Back-Up Rings

Back-up rings (also known as anti-extrusion rings) are rigid, non-sealing components that support primary seals, such as O-rings or U-seals, by preventing them from extruding into the clearance gap between mating metal surfaces under high pressure. Their use significantly extends the life of the primary seal and enhances system reliability in high-pressure applications.
Function and Design
Prevent Extrusion: When system pressure exceeds the capacity of the softer primary seal material, the seal can deform and be forced into the small gap (extrusion gap) between the metal parts. The harder back-up ring fills this gap, providing a barrier and maintaining the seal's shape and position.
High Pressure Performance: Back-up rings are generally recommended for dynamic applications over 100 bar (~1450 psi) and static applications around 950 bar (~13778 psi), although specific requirements vary by application.
Placement: For single-acting applications (pressure from one side), the back-up ring is placed on the low-pressure side of the primary seal. For double-acting applications (pressure from both sides), two back-up rings are used, one on each side of the seal.
Groove Design: O-ring grooves are typically designed to be slightly larger to accommodate the back-up ring(s).
Types of Back-up Rings
Back-up rings come in various forms to facilitate installation and maximize performance.
Solid (Uncut): These continuous rings offer the best anti-extrusion protection but can be difficult to install as they require the system to be disassembled.
Split (Scarf-Cut or Angle-Cut): These have a single cut, allowing them to be opened slightly and installed over a shaft or into a bore without full disassembly. They are popular for easier maintenance but may present a small gap.
Spiral (Multiple Turn): Made from a coiled strip, this design ensures there is no gap along the sealing surface, providing excellent extrusion resistance and ease of installation, especially for non-standard diameters.
Contoured: These rubber rings have a concave shape on one side that mates perfectly with the convex surface of an O-ring, minimizing distortion and providing support.
Materials
Materials are chosen for their high mechanical strength, resistance to extrusion, and chemical compatibility.
PTFE (Polytetrafluoroethylene): The most common material due to its low friction, excellent chemical resistance, and wide temperature range. It is often used in split or spiral designs for easier installation due to its lack of elasticity.
Nitrile Rubber (NBR) (High Durometer): Used for contoured back-up rings, typically at 90 durometer, offering sufficient hardness to resist extrusion while remaining elastic enough for installation.
Nylon (Polyamide PA): Offers excellent mechanical strength, toughness, and good wear resistance, making it suitable for heavy-duty applications.
Other Polymers: Materials like PEEK and Hytrel (a polyester elastomer) are used in extreme conditions for their superior strength, heat resistance, and chemical compatibility.
Common Applications
Back-up rings are essential in industries where high pressure and reliability are critical.
Hydraulic Systems: Found extensively in cylinders, pumps, and valves in construction, agricultural, and mining equipment.
Aerospace: Used in flight controls, landing gear, and fuel systems where extreme pressures and temperature variations are common.
Oil and Gas: Critical for drilling equipment and pipelines that handle high pressures and corrosive fluids.
Industrial Presses and Molding Machines: The ability to handle high pressures and impact loading makes them ideal for heavy industrial machinery.