update? When such small details can ruin an entire engine these measures should perhaps be the way to go at the first opportunity when the service schedule warrants opening of the sump?
 
Pete, that’s a very logical analysis to the problem based on very limited feedbacks and zero hands on.

Your plain bearing explanation is very educational, as interesting as what came out of Kevin Cameron, if not better.

Thanks for sharing.

Phang
 
Pete,
Brilliant!!!!
I’m glad to see there’s still mechanics out there are taking their jobs seriously and have the capability of determining the root cause.
Thank you for sharing this excellent mechanical failure analysis.

Cheers
 
Hey Pete,

Nicely put.

One of the reasons to read / support this site is 'cause one can learn stuff here... I do.

Thanks,

Alex
 
Thanks for that wisdom Pete, very informative.

I :woohoo: Idea of the day:

If one was to add a little electric oil pump that started as soon as the ignition was turned on and pre-pressurised the oil system.... one would theoretically have created a motor with eternal life since there would not be the adverse lack of oil at start up. Maybe same could also be achieved with a little pressure vessel with a bladder to hold a bit of pressure (when the engine is running and a solenoid to release a burst of oil when the starter button is pressed. (This would only help if the engine fired first time which my Bellagio always does!)

Manufacturers would never do this since they wouldn't sell any spare parts at the usual exhorbitant prices.
 
There are engine pre lubrication kits made for car. It stores the pressurized engine oil in a canister and control by a solenoid valve. Shouldn’t be too difficult to implement it on motorcycle :mrgreen:

4x10-b_uzcf.jpg
 
Worth considering. It's hard to come up with an original idea!

Graeme
Bellagio's are best!
 
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