Sorry to go back to the torque tube discussion, but I found this quote from Greg Brown interesting-

I find that when I check the torque tube shaft on my 87 manual,it has moved forward at the 5th gear/input shaft clamp. Every year for the last 5 years since I have had the car I have had to do this. The first time I checked it, the bolt had marks on it from the splines on the shaft.
The torque tube was rebuilt by a previous owner. Maybe I need to go back in there and have a look at the "frequency absorber".

Thank you for starting the thread Mr Benno.

 

From what I've seen rebuilding torque tubes (TTs) for the Porsche models that use them, the slipping of the drive shaft in the 928 5-speed style clamps is a problem in all Porsche drivelines that use the same style clamp. The failure of this style clamp (the 993 TTs use a smaller one) can be attributed to a few different issues. Not correctly torqueing the pinch bolts after maintenance, and using old or damaged pinch bolts are some of the more common issues. But other problems we have found after testing many over the years, is the suspected stretching of the metal in the square clamp around the splined sleeve over time which makes it have a looser fit even when using the correct torque on the pinch bolts. The looseness varies with each clamp and sometimes the internal splines are damaged too, which further adds to the clamp not doing its job properly.

This problem is further exacerbated when the drive shaft splines are also damaged in the sleeve due to the loose fit. The damaged splines are beaten into a knife edge which then can shear off if not changed. The damage splined shaft Michael found in his TT have also been found by owners in their 944 TTs, picture below, damaged one on the left compared to a healthy drive shaft end on the right.




We have also found damaged knife edged splines in 993 TT drive shafts due to incorrect torque of the pinch bolts.

There are a lot of issues with TTs and most can be traced to basic mechanical problems and/or poor maintenance by owners or techs who do not use the Porsche WSMs for proper torque settings and other considerations which are called out in them during their service. Age is now also a consideration for all the TT components, nothing lasts for ever, even if they are expensive to replace.

Harmonics can and do have detrimental affects on drive line components if not taken care of, but they can't be blamed for everything that goes wrong in the drive line. The TT components should be checked to make sure all of them are in good working order, to include the clamps.

HTH.

 

Yeah, I've only got a few hundred hours, few thousand dollars, doing research and taking to engineers that specialize in frequencies about this subject.

And you contribute exactly what, to the 928 community?

 

Agreed!

I just happen to know that "torquing" the hardware correctly doesn't automatically "fix" everything. I've personally torqued rear pinch bolts, marked them, and had them loose again, in 5,000 miles.....many times. Once that driveshaft starts a harmonic vibration inside 5th gear, the clamp isn't going to hold it from doing that. The harmonic vibration/frequency must be absorbed.

 

With the torsional vibration damper on the crankshaft? Or with what component? The housing vibration doesn’t have the energy that the shaft vibration has (some serious fluctuation of potential energy there) and the housing is fairly well isolated from the shaft, so I don’t understand how the housing vibration damper that doesn’t directly touch the shaft could have anything to do with shaft vibration one way or another.

 

Hypothesis? "Reality" is the correct word.

The torque tube housing literally turns into a wave....and since the torque tube shaft is connected directly to the housing with bearings, that wave is directly transferred to the shaft.

Please go back to the transaxle section of Project 928 and read that. Not a hypothesis, but the reality of what Porsche had to solve.

And I'm not a physicist....I don't know or care about the math behind all of this.....I'm just a dumb mechanic trying to fix 928's, the best way possible.

 

No, no, no.

The professional frequency engineers tell me that the frequency problem originates with the torque tube housing and is transmitted to the shaft, not visa versa. They could be wrong, but I've spent a long time with this and talked with several people that specialize in frequency problems and they all tell me the same thing.

I'm guessing that the torque tube's frequency absorber is in contact with the actual tube, for this reason. And why Porsche attached the battery box to the '78 and '79 models (until they came up with the frequency absorber)....to absorb the frequency from the torque shaft tube.

 

My understanding is that vibrations in the actual shaft are at least an order of magnitude stronger than in the housing. A shaft like that is like a wet noodle in my car, for example. Without the support bearings increasing the critical frequency of the system, the shaft would simply break apart at some rpm. Compared to that, the housing vibration is less than a drop in the bucket. Then, furthermore, the housing never actually touches the shaft. The shaft is far isolated from the housing, on the engine side there are the clearances oil film in the main bearings and in the transmission side there is the clearances and lubricant in bearings, gears, etc. So while I’m not an engineer, with high school physics I think some thinhgs can be ruled out.

 

I don't know what to say, except your understanding and my understanding are different.

I certainly have had several of my misconceptions "cleared up" in my discussions with frequency specialists. What I thought I knew turned out to not always be the case.

If you are really interested in talking with an engineer that specializes in frequency problems, I've established a relationship with a few that I can recommend.

 

I don't know if we do.

