Engineers are intensely frustrated when an O-ring is specified for a design and it still fails. This is likely because the O-ring needed a backup ring (BUR), not because the O-ring was not specified correctly. In fact, polymer BURs remain an underrated but critical component in many systems, in industries ranging from food to oil & gas. 

This article looks at the main problem BURs solve, the benefits of opting for polymer BURs, and commonly used polymers. 

O-Rings and Extrusion

The clearance gap is the space between mating parts where the O-ring sits. Under high pressure, the O-ring is going to behave more like a fluid than a solid. It will get forced, or flow, into the clearance gap. When the O-ring extrudes into the clearance gap, it can lead to two different failure modes: nibbling and spiral failure.  Nibbling looks like something has taken multiple small bites from the O-ring. Spiral failure refers to tearing and cracking around the circumference of the O-ring.

This type of failure is irreversible and inevitably leads to weeping or a catastrophic leak. For hydraulic systems operating at high pressure, it will either manifest as a slow leak (weeping) or a catastrophic failure.

How BURs Solve O-Ring Extrusion (and More)

A backup ring (BUR) is installed against the O-ring, downstream of the pressure, to ensure it stays within the clearance gap. In short, a BUR physically closes the clearance gap, giving the seal nowhere to migrate. This not only prevents unwanted leaks but can extend the life of a seal from cycles to years.

BURs are often used in high-temperature or high-pressure applications. BURs can also help to evenly distribute pressure in applications involving varying clearance gaps or pressure fluctuations. Additionally, BURs help to reduce the effects of wear and friction on O-rings and serve as a thermal barrier to protect them from excessive heat.

Benefits of Using Polymer BURs

There are several benefits to using polymer BURs. 

Extrusion prevention – They effectively fill the clearance gap between mating parts, stopping the primary elastomer seal from being forced (extruded) into that gap under pressure, which would otherwise cause premature seal failure.

Extended pressure and temperature range – By protecting the O-ring from extrusion, polymer BURs allow the overall seal assembly to withstand higher pressures and wider temperature swings than an O-ring alone could handle.

Reduced wear on the primary seal – BURs absorb mechanical stress and abrasion so that the O-ring experiences fewer issues with friction and deformation, thus extending its service life.

Design flexibility with looser tolerances – Because the BUR compensates for extrusion risk, designers can use larger clearance gaps. The use of larger clearance gaps, in turn, simplifies the machining process and reduces manufacturing cost.

Compatibility across dynamic and static applications – Different polymers (PTFE, PEEK, nylon, polyurethane, acetal) let engineers tailor the backup ring’s flexibility, hardness, and chemical resistance to match whether the seal is static, reciprocating, or rotating.

Corrosion and chemical resistance – Unlike metal backup rings, polymers can be chosen that will not corrode or react with process fluids. This facet is particularly important in chemical, oilfield, or food-grade applications.

Lightweight and cost-effective – Polymers are generally cheaper and lighter than metal without sacrificing performance in most standard pressure ranges needed for BURs.

Ease of installation – Many polymer backup rings (especially PTFE) are flexible enough to be installed without requiring the use of special tools. This is in contrast to rigid metal rings that may require more complex assembly procedures.

System reliability – By preventing extrusion-related blowouts, polymer BURs reduce the risk of unplanned downtime, leaks, or catastrophic seal failure in critical systems like hydraulics, pneumatics, and downhole tools.

In short, polymer BURs let engineers push seal assemblies harder in terms of more pressure, wider gaps, tougher environments. And they do so while protecting the seal from the system.

Backup Ring Materials

When choosing a BUR material, ensure it is harder than the seal material to effectively prevent migration. They must also have lower coefficients of expansion than the seal and are engineered to be both rigid and tough. 

Polymer BURs are often used in place of elastomeric versions because they offer significantly better resistance to extrusion, deformation, and chemical exposure, as well as reduced long-term creep under high pressure.

Commonly used polymers include ….

In general, PEEK wins over PTFE based on strength and extrusion resistance at high pressure, but PTFE wins on friction and chemical universality. Nylon is more rigid and dimensionally stable than polyurethane, but polyurethane flexes better in dynamic strokes. However, polyurethane degrades faster than Nylon in wet/steam environments. Acetal is often the “in-between” choice because it is more rigid than nylon or Polyurethane, but not as chemically or thermally capable as PTFE or PEEK.

Conclusion

Polymer BURs serve as cheap insurance against otherwise expensive failures. In fact, they are almost indispensable in chemical processing, hydraulics, oil & gas, and aerospace applications. And remember: it is best to decide on the inclusion of a BUR before failure, not after. But whether you need BURs before or after design, contact Advanced EMC for help. Our sealing engineers are ready to put their experience to work for you.