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Inside a strut for self levelling rear suspension

Started by oscar, 27 April 2008, 11:51 PM

oscar

Some photos of what's inside a strut.  Surprisingly, there's no rubber on the piston seal part.  Just two thick black metal parts to make up the piston head with a silver coloured steel sealing ring in between.  There is a split in this ring.  Not sure if this is how it is supposed to be or if it's a fatigue crack.  When surrounded by the struts walls, it would appear that the gap is reduced to nothing so far as both ends touch.

So whether that crack allows oil past the ring or not I don't know.  I've still yet to dismantle the piston head to see if there's any further gaps or valves built into the head.  Ideally I'd like to remove the piston complete without further cutting before I undo the nut that holds the piston head together.  Anyone know how to remove the shaft from the bottom of the strut?

Mercedes and Sach's part numbers


Inside top end


Top of piston


Underside of piston


The split in the ring


Rubber boot removed and inside the cylinder, bottom end.
1973 350SE, my first & fave

WGB

Thanks for that oscar.

Knowledge is becoming more important as these parts become more scarce.

In the 6.9 there is supposed to be a disc with 4 holes in it that is uncovered on compression to allow rapid flow but closes  and meters the flow of oil to provide the damping  on rebound.

?Is there anything like this in there?

Bill

koan


You must have been a bit lost for things to do on weekend oscar!

Your SLS struts look almost exactly like the front struts of the full hydro setup, just missing the leak back line.

On the 6.9 removing the ball joint with a special thin, cranked 22 mm spanner with the protective cup held releases the cup, could be wrong but yours look like they are similar.

Is it a ball joint or a rubber bush on the end?

koan
Boogity, Boogity, Boogity, Amen!

CraigS

Is it a split in the ring, or is it meant to be there like a piston ring ? Is there evidence of a rough break ?
[url="http://s109.photobucket.com/albums/n77/Aegeanfoods/My%20Cars/"]http://s109.photobucket.com/albums/n77/Aegeanfoods/My%20Cars/[/url]

WGB

Yes is it a rubber joint or  a ball joint ?

Also look after those rubber gaiters (boots) that cover the shaft if they are intact.
If they are the same as the 6.9 ones they are $75 each from M-B - I know as I have brought two.

Bill

oscar

Well blow me down, there's holes everywhere.  :o  I just went out and cut the strut in half to get the piston head off.

Firstly, Craigs, that ring appears as though it's supposed to be split like a piston ring.  The black metal head is one piece so the ring has to have a split so it can be fitted.  Not only that, whilst trying to ease the ring over the head, it split in two.  So the original split has smooth machined like ends and the other crack has pitted rough edges like what you'd expect in a metal fracture.  Too hard to get a good photo but the ring is square like in cross section.

Koan, I did everything over the weekend except what I should be doing ;D but yes there's a ball joint on the end pictured further on below.  

And WGB, well the pics tell all.  The strut looks though it will always be full of oil above and below the piston and there doesn't seem that there would be that much resistance for it to flow either way.  Despite the numerous thin but very strong, very stiff washers pictured below, I really don't think these flex to produce any sort of one way valve kind of restriction for backflow.  There's the cavities in the piston's top and underside which leave the holes unobstructed on both sides and act like spacers.  The whole piston was clamped together with that 17mm nut up top with no slack for the washers to move one way or the other and I really doubt they flex to occlude the holes.  Any restriction to fluid movement I think is purely down to the size of the holes in the head and the gap left between the two sets of cavities above and below plus the gap left by the outer perimeter of the largest washers above and below.

Look at the pics and see what you guys think but in my mind it just raises another question.  I'm not sure now how ride height for self levelling is achieved.  It does however fit the observation that when spheres go bad and are completely void of nitrogen and are instead filled with oil that the rear suspension is able to move up and down without any dampening whatsoever.


