joining drive shafts
 

I've read where its reccomended to mark the 2 halfs of a drive shaft when removing from the vehicle so that they are reinstalled in the same relative position later. If I'm using the front half from one shaft and the rear half from another, is there any special directions on how the spline fits together?

I think that's what you call being extra careful. I'm unaware of any good reason for it. But, I would like to hear one if there is.

The drivesghaft is factory balanced. So it is very important to assemble them as they came apart. Mixing a non matched front and back portion of the shafts may or may not cause an issue.

The balance of the new to each other shafts should be balanced on a machine really for best results. Not too hard to pull them again if a vibration is present. Myself I might try balance the front shaft to match the one taken off if I knew the match marking of the original frontshaft. Just duplicate it on the second short shaft and balace it the same as the old one. It would be tricky to do at home though.

When I pulled the driveshaft halves apart on my 240 in 2006 I didn't mark the halves and put it back together without knowing the original orientation. That was 70k miles and 6 years ago, I've never noticed any vibrations. I wouldn't worry about it too much.

Something about that just doesn't sound right. If the shaft is balanced, then it shouldn't make any difference. If the shafts are not balanced, and somehow depend upon their orientation relative to each other to balance... I'm still scratching my head on this one.

The FSM shows that in about 1982 MB got their act together and put alignment marks on the two D shafts - and also say that the alignment marks were not paid attention to in assembly - so please correctly align them in service.

The marks are a triangle on one shaft and a square on the other.

The shafts should be balanced as a pair - a special relationship between the front and the back if you like.

If you take then apart and then refit them incorrectly the special relationship is broken => you could experience prop shaft vibration problems.

Zulfiqar kindly posted the FSM notes which mention both the alignment marks, and the fact that the marks were ignored at assembly time.

In the name of expediency, I have no problem blindly following the FSM advice, that's what I'd do, and what I'd recommend to anyone. But, as a general principle, I try to undertake projects in a mindful and purposeful way. While I understand that it is possible under limited circumstances, to balance shafts in such way. I can't see why anyone would ever think it is a good idea. Most especially an MB engineer. Can you explain why? Are you aware of any consequences for not doing so?

In typing this, I've just realized that like Biodiesel300TD, I have also used mismatched shafts in my TD, and noticed no problem.

Do you mean why they balanced the prop shaft as a whole (two sections together) instead of doing each section individually and then joining the two together (I've learnt recently that this is often called neutrally balanced!)?

Yes, why invite the possibility for error at time of manufacture.

Got any source you can share about neutral balance. I can only find stuff that relates to high RPM racing engines, which doesn't apply to driveshafts.

Phasing U Joints
 

Here's a bit of an explanation:

Driveshafts and U-Joints - Tom Sotomayor

Any two piece shaft should be marked before being taken out so that the two halves go back together in the same relationship, otherwise vibration can be a problem. I just grab a crayon/marker and run a line across the bottom of the shafts where the middle U-joint or splined joint is.
See figure 2 in this page:
http://jeepgarage.free.fr/OWN/drive%20shaft%20alignment.pdf

After a certain year they are marked by bumps on the Casting.

In another thread I speculated that he Prongs on the Yoke had a specife oreintationto the prongs of the Yoke on the opposiet end of the Shaft.
I think that means if there is no marks on the 2 shaft sections there is 3 positions that would work.

I don't have a good pic if the end with the Single Mark on it.

Thanks smiffy, the Tom Sotomayor link cleared it all up for me.

It's not a shaft imbalance problem, it is a limitation of U joints. There's a hint in the name of the u-joint's cousin, the Constant Velocity Joint.

On the W123 we've got those rubber couplings, but they suffer from pretty much the same issue. thanks for the photos.

Does this mean that you believe No one balances Drive Shafts; or that they do nut use the term Neutral Balance where you looked?

Well, yes I am a little more then 1/2 way through the job of changing my Flex Discs; will finish that tomarrow tomarrow.

I only know the term Neutral Balance from the replacing of the Flywheel.

I take the term to mean that a Neutral Balanced Flywheel was one balanced all by itself. Where as the Flywheel that was balanced a long with the Crankshaft attached may or may not be Neutral Balanced; as, they could have drilled out some metal on the Flywheel when the whole Crank and Flywheel assembly was balanced.

