O-ring failures on ...
 

O-ring failures on SSP  

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KW
 KW
(@kw)
Joined: 2 years ago
Posts: 186
2018-06-11 07:49:39  

I maintain a fleet of Daisy 853s for my gun club veterans league.  I am having regular failure of the o-ring between the pump head and the valve.  Its the ring that seals the air reservoir after the pump.

The failed rings look like the skin has been peeled away from a spongy interior.  Not like a cut, but as if there was a flaw in the ring and the air actually got inside and blew the skin off.

I only lube with silicone oil....but these are silicone rings.  Am I degrading the ring by lubing with the same oil?  I can switch to a buna ring if need be.

TIA.

EXPERIENCE, n. The wisdom that enables us to recognize as an undesirable old acquaintance the folly that we have already embraced.


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fivestar45
(@fivestar45)
Joined: 2 years ago
Posts: 26
2018-06-11 08:47:06  

In my experience silicone o rings do scuff easily. You might want to try mil spec urethane.


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nervoustrigger
(@nervoustrigger)
Joined: 2 years ago
Posts: 184
2018-06-11 09:15:27  

Yes, go with a quality polyurethane or even a Buna-N.  Silicone is the worst of all common O-ring materials in terms of abrasion.  It is also the most permeable so it may indeed be that it’s swelling and contributing to the failure.  


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KW
 KW
(@kw)
Joined: 2 years ago
Posts: 186
2018-06-12 12:08:30  

Thanks all for the comments.  Will start swapping out the silicone rings with BunaN as they come through.

The ring that fails is not ever scuffed.  It is a static seal, not one that is moving around.  The only stress on it is the air pressure of the pump action.  I thought it would be safe, but it seems to fail under the pressure.

Thanks again.

K

EXPERIENCE, n. The wisdom that enables us to recognize as an undesirable old acquaintance the folly that we have already embraced.


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nervoustrigger
(@nervoustrigger)
Joined: 2 years ago
Posts: 184
2018-06-12 14:32:14  

Ah okay, yeah I thought you were talking about an O-ring on the piston head so my comment about abrasion was irrelevant. For the silicone to be failing as a static seal, and based on your description, I bet it’s an extrusion failure. One of silicone’s weaknesses is its resistance to tearing so I suspect the material is getting squeezed into the extrusion gap and eventually tears away. Simply changing to a higher durometer will improve things, and going with either polyurethane or Buna-N will improve it further.

Generally 70 durometer is suitable for pressures up to 3000psi based on typical part tolerances (commonly used in PCPs with a 3k fill, for example). So perhaps the Daisy has a slightly oversized extrusion gap but 3000psi is probably 2-3x the pressure developed by a match style single-stroke pneumatic so I expect a 70 durometer Buna-N will do the trick.

Personally, I wouldn’t make the jump to 90 durometer without at least first trying the 70. A harder material needs more pressure to seal completely, and since we’re talking about a SSP where every cycle involves pressurizing from zero, you might get some blow by…i.e. erratic pressure developed from one shot to the next, thus erratic velocities. If the sealing surfaces touched by the O-ring are of very high quality (no machining marks, scratches, or texture), then you could probably get away with it.


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Will Piatt
(@will-piatt)
Member of Trade
Joined: 2 years ago
Posts: 52
2018-06-12 16:27:43  

A couple of years ago I received one bad bag of Buna N o-rings that quickly failed similar to your description in PCP guns. They delaminated. I had to warranty several guns because of this and it was during this process I isolated the problem. Recently I  noticed on  a couple of bags of new o-rings I received the label says "10 year shelf life" and another said "15 year shelf life". Both were Buna N 70 o-rings from McMaster Carr. There was no discernible dating on the label and I had no way to know how long they had already been on the shelf, nor if shelf life includes service life? I would suspect that the less common the size the higher the probability they are on the shelf longer.  


