I've just been re-reading the W123 FSM
 

It is kind of funny in chapter 41-50 para 11 it basically says that they had been putting marks on W123 prop shafts before August 1982 but they had been ignoring them at assembly.

I'm not sure if I should be reading so much between the lines but to me that suggests that they didn't balance prop shafts until 1982 - I mean if they couldn't be bothered to align the marks before August 1982 they couldn't have had a chap in the corner testing them could they - could they?

Is there mention to this in the link you provided?

Either they balanced the shafts, or the manufacturing process always produced shafts that were balanced within specified parameters. The shafts were balanced. I am not talking about a shaft imbalance problem.

No, you have to figure it out. It will be obvious when you get an understanding of the angular velocity problem. Study the constant velocity joint, and the reason for its invention.

Have a look at this animation from wikipedia showing a Double Cardan
http://upload.wikimedia.org/wikipedi...(animated).gif

Now imagine a six point version (our rubber disc) instead of a four point (a 'normal' hookes joint or Cardan joint).

Here's a link which briefly mentions the rubber joints, but nothing about phasing them. Of course, there's the possibility that I am still missing something here. I'm not a mech-E; and though I have some experience, I haven't thought it all through completely.

Universal joints

The elephant in the room here is that the driveline shaft as Army pointed out earlier, has only one Hookes, or Double Cross, or Cardan, or whatever you want to call it. Even though the angle is small, and thus the induced vibration would also be small I still have a hard time believing that a German engineer (having known a few) would design the driveline in such a way.

Another good link.

Pirate4x4.Com - Extreme Four Wheel Drive

http://www.pirate4x4.com/tech/billav...t/Geometry.jpg

Wow you've been busy whilst I've been away cullennewsom!

The thing that I think you're missing is that the universal joint is constrained - it is made out of metal - the cross piece (which also doesn't flex fortunately!) holds the two parts together "rigidly". The W123 flex disc is made out of rubber.

I don't know exactly how Mercedes designed it but I think that the diameter of the flex disc - the pitch circle diamter of the bolts that run through the flex disc - and the elastic properties of the flex disc are combined to ensure that within the design limits (i.e. how much of an angle the propshaft will turn) the joint remains within the required elastic range of the rubber material.

The flex joint is not "rigidly" constrained like the universal joint.

Basically you can't use the same mathematical restriction described by wikipedia =>

Universal joint - Wikipedia, the free encyclopedia

Because the part with the dot product of x1 and x2 being equal to zero does not apply.

EDIT => I tried a cut and past of the line but it didn't work here it is the part where it says

"A constraint on the x1 and x2 vectors is that since they are fixed in the gimbal, they must remain at right angles to each other"

END EDIT

I don't have the mathematical ability / will to sort this one out! It would take me too long to dust off all my old text books and figure it all out - I haven't the time for it sorry... check out my occupation if you don't believe me! I can see where it doesn't work anymore though.

Essentially you're not going to get the same result because rubber isn't as rigid as steel. The flex disc is a resilant element.

I'm sure that the marks put on a W123 prop shaft after summer 1982 are not due to the effect described in this link

Driveshafts and U-Joints - Tom Sotomayor

I think that they are most likely put there to denote how the two halves have been balanced as a whole.

Don't take this the wrong way Army, but you said yourself that you're not keen on the math. if you could take a single Cardan joint, bend it almost to 90 degrees, and then rotate it, you'd see what I'm on about. And yes, I realize, and even mentioned previously that the rubber couplings are rubber and that makes it a bit less straightforward, than if they were rigid, but of course you can't have that same geometry with rigid parts because three points make a plane and what's needed is intersecting axes.

I read the FSM too and noted where it says it is balanced as an assembly. I think the guy that wrote that just didn't know what he was talking about. I've worked with technical writers before too, and while many of them are nice folks they are not always the brilliant engineers that some may liken themselves to be.

For anyone who is still confused about how to install your driveshaft, align the index marks, like it says in the FSM.

I'm still confused.

Back to the original question, can I (real world experience and not theory {no offense intended to those participating in the theory debate}) mix the front half of one shaft with the back half of another shaft if I allign the marks? Am I likely (very likely or highly unlikely) to have balancing problems?

