Things everyone should know about 928 Engine Thrust Bearing Failure
05-31-2010, 08:27 AM
#46
Rennlist Member
I'm not an expert, just a DIY guy, but I installed one of the Superclamps and highly recommend you do-so on the new motor. I had the TT out and wanted the consensus best solution for the reinstall. As you can see in the picture above, when you tighten those allen bolts, that baby locks-down on the shaft. Porsche advised increasing the torque on their arrangement as a service change to increase the pinch. Not enough perhaps. This Superclamp achieves the objective absolutely.
Its running now in an 84 car mated to an 89 S4 driveline transmission.
Its running now in an 84 car mated to an 89 S4 driveline transmission.
05-31-2010, 09:37 PM
#47
One of the problems with the original clamp fitted to S4 Autos is wear. When drive shaft starts to migrate, as the drawout force exceeding the frictional resistance of the clamp and the forward flexplate spring force, it also start to wear the splines inside the clamp and on the end of the drive shaft.
By releasing the clamp and allowing the flexplate to return to its neutral position and reclamping and even increasing torque on by bolt by 10% the clamp will quite possibly continue to migrate until it reaches it maximum migrating position, where it will remain and applying force onto the rear face of the thrust beaing which can cause failure (reason for failure is the engineering art of tribology). It has been noted that the migration distance is not constant between all cars, but it can end up around 3mm.
When the clamp reaches it maximum migration it start to oscilliate around this position, as the frictional resistance between the clamp splines and the drive shaft splines is continuously overcome by the reduction in length of the drive shaft by the angle of twist of the shaft due to the increased torque during acceleration, which pulls the drive shaft aft through of the clamp (by small increments) and when the car has obtained it constant speed the shaft will "unwind" to it original length it will load up the thrust bearing until the frictional force is overcome and this will push the drive shaft back slightly through the clamp. At the stage of maximum migration the flexplate is always bowed out in the aft direction and the oscillation movement commences with acceleration and cruise speed and wear starts or continues, between the clamp splines and the splines on the drive shaft. (Newtons second law of motion, for every action there and equal and contary reaction).
The super clamp will exert a frictional force that far exceed the withdrawal force caused by the reduction of the length of the drive and the forward force applied to the clamp by the flexplate. so the flexplate will now flex as designed and it will stay in the neutral position. When this happens (the flexplate stays in its at rest neutral position there is less load on the thrust bearing than what is exerted with the manual cars with the load caused by the throw out of the cluth).
Locktite will prevent movement as it will fill the worn sections of the spline and lock the flexplate into the neutral position. However, this could cause the DIYer difficulty if the TT ever has to be removed as heat will need to be applied to break this bond.
The PKlamp will also provide sufficient force to increase the frictional resistance between the clamped area on the drive shaft, if the wear is not too great in way of the original clamp. the big unknown here is the extent of wear in the spline area.
The choice of which method to clamp is " the call of the user" on what system if one is installed.
As previously stated my order of preference is:
1. Super Clamp
2. Pklamp
3. Locktite
However, I would still undertake a yealy check for any migration.
I used locktite year ago and there is no migration, but there is a little difficulty in measuring end play of the crankshaft, but I can live with this.
Another aspect I will throw into the ring is the viscosity of the oil used in the engine, when changing to a lower viscosity of oil outside the recommendations of Porsche, is there sufficient oil supplied that can over come the forces applied to the aft side of the TB to preculude boundry lubrication and failure in a degenerative spiril?
Tails 1990 928S4 Auto
By releasing the clamp and allowing the flexplate to return to its neutral position and reclamping and even increasing torque on by bolt by 10% the clamp will quite possibly continue to migrate until it reaches it maximum migrating position, where it will remain and applying force onto the rear face of the thrust beaing which can cause failure (reason for failure is the engineering art of tribology). It has been noted that the migration distance is not constant between all cars, but it can end up around 3mm.
