In dynamic applications that require sealing at low to medium speeds and pressures, design engineers are replacing the poorly performing elastic O-rings with spring-energized PTFE "C-ring" seals.

David Wang, Bal Seal Engineering

When O-rings and other traditional sealing methods fail, diagnostic and drug delivery equipment engineers are adopting a new, more cost-effective method to improve the performance of their existing hardware designs: spring-enhanced PTFE "C-ring" seals.

The C-ring seal was originally developed for a diagnostic tool that uses a piston that runs in a water bath at approximately 100°F and reciprocates at a speed of 5 feet per minute. The operating conditions are mild, but the tolerances are large. The original design required the use of elastic O-rings to seal the piston, but the O-rings could not maintain a consistent seal, resulting in equipment leakage.

The prototype has been built, and engineers have begun to look for alternatives. Due to the large radial tolerances, the U-shaped cups or standard lip seals commonly used in piston applications are not a viable option. In addition, it is impractical to install them on a full-step groove. Excessive stretching is required for installation, resulting in seal deformation and premature failure.

In 2016, Bal Seal Engineering, a seal designer based in California's Foothill Ranch, proposed an experimental solution: an inclined coil spring wrapped in a C-shaped PTFE ring. The seal works exactly as expected. By combining the low friction properties of PTFE with the streamlined sheath geometry, the "C-ring" provides a reliable, consistent seal and smoother and quieter operation than an O-ring. In addition, C-rings can be installed in full-step O-ring glands, and non-elastic materials are generally not recommended. Therefore, no modifications were made to the original hardware design and no special tools were used when installing the C-ring.

The original C-ring seal has been in use for two years. The use of C-rings improves product performance and prolongs equipment service life-reducing downtime and maintenance costs.

Medical imaging devices, insulin pumps, ventilators, and drug delivery equipment usually rely on O-rings to seal short axial spaces. However, when extreme radial deflection capabilities are required, O-rings cannot compensate-usually resulting in wear, permanent deformation and leakage. Despite these shortcomings, engineers continue to use O-rings because other solutions (such as U-shaped cups, lip seals) cannot meet the radial deflection requirements and generally require more axial space than O-rings.

The C-ring is different because it can fit into the small axial space normally provided by O-rings, while the standard seal configuration cannot. In addition, the C-ring is fully customizable, based on the needs of the application. It can be configured with an ultra-thin and flexible lip for low temperature applications, or a thick lip for dynamic applications where the seal requires higher wear resistance.

Since the C-ring adapts to rotation and reciprocating motion, it is a versatile solution that can be used for various products that require medium and low speed sealing, including medical robots, portable medical equipment, and probe/hose connectors. C-rings can accommodate unusually large radial tolerances—at least five times the standard seal configuration of similar cross-sections. The tolerance range depends on the environmental pressure, media type and surface finish conditions. C-rings also work well in static applications, where components need to be protected from environmental pollutants.

By removing the PTFE material from the initial sheath design of the C-ring, engineers were able to enhance its elasticity and flexibility. Therefore, the C-ring has proven to be more stretchable and flexible than originally expected, making it suitable for non-circular applications. C-rings have been used in drug delivery pumps with oval pistons. Since the sealing sleeve can be made of pure PTFE or filled PTFE, the C-ring is a very versatile seal, compatible with hardware made of commonly used metals and plastics.

The original C-ring developed for water-based diagnostic tools included a canted coil spring inside a PTFE sheath. However, it is also possible to use a spiral ribbon spring as an energizer to make a C-ring. Replacing canted coil springs with spiral band springs allows C-rings to provide very high sealing contact pressures, making them ideal for low temperature or static applications.

Bal Seal Engineering cited its ability to provide longer service life in applications where gaps, surface finishes, and other design features vary widely, and referred to its C-rings as "the perfect seal for an imperfect world." Although there is no perfect seal, the versatility and customizability of C-rings undoubtedly make them an interesting and potentially beneficial choice for certain medical and diagnostic equipment. It is a relatively lightweight seal, very suitable for low-pressure (<500 psi) and low-speed (<100 ft/min) applications that require low friction. For this type of environment, C-rings may prove to be a better sealing solution than elastic O-rings or other standard sealing types, giving designers the opportunity to achieve a longer service life without expensive hardware modifications And quieter performance.

David Wang is the global marketing manager of Bal Seal Engineering's medical device business. As an engineer with more than 10 years of design experience, he works with original equipment manufacturers (OEMs) and Tier 1 suppliers to create sealing, connection, conduction, and EMI shielding solutions to help set new standards for equipment performance.

The views expressed in this blog post are only those of the author and do not necessarily reflect the views of MedicalDesignandOutsourcing.com or its employees.

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