Spheres..... New, Re-charged and Pattern Parts

[Pleiades]

Hi All.

I have been reading some posts about spheres, as most of you will know Pleiades was originaly a sphere recharging company that grew onto other hydraulic components. I dont like to "blow my own trumpet" but I do know quite a bit about spheres, so I thought it may be a good time to expell some myths.

All hydraulic systems will need an accumulator of some sort, the accumulator can be seen as a battery for the hydraulics, the pump will charge the accumulator with a given amount of pressure so that the hydraulic system can run from this rather than direct from the pump.

Our Citroens have accumulators for the suspension, they are refered to as spheres simply because they are round and thats what Citroen decided to name them.

Reliability......
New spheres from Citroen are fine and will last a good time but as we know are a tadd expensive

New pattern parts, there are a lot out there, I have seen many boxed as new but they are re-charged ones so watch out, I can however say that GSF amtex spheres are fine

Recharged. Again you do need to be carefull of the scource, I know of one place that thinks that all spheres are the same, so when buying a couple of Xantia fronts, you will get 2 x spheres charged to 55-, but one could be a BX front and the other a Xantia rear.

We call our spheres reconditioned, as we do internally inspect all spheres before charging, we have a reputation to keep.

How long do they last???

A typical new sphere be it Citroen or decent pattern sphere will last well, we have allways reccomended that spheres be changed at approx half pressure, this way it keeps the car as comfy as it should be and with the re-con system it stands you in with a good chance of being cheaper, as your old spheres will not be scrap metal.

A reconditioned or recharged sphere.. Well believe it or not we have living proof that they actually last slightly longer than new, why? Well this is only a theory, but when you cut open an old sphere you can see that the diaphragm forms a seal around the top relying on pressure to keep it there, a bit like a tubless tyre, my guess is that when they are new a certain amount of pressure will have been lost whilst the diaphragm "beds in" when the sphere is re-pressured it has already sealed itsef to the top.

Front spheres. These are under the bonnet and run very hot, as a rule of thumb will lose about 10- of pressure per year Charge pressure 55- Half pressure = 2 1/2years

Main Accumulator. This one runs even hotter as it is placed on most of our cars inbetween the engine and the radiator, it will typically lose about 15- per year. charge pressure 62- Half pressure = 2 1/2 years

Rear Spheres. These run a lot cooler and the loss is about 5- per year, although the Xantia bieng a heavy car somwhat broke tjis theory and have been known to lose pressure more quickly with high miles usage. Charge pressure, saloon 30-, Estate 40- Half pressure 1 1/2 - 2 years

So you can see from this that spheres do need to be changed regulary to keep the citroen running as comfy as Mr Citroen intended.

Why do they lose the pressure?
It's an unfortunate rule of physics called "Grahams law" if you are bored, a quick google will tell you all! But basicly, if you get a helium balloon and stick it in a room it will go up and stay on the ceiling, within a couple of days the balloon will start to sag and drop to the floor, there are no holes so where has the gas gone? Thats Grahams law for you. Gas migration through the molecular structure.

Spheres that fail early.
Of course you can get a duffer, from Citroen, GSF and Pleiades, nobodys quality control is perfect and the odd duffer will slip the net.

Bad recharging.
I have allways said that recharging a sphere with the sort of kit that used to be available was bad, it is engineering on the basis of getting away with it, recharging without inspection is fine if it works, but when it goes wrong, it's normally in the middle of the night and in the middle of knowhere.

Hydraulic Fluid.
If you have a couple of spheres that fail early then I would allways suggest that you check to make sure that nobody has topped your system up with brake fluid, this will kill all of the spheres and every other rubber part in the system. We developed Hydraflush just for this common problem!

How do I check my fluid?
Taste is the answer, sorry chaps but it's tasting time! Stick a clean finger into the resevoir and wiggle it about in the fliud, then touch the tip of your tounge, LHM is very bland with no real taste, but if there is any brake fluid in your tank it will be very bitter on the end of the tounge, sorry but that is the only reliable way that I have found to test it!

Well thats about it except to put an apology in for any bad grammer or spellings.

If there are any questions, I and I am sure that others will try to answer.

Have a good tasting session!!!

Martin.

[Peter.N.]

Thanks Martin. Could you post this on club-xm.com as well.

Thanks, Peter.

