In dynamic applications requiring sealing at low-to-moderate speeds and pressures, design engineers are replacing underperforming elastomeric 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 embracing a new and more cost-effective way to improve the performance of their existing hardware designs: The spring-energized PTFE “C-ring” seal.
The challenge: replacing a failed O-ring
The C-ring seal was first developed for a diagnostic tool employing a piston operating in a water bath of approximately 100°F, reciprocating at a rate of 5 feet per minute. The operating conditions were mild, but the tolerances were large. The original design called for an elastomeric O-ring to seal the piston, but the O-ring was unable to maintain a consistent seal and the equipment leaked.
With prototypes already built, engineers began looking for an alternative. U-cups or standard lip seals, which would typically be used in a piston application, were not a viable option because of large radial tolerances. Also, installing these over a full-step groove wasn’t practical. The installation would require too much stretching, resulting in seal deformation and premature failure.
The successful retrofit
In 2016, Foothill Ranch, Calif.-based seal designer Bal Seal Engineering proposed an experimental solution: A canted coil spring wrapped in a c-shaped PTFE ring. The seal worked exactly as intended. By combining the low friction properties of PTFE with a streamlined jacket geometry, the “C-ring” provided reliable, consistent sealing plus smoother and quieter operation than an O-ring. In addition, the C-ring could be installed in the full-step O-ring gland, which is normally not recommended with non-elastomeric materials. As a result, the C-ring was installed with no modifications to the original hardware design and without the use of any specialized tools.
That original C-ring seal has been in use for two years. Use of the C-ring has resulted in improved product performance and prolonged equipment service life – reducing downtime and maintenance costs.
One seal, many potential applications
Medical imaging units, insulin pumps, ventilators and drug-delivery devices typically rely on O-rings to seal within short axial spaces. But when extreme radial deflection capability is required, O-rings cannot compensate — often resulting in wear, permanent deformation and leaks. Despite these shortcomings, engineers have continued to use O-rings because other solutions (e.g., U-cups, lip seals) are unable to handle radial deflection requirements and normally require more axial space than an O-ring.
The C-ring is different because it can fit into the small axial space typically afforded for O-rings, whereas standard seal configurations cannot. In addition, the C-ring is fully customizable, based on the needs of the application. It can be configured with ultra-thin and flexible lips for cryogenic applications or with one thick lip for a dynamic application where the seal requires more wear resistance.
Rotary, reciprocating and static
Because the C-ring accommodates rotary and reciprocating movements, it’s a versatile solution potentially useful in a variety of products that require slow-to-moderate speed sealing – including medical robots, portable medical equipment and probe/hose connectors. The C-ring can accommodate an unusually large degree of radial tolerance – at least five times as much as a standard seal configuration of a similar cross-section. Tolerance ranges are dependent on ambient pressure, media type and surface finish conditions. The C-ring also functions well in static applications, where components require protection from environmental contaminants.
PTFE density and compound options
By removing PTFE material from the C-ring’s initial jacket design, engineers were able to enhance its elasticity and pliability. As a result, the C-ring has proven to be more stretchable and flexible than originally expected, making it useful for non-round applications. The C-ring is already in use in a drug-delivery pump with an oval piston. And because the seal jacket can be made with virgin or filled PTFE, the C-ring is an extremely versatile seal, compatible with hardware made from commonly-used metals and plastics.
The original C-ring developed for use in the water-based diagnostic tool included a canted coil spring inside the PTFE jacket. But a C-ring can also be manufactured using a helical ribbon spring as the energizer. Substituting a helical ribbon spring for the canted coil spring allows the C-ring to provide very high sealing contact pressure, making it ideal for use in cryogenic or static applications.
Citing its ability to provide longer service life in applications where clearances, surface finishes and other design characteristics vary widely, Bal Seal Engineering has called its C-ring “the perfect seal for an imperfect world.” While no seal is perfect, the versatility and customizability of the C-ring certainly make it an interesting and potentially beneficial option in certain medical and diagnostic devices. It is a relatively light-duty seal, ideal for use in low-pressure (<500 psi), low-speed (<100 ft/min) applications that require low friction. For such environments, a C-ring may prove to be a better sealing solution than an elastomeric O-ring or other standard seal type, offering designers the opportunity to achieve more service life and quieter performance without costly hardware modifications.
David Wang is a global market manager for Bal Seal Engineering’s medical device business. An engineer with more than 10 years of design experience, he collaborates with OEMs and tier suppliers to create sealing, connecting, conducting, and EMI shielding solutions that help set new standards for device performance.
The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of MedicalDesignandOutsourcing.com or its employees.