Small Diameter Spring Energized Seals
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

How Spring Selection Defines Spring-Energized Seal Performance

Most failures blamed on PTFE actually originate in the spring. This blog post discusses the load-management system and key features of spring-energized seals for canted coil springs, V springs, cantilever springs, helical springs, and coil springs.

Canted Coil Springs (Slant Coil Springs)

Canted Coil Springs and Slant Coil Springs from Advanced EMC Technologies

These springs are wound so that individual coils are set at an angle to the longitudinal axis. They are highly versatile and often used for dynamic sealing applications. Their key feature of canted coil springs is the flat load curve they provide. These spring energizers generate a nearly constant force across a wide deflection range. The constant force allows precise control over friction and torque, making these spring energizers ideal for applications where these factors are critical. Canted coil springs are also unlikely to experience compression set.

Canted coil spring energizers work best in moderate to high-speed rotary applications. Beyond sealing, their unique design allows them to serve as mechanical connectors (latching/locking), EMI/RF shields, and multi-point electrical conductors.

V Springs (V Ribbon Springs)

The V spring is a general-purpose, cantilever-type energizer. They offer an excellent balance of performance and cost-effectiveness. In addition, V springs provide a moderate load over a wide deflection range. They function well in both static and dynamic applications, including those involving rotary or reciprocating motion.

V springs are frequently recommended for severe service conditions, including vacuum pressures and cryogenic temperatures. V spring energizers are often a preferred choice for harsh operating environments.

Cantilever Springs (Finger Springs)

Often referred to as finger springs, these spring energizers feature a V-shaped cross-section and are distinguished by a linear load curve, meaning the force increases linearly with deflection.

The load is concentrated at the very front edge of the seal lip, which provides positive wiping action and makes them particularly effective for exclusion and scraping applications. They also generate extremely low friction.

Cantilever spring energizers are well-suited for sealing viscous media. They are typically found in low to medium-speed applications, such as hydraulic cylinders, pumps, compressors, and shocks.

Helical Springs (Helical Flat / Compression Springs)

Helical springs consist of a wound ribbon of metal and are characterized by a high load-versus-displacement curve. Because they produce a very high unit load with a small deflection range, helical springs provide tight, reliable sealing. They are well-adapted for sealing light gases and liquids.

Helical springs are generally limited to static, slow-dynamic, or intermittently dynamic applications because friction and wear are less of a concern than seal reliability. These spring energizers are often used in pipe flanges and crush jackets where the seal must embed into surface irregularities. Experts highly recommend helical configurations for cryogenic applications.

Coil Springs (Spiral Pitch Springs)

When many people visualize a spring-energized seal, they picture this wire coil type. These spring-energizers actually perform best in high-pressure, medium-speed applications and are known for their low friction. 

Spring Materials

The performance of spring-energizers is also dependent on the material selection. The material selection is primarily determined by the chemical and thermal environments involved. At Advanced EMC, we recommend one of the following spring materials: 

  • Stainless Steel (300 Series, 17-7 PH, 301/304): Common for general-purpose and cryogenic applications
  • Hastelloy: Recommended for highly corrosive media
  • Elgiloy: Used for high heat, corrosive environments, and salt water
  • Inconel: Used in severe environments and cryogenic applications

Conclusion

When spring-energized seals fail, the problem is often not the jacket, but the spring. Knowing about load consistency, deflection behavior, and how that force is delivered over time is key to deflection, friction, wear, and whether a seal actually survives its operating environment.

At Advanced EMC, spring-energized seals are engineered as complete systems, not just components. Our team will assist you from spring selection to geometry and material pairing, aligning the seal design with real-world conditions. If you are troubleshooting a failure or designing for demanding service, contact Advanced EMC today.

FEP Encapsulated O-Rings | Advanced EMC Technologies
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Encapsulated O-Rings for Chemical Service

Encapsulated O-rings occupy a narrow but critical niche in chemical service sealing. These O-rings are usually specified when elastomeric solutions lack chemical compatibility against aggressive chemicals, but solid PTFE O-rings lack elasticity. Industries such as chemical processing, semiconductor manufacturing, pharmaceutical systems, and specialty fluid handling all involve problematic combinations of aggressive media, temperature variation, and intermittent pressure. These applications often require an encapsulated O-ring.

This blog post explains why encapsulated O-rings work so well for chemical service, and then continues with a discussion of jack material tradeoffs, why a system-level view of O-rings is important, and then continues with a discussion of permeability, compression set, and the complications involved in vacuum conditions.

Why Encapsulated O-Rings Are Commonly Specified for Chemical Service

Fluoropolymers such as FEP, PFA, and PTFE providea very broad chemical resistance across media such as acids, bases, solvents, and oxidizers. However, when used alone, these materials have low modulus recovery and are extremely susceptible to issues like creep and stress relaxation. Encapsulated O-rings, on the other hand, bridge this gap by pairing a chemically inert fluoropolymer jacket with an internal energizing core that supplies sealing force.