[Mandrake]

Hi Martin,

Thanks for the informative post. Yes I have definately noticed rear spheres don't last long on a Xantia, mine are in need of doing again after 2 years, although I'm quite fussy about the ride.

Do you have any comments on the multilayer spheres used on the front of some Xantia's, in regards to the rate of pressure loss, and the rechargeability of them ?

I've found they seem to last 5 years with no measurable loss in pressure, (less than 5%) and even after 8 years they have only dropped about 10%. What have you found ?

Also, do you attempt to recharge multilayer spheres ? Citroen specifically warn against it and the rumours are that regassing them is not successful, (in the long term) and the reasoning I've heard given is that they last most of their life with a very low leakage rate and then eventually (some time around 10 years old) the middle non-porous layer of the diaphram fails and cracks, leading to a leakage rate comparable to a "normal" sphere, and if they are regassed at that point, they will lose pressure again quickly.

I've also heard the cracked middle layer can lead to failure of the rest of the diaphram in a short time due to physical abrasion.

Interestingly enough - my Xantia which has 3 multilayer spheres on the front, appears to have had the Hydractive centre sphere regassed - it has had a keyway ground on the side of the filler plug and shows definate signs of being regassed - and when I first got the car 2 years ago I measured the sphere and found it had 82 bars in it instead of the 75 stamped on the sphere, whether that was an accidental overcharge or done on purpose by someone trying to cure it's harsh ride, (which was other problems) I don't know.

2 years later and that sphere doesn't seem to have lost any appreciable pressure.

One more question - have you observed any statistically significant difference in the rate of pressure loss with age in spheres from an anti-sink model, compared to models without anti-sink ?

There is quite a bit of debate on this point in the forum but I'm beginning to think that the anti-sink system does in fact hasten the pressure loss of suspension spheres - by keeping them under oil pressure all the time, compared to an older model where the spheres are getting a "rest" any time the car is parked more than a day or so. (With the sphere diaphram button resting on the sealing hole)

Regards,
Simon

[JohnD]

Hi All.


How long do they last???


Front spheres. Half pressure = 2 1/2years

Main Accumulator. 2 1/2 years

Rear Spheres. - 2 years
.

Martin.

Do these life-times also apply to C5's?

[Mandrake]

Hi All.


How long do they last???


Front spheres. Half pressure = 2 1/2years

Main Accumulator. 2 1/2 years

Rear Spheres. - 2 years
.

Martin.

Do these life-times also apply to C5's?

All C5 spheres are multilayer diaphram types, so that would be a no.....their lifetime should be comparable to the multilayer spheres found on the front of some Xantia's - approx 8 to 10 years...

Regards,
Simon

[AndersDK]

Good description Martin -

However most of your lifetime predictions are based on assumptions that work space temperatures accelerates the gas migration. Is that really true that Nitrogen gas migrates faster through any molecular structure at elevated temperatures ?

I thought it to be of less importance, being it only the material itself with its given molecular structure.

Another point is the ongoing discussion that later type multilayer membrane spheres should last a lot longer. I dont think thats the holy grale, as no matter how many membranes there are between 2 cavities with different pressures, there will still be a migration through all membranes at approximately the same rate, as being it only one membrane to pass.
Reason is that any loss migrating through one (first) membrane will instantly start migrating through next membrane - and so on so forth.

I do believe however, that multilayer membrane spheres are more resistant to the odd physical/mechanical failure during its service life - simply because of the physical multilayer construction.

[Pleiades]

Hi All. In answer to a few questions.

Peter.
As you may already know I am a complete computor dinosaur, I dont even know what club XM is, but you are welcome to either point me in the direction or copy the bits that you think they may be interested in. The text above is mainly based on the Xantia.

Simon.

The first multilayer spheres were use on the front of the later Xantia, these were only for the front suspension and the front suspension accumulator on the hydractive models.
It is my theory that they were used as a test for the material, this had some success but due to the shape of the sphere, the failure rate was quite high, after a while with the constant flexing of the diaphragm, the plastic memrane of the multilayer woukd workharden and crack, therfore failing. This was put right by the dealer at a minimum of inconvenience to both customer and dealer as they were easy to change fronts, but this told Citroen a lot about the shpere and the fact that they could not use it.
The solution was to use the same diaphragm but change the shape of the sphere, so out come the grey flying scaucer shaped C5 sphere. These spheres were made much wider allowing a more gentle flex of the diaphragm and failure much less likely.
The early multilayer spheres are easy to spot as they are 450cc sized (slightly longer) and they have 3 dimples around the filler plug, see piccy.

