perspective on O-ring (images added)

[rjlittlefield]

The backstory here has to do with a failed valve on an underground sprinkler system, which eventually traced to an O-ring that somehow managed to blow apart for no apparent reason.

I was documenting the saga for a friend, got some interesting images, and decided to play with them a bit more for here.

This is the final result:



This was shot with an ancient compact camera, Canon A710 IS, using only its built-in macro capability which includes manual focus. The camera lens does close focus only at its short end, focal length 6mm, which produces a nice wide-angle perspective. ISO 80 at f/4.5, 1 second, 8 frames at smallest focus step. The small grid squares are 1 mm; a replacement O-ring turns out to be 1 mm cross-section by 6 mm inside diameter.

The image has had more processing than I hope is apparent. The perspective of the grid was intriguing, but straight out of camera it has a lot of what appears to be barrel distortion. Unfortunately, the distortion is more complicated than just barrel. Attempting to remove it by lens correction in Photoshop could only turn it from an obvious convex distortion to a less obvious but still annoying "gull-wing" wavy pattern. So I pulled the stacked result into PTGui, set what turned out to be 35 control points to define 5 lines that are supposed to be straight, then asked PTGui's optimizer to find the lens a/b/c distortion parameters needed to make the lines actually straight. It succeeded in getting the geometry good to less than 1 pixel error in the web-sized image. Then I had PTGui render a "panorama" with the lens correction applied, producing an image like this:



I pulled that into Photoshop, applied a couple of passes of lens correction filter to get rid of annoying CA, adjusted curves to bring out some more structure in the broken area of the O-ring, and finally rotated and framed to taste.

Here's the setup for the shoot. The arms and clamps and black pipe are part of my new lighting cage, described HERE. The mirror is to provide fill light for the shadowed side of the O-ring.

(Screen image simulated)




All told, a lot of messing around for what is mostly an educational exercise. I do kinda like the image though.

--Rik

[lothman]

nice work on eliminating distortion, but for somebody who earns his living in sealing technology I would say this foto cannot give any hint for an expert in order to find out the root cause for the failure. The camera did not mange the dynamic range of a black O-Ring on white paper. Everything es deep black without structure. If you use a black (or dark gray) paper as background the camera would be able to document the damage.

[rjlittlefield]

...this foto cannot give any hint for an expert in order to find out the root cause for the failure.

Aw, gee, and I was trying so hard to be artistic!

OK, here's a more clinical presentation:





Here's the diagram from the Champion RK-38C official repair kit:



Ironically, although the kit does include a replacement ring, the diagram fails to show where it goes! It goes at the location shown by the red arrow, sealing around that long shaft.

The shaft provides a manual adjustment for maximum flow rate when the valve is actuated. In normal operation, that adjustment is set-and-forget -- the shaft never moves while the valve is electrically controlled.

History of the failure is that the valve was new about 1 year ago, spring 2021. It operated all summer without incident, was shut down for winter, then put back in service this spring. Again it operated without incident for some weeks, but then was found failed, spraying a fine mist of water around the shaft. The shaft would not have been turned between times when it was working OK and then spraying water.

On disassembly, I found the failed O-ring sitting loosely in its groove on the shaft.

I found no burrs on either the shaft or the matching bore.

I installed a new ring, felt nothing odd, disassembled to check, found the new ring looking pristine, repeated that cycle a couple of times, and finally put the valve back in service where it is still working fine.

I would be at least mildly curious to hear thoughts about what caused the failure.

--Rik

[lothman]

I would be at least mildly curious to hear thoughts about what caused the failure.

--Rik

Rik,
for me it looks like the O-Ring was cut and then after a while burst through. On the other hand you told that there are no burs or sharp corners. Is the a cross bore (without chamfers) where the O-Ring has to be pussed over? Those cutting lines would fit perfectly for that.

[rjlittlefield]

...you told that there are no burs or sharp corners. Is the a cross bore (without chamfers) where the O-Ring has to be pussed over? Those cutting lines would fit perfectly for that.

Not quite right -- I said only that I found no burrs. I did not specifically check for sharp corners without burrs, because even if I had found those, I would have considered them part of the manufacturer's design.

Checking my wetware memories (never reliable, but the best info available right now), I think that the bore was not significantly chamfered.

So then, I am wondering if perhaps the design is OK for almost all O-rings, but this one happened to have some small manufacturing flaw that caused it to hang up on the sharp edge and get cut.

BTW, I was impressed by the amount of O-ring material that was simply missing. Is this typical in such failures?

--Rik

[lothman]

BTW, I was impressed by the amount of O-ring material that was simply missing. Is this typical in such failures?

--Rik

I think this is a severe failure ;-)

[BugEZ]

Back in the day I was involved in many o-ring failure investigations on aircraft hydraulic and engine lube oil systems. For a sprinkler valve with only one year of service it looks pretty degraded. More than you would expect in a room temperature water system. I suspect the rubber may have been contaminated, causing it to be both weak and brittle. You describe “mist” spraying out after the failure. Unless this leak persisted for many hours I would not have expected that much o-ring material to erode and carry away. And the remnants should not be brittle unless you exposed the ring to heat to promote disassembly.

Faucet rubber seal washers can erode if there is a defect in the seat and water trickles by slowly at first then faster and faster. I had a faucet valve fail in this fashion in back yard. Quite a surprise when I returned from vacation. But the valve and washer were nearly 50 years old. The washer had become brittle. The seat had eroded beyond being repaired. So I replaced the valve and paid a large water bill…

So I suspect a defective o-ring.

Keith

[blekenbleu]

many o-ring failure investigations on aircraft hydraulic and engine lube oil systems

Some o-rings and other elastomer seals are formulated to depend on e.g. hydraulic fluid to maintain their usefulness.
An o-ring [ab]used to center a projection relay lens in an M42 extension tube
was discovered this morning to have spontaneously disintegrated into multiple short segments:

That sprinkler seal failure may be another instance of inappropriate material selection.