Ambulance propellershaft (moved from 115 project thread)

[scraf]

Do you mind me asking how many driveshaft bearing mounts you have under your chassis ?

Not wanting to sidetrack the thread, but currently having some issues fitting a larger sized driveshaft to a lengthened W114 ambulance.

The original driveshaft was three piece, the new one is two piece.

There were FOUR driveshaft bearing mounts under the thing, two of which we had to remove to fit the new driveshaft. Very curious as to how many there are under a stock W114.

- Richard

[AMG]

The sedan has a 3pc propellershaft, the coupe has a 2pc propellershaft.
these attachments may help you identify parts.

coupe-propellershaft-pic.pdf

coupe-parts-number-list.pdf

sedan propellershaft-pic.pdf

part-number-list-1.pdf

[Bartman4800]

With prop shafts, there are a few things that are very important.

I can sum things up, but have a read through this first:

http://65.170.161.218/~spicerpa/sites/d ... lation.pdf


Regards, Bart

[scraf]

The sedan has a 3pc propellershaft, the coupe has a 2pc propellershaft.

Any idea why the 3pc for the sedan ? Unfortunately closing the gate after the horse has bolted here, as the new drive shaft has been installed, down graded to a two piece due to advice from the guys that renovated / balanced the old one.

( thanks to the admin that took the time to move my post )

[T-Modell]

Hi,
there's two possible mounts underneath my car. As far as my Mercedes Mechanic (he learned on the W115) there were motors with a 2pc and a 3pc shaft. F. i. he told me, that the 230.6 had a three piece shaft.

As the first mount was too far to the front and the rear mount too far to the back, we had to shorten the front part and lengthen the rear part of the SL shaft

Regards
Thomas

[AMG]

The sedan has a 3pc propellershaft, the coupe has a 2pc propellershaft.

Any idea why the 3pc for the sedan ? Unfortunately closing the gate after the horse has bolted here, as the new drive shaft has been installed, down graded to a two piece due to advice from the guys that renovated / balanced the old one.

( thanks to the admin that took the time to move my post )

The propellershaft is a connection between two semi-fixed angles in the (i.e. partially floating or flexible) drivetrain components. Those being the transmission/engine and the differential or final drive. They are not 'fixed' and unmoveable - which is the 'short' answer to the "why?" the shaft needs multiple cardan joints over it's length.

Long answer:
They are both suspended in rubber mounts which allow some flex, to absorb vibration and harshness - to isolate the monocoque from these intrusions.
the propellershaft is the sole link between the two and operates in torsional rotation in two assemblies - it also has a degree of fore-aft movement. So the prolellershaft has to cope with movement in two planes from two different semi-fixed objects at the same time whilst maintaining a stable rotational plane with minimal oscillation or deflection over it's length while allowing those two semi-fixed points to move (relatively) freely.

The idea of the multi-piece shaft is to reduce the length of the shaft sections, which reduces potential oscillation within the shaft, which in turn causes vibration. It has a secondary purpose also - and that is to reduce the operational angle between the transmission output flange and the differential input flange to a minimum. i.e. less than 3º from centre in 2 planes.

When the deflection angle is increased, the oscillation potential increases proportionally to the increase in angle between the shaft and the flange - This is why the shafts are balanced for primary balance and harmonic balance (if done correctly). This is easily proven through a gyroscope model where you try and rotate an object perpendicular to it's axis of rotation. The centrifugal forces can be significant, and when loaded onto a semi-floating component, can cause vibrations / oscillations - this is sometimes referred to as 'period noises' and commonly heard in differential gearsets, as the contact area on the gear teeth move under load/coast. It's also the reason you can feel sometimes a low speed vibration through the floor into the seat at around 105km/hr - this is a common theme in MB's where the propellershaft has been reassembled out of phase (markings on the propellershaft sections show the spline mating points to remain in phase)

The centre bearing(s) are there to further reduce NVH and dampen any further oscillations which occur. Naturally this can occur with any shaft, as the engine throttle results in a torque response from the engine and transmission, which is partially absorbed by the engine and transmission mounts, but transferred longitudinally through the propellershaft to the differential (rotation).

But the main point is, the longer the shaft section, the more critical it is to have a minimal angle of deflection, and a more precise balance over the length of the shaft.

