Spheres and failure modes

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CitroenCrazy
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Spheres and failure modes

Unread post by CitroenCrazy »

I've been working on a couple of Xantias of late and ended up with a pile of spheres that had low or no pressure.
To amuse myself while it was raining, I cut them open on the lathe.

This first picture is of a sphere I replaced because it was down to half its nominal pressure. It was fundamentally healthy, just old.
You can see here just what's inside one of these mysterious green balls.
Sectioned sphere 01.JPG

This shows the metal section that closes the membrane.
The idea is that this metal dome is secured by the screw at the top, creating a gast-tight seal between the edge of the membrane and the upper body of the sphere.
Sectioned sphere 02.JPG

Now we have a comparison between a good membrane and one from an accumulator that was completely dead, probably as a result of heat.
As well as being black, the bad one was also much harder and less flexible than the other.
Sectioned sphere 03.JPG

This shows the split in the membrane
Sectioned sphere 04.JPG

So far, so predictable.....
Next, I sectioned two dead rear spheres; and when i say dead, I mean zero pressure and full of LHM.
When I split them, the membranes were still green and flexible.
The fault lay in the metal domes, which both showed fatigue-induced cracks, starting at the edges of punched holes. As a result the dome has deformed and the seal between the membrane and the sphere body has failed.

This shows a comparison betweem the domes of aftermarket spheres and what I think are genuine Citroen parts (which have the pressure stamped into the shell, in addition to ink-jet printed part numbers).
Sectioned sphere 05.JPG

A slightly blurred close up shows the cracks a bit better. What's not very clear in the photos is there's distortion around the D shaped holes, and rather than the curve of the D being smooth, it comes to a point in the middle. If you were setting out to design a stress-raiser, it would be had to do a better job. The Citroen parts are significantly better finished.
Sectioned sphere 06.JPG

So, mildly interesting ?
People talk a lot about spheres losing pressure over time, but I haven't seen any discussion about fatigue failures before.

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GiveMeABreak
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Re: Spheres and failure modes

Unread post by GiveMeABreak »

Thanks for posting that up. I've often suspected that aftermarket ones are inferior quality and this looks to show just that. The poor finish of the inner metal housing has clearly left the rectangular shape in the membrane and the distorted shape may well of contributed to the cracks and ultimate failure.

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CitroenCrazy
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Re: Spheres and failure modes

Unread post by CitroenCrazy »

Hi Marc
Yes, it's been an interesting afternoon.
In total, I've cut open 5 spheres; these are the membranes.

On the left is the good one; it was in a sphere that still had 20+ bars of pressure.

In the middle are two from Xantia accumulators, where I think the heat has killed them. Both were quite rigid and hard compared to the first.

The 2 on the right are the ones where the metal domes suffered fatigue cracking. They are still flexible, like the the one on the left, though deformed because they've been compressed against the sphere body when there was no gas to act as a cushion.
comparison of membranes 1.jpg

Gibbo2286
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Re: Spheres and failure modes

Unread post by Gibbo2286 »

I think you're kidding us, you've been in Katie Price's dustbin. :)

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white exec
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Re: Spheres and failure modes

Unread post by white exec »

That's a splendid bit of sphere dissection, done really tidily on the lathe.

I opened up a genuine Citroen sphere here some time ago - cut it into cross-section with a thin angle-grinder disc...
DSCF2158.JPG
The punchings and fold-over in the metal cap were also nicely smooth, and would have added strength to the hole, I guess.

I ought to cut open some more, really. Have a few here, all genuine Citroen.
Interesting to see your different membrane materials.
The mechanics' handbook (Xantia-XM-Synergie) lists the membrane type (Multicouche, Desmopan, Urepan) for each sphere, along with its 96....... production number, which is printed on the sphere.

The M diaphragm (the most heat-resistant, iirc) is the subject of a Citroen patent, so I guess aftermarket manufacturers would not be using it.

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CitroenCrazy
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Re: Spheres and failure modes

Unread post by CitroenCrazy »

Nice work Chris, I don't think I could be that precise with a cutting disc