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KW
 KW
(@kw)
Joined: 2 years ago
Posts: 186
2018-06-13 08:04:58  
Posted by: nervoustrigger

Ah okay, yeah I thought you were talking about an O-ring on the piston head so my comment about abrasion was irrelevant. For the silicone to be failing as a static seal, and based on your description, I bet it’s an extrusion failure. One of silicone’s weaknesses is its resistance to tearing so I suspect the material is getting squeezed into the extrusion gap and eventually tears away. Simply changing to a higher durometer will improve things, and going with either polyurethane or Buna-N will improve it further.

Generally 70 durometer is suitable for pressures up to 3000psi based on typical part tolerances (commonly used in PCPs with a 3k fill, for example). So perhaps the Daisy has a slightly oversized extrusion gap but 3000psi is probably 2-3x the pressure developed by a match style single-stroke pneumatic so I expect a 70 durometer Buna-N will do the trick.

Personally, I wouldn’t make the jump to 90 durometer without at least first trying the 70. A harder material needs more pressure to seal completely, and since we’re talking about a SSP where every cycle involves pressurizing from zero, you might get some blow by…i.e. erratic pressure developed from one shot to the next, thus erratic velocities. If the sealing surfaces touched by the O-ring are of very high quality (no machining marks, scratches, or texture), then you could probably get away with it.

I understood all of that!  I must be doing this too long.

One reason I chose silicone rings is that pressure on these guns is so low.  My hope was that the ring would be soft enough to deform quickly and not allow the blow by.  

Will try a couple of buna rings of different hardness and see if I get too much blow by  (and reduced pressures).  

Thanks for the notes!

EXPERIENCE, n. The wisdom that enables us to recognize as an undesirable old acquaintance the folly that we have already embraced.


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KW
 KW
(@kw)
Joined: 2 years ago
Posts: 186
2018-06-13 08:07:11  
Posted by: Will Piatt

A couple of years ago I received one bad bag of Buna N o-rings that quickly failed similar to your description in PCP guns. They delaminated. I had to warranty several guns because of this and it was during this process I isolated the problem. Recently I  noticed on  a couple of bags of new o-rings I received the label says "10 year shelf life" and another said "15 year shelf life". Both were Buna N 70 o-rings from McMaster Carr. There was no discernible dating on the label and I had no way to know how long they had already been on the shelf, nor if shelf life includes service life? I would suspect that the less common the size the higher the probability they are on the shelf longer.  

Shelf life is killing me.  I use natural latex bands and tubes for slingshots.  To save money I buy in bulk...then I dont' use the materials fast enough and they degrade.  I am now storing everything in the dark, wiped with a protective, in a vacuum packed baggie.  Lots of work to save a few shekels.  

EXPERIENCE, n. The wisdom that enables us to recognize as an undesirable old acquaintance the folly that we have already embraced.


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Gratewhitehuntr
(@gratewhitehuntr)
Joined: 2 years ago
Posts: 264
2018-06-13 08:16:37  

I'm in the same boat on a speargun, waiting for the rubber tubing to come back and b**** slap me in the face while loading it.

 

UV and ozone contribute to degradation, and I've got it in the dark and have coated the band in wax (water carrier),  since slippery stuff is a no-go.

Still, every time I load the thing, I fully expect to have narrowly escaped with my eyes..... I swear it must have a 120Lbs draw.... and a big piece of rubber tubing.... freaky.

 

on topic of deformation and extrusion

If you think about how o-rings behave in the hand, it does not take much pressure to deform them. (easier than quoting psi data on o-rings)

Here is a write up on grip strength,  pinch strength in males with manual occupations is 8.5kg. https://www.rehab.research.va.gov/jour/70/7/2/swanson.pdf

That is quite a low threshold for proper pressure under fitment.

 

To be less scientific, o-rings that can be damaged between fingernails are too soft, IMHO, for all but non-critical uses.

I'd rather buy better o-rings than have to go-back.

Will mentioned having to warranty guns..... that is exactly the kind of money sucking BS I strive to avoid ... when the cheap becomes expensive. (no offense to Will, shelf life etc)

There you have it, the world according to John.


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classicalgas
(@classicalgas)
Joined: 2 years ago
Posts: 70
2018-06-15 18:30:48  

I've read that silicone lubes degrade silicon rubber, though i don't remember where.That may be contributing to your problem.


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