Lubrication and Installation of Elastomeric Seals for Fluid Service

[WKTaylor]

Guys... I have a dilemma and need Your advice.

NOTE. All my aircraft’s ‘systems’ have hydrocarbon or synthetic hydrocarbon fluids [primarily fuel, engine-oil, hydraulic-oil]. Lubricating grease is omitted from this question.

Old technicians [and older tech data] taught me to install rubber seals [O-rings, gaskets, etc] after ‘soaking’ [conditioning] the seals in ‘the fluid’ that the system uses… for a few minutes [~15-minutes or more]… or a similar type oil or fluid [clearly compatible with 'the fluid', such as the 'universal fluid' MIL-DTL-5606 hydraulic oil]. After ‘soaking’ the seals… started well-before final component assy… the part was then ready for installation. The rational I have for soaking rubber parts in fluid is that the ‘fluid’ needs time to absorb slightly into the rubber surface pores hence ‘better-wetting’ the part surface and preparing the rubber for [that] fluid service.

Recently I have encountered tech documents where petrolatum is the lubricant of choice. ‘Butter’ lube the part lightly with petrolatum and then immediately install it into the Assy. NOTE: some documents do warn against this practice [using petrolatum] for non-hydrocarbon fluids, such as Skydrol.

Petrolatum is specifically topical and is a grease-like substance that may/may not be 100% hydrocarbon fluid compatible… but appears to be ‘assumed-to-be compatible’ by these documents.

Problem/concern.

A LOT of our elastomer seals [O-rings] are MF Nitrile rubber per AMS-P-5315.

Per ‘AIR786 Elastomer Compatibility Considerations Relative to Elastomeric Sealant Selection’, the -5315 rubber is highly compatible [Satisfactory] for use with hydrocarbon fuels and oils [S rating]; but it is rated ‘Fair’ [F rating] for petrolatum… which sorta confirms/reinforces my discomfort with the petrolatum lube practice… especially when assys may be shelved/left-open indefinitely before use.

What is/are your experiences with seal lubrication for installation into a system, RE this question [and/or etc]?

NOTE. These acft have been transitioned from JP-4 to JP-8 and now are running mostly Jet-A+additives. There have been similar changes revisions] to system hydraulic oils and engine oils.


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[MikeHalloran]

I don't 'do' aircraft.

I have worked on terrestrial systems including aircraft simulators (Skydrol and 5606) and subway cars (Skydrol), assorted industrial hydraulic systems (whatever), and my personal vehicles, fluids as per OEM.

I have come to prefer a light silicone grease for all rubber seals and o-rings, applied by squeezing a few microliters from a tube into the distal joint of my index finger, and rubbing the grease all over the seal, until a light film is visible.

I did do a soak on fork slider seals for a motorcycle once, as per specific instruction in the service manual, and got a complaint from the customer because the fork tubes got wet on the first cycle, which the idiot assumed meant the seals had failed.



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

Silicone can be bad for some rubbers just as petrolatum can be. And not all 'nitrile' rubbers are the same. So you do have to be careful and rely on actual testing. Usually the o-ring manufacturer has done this testing.

I have not seen anyone use a presoak on o-rings. All rubbers will swell to some degree in any fluid. If the rubber swells a lot it is considered incompatible with that fluid. O-rings should be coated with the applicable fluid before installation to lubricate them because dry rubber is usually high friction, and that friction can interfere with proper installation and operation of the o-ring. The main advantage of petrolatum is it will hold o-rings in place during installation. That might explain the advantage of a presoak. Swelling the o-ring before installation might help to hold it in its groove.

 

[btrueblood]

Silicone oils are only bad for silicone and fluorosilicone rubbers. But Skydrol and other phosphate-ester hydraulic fluids will eat nitrile o-rings, so you have to use EPDM seals. Similarly, EPDM seals get eaten by hydrocarbons. So, silicone grease/oil is a pretty good universal o-ring lube in shops that have both types of service. As far as soaking rings in the fluid of use, that sounds great until the fluid in question is something nasty like hydrazine. And, it's not a given that the fluid will provide much lubrication for installation. The swell idea sounds good, but should you really be using an oring material that swells in its fluid service? I mean, you need to swell about 25% to make the ring "stick" in common sized grooves. Sticky silicone greases are available when you need to hold the rings in the groove.

 

[ERAU82]

Will,

I've used a gelled lube product from AFS - they make MS-352B for use with Skydrol and AFS-682 for use with 'red' oils, both mineral based (5606) and the newer synthetics. They are great for lubrication of seals and also for holding stubborn split back-up seals in-place during installation. Parker makes O-Lube and Super O-Lube which works well.

I've seen both the AFS and Parker products referenced in OEM CMM's. All contain some % of silicone and dissolve fully in the hydraulic fluid being used. The only warning is that as a hydraulic MRO shop, you must monitor the silicone level of the test bench fluid. You must change out the fluid when your silicone level exceeds the OEM specified level.

Regards,

Brian Meyer

 

[WKTaylor]

I just downloaded a couple of Parker

http://www.parker.com/

catalogs...

Parker Fluid Power Seal Design Guide Catalog EPS 5370

Parker O-Ring Handbook ORD 5700

and also watched a Parker O-ring lube video

All of these allow/recommend use of system fluid... but seem to emphasize use of 'Parker O-Lube' or 'Parker Super-O-Lube' or petrolatum.

Hmmmm... I suspect my perceptions may have been formed by decades old training that may have obsolete principles/practices... that 'work' but may have evolved to better practices and materials.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]