Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.
Original content can be found on Parker’s Website and was written by Nathan Wells, Application Engineer, Parker Engineered Polymer Systems Division.
My grandpa used to have a rusty, old air compressor in his shop. As a child, when my siblings and I would visit him, he’d use it to power air wrenches, grinders, and inflate flat soccer balls for us. I noticed it had a port labeled “ADD OIL DAILY” that was covered in the same thick layer of greasy dust as all the other unused junk in his shop. Knowing my grandpa, if asked about adding oil he probably would have said, “Oil is expensive. That’s how the companies get ya!” The compressor’s seals leaked so badly, you could hear the hissing even over the loud motor. I was certain one day it would explode.
Pneumatic tools are common in factories, tool shops, and DIY garages around the world. Using compressed air for power is convenient, simple, and — when maintained properly — safe and efficient. However, air treatment costs can add up fast. Traditional rubber seals used in air tools require clean, low moisture, compressed air with the proper amount of lubrication added. Good Filter/Regulator/Lubricator systems (FRLs) cost as much as the tools themselves! So, what would happen if we didn’t have to provide pristine air?
Today we have the technology to create seals for tools which don’t require daily or even yearly upkeep. You’ll find these tools labeled “maintenance-free,” which sounds great to the guy responsible for maintenance. It sounds even better to the guy paying for maintenance … and to engineers designing tools who want to keep warranty costs down.
Seal materials for dry running
Early pressure seals were made out of leather. My grandpa’s compressor probably wasn’t that old, but even since his time, we’ve come a long way.
When I’m asked for seal recommendations in totally dry-running applications, my mind clicks to a material called PTFE (chemical name polytretrafluoroethylene). Most people know PTFE by the brand name Teflon® and are familiar with its use when applied to cookware as a high temperature, slippery, non-stick coating.
PTFE is a semi-hard plastic which feels slick to the touch thanks to its low friction properties. It’s considered self-lubricating because it leaves micro deposits on the sealing surface and reduces friction after just a few strokes. Because of this, it’s good for high-speed sealing and can operate completely dry.
By adding fillers to PTFE, seal manufacturers can tailor materials for greater suitability in meeting performance requirements for a wide range of conditions. String-like additives including fiberglass and carbon fiber increase pressure rating, wear resistance and seal life. Dry lubricant-type additives such as graphite or molybdenum disulfide (MoS2) further increase a seal’s ability to run without lubrication, and at higher speeds and pressures. In pneumatic medical, pharmaceutical, and food processing systems, clean grade mineral-based strengtheners may be used as additives.
PTFE seals for dry running equipment are available in several profile configurations: