Sealing Shielding

Aerospace EPDM Meets NAS1613 Revision 2 and 6, Is There a Difference?

EMG_163_Meets_Both_NAS1613_REV_2&6_PLANE_TAKING_OFF_PARKER_ORING_&_ENGINEERED_SEALS__DIVISIONPerhaps you know Parker’s newest aerospace focused EPDM material is EM163-80. Featuring breakthrough low temperature functionality, resistance to all commercially available phosphate ester fluids, and the ability to be made into custom shapes, extrusions, and spliced geometries, EM163 represents the best-in-class material for applications needing to seal phosphate-ester-based fluids. The latest news is that EM163 meets the full qualification requirements of both Aerospace Industries Association (AIA-NAS) NAS1613 Revision 6 (code A) and the legacy Revision 2 (no code). We’ve been inundated with questions about the specification differences between Revision 6 and 2, enough that it makes sense to devote a blog topic explaining the fluids, conditions, and dynamic cycling requirements which are required to qualify EM163-80 to each specification.  

The easiest part of this comparison is evaluating the areas of Revision 6 which are very much a copy and paste from Revision 2. Compression set conditions, aged and unaged, plus temperature retraction requirements, aged and unaged, are identical. Lastly, both specifications require a test to verify the elastomers will not corrode or adhere to five different metal substrate materials. That is pretty much where the similarities end.  Now for the contrasts.

Specimen size

The first subtle difference is the specimen size. Both specs require testing to measure the change in physical properties and volume following a heated immersion in phosphate ester fluids. For the most part, No Code qualification requires testing to be completed on test slabs or O-rings, while the newer revision, Code A, requires testing on test slabs AND O-rings. Not a big difference, but still, a difference.

The fluid conditions are very similar in both specs, but not identical. There are only two temperatures for the short term 70 hour exposure: 160°F and 250°F. Another similarity is that the longer soaks are at 225°F for 334 and 670 hours. The more difficult A Code also requires 1000 and 1440 hours at 225°F. We begin to see the requirements for the later revision are more reflective of the industry conditions, right?

Fluids

Next, we look at the fluids, which truly are a key difference between the two documents. Revision 2 fluid is exclusively for AS1241 Type IV, CL 2 while revision 6 states the elastomers must meet “all commercially available AS1241 Type IV, Class 1 and 2, and Type V”. Table 1 outlines the AS1241 fluids in context of both NAS 1613 revisions.

 

    Revision 2 Revision 6
Low Density Hyject IV A Plus AS 1241 Type IV class 1   X
Low Density Skydrol LD4 AS 1241 Type IV class 1   X
High Density Skydrol 500B-4 AS 1241 Type IV class 2 X X
Low Density Skydrol V AS 1241 Type V   X
Low Density Hyjet V AS 1241 Type V   X
Low Density Skydrol PE-5 AS 1241 Type V   X

 Table 1: AS1241 fluids

EMG_163_Meets_Both_NAS1613_REV_2&6_ORINGS_PARKER_ORING_&_ENGINEERED_SEALS__DIVISION

Basically, to pass Revision 6, the material must demonstrate compatibility for all six commercially available fluids, while Revision 2 only has one fluid which is must be verified for compatibility. Again, we see Revision 6 is much more comprehensive than Revision 2.

Endurance Testing

Last, we look at the functional testing of the materials, referred to as dynamic or endurance testing. Both specifications require endurance testing on a pair of seals, which have been aged for a week at 225°F. The appropriate fluids are outlined in the table above.

Revision 2 has a gland design per Mil-G-5514. There is a 4” stroke length and the rod must travel 30 full cycles each minute. The rod is chromium plated with a surface finish between 16-32 microinches. PTFE anti-extrusion back up rings are necessary for the 3000 psi high pressure cycling. A temperature of 160°F is maintained for 70,000 strokes and then increased to 225°F for an additional 90,000 strokes.

Revision 6 has a much more demanding endurance test with fives phases and slightly different hardware. The rod must be a smooth 8 to 16 microinches Ra with a cross-hatched finish by lapping, and the cycle is 30 complete strokes per minute but only 3” rather than 4”, which means the speed can be more conservative. A pair of conditioned seals are placed in AS4716 grooves, adjacent to a PTFE back up ring. Similarities to Rev 2 are that there is a pressure of 3000 psi for the dynamic cycling at both 160°F and 225°F, however before and after each high temperature cycle there is a low temperature, -65°F soak. The first soak is static for 24 hours, followed by the 160°F high pressure cycling. The second low temperature soak requires 10 dynamic cycles at ambient pressure followed by 10 cycles at 3000 psi. The final low temperature soak requires one hour static sealing at 3000 psi followed by an 18 hour warm down period. 

EMG_163_Meets_Both_NAS1613_REV_2&6_PLANE_LANDING_GEAR_PARKER_ORING_&_ENGINEERED_SEALS__DIVISIONIf you read carefully through the tests, you begin to see the Revision 6 seals must go through a more rigorous test with harsh low temperature, low pressure conditions. However, Revision 2 is not without its own challenges. The required hardware configuration; ie, low squeeze and more rough surface finish, is far from optimum and not what we recommend in actual service conditions. Added to the difficulty is the longer stroke length and faster speed. The fact that EM163-80 has passed both aerospace specifications proves it is the next generation EPDM seal material ready for flight.

 

For more information on this or other aerospace topics, visit Parker O-Ring & Engineered Seals Division online and chat with our experienced applications engineers or email us at oesmailbox@parker.com 

 

Attending the 2019 Paris Air Show? 

The Paris Air Show is the largest and longest running aerospace trade show in the world, bringing together all the players in this global industry around the latest technological innovations. Visit Parker Engineered Materials Group in Hall 5, Stand C210 to learn more about our solutions for your sealing, shielding and thermal management challenges. 

 

Tetraseal: An Alternate Sealing Solution When An O-Ring Isn't Working, Dorothy Kern

 

 

 

This article was contributed by Dorothy Kern, applications engineering manager, Parker O-Ring & Engineered Seals Division.

 

 

 

 

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O-Ring Sealing for Maximum Service Life in Aggressive HTS Turbine Oils

New Low-Temperature Compound Improves Jet Engine HTS Oil sealing Performance

New Fluorocarbon Seal Material Extends Safe Life of Jet Fuel O-Rings

 

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