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.
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:
- Uni-directional spring-loaded PFTE lip profiles which are good for both linear and slow rotary applications (See Parker's FlexiSeal® Profiles in Catalog EPS5340)
- Dual-acting, O-ring energized "cap" seals for easy installation in linear stroking applications (See Parker's offering in our linear sealing Catalog EPS5370)
- Wear bands which support moving components and prevent contact during shock loading can be made from PTFE for light- and medium-duty systems
For more difficult dry-running applications, tougher engineered plastics such as Nylon, polyester, UHMW-PE, and PEEK are used.
Where cost is the over-riding factor, internally-lubricated rubber elastomer materials are recommended in common, standard profiles:
- For slow speed, lower pressure, completely dry-running applications, a thin, light load lip geometry like Parker's 8400 Profile can accommodate cost, low friction, and low break-away force requirements
- Parker's Standard PolyPak® (SPP) Profile seal tightly at low pressure, and also work great as heavy-duty rod wipers
These common profiles are available in internally-lubricated XNBR (a tougher variant of NBR), which is my go-to rubber for low- and no-lube applications.
Design considerations
An air compressor was one of the first power tools I bought. I knew little about seals back then and just needed something to top off the air pressure in my car tires and power a nail gun. I purchased one advertised as maintenance-free because I am, after all, my grandfather’s offspring. Oil costs money.
Even though dry systems don’t benefit from the protective aide afforded by lubrication, the same set of “good seal design principles” apply whether the system is fully hydraulic, lightly lubricated, or completely dry. Let’s take a look at a few seal design concepts and elaborate on how a lack of lubrication affects them.
Speed and pressure
The sliding movement of seals generates heat through friction. Rub your hands together and they get warm. Rub them faster or press together harder, and they get hot quicker. Excessive heat is the enemy of seals; they begin to soften and wear out faster. We counteract this by using tougher materials which can handle the temperature. Alternatively, we can design methods to cool the system.
Pneumatic systems are unique in that they either pull air from outside (compressors) or vent to atmosphere (tools and cylinders). Expanding gas has a small cooling effect on the latter.
Compressing gas generates heat, so compressors are a good example of a worst-case scenario. In addition, they operate at high speed and pressure. The housing of my compressor is covered with fins to help dissipate heat during long run cycles. Seal materials like PTFE, PEEK, and FKM (energizers) are often used because they can handle hot temperatures.
Surface finish
The Parker catalog recommends a maximum surface roughness of 12 µin Ra for linear and rotary applications. PTFE will benefit from a smoother surface, creating a tighter seal and reducing wear. Applications where leakage is critical, like in cryogenics or when sealing explosive gases, a highly polished finish of 4-6 µin Ra is best.
Surface hardness
A minimum hardness of 25 Rc is recommended for linear and rotary applications. Softer materials can be used in light duty applications, but the surface may wear or become grooved over time, especially in rotary applications.
Some PTFE fillers are abrasive. While tough fillers like fiberglass and carbon fiber provide superior wear resistance, we recommend a minimum surface hardness of 60 Rc to prevent abrasion of the hardware. Mild fillers such as graphite, bronze, and aramid fiber are okay with the minimum requirement of 25 Rc.
Contamination
Dirty air is a problem in all pneumatic systems. If the system is exposed to contaminated air, or to a dirty environment, abrasive particles can accelerate wear of both the hardware and the seals. Using wear resistant materials (like fiberglass-filled PTFE) along with a hardened sealing surface will prolong the life of the equipment.
Seal geometry also plays a role here. Sharp-lipped profiles like our FBD-V FlexiSeal and PolyPak (SPP & DPP) prevent junk from building up under the seal lip and wearing it away. Filling the spring cavities of FlexiSeals with silicone is an option to make the seals easier to clean when used in extremely dirty environments that require equipment washdown.
Fluid compatibility and moisture
Air and non-reactive gases like nitrogen are compatible with all seal materials (so long as they’re used within the material’s temperature capability range). Caustic gases like pure oxygen, chlorine, and hydrogen sulfide (H2S) will degrade many seal materials. PTFE is highly resistant to most forms of chemical attack and is a good choice when compatibility is a concern.
PTFE has one weakness and that’s water. The material is extremely hydrophobic. The protective layer that naturally deposits itself on the hardware gets washed away in wet environments. As a result, seals wear quickly. Carbon fiber-filled PTFE and UHMW-PE plastic are recommended in potentially wet applications.
Conclusion
Having helped design dozens of compressors, the geeky engineer side of me is waiting for the compressor I have at home to break so I can look at the seals. It is entirely possible, however, it will keep working for me for tens of years without failing and just maybe, will someday inspire a blog post from my grandkids.
As technology improves, customers are looking for tools which last longer and cost less to operate. At Parker, they are constantly working and testing to provide tougher seal materials and new designs to aid you in making better products.
Gallagher Fluid Seals is an authorized distributor of Parker. To learn more about how Gallagher Fluid Seals can help you, contact our engineering department at 1-800-822-4063