O-Ring Back-Up Ring: When to Use Them and How They Prevent Extrusion

O-rings are widely used for sealing hydraulic, pneumatic, industrial, automotive, and fluid-control applications. However, when an O-ring operates under high pressure or with a large clearance gap, the rubber can be forced into the gap between mating components.

This can lead to O-ring extrusion, nibbling, and premature seal failure.

For a more detailed explanation of extrusion mechanisms and prevention methods, see our 【O-Ring Extrusion Guide】.

A back-up ring, also called an anti-extrusion ring, is one of the common solutions to this problem.

This guide explains what O-ring back-up rings are, when they are needed, how they work, and what to consider when selecting a back-up ring for an application.

What Is an O-Ring Back-Up Ring?

An O-ring back-up ring is a separate ring installed together with an O-ring to help prevent the O-ring from being pushed into the clearance gap under pressure.

Unlike the O-ring, the back-up ring is generally made from a harder and more extrusion-resistant material, such as PTFE or other high-performance thermoplastics.

The basic arrangement is:

O-ring + back-up ring + clearance gap

When system pressure increases, the O-ring tends to deform toward the clearance gap. The back-up ring occupies this area and supports the O-ring, reducing the possibility of extrusion.

Back-up rings are commonly used in both static and dynamic sealing applications.

Why Do O-Rings Extrude?

An O-ring seals by being compressed between two surfaces. When pressure is applied, the elastomer is pushed toward the lower-pressure side.

If there is sufficient clearance between the mating components, part of the O-ring can be forced into this gap.

As pressure increases, the amount of elastomer entering the gap can increase.

Eventually, the material can be damaged by:

• Extrusion
• Nibbling
• Peeling
• Cutting
• Permanent deformation

Extrusion is affected by several factors, including operating pressure, clearance gap, O-ring hardness, temperature, fluid compatibility, and gland design.

This is why O-ring extrusion should not always be treated as simply a problem with the O-ring itself.

The sealing system needs to be considered as a whole.

How Does a Back-Up Ring Prevent Extrusion?

The function of a back-up ring is relatively simple.

Without a back-up ring, an O-ring under pressure may move toward the clearance gap:

Pressure → O-ring → Clearance Gap

With a back-up ring, the ring provides a physical barrier between the O-ring and the clearance gap:

Pressure → O-ring → Back-Up Ring → Clearance Gap

The back-up ring does not replace the O-ring.

Instead, it supports the O-ring and helps prevent excessive deformation.

This is particularly useful when the combination of pressure, clearance, temperature, material hardness, and dynamic movement makes extrusion more likely.

When Should You Use a Back-Up Ring?

There is no single pressure value that applies to every O-ring application.

The need for a back-up ring depends on several factors.

1. High Operating Pressure

Higher pressure increases the force pushing the O-ring toward the clearance gap.

For high-pressure hydraulic and industrial sealing applications, a back-up ring may be required depending on the actual gland geometry and O-ring material.

Pressure should therefore be evaluated together with clearance and material hardness rather than considered by itself.

2. Excessive Clearance Gap

Even a relatively hard O-ring can extrude if the clearance gap is too large for the operating pressure.

Reducing the diametrical clearance can therefore be an alternative to using a back-up ring.

However, reducing clearance may require tighter machining tolerances, stronger components, or a different mechanical design.

3. Pressure Pulsation

Pressure that repeatedly increases and decreases can be particularly demanding on an O-ring.

Repeated movement of the O-ring into and out of the clearance gap can cause nibbling or progressive damage.

In applications with strong pressure pulsations, back-up rings can provide additional support.

4. Dynamic Applications

Hydraulic cylinders, valves, pistons, and other moving sealing systems can experience extrusion together with friction and wear.

The back-up ring design must therefore be selected carefully for the type of movement.

Static, reciprocating, oscillating, and rotary applications do not necessarily use the same back-up ring design.

5. High Temperature

Temperature can affect the hardness and mechanical properties of elastomeric materials.

An O-ring that performs adequately at room temperature may behave differently at elevated temperatures.

Therefore, high-temperature applications should be evaluated using the actual operating temperature, pressure, clearance, and selected elastomer.

6. Pressure Spikes

A system may normally operate at moderate pressure but occasionally experience pressure peaks.

These short pressure excursions can be important when evaluating extrusion risk.

For applications with significant pressure spikes, an anti-extrusion solution may provide additional protection.

Types of O-Ring Back-Up Rings

Back-up rings are available in several designs.

Spiral Back-Up Rings

Spiral or helical back-up rings are manufactured with a continuous spiral form.

They are commonly used in applications where installation flexibility and dynamic movement are important.

The spiral configuration can make installation easier while maintaining anti-extrusion support.

Split or Cut Back-Up Rings

A cut back-up ring contains a split that allows the ring to be opened slightly during installation.

This can simplify installation in some gland configurations.

The design and cut style should be selected carefully to avoid creating an unfavorable discontinuity in the sealing system.

Uncut Back-Up Rings

An uncut back-up ring is a continuous ring without a split.

These rings can provide a robust anti-extrusion barrier but may require more careful installation because the ring cannot simply be opened around the shaft or inside the housing.