I gave an explanation, from my own experiences, the drive shaft clamps used in 928 and 944 models seem to not be holding the drive shafts as tightly as they could be since the square clamps seem to be stretching with age. If owners are being advised to change the pinch bolts for fresh ones due to stretching and not holding correct torque values, it would stand to reason the square clamp metal would also fatigue and their "pinch" on the drive shaft would not be as strong as when they were new.

So as Michael and others have found, their drive shafts, and possibly other subassemblies like the transmission, get ruined.

Focusing this type of failure to a TT harmonic issue is incorrect. The damage to the drive shaft splines, and in this instance the transmission, was caused by the normal operation of the drive shaft and it being acted upon by the engine harmonics traveling down its length, not from the TT harmonics. If the clamp had held the drive shaft as initially designed, this failure would not have happened. Colin did mention the torque value of the rear clamp pinch bolt seemed to be much less than the specified torque value.

This clamp fatigue also explains what you are finding.

Porsche used a different style drive shaft clamp in the 968 drive line which is a much better design.

 

I'd of course be interested in talking to anyone who's got training on power transfer and vibrations!

That said, if the torque tube housing resonances would have enough energy to do anything to the shaft far isolated from it, how could the torque tube housing fasteners not break off from the clutch side or transmission side?

 

As a data point, Michael's 928 had the harmonic damper installed. The bushings did require force to extract. However, the one bushing did fall off the one end of the damper.
Michael's pinch bolt was loose, but so were most of the bolts holding the setup together. The TT to transmission bolts (6) were really loose 2 or 3 I could've undone with my fingers.
Alternatively, the shaft that was installed (and was in perfect shape with zero spline wear) was in a car with no damper installed, all hardware was fully tightened, and it had had many miles put on it in that orientation. Again with zero spline wear.

 

For me, there's a certain amount of humor involved in these discussions. Actually, there's a whole bunch of humor.

Let's keep in mind that I'm saying one thing (and one thing only). I'm saying that Porsche knew/knows what it took to prevent torque tube harmonics from damaging the car. And I'm simply telling people to retain the engineering that Porsche did. I'm not promoting a product, I've got nothing to sell, I've got zero skin in this game.....I'm just saying to retain the counterweight. I'm assuming that the Porsche engineers that spent hundreds of hours on this problem are way smarter and know a bunch more than a bunch of 928 "arm chair engineers" sitting around discussing how to do it better.

I know that Constantine thinks his bearings are going to do the "job"....and that people can just toss the counterweight in the trash. Yet, he's done none of the testing that Porsche did, none of the frequency study tests. He's like the guy that stayed at Holiday Inn.....suddenly an expert on torque tube frequency problems, with zero data.

There are ways to gather data and prove your theory....it's actually pretty easy, but it does require money to do. I was going to "spring" for the testing, just for my own knowledge, when I decided that it wasn't really my problem....I'm just doing what Porsche did.

From a personal point of view, I have nothing to prove.

Reading what Porsche had to do to just be able to introduce the car tells me that many engineers worked a lot of hours (a couple of years, at least) trying to solve the torque tube problem. Keep in mind that several automotive companies had also worked on this problem....and told Porsche that it simply would not work. That entire "investment" had to cost huge amount of money!

And they didn't just "give up" there. Development went on for years.....and there were several significant changes made to try and make improvements (which may or may not been actual improvements). Heck, the original cars had the Battery box bolted to the back of the transmission to absorb the frequencies.....and then they developed the "frequency absorber" inside the torque tube, eliminating the need to bolt the battery box to the transmission. Every single one of those changes cost Porsche money, both in terms of engineering time and in terms of the costs involved in "changing" the product.

Just throw the frequency absorber inside the torque tube away? Arbitrarily?

What is the difference between doing that and someone making a solid chunk of metal for the front of the crankshaft and telling everyone to throw the harmonic balancer away?

Same thing....tossing away a device that was engineered to absorb damaging harmonics is a funny thing to consider.

 

Wow...!

So slandering is okay on here then?

Or is this statement considered an opinion, cause I sure have lots of them too...

 

The question isn't whether Porsche engineers though that dampener was worth it or not, since it's in the car it was considered worth it.

What's the question being debated here is whether it was there for noise-vibration-harshness reasons or driveline reliability reasons.

The crankshaft torsional vibration damper is there mainly for driveline reliability reasons. Crankshaft counterweights are there for both driveline reliability reasons and NVH reasons. The drive shaft support bearings are there mainly for driveline reliability reasons. What you'll notice here is that vibration dampers, counterweights, and supports that are there for driveline reliability reasons are either rotating themselves or in direct contact with rotating components.

The facts that the torque tube vibration damper doesn't touch the rotating components and that it was omitted from the club-sport models both point towards that dampener being there for NVH reasons and not for driveline reliability reasons.

If someone sees the torque tube housing breaking off from the engine or from the transmission because the torque tube vibration dampener has been omitted, then I'd reconsider the possibility that it may be there for driveline reliability reasons.