When the clamp reaches it maximum migration it start to oscilliate around this position, as the frictional resistance between the clamp splines and the drive shaft splines is continuously overcome by the reduction in length of the drive shaft by the angle of twist of the shaft due to the increased torque during acceleration, which pulls the drive shaft aft through of the clamp (by small increments) and when the car has obtained it constant speed the shaft will "unwind" to it original length it will load up the thrust bearing until the frictional force is overcome and this will push the drive shaft back slightly through the clamp. At the stage of maximum migration the flexplate is always bowed out in the aft direction and the oscillation movement commences with acceleration and cruise speed and wear starts or continues, between the clamp splines and the splines on the drive shaft. (Newtons second law of motion, for every action there and equal and contary reaction).
The super clamp will exert a frictional force that far exceed the withdrawal force caused by the reduction of the length of the drive and the forward force applied to the clamp by the flexplate. so the flexplate will now flex as designed and it will stay in the neutral position. When this happens (the flexplate stays in its at rest neutral position there is less load on the thrust bearing than what is exerted with the manual cars with the load caused by the throw out of the cluth).
Locktite will prevent movement as it will fill the worn sections of the spline and lock the flexplate into the neutral position. However, this could cause the DIYer difficulty if the TT ever has to be removed as heat will need to be applied to break this bond.
The PKlamp will also provide sufficient force to increase the frictional resistance between the clamped area on the drive shaft, if the wear is not too great in way of the original clamp. the big unknown here is the extent of wear in the spline area.
The choice of which method to clamp is " the call of the user" on what system if one is installed.
As previously stated my order of preference is:
1. Super Clamp
2. Pklamp
3. Locktite
However, I would still undertake a yealy check for any migration.
I used locktite year ago and there is no migration, but there is a little difficulty in measuring end play of the crankshaft, but I can live with this.
Another aspect I will throw into the ring is the viscosity of the oil used in the engine, when changing to a lower viscosity of oil outside the recommendations of Porsche, is there sufficient oil supplied that can over come the forces applied to the aft side of the TB to preculude boundry lubrication and failure in a degenerative spiril?
Tails 1990 928S4 Auto
06-01-2010, 06:51 AM
#48
Burning Brakes
Perhaps a bit of a silly question; but one factor I never see in the TBF discussions is driving style.
If I understand correctly the sliding of the shaft is caused by the engines torque, which explains why the more powerful and torqier models suffer more frequently from TBF.
If this is the case, it seems to me that cars which are driven hard should be more likely to suffer from TBF than cars which are driving around town cruising around?
Off course I know it is a Porsche which should always be driven hard
, but I was just wondering.
I released tension on the front clamp of my 86.5 immediately after I bought the car, but noticed no pre-tesnion. Will check it again this weekend.
As I understand the 86.5 also has relatively lower Power/torque output, could this explain why my car does not seem to have pre-tension problems??
If I understand correctly the sliding of the shaft is caused by the engines torque, which explains why the more powerful and torqier models suffer more frequently from TBF.
If this is the case, it seems to me that cars which are driven hard should be more likely to suffer from TBF than cars which are driving around town cruising around?
Off course I know it is a Porsche which should always be driven hard
, but I was just wondering.I released tension on the front clamp of my 86.5 immediately after I bought the car, but noticed no pre-tesnion. Will check it again this weekend.
As I understand the 86.5 also has relatively lower Power/torque output, could this explain why my car does not seem to have pre-tension problems??
06-01-2010, 07:12 AM
#49
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Driving style does have an effect on the drive shaft moving from the front coupler.
The 1986.5 models have suffecient torque to have this issue come up.
Most cars have pre-load some do not, just keep an eye on it for any forward bow of the flex plates in the future.
One thing you should do soon is get a base line of your crack end play established for future reference.
Cheers,
The 1986.5 models have suffecient torque to have this issue come up.
Most cars have pre-load some do not, just keep an eye on it for any forward bow of the flex plates in the future.
One thing you should do soon is get a base line of your crack end play established for future reference.
Cheers,
06-01-2010, 09:20 AM
#50
During hard acceleration from rest or when cruising along, Newton's 1st Law of motion comes into effect.
First Law: Every body will persist in its state of rest or of uniform motion (constant velocity) in a straight line unless it is compelled to change that state by forces impressed on it.
Basically this means when you floor the accelerator the car will continue in its state of rest or at the same speed until the torque imparted into the drive shaft,( measured by the angle of twist of the drive shaft) reaches the force required to overcome the moment of inertia and the sheering of the oil in the torque converter to get the car moving and accelerate or just accelerate say in passing another car.