In the case of the Drive Shaft I believe it would mean that each separate section of the Drive Shaft was balanced individually.

This is what is in part of the manual:
"The components of the propeller shafts are individually pre-balanced at the factory. To eliminate any residual unbalance which might still remain, re-balance the complete propeller shaft including the companion plates. For this reason, when dissembling the complete propeller shaft (e.g. when changing propeller shaft intermediate bearing) mark individual components in relation to each other (if no marks are in place) reassemble to the original Shape."

"On vehicles up to Jul 1982, mark the individual propeller shaft components in relation to each other.

On vehicles starting August 1982 the front and the rear propeller shaft ar marked in relation to each other.

Note: The identification has been partially already in place on propeller shafts prior to August 1982, but has no been taken into consideration during assembly. For this reason, propeller shafts on which the marking is not in agreement, must be identified prior to removal and the 2 parts must be plugged together following this identification."

The companion Plates are the Flex Discs.

So on one hand the Factory has the parts are pre-balanced but at the same time it recommends balancing "To eliminate any residual unbalance which might still remain".

So you assemble your Drive Shaft and hope for the Best. If you experience vibration and it bothers you find some way to Balance the Driv Shaft.

Of course they balance driveshafts. I was doubting the implied claim that the balance was dependent upon some Rube Goldberg arrangement of two shafts together with a carrier bearing.

When I googled neutral balance, I came up with a lot of links taking about dual camshafts and counterbalances for them, none of it looked related. I didn't spend much time looking though them though.

Weighted Balance
 

There are also notes in the FSM (for some models) on how to correct imbalance
at the Flex Discs (Of the Flex Discs) by the use of weighted washers.

I've seen directions in a truck FSM where recommended procedure is to clamp 2 worm gear hose clamps to the drive shaft. If it is out of balance, move the relative location of the clamps until the shaft is balanced. I think a Dodge Ram with its poor quality control uses that method. 01 FSM is at pdftown for reference.

OP stated that he was using 2 halves but didn't say why. Perhaps he wants to take his chances with the parts on hand. In which case, a method of at least checking balance would be useful.

Sorry not to have replied sooner - I've been asleep (check out my location)!

Answers for neutrally balanced =>

http://www.peachparts.com/shopforum/diesel-discussion/311692-terminology-problem-meaning-neutrally-balanced.html

Essentially the situation of balancing a drive shaft is different from the situation of balancing a crank shaft and a flywheel.

I'll try not to confuse the issue too much by going on about a crankshaft and a flywheel - I'll stick to the drive shaft first.


You can indeed (and they probably do) balance each individual part of the shaft (to a certain extent!). The first stage of balancing is to statically balance the part(s). This is making sure that when you spin the part it does not spin and stop at the same spot each time. Think of a bicycle wheel with a heavy valve stem - as long as the wheel bearings are in good shape the wheel will stop spinning at the same point (more or less). Static balancing is correcting for this so that the wheel will stop at random points instead of just one.

My guess is that the two parts of the W123 and W124 propshafts were individually statically balanced. I don't know this for sure - but I think they probably did it by perfecting their manufacturing process to such an extent that a static balance was more or less ensured.

Dynamic balance is correcting effects when the parts are spinning. Typically "everyday car parts" will be balcanced to reduce the effects of the first largest mode of vibration. If you consider a prop shaft to be much like a beam then you can see in the right hand side of the picture below that the first mode of vibration is a whipping of the centre of the beam (as it is being held in place at either end)

http://www.eng.buffalo.edu/~abani/fem/dyn/beam.gif

For the case of a two part prop shaft you can only balance this (sucessfully) when the two parts are joined together => it is a function of the over all length of the beam / prop shaft


Now I'm not 100% sure whether the W123 / W124 prop shafts are balanced to reduce the first or second (also shown in the picture) longitudinal mode of vibration - I guess you'd have to measure it to see which is most dominant. The biggest problem would be tackled.

The centre support complicates the behaviour of longitudinal vibration as does the situation when the ends of the prop shaft are not at the same heights. Generally speaking these effects are probably going to be ignored / included in the measurement.

Going back to the crankshaft and flywheel situation you can see that the longitudinal vibration characteristics aren't really altered that much because the overall length of the crankshaft and the flywheel are about the same as the length of the crank - there's only a little difference. (And besides the flywheel will be balanced for a different mode of vibration - not longitudinal) So you can see that the prop shaft sitution is different 'cos it gets much longer when the two parts are joined together.



