Extrusion and Nibbling of the o‑ring is a primary cause of seal failure in dynamic applications such as Hydraulic Rod and Piston seals.
This form of failure may also be found from time to time in static applications subject to high pressure pulsing which causes the clearance
gap of the mating flanges to open and close, thereby trapping the o‑ring between the mating surfaces.
In general, EXTRUSION and NIBBLING are caused by one or more of the following conditions:
- Excessive clearances.
- High pressure (in excess of system design or high pressure excursions).
- O‑ring material too soft.
- Degradation of o‑ring material by system fluid (swelling, softening, shrinking, cracking, etc.)
- Irregular clearance gaps caused by eccentricity.
- Increase in clearance gaps due to excessive system pressure.
- Improper machining of o‑ring gland (sharp edges).
- Improper size o‑ring installed (too large) causing excessive filling of groove.
Suggested solutions to the causes of Extrusion and Nibbling listed above are as follows:
IDENTIFICATION OF EXTRUSION FAILURE
- Decrease clearance by reduction of machining tolerances.
- Consider use of back-up devices
- Check o‑ring material compatibility with system fluid.
- Increase rigidity of metal components.
- Increase durometer (hardness) of o‑ring material.
- Break sharp edges of gland to a minimum radius of 0.002 in.
- Insure installation of proper size o‑rings.
Consider use of alternative seal shapes. For example, in some long stroke piston or rod
applications, the Parker T-Seal, with its built-in back-up rings may be useful and also prevent "Spiral Failure."
Below is a typical example of o‑ring extrusion. The edges of the ring on the low pressure or downstream side of the gland exhibit a "chewed" or "chipped" appearance.
In an o‑ring which has failed due to nibbling, it may have the appearance of many small pieces removed from the low pressure side.
In some forms of extrusion, more than 50% of the o‑ring may be destroyed before catastrophic leakage is observed.