This is one of the reasons that the stall speed of the torque converters are tested to ensure they are functions efficiently to ensure we get the best acceleration if required as our cars are designed for spirited driving.
In very large steam ships they measure the angle of twist of a section the propeller line shaft to determine the horsepower developed by the main turbine engines. In diesel engine ships they calculate the horsepower by taking indicator card of the pressures direct from the combustion chamber of the engine. Today this is done by a pressure sensor via computer program which calculates the horsepower which is displayed in real time. Some ships develop over 70,000 HP, so twistup and reduction in length of the shaft is well known.
Mr Newton in his 3rd law said that for every action there is an equal and oposite reaction, so as the car accelerates very hard the drive shaft twists up to the level of horse power being developed by the engine and at the same time it tries to untwist, so when the car reaches it speed the drive shaft will have returned to it normal say cruise position with a small angle of twist to maintaine this speed. However the car speed is never fully uniform, so the drive shaft will twist up and untwist continuously in small quantities, so the flexplate moves backwards and forwards slightly. Engine breaking will cause the twistup to be in the opposite direction.
When the car reaches it state of uniform motion or cruise speed there will always be some angle of twist inparted into the drive shaft to overcome the friction of the drive chain components. This is the reason the motor always produces more horsepower at the flywheel than at the drive wheels.
So hard acceleration causes maximum twist up, but when you install a super or turbo charger the angle of twist will be greater, so in the case of the S4 and GTS auto the increments in the migration, due to the inability of the flexplate clamp to impart sufficient frictional force that is greater then the drive shaft withdrawl force, will be proportional to the quantity of HP being thrown at the drive wheels.
Driving style definately has a bearing on the time it take for the migration of the flexplate clamp to reach its maximum forward migration position.
This is a simple explanation of very complex physis and in itself it is a very complex expanation to Rexpontius's question.
One could ask why did the Porsche design people ever remove the circlip at the front end of the drive shaft.
P.S. In my previous post it is Newtons 3rd law of motion not his 2nd law.
Tails 1990 928S4 Auto
First Law: Every body will persist in its state of rest or of uniform motion (constant velocity) in a straight line unless it is compelled to change that state by forces impressed on it.
Basically this means when you floor the accelerator the car will continue in its state of rest or at the same speed until the torque imparted into the drive shaft,( measured by the angle of twist of the drive shaft) reaches the force required to overcome the moment of inertia and the sheering of the oil in the torque converter to get the car moving and accelerate or just accelerate say in passing another car.
This is one of the reasons that the stall speed of the torque converters are tested to ensure they are functions efficiently to ensure we get the best acceleration if required as our cars are designed for spirited driving.
In very large steam ships they measure the angle of twist of a section the propeller line shaft to determine the horsepower developed by the main turbine engines. In diesel engine ships they calculate the horsepower by taking indicator card of the pressures direct from the combustion chamber of the engine. Today this is done by a pressure sensor via computer program which calculates the horsepower which is displayed in real time. Some ships develop over 70,000 HP, so twistup and reduction in length of the shaft is well known.
Mr Newton in his 3rd law said that for every action there is an equal and oposite reaction, so as the car accelerates very hard the drive shaft twists up to the level of horse power being developed by the engine and at the same time it tries to untwist, so when the car reaches it speed the drive shaft will have returned to it normal say cruise position with a small angle of twist to maintaine this speed. However the car speed is never fully uniform, so the drive shaft will twist up and untwist continuously in small quantities, so the flexplate moves backwards and forwards slightly. Engine breaking will cause the twistup to be in the opposite direction.
When the car reaches it state of uniform motion or cruise speed there will always be some angle of twist inparted into the drive shaft to overcome the friction of the drive chain components. This is the reason the motor always produces more horsepower at the flywheel than at the drive wheels.
So hard acceleration causes maximum twist up, but when you install a super or turbo charger the angle of twist will be greater, so in the case of the S4 and GTS auto the increments in the migration, due to the inability of the flexplate clamp to impart sufficient frictional force that is greater then the drive shaft withdrawl force, will be proportional to the quantity of HP being thrown at the drive wheels.
