Dynamic O-ring failure Short Stroke

[nickjk]

I am working on a project that is experiencing o-ring seal failure during life testing.
Any suggestions or thoughts to improve seal life would greatly be appreciated.
The seal design applied has been used in other very similar product with great success
(smaller in bore size)
Note: all dimensions are metric "mm"
Current Design:
Type of application Dynamic outer seal, single acting
O-ring Size 3.55 x 36.5 ISO 3601 #C0365G
O-ring Material 70 Dur. NBR
Bore Size 44.020 +/-.020
Piston Size 43.931 +/-.020
Groove Dia 37.77 +/-.03
Total Diametral Clr. .089 +/-.039
Gland Depth 3.125 +/-.025
Gland Width 4.42 +/-.13
Stretch 3.71%-5.62%
Squeeze 8.18%-14.6%
Gland Fill 65%-78%
Close Inspection of the bore indicated that the surface finish was not uniform around the bore.
I took Ra measurements every 90 degrees with micro inch readings of 8.93Ra, 9.69Ra, 8.08Ra, 21.66Ra.
The bore checked to be .033 TIR out of round. It is believed that the bore went out of round prior to the roller burnish and for that reason there is an area when measuring the finish that is rough.

Piston stroke is very small 1.00mm

Attached you will find a pic of the failure. it appears that the o-ring is shredding. Material can be found on top of the o-ring in the groove.

I believe that because of the small stroke and the single bore area with high surface finish the O-ring is rolling (static instead of dynamic) during the operation and as the o-ring rolls it rubs against the gland walls and groove which are rougher causing the material to shred.

We were able to alter the bores to improve surface finish for future testing.

My question is, has anyone seen this condition before with a short stroke application?
Would a 90 Dur. O-ring help to prevent the o-ring from rolling if that is the problem?
Although the stretch, squeeze and gland fill are what is recommended by the manufacture do these values change for short stroke applications that may be subject to the o-ring rolling?
Although the stretch, squeeze and gland fill are what is recommended by the manufacture do these values change as the bore increases (more surface friction)?

Again, any recommendations would truly be appreciated

Thank you,
nickjk

 

[btrueblood]

Squeeze seems a bit high for dynamic seal. What max. pressure differential is the o-ring operating at in each direction? Higher durometer might help, but if the ring is partially extruding (which it sounds like), you may want to try backup rings.

 

[nickjk]

Btrueblood,

Operating pressure 1015psi max 362psi min. one direction
Single acting application 0 psi opposite direction.

8.81%-14.6% squeeze is in the range of what is recommended for a dynamic reciprocating o-ring.
(Parker spec's 9%-16% Treleborg spec's 6%-20%)

The diametral clearance is also .002"-.005" which is what Parker recommends.
Not sure what would be causing the oring to extrude, pressure is low and clearance is good.

Thanks for your response

Nickjk

 

[gerhardl]

Must the O-ring have the O -form? Other physical form or (additional sealing ring) support for O-ring and/or other form of groove? Better material? Strokes per time unit? Temperature? Could high temperature (also caused by high number of strokes per time unit) lead to friction and higher temperature again leading to shredding? Using o-rings suited for high temperature?

 

[Dougt115]

Your Ra number of 21 is more than twice the other Ra numbers this may be indicating that you have a localized issue. Even an Ra of 8 is on the high end. Also check what process is used to create the surface. Is the resultant surface a saw tooth, wavy, random? We have our parts electropolished just to have the sharp edges removed.