Deep breath!



As for effects - if a prop shaft is not "dynamically balanced" then it is possible that the whipping motion of the prop shaft (longitudinal modes of vibration) will induce too much flexing of the flex discs and too much motion at the centre bearing.

Converting an '87 190DT from auto to manual using a donor '85 2.3. Transmissions are different lengths. My readings indicate the rear halfs are the same length with the difference in the front halfs. Trying to perform the swap over a weekend. Would be a bit quicker I think to swap on the front half of the shafts rather than to change out the entire drive shaft. All this assumes everything bolts together of course. If it has the potential to cause problems, then I'll swap the entire shaft. Will probably need to swap the differentials eventually (again from my readings) but I don't know if time will allow over the weekend.

Thanks you for the indepth discussion of drive shaft balancing. I'm still not clear if there is a consensus as to whether swapping the front halves would create a problem though?

Some of you folks are missing the important point here. The index marks are not for balance. They are there because of a limitation in the design of U-joints. Everything you need to know is explained in the link from smiffy6four.

The vibration smiffy is talking about is not due to imbalance. It is due to the fact that on a U-joint, angular velocity in only equals angular velocity out, when there is zero deflection in the U joint. It's enough to make you ask, 'What good is a u-joint then?' In the first link smiffy provided, they point out that you need two joints, 'phased' properly so that the angular velocity differences cancel each other out.

We're not talking about balancing. Driveshaft orientation does not affect balance. That would require a Rube Goldberg eccentric weighted balancing scheme.
It will not be a problem, so long as you pay attention to the relative orientation of the joints. After reading this, I give it a fair chance that my TD has the shafts phased incorrectly. I have noticed a very slight whine, like my differential my be worn, but we don't drive it a whole lot. YMMV I think I'll probably check mine this weekend.

Can you explain further how to do this?

OK I've got a question now
 

How many UJs are fitted to your car?

My W123 has one.

The information contained in this fantastic article...

Driveshafts and U-Joints - Tom Sotomayor

...does not seem applicable to my car as the article considers cars fitted with prop shafts with two UJs and a splined shaft in between.

(Really nice article by the way smiffy6four I've been looking for an article on line like that for a while)

I'm not sure that I can do better than Tom Sotomayor. Let me think about it for a minute. Anybody else feel free to jump in any time.

Really? My W123 has one rubber coupling at the transmission output shaft, and one at the differential.

http://w124-zone.com/downloads/MB%20CD/W124/w124CD2/Program/Chassis/41-050.pdf

Cullennewsom - The above shows a picture of a 190 propeller shaft and there only appears to be a single universal joint in the shaft along with the (2) rubber flex discs. I'm assuming the flex discs don't factor into the equation. My confusion is I'm unclear as to how the front half's orientation to the rear half would affect the universal joint as referenced in the Tom Sotomayor article?

It does say in the FSM on pg 4 "If seperation of the propeller shaft is required, mark the respective parts in relation to each other, since the propeller shaft has been balanced in assembled condition"

Per this info, if front and rear halfs are mixed and matched then the possibility of an unbalanced shaft is the only concern?

There is a U joint right at the center carrier.. sorry about not posting earlier but your question to me was answered by diesel 911.

My W123 prop shaft has one universal joint, two flex discs and one "centre" bearing support. The flex discs and the centre bearing support are resilient elements that I assume are there to counteract vibration.

The flex discs suffer the same limitation as a U joint. There is a difference though. The U joint (Hooke's joint) has the following property: The MB flex joints on my W123 have the same characteristic, but with three cycles per revolution instead of two.

The give and take of the rubber will help some in the event of a mismatch at assembly time, but the problem still exists.
Have a look at the following links:
http://en.wikipedia.org/wiki/Constant-velocity_joint

http://www.sdp-si.com/D757/couplings3.htm

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)

Thanks again for all the input.

Appears the consensus is that each half of the propeller shaft is more than likely independantly balanced and could be mixed and matched with other shafts.

This debate reminds me of the one concerning whether the flywheel is neutrally balanced on an OM617. The FSM indicates its balanced with the crank but real world repowers have proven its rarely a problem to connect a 617 out of an auto tranny setup to a neutrally balanced flywheel in front of a manual tranny.