The result is not a chemically inert O-ring, but rather a hybrid sealing system. The resulting system, when correctly engineered, can help strike that delicate balance between chemical compatibility, consistent contact stress, and dimensional stability.

Encapsulated O-Rings: Jacket + Core System

Encapsulated O-rings are composite assemblies: the fluoropolymer jacket provides chemical isolation and surface characteristics, while the core is responsible for contact stress, recovery, and dimensional tolerance.

The jacket limits the core’s ability to energize the seal, which is the primary factor that makes these O-rings extremely effective. The core compensates for factors such as jacket creep, surface irregularities, and pressure fluctuations. 

Failure to account for the interaction between the core and the jack leads to seals that are chemically compatible but mechanically unstable, however. A system-level view like this is essential when selecting encapsulated O-rings for chemical service, particularly in long-duration or safety-critical applications.

Jacket Material Tradeoffs: FEP, PFA, and PTFE

The jacket material serves as the main barrier between the process media and the internal core. In addition, it exerts a dominant influence on seal stiffness, installation force, and recovery behavior.

FEP is widely regarded as the industry standard for encapsulated O-rings. This material offers a combination of excellent chemical resistance, relatively low stiffness, and good flexibility at moderate temperatures. These characteristics of FEP make it an excellent choice for general chemical service where sealing force is limited and good dimensional compliance is necessary.

PFA  is another option that provides similar chemical resistance to FEP but also provides better thermal stability and resistance to fatigue cracking at elevated temperatures. FEP has a higher melt strength and reduced permeability, both of which make it attractive for hot chemical service or applications with repeated thermal cycling. However, these benefits come at the cost of increased stiffness compared to FEP.

PTFE jackets offer the highest temperature capability and the lowest permeability among the three materials discussed. These jackets also exhibit the greatest stiffness and lowest compliance. PTFE-encapsulated O-rings usually require significantly higher gland squeeze to achieve the necessary sealing force. These factors can be problematic in low-pressure systems or designs with limited hardware stiffness.

Also, keep in mind that as jacket stiffness and thickness increase, chemical impermeability and vacuum resistance improve. At the same time, the ability of the core to energize the seal is going to be significantly reduced. Jacket hardness, wall thickness, and gland geometry must be accounted for together to avoid under-energized or over-compressed seals.

Permeability

Fluoropolymers are chemically resistant but not impermeable. Small molecules can diffuse through them over time because of concentration gradients, temperature differentials, and pressure differentials.This is problematic in applications involving solvents, gases, or sustained exposure at elevated temperatures. Among the jacket materials discussed, PTFE generally exhibits the lowest permeation rates, followed by PFA, with FEP being the most permeable.

Permeation can lead to core swelling, blistering during depressurization, or distortion of the jacket. In some cases, these effects are merely cosmetic. In others, they compromise the geometry of the seal and contact stress, leading to leakage or mechanical failure. Fortunately, there exist design strategies to manage permeation. These strategies include selecting lower-permeability jackets, increasing wall thickness, limiting temperature exposure, or transitioning to spring-energized cores that are less sensitive to swelling.

Compression Set 

The core is responsible for maintaining the sealing force as the jacket creeps and relaxes. Elastomeric cores (e.g., silicone or fluoroelastomer) are often used because of their resilience and ease of manufacture. There is another option: spring-energized cores that replace elastomer recovery with mechanical force. The use of metal springs leads to a highly consistent load across wide temperature ranges and eliminates the problem of compression set. Spring-energized O-rings are effective in high-temperature chemical service, long dwell applications, and systems that experience frequent pressure cycling.

Vacuum Conditions

Encapsulated O-rings face some unique challenges in vacuum service. Outgassing from elastomeric cores can contaminate vacuum environments. Permeation through the jacket, usually driven by high pressure differentials, can lead to leaks. Then, at low differential pressures, stiff fluoropolymer jackets may collapse or lose conformity to the gland walls. 

Proper gland geometry is essential. 

Excessive squeezing will increase both creep and cold flow. Insufficient squeeze, though, results in a loss of contact stress under vacuum. Encapsulated O-rings can perform well in moderate vacuum environments, but high and ultra-high vacuum applications often require alternative sealing technologies or design modifications such as spring-energized PTFE seals.

Conclusion

Encapsulated O-rings are not drop-in replacements for elastomer O-rings. They are complex sealing systems whose performance depends on considering the interaction between jacket material, permeability, core behavior, and the operating environment. 

 By engaging seal specialists early in the design process, it becomes much easier to align material selection, gland geometry, and operating conditions for a successful O-ring seal.

For applications involving aggressive chemicals, thermal cycling, or vacuum exposure, Advanced EMC is an expert in evaluating encapsulated O-ring designs and navigating the tradeoffs inherent in chemical service sealing. Contact us today!