When an elastomer seal runs out of chemical resistance, temperature capability or friction performance, engineers often turn to a PTFE spring energized seal. This article explains how these seals are built, how they work, where they fit and what has to be defined before one can be selected.
What a spring energized seal is
A spring energized seal has two parts: a jacket machined from PTFE or a filled PTFE compound, usually with a U-shaped cross-section, and a metal spring fitted inside the jacket.
PTFE has low friction and broad chemical resistance, but it is not elastic in the way rubber is. Once deformed, it does not spring back, so a plain PTFE ring cannot maintain contact with the sealing surfaces on its own. The spring supplies the missing elasticity.
How it seals
At low pressure, the spring pushes the jacket lips against the mating surfaces and provides the initial sealing load. As pressure rises, the fluid enters the open side of the jacket and adds to that load, so sealing force increases with system pressure. For this reason the seal is installed with the open side of the jacket facing the pressure.
The spring also takes up small amounts of lip wear, minor misalignment and the dimensional changes that come with temperature, which helps the seal keep working as conditions vary.
Jacket materials and spring types
Jacket
Unfilled PTFE offers the lowest friction and the widest chemical compatibility. Fillers such as glass, carbon, graphite or bronze are added to improve wear resistance and to reduce deformation under load. The filler affects more than wear life: it can change chemical compatibility, and some fillers can be abrasive to softer mating surfaces. Jacket selection therefore has to consider the shaft or bore material as well as the media.
Spring
Several spring forms are in common use, including cantilever (V-shaped), helical and canted-coil types. Each has a different load and deflection characteristic, and the choice depends on whether the duty is static, reciprocating or rotary and on how sensitive the application is to friction. The spring material is chosen for compatibility with the media, since the spring may be exposed to the fluid.
Where they are used
Spring energized seals are typically considered when one or more of the following applies:
- the media would attack common elastomers;
- the temperature is too high or too low for an elastomer seal to remain serviceable;
- low or consistent friction is needed, or stick-slip must be avoided;
- lubrication is poor, or the seal must run dry;
- long periods of standstill would cause an elastomer to stick or take a permanent set.
They are found in pumps, valves, compressors, actuators and process equipment across many of the industries we serve. Suitability for a particular temperature, pressure or speed depends on the jacket compound, the spring, the seal design and the application, and should be confirmed against the supplier's data. Related guidance is in our article on seal selection for high temperature applications.
Limitations and design points
- Installation: a PTFE jacket cannot be stretched or folded like a rubber seal. Depending on size and section, the housing may need to be split or open, or installation tooling may be required.
- Surface finish: PTFE does not conform to surface imperfections as readily as rubber, so the finish and hardness of the mating surface matter more.
- Handling: a scratch across a sealing lip can become a leak path. Lead-in chamfers and careful handling are important.
- Housing: these seals are usually made to suit a specific groove, so the housing dimensions must be known or designed together with the seal.
- Cost: they cost more than standard elastomer seals, so they are normally used where the duty justifies it.
Share your application details
We supply PTFE spring energized seals, including customized sizes sourced to suit your housing. To help our sales engineers recommend a suitable design, send the following through the enquiry form:
- the type of motion: static, reciprocating, rotary or oscillating;
- shaft or rod diameter, bore diameter and groove width, or a housing drawing;
- the media, temperature range, pressure and speed;
- the mating surface material and its condition;
- a sample of the existing seal, if there is one.
