Backup Rings: Preventing Extrusion in High-Pressure Systems

Introduction to Backup Rings

Backup rings, also known as anti-extrusion rings or support rings, are precision-engineered components that prevent elastomer O-rings and other seals from extruding into the clearance gap between mating hardware surfaces under high-pressure conditions. Without backup ring protection, high system pressures force the soft elastomer seal material into the gap between the piston and cylinder bore (or rod and gland), causing the seal to develop a characteristic nibbled or chewed appearance that quickly leads to seal failure and system leakage.

Extrusion is one of the most common failure modes for O-ring seals in hydraulic and pneumatic systems operating above 1,500 psi (100 bar). As system pressures increase, the clearance gap tolerances tighten and the need for backup ring protection becomes critical. Understanding when backup rings are needed, how to select the correct type, and proper installation practices is essential knowledge for anyone involved in high-pressure sealing system design or maintenance.

When Are Backup Rings Needed?

The decision to use backup rings depends on the relationship between system pressure, clearance gap, O-ring hardness, and whether the application is static or dynamic. General guidelines for backup ring usage include:

• Below 1,500 psi (100 bar): Backup rings are generally not required for standard 70 Shore A O-rings with normal clearance gaps
• 1,500-3,000 psi (100-200 bar): Single backup ring recommended on the low-pressure side of the O-ring
• 3,000-5,000 psi (200-350 bar): Backup rings on both sides of the O-ring recommended for dynamic applications
• Above 5,000 psi (350 bar): Backup rings on both sides required; consider harder O-ring compounds (90 Shore A) and tighter machining tolerances
• Higher O-ring hardness (80-90 Shore A) increases the pressure threshold before extrusion occurs
• Larger clearance gaps reduce the pressure threshold and increase the need for backup rings
• Dynamic applications have lower extrusion thresholds than static applications due to pressure spikes and gap variations during movement

Backup Ring Materials

PTFE (Polytetrafluoroethylene)

PTFE is the most widely used backup ring material due to its combination of low friction, chemical inertness, and excellent dimensional stability. Virgin (unfilled) PTFE provides universal chemical compatibility and very low friction but has relatively high cold flow (creep) under sustained loading. Filled PTFE compounds offer improved mechanical properties:

• Virgin PTFE: Universal chemical resistance, lowest friction, suitable for most applications up to 5,000 psi
• Glass-filled PTFE (15-25% glass fiber): Improved wear resistance and reduced cold flow for dynamic applications
• Carbon-filled PTFE: Enhanced wear resistance and thermal conductivity for high-speed dynamic sealing
• Bronze-filled PTFE: Maximum wear resistance and thermal conductivity for heavy-duty hydraulic cylinders
• PEEK-filled PTFE: Superior creep resistance and mechanical strength for ultra-high-pressure applications

Nylon (Polyamide)

Nylon backup rings offer higher mechanical strength and better extrusion resistance than PTFE at a lower cost. Nylon is commonly used in hydraulic systems with petroleum-based fluids where its higher stiffness provides effective gap bridging. However, nylon has limited chemical resistance compared to PTFE and is not suitable for applications involving aggressive chemicals or fluids outside its compatibility range. Nylon backup rings are typically used in standard hydraulic applications up to 7,000 psi.

Other Materials

• PEEK (Polyetheretherketone): Premium high-performance material for extreme pressure and temperature applications
• Acetal (Delrin/POM): Good dimensional stability and machinability for moderate-pressure applications
• Leather: Traditional backup ring material still used in some legacy hydraulic cylinder designs
• Elastomer backup rings: Softer backup rings for low-pressure applications or as secondary support elements

Backup Ring Types and Configurations

Solid (Continuous) Backup Rings

Solid backup rings are machined or molded as complete rings without any gap or split. They provide the maximum gap-bridging capability and are the preferred choice for the highest-pressure applications. However, solid backup rings can only be installed over the end of a shaft or piston, which limits their use to applications where the hardware allows end installation. Solid rings must be sized precisely to the groove diameter, as they cannot expand or contract to accommodate assembly.

Spiral (Wound) Backup Rings

Spiral backup rings are manufactured by winding a flat strip of PTFE or other material into a multi-turn spiral that can be expanded for installation into internal grooves or compressed for installation onto shafts. The spiral construction allows installation without disassembly, making them ideal for field maintenance and applications where solid rings cannot be installed. Spiral backup rings typically have two to four turns of material, providing effective gap bridging with the flexibility of expandable installation.

Split (Cut) Backup Rings

Split backup rings are solid rings with a single cut (gap) that allows them to be opened for installation around a shaft or into a bore. The gap must be properly oriented relative to any pressure ports or features that could cause the gap to open and allow extrusion. Split rings are simpler and less expensive than spiral rings but provide less effective gap bridging at the split location. They are suitable for moderate-pressure applications where the gap can be positioned away from the highest-pressure zone.

Gap Configuration and Orientation

The gap in split and spiral backup rings must be properly configured and oriented for effective extrusion prevention. For split rings, the gap is typically cut at a straight angle (butt cut) or a diagonal angle (scarf cut). Scarf cuts provide better gap coverage than butt cuts by creating an overlapping joint. Spiral rings inherently provide overlap coverage through their multi-turn construction, eliminating the single-point gap weakness of split rings.

When installing split backup rings, the gap should be positioned at the top of the groove (12 oclock position) in horizontal cylinder applications, or rotated 90 degrees from any pressure ports or features that could cause localized pressure spikes at the gap. In applications where two backup rings are used (one on each side of the O-ring), the gaps should be offset by 180 degrees to ensure that no single circumferential location has gaps on both rings simultaneously.

Installation and Sizing

Backup rings must be properly sized and installed to provide effective extrusion prevention without interfering with O-ring function. The backup ring is installed in the O-ring groove on the low-pressure side of the O-ring (the side away from system pressure). The combined cross-section of the O-ring and backup ring must fit within the groove without excessive compression of the O-ring, which would increase friction and accelerate wear. Standard groove design guides from O-ring manufacturers provide recommended groove dimensions for O-ring plus backup ring combinations.

• The backup ring must fill the gap between the O-ring and the groove wall on the low-pressure side
• Groove width must be increased from standard O-ring groove width to accommodate the backup ring thickness
• For dual backup ring installations, groove width must accommodate both backup rings plus the O-ring
• Backup ring inside diameter should closely match the groove bottom diameter to minimize any residual gap

Tereefek Solutions: Backup Ring Supplier

Tereefek Solutions stocks a comprehensive range of backup rings in PTFE (virgin and filled grades), nylon, and specialty materials. Our inventory covers AS568, metric, and custom sizes in solid, spiral, and split configurations. Whether you need standard backup rings for routine hydraulic cylinder maintenance or specialty anti-extrusion solutions for ultra-high-pressure systems, our product range and technical expertise can support your requirements. Our team provides guidance on backup ring selection, groove design, and installation procedures to ensure optimal high-pressure sealing performance. Contact Tereefek Solutions for your backup ring needs.