Repeated o-ring failures

[jcliburn]

I acquired and installed a Gator Attachments 60" light duty grapple for my Kubota MX5100 with LA844 FEL recently, and I used it heavily to clear some overgrown ground this three day weekend. I blew out 4 o-rings in the process, all in the same QD connector -- the FEL curl circuit connector at the FEL valve. Three of the 4 failed o-rings are shown in the picture below.

As far as I can tell, they weren't damaged during installation because I was extra gentle inserting them into the groove, I was careful reconnecting the QD each time, and I used the grapple for hours at a time before springing each new leak. My thinking is that the curl circuit has never seen this level of exertion before -- I used the grapple teeth to dig under and pry up on root balls -- a lot of 'em. (Not big ones, but big enough not to want to come out easily.)

The o-rings came from a local supplier a couple of years ago, and I've kept them in a zip-lock bag in a tool cabinet for storage. Thank goodness I had 6 on hand when all this started.

I'm curious: Are o-rings pressure rated? When I go buy some replacement o-rings, should I ask specifically for use in a tractor hydraulic system?

I'd appreciate any insight.

Thanks,
Jay

 

[westcliffe01]

How did you determine what size to buy ? They look like they were either sheared when the coupling was inserted, or were extruded through a gap that was too large.

So here are the variables:
ID
Cross section
Material (Viton ?)
Shore hardness (at least 70)

You are sure that the 2 parts of the QD match each other ? I know in Pneumatics there are many QD's that look the same but have a slightly different pattern.

You are sure you are putting it in the right groove ? Was there perhaps a back up ring in the groove to prevent the O ring from being extruded ? If the O ring is not sufficiently pre-loaded and its volume not sufficiently constrained, the hydraulic pressure has no problem squishing it where it is not supposed to go.

 

[jcliburn]

The QDs match because they came with the tractor.

The cross section is 1/16". The ID is either 3/4 or 13/16. I wrote those numbers down on a 3x5 card when I searched for a replacement for the factory original when it failed a couple of years ago. I stuck the card -- with the failed factory original o-ring taped to the back of it -- in the ziplock bag with the spares. At the supplier back then there was only one size that was a clear match; everything else he had was obviously wrong.

I don't know what the material is.

I tried to get a replacement from a Kubota dealer when the factory original failed, but they'd sell me only a whole QD connector.

Oh, and there's only one groove inside the coupler; there's no other place to insert an o-ring.

 

[moship]

jcliburn,

The o-rings looked like they failed from extrusion. There are a number of possible causes. 1) Metal parts machined incorrectly 2) wrong size o-ring (could be metric size) 3) missing backiing ring(s) if used.

If the same QC is failing, I'd try swapping it with another one to see if the problem goes away. Then again, it could always be associated with the male connector and not the female connector.

Higher durometer o-rings are a bit more tolerant to extrusion, but not a cure all. It's likely the ones you purchased were approximately 70 durometer. Maybe try 90 durometer if you are into changing o-rings versus the entire connector.

Material selection is typically based on fluid compatibility and expected operating temperature range.

For all the o-ring information you ever wanted take a look at Parker's Handbook. Google Parker o-ring handbook and look for the PDF.

 

[jcliburn]

Thanks for the info, guys.

There's a white plastic lining inside the groove. Is that the backing ring?

Swapping connectors is a good idea. The easiest way is to move the dump coupler to the curl male, and the curl coupler to the dump male. Of course I'll have to relearn the joystick.

Or maybe it's time for a new coupler...

 

[westcliffe01]

I have never had cause to examine the QD's on my bobcat, since I have no hydraulically powered accessories. Does the O ring go on the male part of the QD or the female part ? A backing ring is typically made in spiral fashion (almost like the rings that you thread your keys onto on a key ring). The backing ring would go on the opposite side of the O ring to the side that will see the hydraulic fluid. For heavy service they may be much thicker and be a split C shape with a very tiny gap. Often they are PTFE (teflon).

Here is an image:

Make sure the section of the O ring is right. 1/16th is a pretty thin O ring. Better suited to permanent assemblies, vs something that goes together and comes apart over and over. Also, if in doubt, be sure that if it is an internal groove (in female part) that the O ring interferes on the OD when inserted. If it goes on a male part, it should interfere on the ID. This initial interference helps prevent the O ring from rolling from friction when inserted and avoids damage. If it is dry when assembled, lube it with petroleum jelly or grease, since it is very easy to damage a dry o ring on assembly.

 

[Willl]

The o-rings came from a local supplier a couple of years ago

Just to take these rings out of the equation (that and their cheap), a new supply is in order. Break the bank, get the expensive ones.

 

[oldnslo]

Thanks for the info, guys.

There's a white plastic lining inside the groove. Is that the backing ring?

...

Yes that is the back-up ring and it goes on low pressure side of the O-ring. I.e. the Back-up should be towards uou when looking into the end of the QD. These will prevent the O-ring from extruding.

Roy

 

[jcliburn]

Swung by the dealer today and took a look at a new coupler, and sure enough it's just like Roy/oldnslo said it should be. The white teflon backup ring is closer to the coupler opening, with the o-ring underneath it and in the same groove.

I've been installing my o-rings incorrectly, with the o-ring closer to the coupler opening and the backup ring sitting in the bottom of the groove (thanks to gravity). I'll yank it out and install it correctly tonight after work.

Thanks a million, gents.

Jay