Choosing the right polymer backup ring can be challenging, but having a basic understanding of how they work, combined with some guidelines, can make the design decisions much easier. This blog post provides a brief overview of what backup rings are, what distinguishes polymer backup rings, and how to choose the right one for a specific application.
Backup Rings
Backup rings are supportive elements typically used with O-rings in sealing systems to help prevent extrusion in the clearance gap of a seal. Often used in high-temperature or high-pressure applications, these rings have lower coefficients of expansion and are engineered to be very rigid and tough. Backup rings can also help to evenly distribute pressure in applications involving varying clearance gaps or pressure fluctuations. Additionally, backup rings help to reduce the effects of wear and friction on O-rings and serve as a thermal barrier to protect them from excessive heat.
Why Polymer Backup Rings?
Polymer backup rings are often used in place of elastomeric versions because they offer significantly better resistance to extrusion, deformation, chemical resistance, and long-term creep under high pressure. Materials like PTFE, PEEK, or Nylon tolerate wider temperature swings, aggressive fluids, and high-cycle dynamic motion without breaking down. The result is a more stable sealing system with significantly reduced risk of issues such as blowouts, spiral failure, or premature wear.
Choosing the Right Polymer Backup Ring
There are several different factors that go into selecting the right polymer backup ring.
Clearance Gap
The clearance gap is one of the most important factors when specifying a polymer backup ring. It directly determines how much support the O-ring needs to prevent extrusion. As the gap increases, the load on the elastomer rises, and the likelihood of extrusion rises sharply. This is especially true at higher pressures or temperatures.
Keep in mind that larger gaps typically require a stiffer polymer (such as PEEK instead of PTFE), a thicker or contoured backup ring profile, or even double backup rings for bidirectional pressure. Small, well-controlled gaps, however, may allow the use of thinner PTFE rings. In short, the wider the clearance gap, the stronger and more dimensionally stable the backup material must be..
Pressure and Temperature
Environmental factors that influence the selection of the right backup ring include system pressure, as higher pressures require harder, more extrusion-resistant materials. System temperatures, which impact dimensional stability, can limit the choice of materials based on their operating temperature limits. High temperatures can make materials more pliable, while low temperatures can lead to brittle behavior. Temperature also affects dimensional stability, as thermal expansion can increase the clearance gap and increase the likelihood of extrusion.
Static or Dynamic
Another key factor in selecting a backup ring is whether the sealing system will operate in a dynamic or static environment. Static systems will involve less mechanical stress on the backup ring, allowing a wider range of polymer materials to be suitable. In contrast, dynamic applications that introduce continuous motion, friction, and side loading require tougher, more wear-resistant polymers with excellent dimensional stability, such as PEEK or certain PTFE blends.
Backup Ring Shape
The two most commonly used backup ring shapes are the scarf-cut and solid types. Scarf-cut backup rings feature a single angled split that allows them to flex during installation. They can also conform more easily to the groove and reduce the risk of damage during assembly, since the ring can be expanded or compressed without overstressing the material. Because they can open slightly, scarf-cut rings are often chosen when hardware tolerances are tight or when installation must be done without special tools.
Solid backup rings do not have a split and must be made from a more flexible polymer that allows them to be stretched or compressed into place without cracking. While they do not offer the same level of extrusion resistance as scarf-cut designs in extreme conditions, their continuous, unbroken profile provides uniform support and is useful in moderate-pressure systems where ease of installation and stable sealing performance are priorities.
Back-Up Ring Material
PTFE is an excellent option for backup rings due to its extremely low coefficient of friction and extensive chemical compatibility. Applications requiring dynamic sealing or very low temperatures are excellent opportunities to use PTFE. PTFE does have its limitations, however. There is the possibility of cold flow under sustained loads, which can make it unsuitable for extreme pressure conditions. Virgin PTFE is limited to about 3,600 psi. Filled PTFE (e.g., glass-filled, carbon-filled, graphite-filled, bronze-filled) is effective for pressures up to 5,800 psi.
PEEK possesses high mechanical strength and excellent resistance to extrusion, with thermal stability up to 480°F and exceptional resistance to extrusion. PEEK can be used at pressures up to 20,000 psi and a maximum temperature of 500°F. These properties make PEEK an excellent option for both high-pressure and high-temperature applications. Additional strength and hardness can be achieved by using various fillers. Finally, PEEK is compatible with a wide range of chemicals and exhibits very low friction.
Nylon works very well in moderate conditions and is more economical compared to PEEK and PTFE. However, it does have some critical limitations that include swelling and water absorption, both of which can heavily impact tolerances. Nylon 6,6 works well at pressures below 10,000 psi and a maximum temperature of 186°F. Nylon 6,6 filled with Molybdenum Disulfide (MoS2) to reduce friction is commonly used for backup rings. However, one of the drawbacks of Nylon 6,6 is its water absorption, which can range from 0.5% to 1.4% per 24 hours.
Conclusion
Choosing the right polymer backup ring is ultimately about understanding the pressures, temperatures, clearances, and motion your system must withstand. When those variables are matched with the correct geometry and material, you will have a more reliable sealing system with fewer failures and a longer service life. If you need help evaluating your application or choosing the optimal backup ring design, Advanced EMC’s engineering team is ready to assist. Contact us today to discuss your requirements and get expert guidance on the best polymer sealing solution for your equipment.