Concave Back-Up Rings

Some back-up rings use a concave profile designed to support the O-ring more effectively.

The correct profile depends on the O-ring geometry, gland design, pressure direction, and application.

Should You Use One or Two Back-Up Rings?

This depends on the pressure direction.

For a single-direction pressure application, one back-up ring may be sufficient if it is installed on the correct low-pressure side of the O-ring.

For applications where pressure can act from either direction, two back-up rings are commonly considered so that the O-ring is supported on both sides.

The actual design should still be verified against the gland geometry and operating conditions.

What Materials Are Used for Back-Up Rings?

Back-up rings are commonly manufactured from materials that have higher resistance to extrusion than the elastomeric O-ring.

Typical materials include:

• PTFE
• Filled PTFE
• Thermoplastics
• Engineering plastics
• Selected elastomeric materials for specific applications

PTFE is widely used because of its chemical resistance and low friction characteristics.

However, material selection should not be based on PTFE versus another material alone.

The application should also be evaluated for:

• Operating temperature
• Pressure
• Fluid compatibility
• Sliding velocity
• Friction
• Wear
• Installation method
• Chemical exposure

The O-ring material and hardness are also important factors in extrusion resistance. For more information, see our 【O-Ring Material Selection Guide】.

Back-Up Ring vs. Harder O-Ring

When extrusion occurs, there are several possible solutions.

One option is to use a harder O-ring material.

For example, changing from a softer elastomer to a harder compound may improve extrusion resistance.

Another option is to reduce the clearance gap.

A third option is to add a back-up ring.

In some applications, more than one solution may be appropriate.

The correct choice depends on the complete sealing system.

Simply selecting the hardest possible O-ring is not always the best solution because hardness can affect installation, compression, friction, low-temperature performance, and other sealing characteristics.

Back-Up Rings and O-Ring Groove Design

Back-up rings should not be selected separately from the O-ring gland.

The groove must provide sufficient space for the O-ring and back-up ring while maintaining the required compression and preventing excessive filling.

For more information about gland dimensions and machining considerations, see our 【O-Ring Groove Design Guide】.

The available gland volume should also be checked against the required O-ring compression. Our 【O-Ring Squeeze Guide】 explains how compression affects sealing performance and gland design.

This is particularly important because adding a back-up ring changes the available groove volume and may require a different gland width.

For this reason, the following parameters should be considered together:

• O-ring cross-section
• O-ring inside diameter
• Groove depth
• Groove width
• Clearance gap
• Pressure direction
• Back-up ring thickness
• Back-up ring material
• O-ring hardness
• Temperature
• Dynamic movement

Common Problems When Using Back-Up Rings

A back-up ring can help prevent extrusion, but incorrect installation or design can create other problems.

Incorrect Back-Up Ring Size

A back-up ring that is too small or too large may not properly support the O-ring.

The dimensions should match the selected O-ring and gland.

Before selecting the back-up ring, the correct O-ring size should also be confirmed. See our 【O-Ring Size Guide】 for information about O-ring dimensions and sizing.

Incorrect Pressure Direction

For a single-acting pressure application, the back-up ring needs to be positioned on the correct side of the O-ring.

Installing it on the wrong side can significantly reduce its effectiveness.

Insufficient Groove Space

Adding a back-up ring without checking gland volume can result in excessive groove fill.

This can increase O-ring compression and installation difficulty.

Sharp Edges

Sharp gland edges can damage both the O-ring and back-up ring during installation or operation.

The gland should be properly machined and deburred.

Incorrect Material Selection

A back-up ring material that is chemically incompatible with the operating medium or unsuitable for the temperature or movement can fail even if its dimensions are correct.

How to Select an O-Ring Back-Up Ring

When requesting a quotation for an O-ring and back-up ring assembly, provide as much application information as possible.

A supplier will typically need:

1. O-ring size
2. O-ring material
3. O-ring hardness
4. Groove dimensions
5. Clearance gap
6. Operating pressure
7. Pressure direction
8. Operating temperature
9. Medium or fluid
10. Static or dynamic application
11. Type of movement
12. Required quantity

If the application is dynamic, also provide the approximate stroke, speed, or rotational conditions.

This information allows the seal manufacturer to recommend a suitable O-ring and anti-extrusion solution rather than simply supplying a standard ring.

Final Thoughts

An O-ring back-up ring is a relatively simple component, but it can play an important role in high-pressure sealing systems.

When extrusion is caused by the combination of pressure and clearance, a back-up ring can provide additional support and reduce the risk of O-ring damage.

However, a back-up ring should not be treated as a universal solution.

The correct sealing design should consider the complete system, including O-ring material, hardness, groove dimensions, clearance, pressure, temperature, fluid compatibility, and dynamic conditions.

At DRFPART, we can support customers with custom O-rings, rubber seals, and sealing components based on specific application requirements.

If you have an existing O-ring that is extruding, leaking, or failing prematurely, you can provide the O-ring dimensions, material, operating pressure, temperature, fluid, and groove information. We can help evaluate the possible causes and discuss suitable sealing solutions.