TBF - Whoa There!
Thrust Bearing Failure - Torque Tube
Update from Paul Jaeger
From Glenn Evans;
Bruce, Peter, Jes and others! The 928 engine seems to be subject to two totally unrelated types of bearing
failure. The first is failure of the crankshaft bearings due to oil starvation, as
happened to Charles' GT.
There are two factors which, separately or together, cause the oil starvation. The shameful one, for Porsche, is that
the design of the crankshaft lubrication was FUBAR'ed (F*cked Up Beyond All Recognition). The delivery of oil to the crankshaft journals in the 4.7
and later engines works against centrifugal force, so the faster the engine revs, the more difficulty the oil has getting to the journals - not what you
want. If you should happen to rev your engine to 7200 rpm for more than a few seconds, the bearings will run dry and fail.
Mark Anderson (928 Specialists; 500 hp 928 racing car) destroyed eight engines (from memory)
before he realised, or was told, that the oil galleries had to be redrilled and reoriented to assist the delivery of oil to the crankshaft journals as
RPM increases.
Devek and other aftermarket crankshafts have this modification. Apparently, the oil galleries in the 4.5 litre engine's
crankshaft are oriented at 90 degrees to what they should be, so the oil is not being fed against centrifugal force but neither is it being assisted, as
ideally it should be. (Don't ask me for more detail; I've never seen a naked 928 crankshaft, or even a diagram of the problem.)
The second cause of oil starvation occurs when cornering hard. Centrifugal (cornering) force causes the oil to be thrown up against the side of the
sump; if the force is enough, the oil pickup is uncovered and sucks air, which is not up to the job of lubricating the crankshaft bearings, and they
fail. Charles' failure occurred above 6000 rpm, while he was cornering hard, so both oil starvation factors came into play. The GTS engine had
extra baffles in the sump to reduce the likelihood of oil starvation when cornering but, if the car is going to be raced seriously, the best remedy is
to dry sump the engine (the oil reservoir is a tank, not the sump, designed so that the oil pickup cannot possibly be exposed.
911's have dry sumps. 996's do not, and some have suffered bearing failures when they are competed
on the track with fat tyres, which enable cornering forces high enough to cause oil starvation to be achieved.
The bearing failure which only recently has come to our attention is the thrust bearing failure.
The thrust bearing is in the middle of the crankshaft and its purpose is to carry any fore/aft crankshaft loads
(forces) ie this part of the crankshaft and the part of the crankcase which carries it are designed to be able to push sideways against each other, whereas the other main bearings are designed to carry only loads caused
directly by the rotation of the engine. Normally, any fore/aft loads are intermittent and (I would guess) not high. The thrust bearing would not be
designed to carry a constant load.
The driveshaft (inside the torque tube) in automatics is bolted to the flywheel via the "flex plate". I assume that the flex plate is designed to
cushion fore/aft loads.. (In a manual, the front end of the driveshaft is splined, which allows fore and aft movement without transmitting loads directly from the drivetrain to the crankshaft (I hope!). For the
engineering pedants, some load will be transferred via friction of the splines while any movement is being taken up.)
The problem appears to occur when there is a preload on the thrust bearing from the drivetrain. My earlier post contained James Morrison's theory
that this is caused by incorrect installation of the automatic transmission ie the transmission and drivetrain are installed so that they push constantly against the flex plate, which places a constant load on the
thrust bearing. This may occur if the crankshaft endplay (fore/aft movement) is not checked when the transmission is installed.
James Morrison mentions the torque converter expanding (in the fore/aft direction) under load. Possible; I don't really know. Over to Eddy.
Paul Jager, an engineer ("Jager Engineering Group"), states on his web page http://www.jageng.com/auto_1.htm
that he thinks that:
"... For axial loading to occur the central shaft must somehow "grow" by a mm or
more in length, whilst being connected at each end. This could actually happen as the central shaft is severely flexed, or twisted. Each time the
shaft twists, the metal does not return to its original state and "necks" or deforms causing an increase in length. This might happen for defective or
abused shafts, ones that have suffered repeated driveline shocks. Perhaps only for S4 cars or better as they are capable of increased torque. We doubt
this would happen on earlier models ..."
I cannot accept that the bit about the "central shaft" (driveshaft carried inside the torque tube) growing is anything but a CROCK OF SHIT!
For steel to be permanently deformed/stretched/twisted, it must be loaded beyond a known limit. Engineers simply do not design components to have to carry
these sort of loads; any conceivable load will be well under this known limit. Below this limit, steel is elastic; load it and it will stretch,
compress or twist, depending on the type of load; release the load and it will return to its original length and shape.
Barring gross incompetence on the part of the design engineer (and he probably would be told by his
draftsmen and tradesmen that the part is too small for the load), the strength limits of materials are
approached only in aircraft, racing vehicle and spacecraft design (and even then, a factor of safety is used in
strength calculations).
"Necking" is the term used to describe the permanent narrowing of the component when it is grossly overloaded by stretching (think of how a piece
of plasticine narrows when you pull it apart). There is just no way that a driveshaft would "neck" due to any torsional (twisting) load which it would
experience.
Perhaps Porsche contributed to the thrust bearing problem with some design change in later cars, as Jager suggests. One would hope that no car left
the factory with insufficient crankshaft endplay. I suspect, though, that we are seeing another manifestation of ignorant spanner-wielders being let
loose on our cars.
Glenn Evans,
'80 928 Petrolblaumetallic 'S' look 5 speed
Update from Paul Jager
We had the flexing part right, it’s just that the drive shaft pulls out of the collar, and not the shaft itself getting longer – although it certainly
bends up. Doesn’t really matter in the end anyway, the remedy is the same.
We were the first to explain and offer a solution to the Rennlist group, and saved a few bearings.
So it’s nothing to do with settings at the factory or anything Porsche did, other than the connection is undersized for the driveline loads that
occur. Drive shafts do neck, fatigue, bend and break. A ski club busted their drive shaft on their Ski
Nautique, same conditions as the
928.
We shall post an update in our manual for download.
G’day & Cheers, Paul Jager, P. Eng.
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