O-Ring Back-Up Rings Designed for High Pressure Applications to prevent O-Ring Extrusion
Back-Up rings are used for both static and dynamic fluid power applications and prevent the migration or extrusion of an O-Ring in the gap as pressurization and de-pressurization/pressure cycling occur. The profile shape of the Back-Up rings is usually rectangular but may also be concave at the face mating with the O-Ring. Manufactured in various PTFE compounds, Buna-N 90 Durometer, FKM 90 Duro, thermoplastic polyester elastomer TPE ((Hytrel®) or less common urethane Back-Up Rings.
Pressure applied evenly to both sides of a seal normally has no effect on sealing performance. When a pressure difference is anticipated, elastomer selection must also consider differential pressure resistance. High differential pressures will cause improperly specified O-Rings to extrude, resulting in seal damage and eventually failure.
Standard O-Ring groove and gap dimensions cited in MIL-G-5514 and AS4873 generally provide adequate sealing for differential pressures to 1500 psi for all elastomers. Substantial improvement in extrusion resistance is attainable by 1) using harder O-Rings, 2) decreasing the diametral clearance, or 3) using contoured hard rubber or plastic back-up rings. O-Rings with high modulus and hardness are better able to resist extrusion. The higher the modulus of a material, the greater the force required to stretch it. Similarly, the harder the material, the greater its resistance is to indentation.
Extrusion failure is one of the most common types of O-Ring failure. When the internal pressure of an application becomes too great, the O-Ring will actually extrude into the clearance gap. This extrudate will quickly be nibbled away causing a loss of material, and once sufficient material is lost, seal failure will quickly follow. There are three options to prevent this, the first of which is to reduce the clearances to lower the extrusion gap. This is obviously an expensive option, so a cheaper solution is to raise the durometer of the O-Ring. Even though a higher durometer O-Ring offers superior extrusion resistance, this is often not a feasible solution due to material availability, and to the fact that harder durometer materials have limited low pressure sealing ability. The last and best option is the addition of a Back-Up ring. A Back-Up ring is a ring of hard, extrusion resistant material such as high durometer Buna Nitrile, FKM Flourocarbon, TPE or PTFE. In a sealing application, Back-Up rings provide added extrusion resistance, and prevent damage to O-Rings and seals when subjected to high pressures, larger extrusion gaps, and/or high temperatures.
A Back-Up ring is designed to fit between the O-Ring and the extrusion gap and prevent the extrusion of the O-Ring. Depending upon the direction of pressure in the sealing application, you can either use 1 Back-Up ring or 2 Back-Up rings. If unsure, it is always best to use two Back-Up rings per 1 O-Ring.
O-Ring Back-Up Rings Designed for High Pressure Applications
- Back-Up Rings used with O-Rings at pressures > 1500 psi or big extrusion gaps
- Back-Up Rings are designed to limit O-Ring clearance gap extrusion.
- Available in elastomeric compounds, TPE and PTFE Back-Up Rings.
- PTFE Back-up rings can be solid, split, spiral, or custom for ease of install.
- PTFE Back-up Ring size standards AN6244, AN6246, MS27595, MS28774, MS28782 and MS28783.
- Elastomer Back-up Rings are typically contoured to conform to an O-Ring.
- Elastomer Back-up Rings fit standard USA and custom O-Ring sizes.
- Back-up Rings require wider glands to accommodate.
Back Up Rings are flat, split, spiral or contoured washer type fittings used to protect an O-Ring seal from extrusion at higher pressures and temperatures. A Back Up Ring works by blocking or reducing the extrusion gap, thus preventing the O-Ring from extruding. The use of a back-up ring increases the pressure and temperature capability of O-Rings. Back Up Rings can be solid, split, spiral or contoured. The split and spiral back-up rings are easier to install, especially in small diameters. The chart to the right depicts the higher pressure capabilities of using Back Up Rings for a given clearance gap dimension. Back Up Rings can be used on both sides of the O-Ring if pressure is bidirectional. Gland widths should be widened to accommodate one or two Back Up Rings.
Teflon (PTFE) Backup Rings
We offer a large selection of PTFE Backup Rings including; Solid (uncut) and Single Turn (scarf cut) PTFE Backup Rings.
TPE (Hytrel®) Backup Rings
TPE backups are manufactured from high quality thermoplastic polyester elastomer (TPE). This tough material is long wearing with very good abrasion resistance in both dynamic and static applications and improved extrusion resistance compared to standard FKM or NBR backups. It has high temperature capabilities up to 275°F (135°C) along with a broad range of fluid compatibility including water based fluids. Molded of 55D Durometer TPEl, these back-up rings provide excellent extrusion resistance and tolerance to fluids such as phosphate esters, hydraulic oil and water. Stretched for installation, they return quickly to their original size for easy installation.
- 000 Series O-rings 0.070" - .050" Thick
- 100 Series O-rings 0.103" - .050" Thick
- 200 Series O-rings 0.139" - .050" Thick
- 300 Series O-rings 0.210" - .070" Thick
- 400 Series O-rings 0.275" - .105" Thick
NOTE: Sizes (Dash No.) corresponds to O-ring numbers.
Buna-N 90 Contoured Backup Rings
Elastomeric Contoured back-ups are routinely produced in a 90 durometer Buna Nitrile and FKM. This provides sufficient elasticity to permit stretching over the major diameter of a piston and then snapping back into the gland groove cut into the piston. 90 durometer has sufficient hardness to resist extrusion of the softer elastomeric O-Ring performing the actual sealing function against the high pressure liquid or gas.
- -000 Series O-Rings 0.070" ±0.003" - .045" Thick
- -100 Series O-Rings 0.103" ±0.003" - .045" Thick
- -200 Series O-Rings 0.139" ±0.004" - .040" Thick
- -300 Series O-Rings 0.210" ±0.005" - .060" Thick
- -400 Series O-Rings 0.275" ±0.006" - .096" Thick