SharkSkin

Here's another example of a blown TT shaft. I took this pic at Sharktoberfest '04, and have no clue what the history is... but it's another example of the way these things tend to come apart. Yeah, it's not exactly like edco's but you can see the tendency to split down the length.

SharkSkin

And yes, Bill rocks!

bigs

Does anyone else think the drive shaft is a little small in diameter for a car having as much power and torque as a shark does?

Bill Ball

Maybe we need THREE shafts like the one above the broken sample in Dave's pic.

Tony

These pics are giving me the heebee geebees
Some sticky tires and the drag strip friday night

SharkSkin

Not really.... the halfshafts are about the same diameter, and they see far more twisting load in first gear than the TT shaft ever sees. They make up for it somewhat by being shorter, but still.... I think if it were really inadequate we'd have heard about it a long time ago. Surfing Rennlist will tend to make it seem like problems happen a lot more often than they really do, IMHO.

daveo90s4

Bill, I just saw your photos. I tried posting my photos last night, but file was too big. Anyway, my photos look exactly the same as yours - precisely the same 'wire hawser' type effect, break at exactly the same place, close inspection of the linear fractures showing bright shiny new (?) factures and ochre red coloured old (?) factures. In fact, when I first saw them I thought they were my photos!!. When you get the shaft out and look at it end on, you may also see what appears to be two 'layers' as if had already been highly stressed and in the recent event the last bit let go. And dark arc welding coloured blue right in the centre, where the very centre points of the two ends were touching and melted at the last moment. Interesting thing is I'd just the week before checked the flex plate end thrust / crank end float so had loosened the clamp and reclamped it. No visual sign of any metal fatigue when I did that. A week or so later the central shaft snapped. Had your/Matt's shaft also been adjusted a short while before it snapped? Is our fear of thrust bearing failure and our preventative maintenance somehow a cause for broken central shafts??

Dave
Australia

edco

Hey Dave,

Yes I had checked Thrust Bearing with Bill and it moved 2 mm but everything looked fine.

Mrmerlin

Hi , that driveshaft is pretty rusty, if you look at the first photo of the snapped drive shaft look at the top you will see a hole this is the place where the clutch pivot bolts for the clutch release arm this hole is on the top of the bellhousing, this is where water can enter and start rusting your drive shaft, it usually happens from washing the motor as water is rinsed from front to back on the top of the motor, it is a good idea to plug this hole with a rag when washing the motor. Not sure why Porsche left this part open, if you are running a clutch instead of the auto then water will rinse down this hole and get into the throw out bearing as well as the drive shaft and ruin the the TOB and small shaft and pilot bearing, Stan

Bill Ball

Yup. This car was the proverbial dentist's wife's car. Original owner. Every piece of paper since new. By-the-book dealer service. No indication they had ever touched the TT. There is no call for it unless you pull the motor, according to Porsche. Anyway, the day Matt bought the car 3 weeks ago, we did our Rennlist duty and opened up the lower bellhousing, loosened the TT clamp and watched it move less than 2mm. After seeing that we chose not check the crank end float. Everything looked fine. Tightened the clamp and buttoned it up.

It is scary to see this in a car with no signs of abuse. Guess it's some kind of crap shoot and that driving like a dentist's wife isn't necessarily going to prevent catastrophes. So, I'm not going to think about it anymore. It's like heart attacks and genetics - there is little you can do about it, so you might as well enjoy life.

Bill Ball

Stan: Thanks for this insight. There is a rather large opening at the top of the bellhousing. Also, the intake valley has a drain that exits right above the main seal and could splash on the shaft. Still, the shaft did not look at all rusty last week, and the rust color is exaggerated by Matt's camera. It looks awful, like it had sat out in a field exposed to the elements for many years, not like fresh break of high strength steel.

byrdman454

Bill,
Any chance I can persuade you to take alot of pics and do a TT change writeup? I have this job to do soon.

Thanks

Fogey1

I picked up a couple of broken driveshafts a few years ago - souvenirs from 928Intl, I use them as spud bars now.

Your "over-torqued bamboo" failure is the precise failure on one. The other shattered differently, but you can still see the torque.

StevenB

I saw the shaft as well that day. My recollection is the same as Bill's and Matt's - no visible signs of rust or anything out of the ordinary. Could some of the rust be oxidation of any newly scored/broken metal? Freshly exposed metal will rust rather quickly. Just a thought.

Tails

David A photograph is typical of a torque failure that I have seen in crankshafts and drive shafts of much larger engines and shafts.

You have a very good view of the tapered down section of the drive shaft to the spline that fits into the forward flexplate hub and clamp (no undercut through the splines to take the clamp bolt) on the drive shaft just below the one that has failed.

When manufacturing drive shaft similar to the TT drive shaft it is imperative that there is a good radius between the smallest diameter of the tapered area to the spline. As all shafts are machined and usually the machinest sharpenes his own lathe tools there is always a possibility for a slight error in the form of the radius which may not be picked up by quality control. If there is a sharp notch section, small groove in this radius it can set up a stress riser that could cause cracking to start at the lower point of the groove radius and these cracks could spread longitudionally/axially along the shaft due to the torque. At the juncture of the radius to the raised splines, this area will initially be stronger than at the lowest section of the radius, so this could be the reason for the longitudional fingers of metal along the shaft as shown in the photographs of Matt's failure.

The shaft may not have broken instantously, so as the cracks grew along the shaft there could have been rubbing along the sides of the cracks, as the drive shaft was torqued up and torqued down under load whcih may explain the rusted appearance. This rubbing against may have produced very fine particals of metal which may have taken on the rusted look when exposed to the atmosphere around the drive shaft.

As the cracks probably probogated along the shaft until they reached a point where the shaft failed under load.

This is only a hypothisis and as I suggest earlier in this thread it would be great to get a metalurgical opinion as to the reason for the failure.

There is various types of failure in shafts caused by torque stress, notch stress, resonating vibration causing fatigue failure, shuttling vibration of the shaft (when the shaft moves backwards and forwards whilst under torque) that could be the start of the shaft failure. All of those mentioned above can happen in the TT drive shaft and can happen at the same time.

Just my thoughts on the subject.

Tails 1990 928S4 Auto