Green Multilayer Sphere.....



C5 Type Sphere.
These will last a long time due to the plastic part of the diaphragm having a tighter molecular structure, therefore duffusion is much less.
But, inside the top of the sphere, Mr Citroen decided to build in a self destruct button, this would stop poeple like myself from recharging spheres. There are 3 sharp tangs that hang down inside, when the pressure gets low, these tangs will puncture the diaphragm, making your sphere srap metal, See piccy...

Inside the C5 Sphere.



It is an interesting theory about the anti sink system keeping the spheres in the use mode therefore loosing pressure faster. I think that you are spot on with that one, I had never realy given it any thought.

So the C5 sphere will last a long time but when it's gone, it's scrap, we do not recondition these, we could but we dont because of the age of the sphere when it gets to that stage in it's life will mean either a weakened plastic membrane or a stabbing from the tangs.

Anders.
Time has told us that front spheres lose pressure faster than rears and accumulators faster than fronts, the only reason that I can see for this is temperature related.
The gas can indeed migrate throgh the 2 rubber layers but the plastic in between them has a tighter molecular structure.

It may be worth stating that the old DS pre 1966 that used LHS2 hydraulic fluid had narural rubber for the sphere diaphragms, due to the molecular structure of natural rubber being tighter, the ols LHS2 spheres last a lot longer that the LHM style sphere in use post 1966.

Thats all for now as I am of to London to collect a new Kayak.

Happy Days........

Martin.

[Pleiades]

OOPS!!!!

Sorry about the photo's. They have come out a bit big, it's beyond me of how to make em smaller.

Martin.

[citronut]

i would have thought the more gas has permiated the membrain, it will deteriarate so letting the gas migrate even quiker
regards malcolm

[Pleiades]

Hi Malcolm.

Everything is made from molecules, we learnt this at school (a few years back) the gas diffusing through the diaphragm is just one of those things that happen, you cannot stop it, as said previous, it's grahams law of physics.

The fact that it has happend once does not in any way harm the diaphragm, the sphere can be recharged for a full life over and over again, as long as the diaphragm is checked for damage.

Think of the balloon example, when the helium ballon has been around for a week and lying in a saggy state on the floor, you can add more helium and up it goes again for another couple of days.

I allways use the helium balloon example because everybody understands it better this way.

The problems happen when a sphere has been run to near "flat", this can damage the diaphragm to a point where the the sphere will hold a charge of presure but not for very long, this is the reason why I never did like the on car charging method, as it's a bit of a blow it up and see what happens type of repair and when they fail this makes people think that all recharged speres are naff!!

Martin.

[aerodynamica]

There's a film included in the Citroen car club's collection of videos about sphere recharging in the Netherlands in the early 80s. The rubber diaphragms were scientifically tested for age- related deterioration over 10 years. They then tested the rubber to destruction in the 'elongation test' - taking a test strip and stretching it until it snaps and a hardness test. Amazingly, both tests were more favourable for the 10 year old rubber - it was even found to be slightly softer after 10 years since the presence of oxygen is very much reduced bu having dry nitrogen on one surface and LHM on the other.

The original spheres also had a self destruct set of barbs in them according to this film.

[Mandrake]

Another point is the ongoing discussion that later type multilayer membrane spheres should last a lot longer. I dont think thats the holy grale, as no matter how many membranes there are between 2 cavities with different pressures, there will still be a migration through all membranes at approximately the same rate, as being it only one membrane to pass.
Reason is that any loss migrating through one (first) membrane will instantly start migrating through next membrane - and so on so forth.

Hi Anders,

The key to the effectiveness of the multilayer spheres is that as Martin mentions, two different materials are used - the outer two layers are the same material as standard spheres - Desmopan or Urepan, both synthetic rubbers I believe, while the middle layer is a thin layer of a non-porous (relative to rubber anyway) plastic like material. I actually found the name of this material at one point but I'm not sure where to find it again now.

Suffice to say, the material of the middle layer is too brittle to flex if it were as thick as a normal diaphram, and too weak to be there on it's own at its current thickness - so the outer rubber layers provide the strength and flexibility to support it, and the middle layer acts as a thin non-porus layer.