Cavity below head


Top of head and three cavities


17mm nut removed


as much as I could pull off the shaft.


head parts laid out, left to right is bottom to top


Top side of head


Underside of head


Ball joint at the bottom of the shaft and cracked ring which was one piece before I accidentally broke it.



1973 350SE, my first & fave

oscar

Ah geez, I'm wrong aren't I regarding the one way valve controlled back flow thing.  It wasn't obvious till I saw the photos up close and comparing the top and bottom views of the head but yes, the washers do flex if only a little.  The 6 smaller holes allow oil into the bottom of the strut when compressed, then close as the 3 large holes allow oil back into the top half of the strut when the strut extends.  There'd be some degree of dampening both ways but I still can't wrap my head around how ride height is achieved for a car with more weight on the back when more oil is pushed into the top half of the shaft through the one inlet.
1973 350SE, my first & fave

Niclas

Great pics Oscar!
I think the ride height is basicly set by the coil springs. If they are compressed for a long time oil will flow via the level control valve and increasing the pressure inside the whole strut. Since you have a piston rod comning out on one side the areas of where the pressure works at are different. This will generate a force on the piston rod, the force will be higher if the level control valve lets in more oil in the system and that way helping the coil springs back to the desired ride height.
Again, this is how I think it works..

What was wrong with the strut? Can anything else be wrong with them than leaking oil where the piston rod comes out???

Niclas

koan

I agree with your last comment oscar, there's far too many holes and washers for it not to be a valve. That triangular shaped piece is designed to control the flex of the washers, is it on the upper or lower side of the piston when in the installed position?

I'm with you with the mystery of how it manages to self-level, it's the same with the full hydro setup. I've always thought that one side of the piston was free of oil but as WGB pointed out to me some time ago the description talks of a valve disc and holes. Both sides of the piston must be oil filled, if there was any air on one side and it was displaced it couldn't be replaced.

On the full hydro struts there is a leak back line to return oil from the non pressure side of the piston, it's small bore nylon tube about 3mm ID. I've assumed this just returns any oil that leaks past the piston but that can't be the case, it must be returning relative large quantities of oil on every rebound.

In your case could it be that the washer valve only lets oil past the piston under extreme compression but lets the oil back relatively easily? It would have to be something like that or the oil on the wrong side of the piston would lock the strut solid.

The precise operation of both systems is a bit of a mystery.

koan
Boogity, Boogity, Boogity, Amen!

koan

Quote from: Niclas on 28 April 2008, 04:25 AM

Since you have a piston rod comning out on one side the areas of where the pressure works at are different. This will generate a force on the piston rod,


Good point, I'll have to work on that idea.

koan
Boogity, Boogity, Boogity, Amen!

oscar

#10
Niclas, you're a champion.  That makes complete sense, I get it.  It's all about pressure increase, not so much the piston head forming a fluid tight seal and being pushed out like an ordinary hydraulic ram.  So basically, the strut is a rigid container and if the pressure increases inside to overcome the force compressing it, something has to give. 
In other words, the pressure increase forces the shaft out of the strut. 8) 
Quote from: Niclas on 28 April 2008, 04:25 AM
What was wrong with the strut? Can anything else be wrong with them than leaking oil where the piston rod comes out???
Nothing really.  I pulled two struts from a wreck I dismantled.  This particular strut had a crappy rubber boot that was cut and some oil had leaked out of it but that was all.

Quote from: koan on 28 April 2008, 04:30 AM
In your case could it be that the washer valve only lets oil past the piston under extreme compression but lets the oil back relatively easily? It would have to be something like that or the oil on the wrong side of the piston would lock the strut solid.
Yes, I believe so.  You may or may not remember that when I first removed these struts from the wrecked yellow 280, I asked a question about strut operation because I compressed the strut relatively easy by hand to get the fluid out.  It was then that whilst extending it I realised there was oil in the bottom part of the strut that squelched into the top part of the strut.  I compressed it again and fluid went flying.  I extended it again and found yet more oil in the bottom half of the strut.  Add to this that extending the strut was much harder then compressing it i think what you say in the above quote is spot on. EDIT, EDIT Sorry koan, I just realised the above is opposite of what you said.  Tomorrow I'll fill the intact strut with oil, seal it, then determine which direction is easier to move it, extend or compress.