Thanks again for all the input.

IMO, very unlikely to have balancing problems, especially if you follow the procedure in the FSM. Do your shafts have indexing marks as mentioned in the FSM?

I would say you are somewhat unlikely to have balance problems. That having been said, it would not surprise me if you did. I would hazard a guess that the balancing thing is stated the way it is in the manual as a liability thing, taking at least one variable off of the table. If you maintain the alignment, it helps to ensure that any imbalances would tend to cancel themselves out rather than make things worse. Additionally, if balance were such a huge issue, there would be some procedure for balancing the flex discs when they are replaced.

Obviously it's best to keep things as they are from the factory, but it's very likely that it will be ok.

I would go one step further and say that if you had a small balance problem it would be worth it to swap the two shafts 180 degrees relative to each other and test before going to the hassle of balancing.

Now, if you have to cut and weld the shaft to make it work, you would have no choice but to get it balanced.

I offer up one small perhaps irrelevant datapoint for your consideration. When I did the swap on my car, I needed to weld 4 "teeth" to the drive shaft to provide trigger for my electronic speedometer. (entire thread here - http://www.peachparts.com/shopforum/diesel-discussion/312801-mechanical-electronic-speedometer-conversion-2.html) I did everything I could to make sure the teeth were the same weight and the welds the same, but obviously there was no way to ensure that I kept things in balance. It turned out just fine (luck, planning or execution? who knows)

http://www.peachparts.com/shopforum/...on-trans27.jpg

Like you, I was prepared to pull things apart to get them balanced if needed, but not necessary in practice.

My vote is for "unlikely to be a problem".

How do we know the parts are as they left the factory? after all these are 27 to 30+ yr cars, and have gone through various owners and untold number of mechanics, both Professional and Shade Tree. there can be a whole load of miss matched mixed up parts by this time in the vehicles journey through it life.

Charlie

My experience of technical writers has been somewhat different. They strike me as being a similar breed to the pedantic selfrighteous lot who work in quality control! In my experience they are mostly very careful about what they write and adopt an almost lawyerly manner. It makes for very dry reading.

As for the FSM I've found it to be the ultimate argument stopper. It contains extraordinarily good technical content in my opinion. The translation to English is a bit rubbish in some places but nevertheless it puts lesser texts like Haynes and ATSG to shame. My only gripe with the FSM is that it says you MUST use the super expensive special tools...

...I see no reason to disbelieve the FSM in this case. In fact so far for the W123 FSM I've only come across one bit of information that is arguably a bit iffy.

I don't think you are looking at the big picture.

The drive train design of the W123 is not some horse-cart-sprung-live-axle (endorsed by an old race car driver) set up.

The prop shaft does not move up and down with the motion of the wheels traveling over bumps. The engine is mounted on resilient mounts; as it the transmission; the subframe; and the differential. The propshaft is also supported at the "centre" mount. Under normal conditions the UJ in the prop shaft is kept straight where (as you have seen) the difference in output speed from the joint is minimal to non existent. The biggest angle a W123 UJ should see is when it is being installed or removed.

The majority of the movement in the W123 drive train is in the axles. Consequently you'll see more axle related problems on the forum than in normal service prop shaft problems. Most of the prop shaft questions seem to be about changing the drive train rather than repairing it.

I think the design has proven itself to be a rugged dependable design that only has problems at super high mileages or when the rubber parts in the system deteriorate. These rubber parts are key to the efficient and proper operation of this design.

Sorry Walkenvol - here are my impressions of the practical side of things

1) If you have marks on the prop shaft you should align them - in the absence of professional balancing equipment that is the best you can do

2) If you can't find marks on your prop shaft then I wouldn't get too worried about it. The information in the FSM suggests to me that Mercedes wasn't too worried about it before August 1982!

3) If you get drive train vibration after fitting a prop shaft I'd make sure that the gearbox mount / rear engine mount has been adjusted in accordance with the FSM and that ALL of the resilient components in the drive train are in good shape. (So that means engine mounts, gearbox / transmission mounts, flex discs, prop shaft centre mount, sub frame mounts differential mount)