As Martin mentioned - they do have a tendency for the middle layer to crack due to the extreme angles the diaphram must flex over (although typically after many more years than a conventional sphere will last without regassing) - hence the decision to change to a wide flat sphere from the C5 onwards to minimize the amount of sharp flexing of the more brittle middle layer of the diaphrams.

Also regarding the pressure differential across the diaphram - there isn't a pressure differential (at least not a static one) Any time the diaphram is sitting within its operating range the gas pressure and oil pressure are in equilibrium. Any gas loss is due to migration of the gas particles unrelated to pressure differential. (I want to say osmosis, but I don't know if that is the correct term)

I do believe however, that multilayer membrane spheres are more resistant to the odd physical/mechanical failure during its service life - simply because of the physical multilayer construction.

It actually seems that they're less resistant to mechanical failure than a conventional sphere, (hence the change in shape in C5 spheres to try and combat that) but ONLY if the conventional sphere is regularly regassed.

With no regassing a conventional sphere will fail first because low gas pressure in operation will eventually cause the diaphram to fail due to excess flexing, but with regular regassing the conventional sphere could last a very long time. (maybe even 15+ years)

On the other hand a multilayer sphere will last a long time, but only because it's lack of leakage prevents it being operated at low pressure, but eventually the middle layer will crack, the sphere will lose pressure quickly, and it will then fail - and in a way that makes regassing impossible.

(In other words a multilayer sphere is a long lasting but ultimately disposable/throwaway item which can't have its life significantly lengthened by regular "maintainance")

Regards,
Simon

[AndersDK]

Thanks for discussing this Simon -

Yes - as long as there is a working pressure present - there is NO differential pressure over the sphere membrane(s) - and hence nil or at least only neglegible diffusion.
Its in the "off" state where the suspension system is deflated you have the largest amount of diffusion - because of the MUCH larger charge pressure inside the sphere.

This fact alone also states that a working sphere under "load" (i.e. with a suspension pressure present) will last much longer, as no pressure differential exists to allow for diffuson or any ever so small mechanical leak.

This leads us back to the old discussions also on sphere shelf/storage times.
But here again there is another situation inside the sphere : the membrane(s) are pressed really hard against the sphere's inner steel surface (by its own charge prerssure). And the Nitrogen gas wont diffuse through the steel.
Only the small orifice in the bottom would allow for a smallish diffusion to take place.

[xantia_v6]

I have a theory for why diaphragms leak more when the suspension is pressurised.

When the suspension is pressurised, there is (practically) equal pressure on each side of the diapragm, and (due to the slightly porous nature of the diapragm) within the diapragm. The rubber is in a relaxed state.

When the suspension is de-pressurised, there is now a large pressure gradient across the diapragm, and it gets pushed against the wall of the sphere. The rubber is now slightly stretched laterally, but is highly compressed across its cross section, due to the pressure differential.

The compressed rubber has smaller holes in its molecular structure than relaxed rubber, and thus leaks less.

[Mandrake]

I have a theory for why diaphragms leak more when the suspension is pressurised.

When the suspension is pressurised, there is (practically) equal pressure on each side of the diapragm, and (due to the slightly porous nature of the diapragm) within the diapragm. The rubber is in a relaxed state.

When the suspension is de-pressurised, there is now a large pressure gradient across the diapragm, and it gets pushed against the wall of the sphere. The rubber is now slightly stretched laterally, but is highly compressed across its cross section, due to the pressure differential.

The compressed rubber has smaller holes in its molecular structure than relaxed rubber, and thus leaks less.

I think it's simpler than that - as Anders points out, when the sphere is not under oil pressure the diaphram is pressed out against the steel shell and no nitrogen loss can take place through the steel, and the pressure of the gas pressing on the diaphram will equal the pressure of the steel pressing back on the diaphram.

In this "unused" state the only possible source of gas leakage is through the neck of the sphere, (ignoring the possibility of leakage past the filler plug o-ring) and in the middle of the diaphram is a small metal button approximately 10mm in diameter which, with no oil pressure, rests against the steel orifice in the neck of the sphere effectively sealing it off.

If this metal to metal seal was perfect, there would be no loss of gas pressure and the shelf life of a sphere would be almost indefinate. (Apart from possible deterioration of the membrane ?)

As a metal to metal seal won't be perfect, and may vary a lot from sphere to sphere, the shelf life of different batches of spheres is likely quite different due to manufacturing tolerances, but in all cases is likely to be considerably longer than the life of the same sphere kept pressurized and in operation.

Regards,
Simon