Driving style definately has a bearing on the time it take for the migration of the flexplate clamp to reach its maximum forward migration position.
This is a simple explanation of very complex physis and in itself it is a very complex expanation to Rexpontius's question.
One could ask why did the Porsche design people ever remove the circlip at the front end of the drive shaft.
P.S. In my previous post it is Newtons 3rd law of motion not his 2nd law.
Tails 1990 928S4 Auto
06-01-2010, 09:41 AM
#51
Addict
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Mechanics didn't do very good job when they installed flex plates with shims. They installed flex plates with wrong preload. This caused TBF cases even though shims were used. Which in turn caused warranty claims for Porsche while real cause was just that WSM instructions were not followed. To make installation easier for their dealer mechanics Porsche simply decided to leave shims out. Which in turn caused many more TBF cases than before '85 MY. Porsches view is that leaving shims out will not cause problems. They didn't even saw any need for possibility of using shims when TT was "upgraded" to 28mm shaft in '88 MY.
06-01-2010, 11:39 AM
#52
Rennlist Member
During hard acceleration from rest or when cruising along, Newton's 1st Law of motion comes into effect.
First Law: Every body will persist in its state of rest or of uniform motion (constant velocity) in a straight line unless it is compelled to change that state by forces impressed on it.
Basically this means when you floor the accelerator the car will continue in its state of rest or at the same speed until the torque imparted into the drive shaft,( measured by the angle of twist of the drive shaft) reaches the force required to overcome the moment of inertia and the sheering of the oil in the torque converter to get the car moving and accelerate or just accelerate say in passing another car.
This is one of the reasons that the stall speed of the torque converters are tested to ensure they are functions efficiently to ensure we get the best acceleration if required as our cars are designed for spirited driving.
In very large steam ships they measure the angle of twist of a section the propeller line shaft to determine the horsepower developed by the main turbine engines. In diesel engine ships they calculate the horsepower by taking indicator card of the pressures direct from the combustion chamber of the engine. Today this is done by a pressure sensor via computer program which calculates the horsepower which is displayed in real time. Some ships develop over 70,000 HP, so twistup and reduction in length of the shaft is well known.
First Law: Every body will persist in its state of rest or of uniform motion (constant velocity) in a straight line unless it is compelled to change that state by forces impressed on it.
Basically this means when you floor the accelerator the car will continue in its state of rest or at the same speed until the torque imparted into the drive shaft,( measured by the angle of twist of the drive shaft) reaches the force required to overcome the moment of inertia and the sheering of the oil in the torque converter to get the car moving and accelerate or just accelerate say in passing another car.
This is one of the reasons that the stall speed of the torque converters are tested to ensure they are functions efficiently to ensure we get the best acceleration if required as our cars are designed for spirited driving.
In very large steam ships they measure the angle of twist of a section the propeller line shaft to determine the horsepower developed by the main turbine engines. In diesel engine ships they calculate the horsepower by taking indicator card of the pressures direct from the combustion chamber of the engine. Today this is done by a pressure sensor via computer program which calculates the horsepower which is displayed in real time. Some ships develop over 70,000 HP, so twistup and reduction in length of the shaft is well known.
If anyone can help direct me to this equation I would greatly appreciate it. I have a few theories of my own on flex plate coupler creep and would like to understand better how this shaft shortening comes into play.
Mr Newton in his 3rd law said that for every action there is an equal and oposite reaction, so as the car accelerates very hard the drive shaft twists up to the level of horse power being developed by the engine and at the same time it tries to untwist, so when the car reaches it speed the drive shaft will have returned to it normal say cruise position with a small angle of twist to maintaine this speed. However the car speed is never fully uniform, so the drive shaft will twist up and untwist continuously in small quantities, so the flexplate moves backwards and forwards slightly. Engine breaking will cause the twistup to be in the opposite direction.
When the car reaches it state of uniform motion or cruise speed there will always be some angle of twist inparted into the drive shaft to overcome the friction of the drive chain components. This is the reason the motor always produces more horsepower at the flywheel than at the drive wheels.
When the car reaches it state of uniform motion or cruise speed there will always be some angle of twist inparted into the drive shaft to overcome the friction of the drive chain components. This is the reason the motor always produces more horsepower at the flywheel than at the drive wheels.
