928 Flexplate Migration

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928 Flexplate Migration
I believe that setting the clamp on the front flexplate with any preload or clearance etc was superceded by Porsche technical bulletin number 9023 dated May 5th 1992 relating to installation procedure of the central tube.
Under repair information within this technical bulletin it states: "After the cnetral tube is bolted to the forward housing, first tighten the six (6) drive plate bolts (A in Figure 1 [which refers to the flexplate bolts to the flywheel] to 32-39 Nm (23-28ft lbs). Then tighten the clamping screw (B in Figure 1 {which refers to the clamp set bolt] to 75 to 85 Nm (54-62 ft lbs). following this procedure will ensure necessary running clearances for the crankshaft thrust bearings".
There is no mention of setting any pre load forward or aft on the thrust bearing via the flexplate, so one can assume that this has become redundant?
In tightenting the clamping screw by 10% over the recommended tension would, under the law of physics, increase the frictional resistance between the clamp and the spline of the drive shaft, that is if the elastic limit of the clamp or bolt is not exceeded.
Even increasing the tension of the clamp bolt did not stop the migration of the flexplate clamp along the spline of the drive shaft on my car.
As Constantine pointed out there are 3 methods in trying to improve the clamping the flexplate clamp to the drive shaft:

1. Increase the torque on the clamp screw by 10%

2. Using Locktite

3. Using his redesigned clamp.

Now 4 with the PK clamp.

1. Does not appear to work in my case as the clamp still migrated forward along the spline, which would indicate insufficient frictional resistance can be imparted between the clamp and the spline to overcome the axial forces involved that caused the migration.

2. By using loctite. I have used loctite 290 and currently after 12 months use, and checking, no movement, so it would appear that the locking effect of the loctite has secured the clamp in place on the drive shaft spline so that the axial forces are now being absorbed by the flexplate thereby only imposing cyclic axial forces on the thrust bearing. Loctite's product description sheet states that if there is difficulty removing apply localised heat to 250 degrees C and remove when hot. Brake away torque on a M10 steel bolt and nut tensioned to 5Nm is 30Nm, so it looks like the loctite works.

3. By using Constantine's collar clamp. As he stated no migration has been experienced. This definately looks the best fix and is a great engineering design.

With regards to whether the crankshaft is set forward or aft on the thrust bearing faces is still a matter of conjecture, as there is no definative reason that has been given that all can agree on.
I can bring to mind three reasons given for the excessive loading on the flexplate/thrust bearing:

1. Torque converter balooning

2. Drive shaft length increase, and

3. Torque twist up of drive shaft reducing it length momentarily during hard acceleration causing a incremental migration of the drive shaft out of the clamp in the aft direction.

I personally subscribe to the 3 point based on my engineering experience.
Whatever the reason the clamp migrates along the spline of the drive shaft in the forward direction. This migration can ultimately cause excessive continuous load on the aft thrust bearing face via the flexplate on the automatic transmission cars. This I believe has been proven by the simple paint method of monitoring which has verified this forward migration of the clamp along the spline of the drive shaft, for whatever reason.
By returning the flexplate to its neutral positon by releasing the clamp bolt and then reclamping basically returns the constant axial forces acting upon the thrust bearing to cyclic axial loading only as designed.
This cyclic loading only occasions in the manual transmission models due to the operation of the clutch as explained above.
The reason for the thrust bearing failure when the constant load imparted by the flexplate due to clamp migration can be explained by tribology, that is the study of the science of friction, lubrication and wear.
Tails 1990 928 S4 Auto.

Whilst you are naval gazing relating to whether the crankshaft should be pushed forward against the aft end of the thrust bearing or pulled aft against the forward end of the thrust bearing before re-clamping you and the other learned folk may like to consider the following:

Do we agree that when the clamp bolt is released, the flexplate clamp moves aft along the spline of the drive shaft to its neutral position?
If we agree that yes this is what actually happens, then this means that the drive shaft (under load) is pulled aft out of the flexplate clamp. I would suggest this migration is incremental as I have measured my migration at different time and it had not returned to the original 3mm mark, as I kept releasing it and letting it return to its neutral position before it got there.
This incremental migration occasions when the axial aft drawing force of the drive shaft (say by torsional twisting/wind up or whatever) exceeds the opposite forward acting spring loaded force exerted by the flexplate and this force draws the clamp along with it when the frictional resistance between the flexplate clamp and the spline of the drive shaft is exceeded. This is why I suggest that the an incremental movement takes place.
This process continues until the flexplate clamp reaches around 3mm at which time the clamp would actually go into a small forward and aft oscellation movement on the spline as the frictional resistance of the clamp is exceeded in both directions.
I believe that this small oscellation movement start to occasion when the flexplate clamp has moved to its maximum extent on each late model automatic car (ever car actions and reactions are different, so movement of the clamp will not be exactly the same). Under heavy load conditions when maximum torque is reached on the drive shaft and is then removed, say when the car has reached it cruise speed, the reaultant force exerted by the flexplate in the forward direction is countered by the force exerted by the aft side of the thrust bearing. When this force exceeds the frictional resistance of the clamp, the clamp will move slightly aft ( this is Newtons 2nd Law of motion, "ever action has an equal and opposite reaction"), however, there will always be a resultant forward axial load/force, which is equal to the frictional resistance imparted by the clamp on the spline when the clamp has reached its maximum movement.
When the clamp is secured either by lictite or Constantine's modified clamp in the neutral position or by some other method, then the flexplate will oscellate as designed and will cause the thrust bearing to be loaded and unloaded against its forward and aft faces, but will never have the resultant constant force applied against the aft face of the thrust bearing.
If this clamp migration resultant constant force is not removed by releasing the clamp and firmly locking it in the neutral position it can cause TB failure, as we have seen or heard of .
This TB failure is caused by the breakdown of the oil film between the faces of the thrust bearing and the crankshaft thrust collar to boundry lubrication which causes friction by asperity contact. This micoscopic frictional contact generates heat and the oil film will become less and the bearing subsequently enters into a degenerative spiral with eventual total thrust bearing failure and engine failure.
Now the question to contemplate is why do some thrust bearing fail and some don't?
Constantine drew our attention to the web site which explained the correct fitting of the thrust bearing in its pocket to ensure that there is full contact with the thrust bearing faces to the crankshaft thrust collar and also the quality of the surface finish of the crankshaft thrust collar.
Unfortunately the thrust bearing fitted to engines of automobiles is the most basic type of thrust bearing and it is not designed to take a constant load. Thrust bearing which have a constant load like in the marine field are usually fitted with a thrust collar on the propeller shaft and the thrust bearing is made up of tilting bearing pads. When the shaft turns the pads tilt in the rotating direction and an oil wedge is formed between the collar and the tilting pad which ensures that the oil film is maintained and no boundry lubrication is possible. Heat is still generated in this type of bearing, however on large vessel the bearings are water cooled.
The reason for TB failure, as I mention before, is covered by the study of Tribology and its science of friction, lubrication and wear, so this can equate to the bearing setup, the surface finish of the bearing faces, the oil used and its ability to maintain the oil film, the actual friction resistance of the clamp and the force of the subsequent constant load on the thrust bearing, its friction etc.
Bill, I hope that you have given due consideration to the question posed at the beginning of this thread, so if we agree that when when the clamp is released and the flexplate return to its neutral position and subsequently commences migrating in a forward direction I would suggest that you would get around an 0.008 inch gain by moving the crankshaft forward against the aft end of the

TB in the cold static flexplate neutral position.

Tails 1990 928 S4 Auto

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