The Role of Backup Rings in Hydraulic and Pneumatic Sealing Systems

What is a Backup Ring?

A backup ring is a rigid, typically polymer-based ring placed on the low-pressure side of a primary seal (or on both sides in double-acting systems). Its primary function is to prevent the seal material from extruding into the clearance gap between the piston and cylinder bore, or between the rod and gland.

When system pressure rises, the soft elastomeric seal is pushed against the clearance gap. If the gap is too large or the pressure is too high, the seal material can be forced into the gap, causing permanent deformation, nibbling, and eventual leakage. The backup ring acts as a barrier, bridging the gap and supporting the seal.

How Backup Rings Work

1. Gap bridging: The backup ring sits between the seal and the clearance gap, preventing the seal from being extruded.

2. Load distribution: It distributes the pressure evenly across the seal, reducing localized stress.

3. Wear protection: In dynamic applications, backup rings can also reduce wear on the primary seal by acting as a bearing surface.

Backup rings are typically used in:

• High-pressure hydraulic cylinders

• Pneumatic actuators

• Rotary and reciprocating shafts

• Static sealing applications with large clearances

Common Materials for Backup Rings

The choice of material depends on pressure, temperature, and fluid compatibility.

PEEK and filled PTFE are the most common choices for demanding hydraulic applications. POM and nylon are often used in pneumatic systems and lower-pressure applications.

Types of Backup Rings

Backup rings are available in several designs:

• Solid backup rings: A continuous ring, used in static or low-speed dynamic applications.

• Split backup rings: A ring with a diagonal cut, allowing easy installation over the shaft or piston.

• Spiral backup rings: A coiled design that provides flexibility and easy installation.

• Contoured backup rings: Shaped to match the profile of the primary seal, providing better support.

The choice of design depends on installation constraints and the type of primary seal.

Selection Criteria for Backup Rings

When selecting a backup ring, consider the following:

1. Pressure: Higher pressures require harder, more extrusion-resistant materials such as PEEK.

2. Temperature: Ensure the material can withstand the operating temperature range.

3. Fluid compatibility: The backup ring material must be compatible with the hydraulic fluid, water, or chemicals.

4. Clearance gap: The larger the gap, the more critical the backup ring.

5. Motion type: Static, reciprocating, or rotary applications may require different backup ring designs.

6. Installation: Split or spiral backup rings are easier to install in closed grooves.

Installation and Maintenance

• Correct placement: Install the backup ring on the low-pressure side of the primary seal (or both sides for double-acting systems).

• Avoid sharp edges: Ensure the groove is free of burrs or sharp edges that could damage the backup ring.

• Lubrication: Lightly lubricate the backup ring during installation.

• Inspect periodically: Check for wear, cracking, or extrusion. Replace if damaged.

Conclusion

Backup rings are small but essential components in hydraulic and pneumatic sealing systems. By preventing seal extrusion, they extend the life of primary seals, reduce downtime, and improve system reliability. Selecting the right backup ring material and design—and installing it correctly—is critical for demanding applications in construction, agriculture, manufacturing, and beyond.

At FOREVER SEALS, we supply a wide range of backup rings in PTFE, PEEK, POM, and other materials. Contact our engineering team for help in selecting the right backup ring for your application.