So hard acceleration causes maximum twist up, but when you install a super or turbo charger the angle of twist will be greater, so in the case of the S4 and GTS auto the increments in the migration, due to the inability of the flexplate clamp to impart sufficient frictional force that is greater then the drive shaft withdrawl force, will be proportional to the quantity of HP being thrown at the drive wheels.
06-01-2010, 06:46 PM
#53
Rexpontius question is post # 48 above.
HP/PS etc is the power developed at the flywheel (in foot-pounds per second) and is usually measured by a dynometer, and the dynometer measures the brake force applied back to the motor that then displays the HP being developed, so HP is the force lever working on the drive shaft, that is torque. The torque transmitted into the drive shaft is not equal at both ends due to the frictional resistance of TT bearings, however the angle of twist of the drive shaft at verious HP is different, more HP developed the more angle of twist of the drive shaft.
HP produces the torque that twistup the shaft.
Tails 1990 928S4 auto
HP/PS etc is the power developed at the flywheel (in foot-pounds per second) and is usually measured by a dynometer, and the dynometer measures the brake force applied back to the motor that then displays the HP being developed, so HP is the force lever working on the drive shaft, that is torque. The torque transmitted into the drive shaft is not equal at both ends due to the frictional resistance of TT bearings, however the angle of twist of the drive shaft at verious HP is different, more HP developed the more angle of twist of the drive shaft.
HP produces the torque that twistup the shaft.
Tails 1990 928S4 auto
06-01-2010, 08:43 PM
#54
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I am wondering......are most TBF failures on cars without the larger size driveshaft. If the shaft size was increased, has the problem been overcome, and if so, are we really saying that only a few models of the S4 were the most vulnerable to TBF.
Could we all overcome the problem by fitting the larger driveshaft?
As stated by Villhuer ".......when TT was "upgraded" to 28mm shaft in '88 MY."
Could we all overcome the problem by fitting the larger driveshaft?
As stated by Villhuer ".......when TT was "upgraded" to 28mm shaft in '88 MY."
06-01-2010, 09:40 PM
#55
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I am wondering......are most TBF failures on cars without the larger size driveshaft. If the shaft size was increased, has the problem been overcome, and if so, are we really saying that only a few models of the S4 were the most vulnerable to TBF.
Could we all overcome the problem by fitting the larger driveshaft?
As stated by Villhuer ".......when TT was "upgraded" to 28mm shaft in '88 MY."
Could we all overcome the problem by fitting the larger driveshaft?
As stated by Villhuer ".......when TT was "upgraded" to 28mm shaft in '88 MY."
This increase in drive shaft diameter was Porsches answer to this whole pull out problem under the guise of doing so for the added torque of later S4 models. This reasoning at first seems plausible until you notice no such increase in drive shaft diameter was done for the 5 speed models.
We know of more than a handful of fat drive shafted 928s that suffered TBF.
Not too mention the introduction of drive shaft shear points at both of the necked down areas of the 28mm drive shaft where they reduce down to 25mm to fit the front and rear couplers.
Makes you wonder what Porsche was thinking...
Cheers,
06-01-2010, 09:51 PM
#56
Rennlist Member
Rexpontius question is post # 48 above.
HP/PS etc is the power developed at the flywheel (in foot-pounds per second) and is usually measured by a dynometer, and the dynometer measures the brake force applied back to the motor that then displays the HP being developed, so HP is the force lever working on the drive shaft, that is torque. The torque transmitted into the drive shaft is not equal at both ends due to the frictional resistance of TT bearings, however the angle of twist of the drive shaft at verious HP is different, more HP developed the more angle of twist of the drive shaft.
HP produces the torque that twistup the shaft.
Tails 1990 928S4 auto
HP/PS etc is the power developed at the flywheel (in foot-pounds per second) and is usually measured by a dynometer, and the dynometer measures the brake force applied back to the motor that then displays the HP being developed, so HP is the force lever working on the drive shaft, that is torque. The torque transmitted into the drive shaft is not equal at both ends due to the frictional resistance of TT bearings, however the angle of twist of the drive shaft at verious HP is different, more HP developed the more angle of twist of the drive shaft.
