Backup O-Rings By Parker Eliminate O-ring Extrusion

Parbak Backup O-Rings Prevent Extrusion

Perfect fit is needed for an O-ring to properly do its job, but pressure at high temperatures can make the O-ring weak. It can force it to extrude beyond the groove in which it rests into the clearance gap where two parts come together. As material from the O-ring is lost, the seal it is meant to create can fail. When engineers combat this problem, they often look toward backup O-rings to keep the original one strong.

Why O-Rings Fail

O-rings are designed to rest in a groove. When the opposing part first places force on the O-ring in a perpendicular direction, the ring continues to provide a good seal until the force exceeds its limits. At this point, the ring deforms to the point where some material is caught in the gap when more pressure comes down upon it. In designing the O-ring assembly, the groove has some room for expansion but pressure can ultimately push it past its limits. The point at which the O-ring needs an anti-extrusion device (a backup O-ring) depends on the type of elastomer, the pressure, the Shore hardness, the size of the clearance gap, and the amount of 'breathing' that occurs in the middle parts.

Preventing O-Ring Extrusion

There are several solutions to preventing this weakening of O-rings through extrusion:

  • The clearances can be reduced so that there is less extrusion gap, a very costly solution.
  • The O-ring can be made of a material with a higher durometer or degree of hardness. While a harder seal is less likely to extrude under pressure, this type of O-ring does not seal as well under pressure.
  • A backup ring made of hard extrusion-resistant material can be placed between the O-ring and the extrusion gap to prevent O-ring material from being pushed into the gap. Backup rings are made of a much harder material with increased resistance to pressure. When used in conjunction with an O-ring, a backup ring helps create zero clearance in the gap.

 

Common materials for backup O-rings include high durometer nitrile, nylon, Hytrel, PTFE, or leather, while the shape of the ring can be circular or crescent shaped, a broken or unbroken ring, flat or contoured.

Parbak Backup O-Rings From Parker

The Parbak line of O-rings made by Parker:

  • Are easily stretched into place.
  • Are continuous for uniform contact with all surfaces subject to pressure, which eliminates wear spots.
  • Are quick and easy to assemble to minimize assembly costs.
  • Are designed to fit securely in the O-ring groove.
  • Are hopefully trapping lubricant, which reduces friction, while preserving the O-ring.
  • Are available in standard or custom sizes.

 

Parbak rings are often used in pairs, one on each side of the O-ring. The most common material is hard rubber (high durometer nitrile), which functions in temperatures from -40°C to 120°C (-40°F to 250°F.) For higher temperatures, rings are often made in polytetrafluoroethylene (PTFE), which can withstand up to 204°C (400°F).

To complement our line of Parker O-rings, Performance Seals also offers Parbak backup O-rings and hydrometer materials to enhance the performance of elastomers. For assistance in finding the right backup O-ring, you can view the catalog, contact one of our technical representatives, or visit our website.

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