BTW, there's two triangular pieces, one clamping the top set of washers, the other clamping the bottom set.

The mystery of self levelling - I think I got it after what niclas posted.  I'm thinking that yes, the springs are part of it.  When I released these struts from the wreck, I had jacks under the subframe/axle.  The rear of the body sprung upwards a bit and when i bounced the boot section, there was this almost perpetual springing motion but it came to rest higher than before. 

So the springs give the vehicle a natural ride height on their own.  Add struts now and put the car on the ground.  Thinking purely in terms of pressure rather than fluid movement, imagine the engine running and a heavy load put in the back.  The levelling valve is actuated thanks to the sway bar turning and causes a pressure increase to the strut, the shaft is forced out of the strut raising the vehicle until the levelling valve is returned to a neutral postion.
Take the weight out of the back, the rear of the car lifts up, the levelling valve releases the pressure, shaft moves back into the strut, rear of the car lowers until the levelling valve reaches the neutral position again. 

So, if that makes sense which I hope it does, how does this relate to the 6.9 full hydro setup?
1973 350SE, my first & fave

TJ 450

What an awesome post! These struts appear extremely well engineered, I'm very impressed.

Koan, on the 6.9, does the balljoint have flats on the shaft before the protective cup? Do you think it would be removeable in-situ and can the cup be held with conventional tools without damage?

Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

koan

Quote from: TJ 450 on 28 April 2008, 05:40 AM

Koan, on the 6.9, does the balljoint have flats on the shaft before the protective cup? Do you think it would be removeable in-situ and can the cup be held with conventional tools without damage?


I've changed a ball joint on the rear which doesn't have the cup, there are flats on both the ball joint  and on the "ram". I bought some cheap 22mm spanners from Bunnings and ground them down, they have to be thin.

As for the front there is special tool with pins that fit into the base of the cup to hold it and use the same thin spanner on the ball joint.  Never done it but holding the cup on the car without the tool might be the hard bit.

On both front and rear the flats on the ball joint are above the rubber cover. The cover might need to be pushed down on to the joint a bit.

koan

Boogity, Boogity, Boogity, Amen!

koan


The different forces on the faces of the piston because the oil pressure acts on different areas makes a lot of sense. On SLS the springs take the load and the strut functions both as a damper and as a level control ram.

Just re-read the 6.9 hydro strut description, there is a high pressure seal and a low pressure seal in the bottom of the strut which implies there is high pressure below the piston. The leak back line connects between the high and low pressure seal and  returns fluid that has got by the high pressure seal.

There's one thing that had me  puzzled, and this applies to both systems.

When the strut is compressed it forces oil out into the sphere, while this happening the volume below the piston is increasing, where does the oil come from to fill the increasing volume? if it just passes through the piston valve then what goes out to the sphere?

But looking at the diagrams of the full hydro setup the 6.9 piston rod looks much bigger in than in oscar's pictures.

The volume change in the upper compartment is much greater than that in the lower compartment because of the size of the piston rod. On compression some fluid passes through the piston but some goes to the sphere, a lot more in 6.9 case because of the bigger piston rod.

koan
Boogity, Boogity, Boogity, Amen!

WGB

What I appear to be seeing are ball joints on the end that look very similar to the ball joints on a 6.9 strut.

Does this mean we may have an inexhaustible supply of relatively inexpensive ball joints for 6.9's - must check on the EPC.

I can understand the hydraulics in the self levelling now as well as why the damping fails when the sphere's fail even though the damping is not strictly done by the spheres.

My understanding of the 6.9 strut is that there is a solid teflon ring inside not a split piston ring type arrangement so maybe the pressure distribution is different.

The 6.9 strut is also threaded together so is theoretically serviceable.

Bill