Spring Energized PTFE Seal
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Backup Rings in High-Pressure Sealing Systems: Preventing Seal Extrusion and Extending Service Life

Backup rings in high-pressure sealing systems address face extrusion failure in both dynamic and static seals found in high-pressure systems. Extrusion in sealing occurs when pressurized fluid forces the seal material into the clearance gap between mating surfaces, compromising seal performance. However, backup rings are a simple, low-cost (but critical) solution for extending the service life and maintaining the performance of extreme sealing conditions.

The Challenge of Seal Extrusion in High-Pressure Applications

Seal extrusion happens when pressurized fluid forces part of a sealing element into the clearance gap between mating hardware surfaces. Under high pressure, the soft material deforms plastically and begins to “flow” into this gap, where it can be pinched, torn, or permanently deformed. The result from seal extrusion is nibbling damage along the edges, rapid loss of sealing capability, and, in severe cases, catastrophic leakage.

Extrusion not only shortens seal life but also accelerates equipment wear, drives unplanned downtime, and raises operating costs in hydraulic, pneumatic, and process systems. The causes of seal extrusion are typically high pressure differentials ( > 1500 psi) and/or significant clearance gaps, exacerbated by the use of elastomers or polymers that deform plastically under a load. Seal extrusion can be difficult to avoid under certain circumstances, but that is where backup rings come in.

Backup Rings in High-Pressure Sealing Systems: Purpose and Function

Backup rings in high-pressure sealing systems are annular support components that are installed next to a seal, such as an O-ring or a spring-energized seal. They act as an effective barrier to keep the seal from being forced into the clearance gap.

Backup rings come in different configurations, including single-ring, double-ring (for applications with bi-directional pressure), as well as split vs solid rings. Regardless of the configuration chosen, it is critical to achieve a precise fit because too loose undermines support, while too tight causes problems with assembly.

Material Considerations for Backup Rings in High-Pressure Sealing Systems

The three most commonly used materials for backup rings are PTFE, PEEK, and Nylon. However, other materials such as  UHMW-PE, filled PTFE blends for wear resistance, reinforced polymers for high PV limits, may be used.

PTFE

PTFE is an excellent option for backup rings with its extremely low coefficient of friction and extensive chemical compatibility. It works exceptionally well for applications requiring dynamic sealing or very low temperatures. Its primary limitations are the possibility of cold flow under sustained loads, so it might not always be suitable for extreme pressure conditions.

PEEK

PEEK is another good option for use as a backup ring with its high mechanical strength and excellent resistance to extrusion, as well as its thermal stability up to ~250°C. It also possesses exceptional resistance to extrusion. While it may be a more costly option compared to other polymers, it has found widespread application in industries such as aerospace, oil and gas, and high-performance hydraulics.

Nylon (PA)

Nylon works extremely well in moderate conditions with its strength, and it has a more economical price compared to PEEK and PTFE. However, it does have some critical limitations that including swelling and water absorption, both of which can heavily impact tolerances.

Design and Geometry Options

Solid backup rings provide the best extrusion resistance but can be challenging to install. Split rings can simplify the assembly process, but may allowed extrusion under extremely high loads, which essential defeats the purpose of having a backup ring. Another alternative is the use of spiral cut designs, which balance easier installation (without requiring excessive stretching) with maintaining good support for the seal. Contoured cuts such as scarf of step joints further reduce weak points found at splits.

For pressure direction, a single ring works when the load comes from one side, while double rings are necessary for bidirectional pressure. In every case, the trade-off is clear: easier installation often means slightly higher extrusion risk at the joint.

Engineering Considerations for Integration

Housing tolerances are critical for backup rings. Excessive clearance gaps increase the risk of extrusion, while precise fits provide reliable support. Single rings work well when pressure comes from one direction. However, double rings are required when it fluctuates or is bidirectional. Temperature adds another layer of complexity. Heat accelerates creep and changes dimensions through thermal expansion, which weakens long-term performance. Material compatibility is also important. Chemicals, lubricants, or swelling agents can reduce hardness and shorten service life. In failure analysis, extrusion often appears as edge nibbling or shearing. Compression set, on the other hand, leaves the ring permanently deformed. Recognizing the difference is key to preventing repeat issues.

Conclusion

Backup rings extend seal service life by preventing premature failure and protecting against extrusion. They reduce downtime and maintenance costs, which is especially valuable in high-value systems. Reliability and safety improve in critical applications where failures aren’t an option. They also make it possible to use softer elastomers for better sealing performance without increasing the risk of extrusion.

Investing in the right backup ring extends seal life, reduces failures, and ultimately saves money and downtime. Contact us at Advanced EMC to learn more about backup ring solutions.

