Polon® Parker Prädifa anti-extrusion rings (back-up rings) are used for static and dynamic applications in connection with O-rings, to prevent extrusion of the O-ring into the diametral clearance.

The single-cut and spiral design rings are easily installed into closed grooves while the single solid design, recommended for highest pressures, is for installation in open groove.

Polon®anti-extrusion rings for O-rings are available in three versions:

• XA: Single turn continuous
• XB: Single turn scarf cut
• XC: Spiral cut

Range of application

We recommend to specify anti-extrusion rings when at least one of the following working conditions is present in cases where the intention is to seal with O-rings only:

• Pressure above 70 bar (7MPa)
• Diametral clearance exceeding 0.25 mm at p > 10 bar (1MPa)
• High stroke frequency
• High temperatures
• Contaminated medium
• Strong pressure pulsing or pressure changes

Compounds

Polon® Parker Praedifa 001, virgin PTFE.

Installation

In case of single-acting O-rings, it is sufficient to install only one anti-extrusion ring on the lee side of the O-rings. In case of double-acting sealing, two anti-extrusion rings are required.

Installation grooves should basically be produced with a rectangular cross-section (parallel side walls). If this for machining reasons is impossible, max. deviation of 5°  is allowed.

A spiral anti-extrusion ring consists of two spiral windings which are cut at the ends and allows for compensating for large temperature variations and tolerances by means of a screw-like elongation or contraction. Only to be used in reciprocating applications.

In case of single-acting O-rings, it is sufficient to install only one anti-extrusion ring on the downstream side of the O-ring. In case of a double-acting sealing function, two anti-extrusion rings are required at each side of the O-ring.

Installation grooves should preferably be made with parallel sidewalls. If for machining reasons this is not possible, a maximum deviation of 5° is allowed.

Anti-extrusion rings with concave face on the O-ring side are recommended in case of wide or out-of-tolerance bores and shafts. Additionally, this type of anti-extrusion ring extends the operating pressure of the O-ring and its specific shape tends to keep the O-ring round, even under high-pressure, resulting in better seal performance.

• Insensitive to pressure
• High extrusion resistance.
• Excellent media resistance in case of suitable compound
• Suitable compounds available for special requirements of the chemical process industry.
• Suitable compounds available for special requirements of the food processing
• Any desired nominal diameter available due to use of machining
• Installation in closed and undercut