When should you use PTFE seals?

Being the fluid power professional you are, you know that Teflon is used for more than making pans slippery. However, you may not know that Teflon is a registered trademark of DuPont, discovered accidentally many decades ago and associated with non-stock coatings. PTFE is an acronym for polytetrafluoroethylene, which is also a fluorocarbon. DuPont trademarked another popular fluorocarbon, Viton.

PTFE offers unique advantages for fluid power applications when standard elastomer seals cannot offer a sealing solution. The material is practically impervious to any form of harm a hydraulic system can throw at it. Heat, contamination and friction are no match for a PTFE seal, especially with the already limited bounds hydraulic systems play within.

Polytetrafluoroethylene (PTFE) polymer, chemical structure

PTFE shrugs off excess heat to the tune of 500°F, which is well beyond the needs of any hydraulic system or actuator not literally exposed to open flame. Hydraulic fluid prefers to remain on the cooler side of 140°F, where it remains at an appropriate viscosity and without oxidation. However, specific applications may allow fluid to approach 200° or more using specialty fluids, requiring PTFE sealing technology.

PTFE is also the seal of choice for high-velocity applications, such as with high-speed cylinders. PTFE offers one of the lowest friction coefficients of any known material, so their use with high-velocity cylinders is widespread. In addition, their lack of static friction means they work well for motion control applications, where the piston must accelerate quickly and predictably.

Just as with heat, polytetrafluoroethylene is impervious to every hydraulic fluid designers could expose to the material, such as phosphate ester. The same can be said about environmental exposure, where seals such as a cylinder’s rod wiper may be exposed to a caustic or corrosive fallout. This natural resistance offers the seal material unlimited shelf life, while other materials may degrade when left exposed to heat, oxygen or light for an extended period.

The downside to PTFE is its plastic nature, which makes the material prone to leakage or bypass when not applied correctly. Rubber energisers are often required to improve contact with its sealing surface, which reduces the tendency to bypass. Also, because of the plastic nature of PTFE, it can sometimes make installation difficult, so intelligent and thoughtful design of the parent component’s sealing pockets and cavities should be considered. For example, a flange or retainer to hold a cylinder’s rod seal makes installation possible compared to a machined pocket requiring the seal to be twisted and bent before installation.

Source: Fluid Power World 

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