Longer shafts are far more susceptible to these issues - be they hollow or solid, the fact remains that an unsupported section is able to oscillate within the rotational phase of the shaft, which is commonly referred to as 'whip' and this oscillation can destroy a shaft - through breakage in the unsupported section or at the ends where it is supported.

Hence the short shaft sections.

There's other science to the propellershaft too - hollow shafts are stronger than solid for given diameter and material type, wall thickness is important, mass is a factor and the 'weld' flange diameter and type is also a contributor.

What you may not be aware of, is that MB propellershafts are mostly friction welded at the yokes. You can see this by noticing the two concentric rings near the flange joints. that is the weld itself. Tremendously light and strong, yielding an advantage over conventional welds when primary and harmonic balancing is concerned - minimizing mass concentration and keeping the shafts concentricity to the weldyoke is a precision job..

Also, MB propellershaft material is extremely high quality. I have had shafts remade and even the professionals say that the original material is a higher quality than what can be obtained for their industry in general. It's not 300M, but it's definitely a high quality tensile steel of at least 4140 grade, and obviously the friction welding is a good match for this material type.

With an aftermarket shaft, I've had a 4140 thickwall shaft of a slightly larger dimention made, to cope with severe torsional load and minimal oscillation (shaft section had to be lengthened) and while stronger, the mass increase alone is enough to negate any attempt to minimize oascillation. It's a very difficult thing to get right - and then you still have the suspended components at each end of the shaft which can complicate the process.

[scraf]

I'd forgotten how awesome you guys are. Thanks AMG, hope that wealth of information helps other in the future as much as it has helped me, and apologies Bartman, that link you posted was well out of my league.

Current state of play with my ambulance .... did a test drive today, vibrations at certain speeds, " upgrading " to a two piece differential was probably definitely bad advice from, as we call them in the Netherlands, the " axle farmer ". The new axle has a greater circumference so may have to enlarge the tunnel at a few places, there was also a very worrying " tick tick tick ", which, if we diagnosed it properly, was due to the balancing weight on the axle hitting the gear selector.

Replaced the mounting rubber on the gearbox, has, as of now, has kept it driveable.

Been an expensive repair for once was originally just a burnt out center bearing of the propeller shaft.

€ 550 for the renovated " upgraded " and balanced axle, new flex discs and € 200 for a secondhand diff, haven't dared to ask my mechanic ( shade tree ) yet what his costs are going to be.

No time to be working under a car in a pit without heating in the current climes, so if a solution is to come for our rather sloppy " upgrade " ( thinking an additional universal joint at the gearbox end of the driveshaft ), it will be a project for next summer, or winter in OZ land.

[scraf]

An update for prosperity, as in maybe this will help someone in the future ...

Mild spell of weather has made ( at an old age ) crawling into a unheated pit a possibility, fortunately not for too long as the days couldn't be shorter.

Dropped the propeller shaft on the central bearing yesterday and got the angle grinder out to enlarge the tunnel. First attempt was rather flawed as we didn't take into consideration the the driveshaft isn't centered in the tunnel. Long story short, gaping hole in the tunnel to be welded up at a later date.

Test drive #2 yesterday, a lot of vibration, far too much, emanating from the rear at around 50 - 60 kmh, third and fourth gear.

Researching on the web, it looks like " Shady " who I'll affectionately name my " shade tree mechanic " shouldn't have tightened, or loosened the large nut on the spline to make the driveshaft as long as possible. Looking at the .pdf that Bart posted above confirms what I'd read on hotrod forums, a driveshaft should be " capable of changing lengths ", thus next step is to shorten the driveshaft, giving it 3/4 to an inch of play. ( please comment if I'm mistaken there )

If that doesn't sort out the vibration problem, it's going to be getting one of those digital angle meters and looking into the angles both ends of the ( two piece ) driveshaft and adjust the heights of the diff and the gearbox to get it straightish, or within recommended parameters.

NOT stopping till we get this right, have spent almost the cost price of the car on this " upgrade ", would be a huge shame if the thing vibrates apart due to dodgy mechanics. On the plus side, learning a heck of a lot about drive trains.

[scraf]

.... um, just another thought.

The central bearing mount, the "nut" is a " wedge shaped piece of metal" in a small " cage".

Could it be that that mount shouldn't be tightened too much to allow for fore and aft movement ? Or is that " wedge shaped nut " just to allow for correct positioning ?