HP produces the torque that twistup the shaft.
Tails 1990 928S4 auto
Torque Angle =(TL)/(JG) EDIT twist angle is in radians to convert to deg use (TA radians)*360/(2*pi)=(TA deg)
r = radius
T = torque (N·m or ft·lbf).
L = length of the shaft.
G = shear modulus ~80GPa~11600ksi .
J = torsion constant for the section. for round centralizedaxis = pi/2 * r^4
I agree that the TTbearings will induce a small amount of drivetrain loss but is most likely negligable when compared with the losses associated with the TC, trans gear friction, trans bearings, viscous drag from trans oil, wheel bearings, CV joint friction, tire loss...
06-01-2010, 10:24 PM
#57
Rennlist Member
Just a reference piont I calculated the twist angle (TA) of the shaft using a few rough asumption, and here is what I found.
Data and assumptions
old style shaft D=25mm r=12.5mm~.492"
new style shaft D=28mm r=14mm~.551"
G=11600000 psi value for 4130 other steels are similar
J for round 25mm shaft = 0.09214 in^4
J for round 28mm shaft = 0.14498 in^4
I then assumed a peak torque of 1200 lbft I chose this number based on louis' ~600lbft and then multiplied by 2 to account for dynamic loads ie. inertia...
T=1200 lbft=14400 lbin
when using the equation in my previos post I got a TA of:
25mm shaft: 0.808 radians = 46.3 deg
28mm shaft: 0.531 radians = 29.4 deg
Data and assumptions
old style shaft D=25mm r=12.5mm~.492"
new style shaft D=28mm r=14mm~.551"
G=11600000 psi value for 4130 other steels are similar
J for round 25mm shaft = 0.09214 in^4
J for round 28mm shaft = 0.14498 in^4
I then assumed a peak torque of 1200 lbft I chose this number based on louis' ~600lbft and then multiplied by 2 to account for dynamic loads ie. inertia...
T=1200 lbft=14400 lbin
when using the equation in my previos post I got a TA of:
25mm shaft: 0.808 radians = 46.3 deg
28mm shaft: 0.531 radians = 29.4 deg
06-01-2010, 11:01 PM
#59
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Hi sharkies,
hi Constantine,
after spotting some TBFs I decided to buy Roger's PKLAMP - thought its a good solution, at least until I'll have a chance to switch to Constantine's SUPERKLAMP.
Anyway, the PKLAMP install instructions call for the flywheel to be levered away from the engine before tightening the original clamp and new PKlamp. This seems intuitively correct.
However, Porsche changed in the WSM that advice, perhaps because the drive shaft's twisting (and shortening) will cause a rearward pull on the flexplate.
I am concerned that if you pull the flywheel back as far as it will go as suggested before tightening up, the end thrust bearing will be in contact with it’s rearmost surfaces. Then driveshaft shortening will produce pressure on the bearing right from the beginning.
So, I assume it might be better to position the flexplate and the flywheel at their centers without any pretension?!
Thank you guys; Any input welcome
Cheers
Marc
hi Constantine,
after spotting some TBFs I decided to buy Roger's PKLAMP - thought its a good solution, at least until I'll have a chance to switch to Constantine's SUPERKLAMP.
Anyway, the PKLAMP install instructions call for the flywheel to be levered away from the engine before tightening the original clamp and new PKlamp. This seems intuitively correct.
However, Porsche changed in the WSM that advice, perhaps because the drive shaft's twisting (and shortening) will cause a rearward pull on the flexplate.
I am concerned that if you pull the flywheel back as far as it will go as suggested before tightening up, the end thrust bearing will be in contact with it’s rearmost surfaces. Then driveshaft shortening will produce pressure on the bearing right from the beginning.
So, I assume it might be better to position the flexplate and the flywheel at their centers without any pretension?!
Thank you guys; Any input welcome
Cheers
Marc
06-01-2010, 11:14 PM
#60
Rennlist Member
in bending and tension you are correct, however if you look at a surface of the shaft and take a small square unit as reference you will se that in tension/compression the unit becomes a rectangle. this is the action controlled by the Young's modulous. When looking at the surface of the shaft in torsion the unit becomes a parallelgram this situation applies to the shear modulous.