Hydraulic U seal installation info needed...for starters, "pre tension element" ??

Hydraulic U seal installation info needed...for starters, "pre tension element" ??

Single acting pancake cylinder (releases drawbar on VMC)... the original seals were so deterioated they basically fell apart during removal so I wonder about two things before installing the new seals -

1. As there was no "pre tension element" (simply an 0 ring ?...see 2:07 in below video ) in the original setup, should I perhaps add one now, regardless ? I did think it odd that the ID where the seals mount, is larger than the OD of the seal. So now I wonder if the pre tension element was left out and that is the reason the seals failed ??

2. As the original seals were toast it didn't occur to me to pay attention to the orientation as I removed the remnants... should the seal lip angle toward the pressure, or away from it ? The video at 4:32 would indicate that the lip angles toward the pressure...which makes logical sense...but just to be sure, you tell me.

It's possible that the groove is shallower than the seal cross-section, which would force the OD to be in contact with the bore when installed.

O-Rings are a common "energizer" element for non-elastomer seals. That video is for Turcon, which is a trade name for a specific blend of PTFE, which does not have its own elastic memory. To make it usable, a compressible rubber element is used to ensure that the sealing lips are pressed against the faces to be sealed. If your seal is a u-cup that is already made of a compressible rubber, it's not as important because it will already be in contact from its own resilience.

Seal lip orientation should always face the pressure. The fluid gets between them and forces them to contact the bore and shaft more firmly. The other way, the pressure would just bend them towards center and make a leak path.

What did the seals look like? Did they fall apart when you pulled them out, or did they look all torn up when they were still in the assembly before you touched them?

BoxcarPete said:

What did the seals look like? Did they fall apart when you pulled them out, or did they look all torn up when they were still in the assembly before you touched them?

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The latter... and fell apart even further when removing with dental pick... was lucky to find a complete remnant where I could read the part number via microscope. Most curious on a 2011 machine with low hours...wonder if the seal manuf had a bad run of material back in 2010 or what..

Re the tension element, so if seals are normal polyurethane no need for the O ring, yes ?

Milacron said:

The latter... and fell apart even further when removing with dental pick... was lucky to find a complete remnant where I could read the part number via microscope. Most curious on a 2011 machine with low hours...wonder if the seal manuf had a bad run of material back in 2010 or what..

Click to expand...

If they're all chewed up inside the cylinder, the most likely failure mode is excessive pressure for the rubber's physical properties. It's possible that your assembly was made with the wrong hardness rubber, or the clearance gap between the piston and the bore is too large. If it continued to fall apart as you disassembled it, it's also possible that the oil and rubber were incompatible with each other, and it broke down the rubber. I assume there's nothing special about the hydraulic fluid you have in the system?

According to the website of the manufacturer you listed in your other thread, the part number is the same for all the different rubbers they make that part, so we may never know for sure what it really was...

Yes, on U section or V style section, the grove - lip side wants to see the high pressure. Rubber energizers are common in the grove of the seals. Pays to fit the seals in a warm condition too, so there nice and flexible, a saucepan of nice warm - hot to the touch water makes it go a lot easier.

Again, all polyurethane seals get cast from 2 component resin, incorrect mix ratios and impurities can cause the seals to disintegrate in just a few years though seam fine on original assembly.

Some polyurethane seals do have Energizer o rings, its common on rod seals in particular, o ring simply wedges the seal lips apart, can really reduce seepage rates on low pressure hydraulics but at the expense of added seal friction. Lots of piston seals have a mix of different compounds and often parts, acting both as seals and guide elements + energizing elements. Normally most seals that need orings to energize them come with them, often assembled in the pack. That said, its pretty common to then pop them apart and add the parts separately not just try and stretch the whole piston seal on in one go. Smaller seals are always the hardest, big 8" piston seals are dead easy same with large rod seals.

Just an update that as it turns out the groove depths were exactly correct for the 6mm wide seals such that there is no way I could have added "pre tension elements" even if I wanted to. As of yesterday, all back together, cylinder now works properly, tool is finally released from the drawbar.

Milacron said:

Just an update that as it turns out the groove depths were exactly correct for the 6mm wide seals such that there is no way I could have added "pre tension elements" even if I wanted to. As of yesterday, all back together, cylinder now works properly, tool is finally released from the drawbar.

Click to expand...

Just out of curiosity, did you end up doing a direct replacement with an NBR U-Cup, or did you change it over to a urethane polyseal?

BoxcarPete said:

Just out of curiosity, did you end up doing a direct replacement with an NBR U-Cup, or did you change it over to a urethane polyseal?

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Different manufacturer, but direct replacement. The standard seals should last for decades in this application.....all I can figure is Valqua happened to have a bad run of material back in 2010 or 11.

Photo below of original seals... besides the rarity of becoming so brittle so quickly, it is curious the black crud on the seals...the cylinder rod/walls and fluid were clean/normal in appearance....the crud was only on the seals.

The crud may have been assembly lube.
I have seen some that looked black grease with lots of moly.