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Designing Polymer Seals for Dynamic Applications: Balancing Wear, Friction, and Thermal Expansion

Designing polymer seals for dynamic applications can be a challenging task. Polymer seals have proven vital in dynamic applications such as rotary shafts, reciprocating pistons, and oscillating systems. However, dynamic conditions can introduce challenges that are not found in static conditions. These challenges include continuous motion, heat buildup, wear mechanisms, and variable pressures.

This blog post examines three key challenges involved in dynamic sealing: wear, friction, and thermal expansion.

The Role of Polymers in Dynamic Seals

Engineering polymers such as PTFE and PEEK offer several advantages over both traditional metals and elastomeric seals in dynamic systems. Such benefits include outstanding performance even in operating environments that include extreme temperatures and require excellent chemical compatibility and extremely low friction. And engineers can further enhance the most desirable features of these polymers through the use of fillers and blends (e.g., graphite, carbon, bronze, glass, and even PTFE).  Polymer seals are also lightweight and ideal for compact systems where space is limited.

Balancing Wear Resistance

One of the most limiting factors in dynamic seal applications is wear. The three most common wear mechanisms involved are adhesion, abrasion, and fatigue. 

  • Adhesive wear happens when the seal momentarily sticks to the counterface, thus tearing material away from the surface and resulting in material transfer or scoring.
  • Abrasive wear occurs when hard (abrasive) particles or rough surfaces cut into the polymer, creating grooves and accelerating material loss.
  • Fatigue wear takes place when the seal is subject to repeated cyclic stresses that form micro-cracks, eventually leading to surface flaking or spalling.

Polymers can effectively address wear issues. PTFE effectively combines extremely low dynamic friction and excellent self-lubrication. This combination makes it well-suited for high-wear dynamic applications such as piston rings in gas compressors. Another example is the use of PEEK seals in aerospace actuators, where its high resistance and ability to maintain mechanical strength at high temperatures make it an excellent choice for applications involving cycling under high loads.

One of the most effective ways to further improve the wear resistance of PTFE and PEEK dynamic seals would be the use of filled composites, the use of appropriate surface finishes on countersurfaces, and wise design choices that minimize localized stresses.

Managing Friction

Friction is particularly problematic in dynamic seals, as it leads to heat generation, energy loss, and accelerated degradation. This problem leads to a trade-off between achieving an effective sealing force and maintaining low friction. 

PTFE is an excellent example of how low-friction engineering polymers can help achieve this balance. PTFE has the lowest coefficient of friction of any engineering polymer, and is far less than that of metal or elastomers. Its self-lubricating nature keeps friction very low at the shaft-seal interface, which will minimize heat buildup and lost energy. In fact, it can even reduce energy loss during dry running conditions. The strength and modulus of elasticity of PTFE can be modified through the use of fillers and hybrids.

Spring-energized seals, which use a metallic energizer to keep the seal lip in contact with the sealing surface and generate a predictable, consistent load to compensate for problems such as wear, thermal expansion, and pressure changes. As the load is kept within a predictable range, the friction is also kept at consistent levels over a well-distributed sealing force.

Thermal Expansion Considerations

Polymers indeed possess a higher coefficient of thermal expansion when compared to metals and most elastomers. Changes in dimensions can impact clearance, sealing performance, and contact pressure in dynamic sealing applications. In aerospace and automotive applications, for example,  there can be an abundance of extreme temperature cycling, which is going to be especially problematic in rotary shaft seal designs. 

There are several approaches to minimizing the impact of thermal expansion, starting with customized PTFE or PEEK polymer blends with materials that will lower the coefficient of thermal expansion without compromising wear resistance or friction.

The use of spring-energized seals allows the polymeric sealing lip to remain in contact with the sealing surface despite changes in geometry or alignment, whether they are due to wear, thermal expansion, or thermal contraction in the presence of extreme temperature cycling. 

Note that both of these approaches can be further enhanced through predictive modeling of how the seal will deform under thermal stress.

Polymer Seals for Dynamic Applications: Design Best Practices

Here are some straightforward design best practices related to dynamic sealing challenges:

  • Always match the seal geometry to motion type (i.e., rotary vs reciprocating).
  • Carefully consider the allowable surface roughness and hardness of mating surfaces.
  • Respect the PV limit (pressure × velocity) when selecting a polymer.
  • Remember the importance of predictive modeling (finite element analysis for thermal and tribological performance).
  • Always test under real-world operating conditions before full-scale deployment.

Conclusion

Dynamic sealing requires balancing wear, friction, and thermal expansion, with no single solution that fits all. Fortunately, advances in polymer science and composites make it possible to design seals that meet increasingly demanding requirements. However, engineers must still carefully match polymer formulations, energizers, and geometries to the unique conditions of each application.

If you need a dynamic seal for an application, contact the experts at Advanced EMC. Our engineers are very experienced and highly knowledgeable, able to take you all the way from seal design and material selection to testing.