Spring Energized Teflon Seals
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Inside PEEK Bearing Materials: High Load Capacity, Low Wear

Engineers are replacing metal bearings too often or are dealing with corrosion, weight, or lubrication failures. PEEK is a high-performance polymer alternative that is changing bearing design. In this article, we look at what PEEK is, why it performs well in many bearing applications, and where it genuinely makes sense to use it.

What Is PEEK and Why Does It Matter for Bearings?

PEEK (Polyether ether ketone) is a semi-crystalline thermoplastic that has proven an excellent solution to many bearing problems. It offers a high strength-to-weight ratio, excellent chemical resistance, a low coefficient of friction, and thermal stability up to 250°C continuous. This combination of properties leads to effective bearing performance, offering PEEK an advantage over both metal and many standard polymer solutions. 

High Load Capacity

The standard performance-bearing metric is the PV rating (pressure × velocity). If we look at the PV ratings of commonly used polymer bearing materials, we see that PTFE is severely limited despite its low friction, and nylon and acetal do not do much better. Unfilled PEEK has the best PV ratio, and when filled with carbon fiber, it has a significantly higher PV rating than other engineering polymers. This alone is an excellent reason to consider using PEEK in bearing applications. And note that filled PEEK grades are typically the main choice for demanding applications, while unfilled PEEK is more commonly used where chemical purity or machineability is the priority.

Low Wear Performance

One of the most interesting features of PEEK as a bearing material is its wear resistance, which is critical for many applications. PEEK’s molecular structure resists abrasion under dynamic loading, and it offers excellent performance in dry-running applications because it is self-lubricating, offering an excellent option when lubricants are discouraged or prohibited. Bearing-grade PEEK filled with carbon or graphite further enhances wear performance by significantly reducing wear rates. The low wear performance of PEEK bearings also leads to reduced downtime and longer maintenance intervals. 

Where PEEK Bearings Make Sense

PEEK has excellent properties, but they are not an ideal fit for all bearing applications. PEEK bearings are commonly used for applications such as the following:

  • Medical devices and surgical equipment, where biocompatibility and sterilization resistance prove critical
  • Food and beverage processing, where the self-lubricating property reduces contamination risks
  • Aerospace and defense, where the high strength-to-weight ratio leads to weight savings, reduced energy requirements, and SWaP-friendly designs
  • Chemical processing, where resistance to acids, solvents, and aggressive media exposure is necessary
  • Semiconductor and cleanroom environments, where PEEK exhibits no outgassing 

There are areas where PEEK is not recommended, however. It should not be used in ultra-high load static applications, where steel still dominates. In addition, it’s not well-suited to very high-temperature environments, which are likely beyond PEEK’s thermal ceiling. Finally, PEEK is not a good option for budget-sensitive, low-performance applications where cheaper plastics work well.

Conclusion

PEEK bearings are not a universal replacement for other materials, but rather a viable option for demanding operating environments where their unique properties justify the cost. If you are looking for a sealing solution that sounds like a great application for PEEK, contact the sealing experts here at Advanced EMC and put their knowledge to work for you.

Metal seal
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Using Metal Seals Correctly

Using metal seals can be a viable alternative to polymer and rubber seals. They are used extensively in extreme applications, such as rocket engine nozzles and missile guidance housings, as well as in ultra-high-vacuum equipment and cryogenic transfer systems. This article focuses on where metal seals are used and the pros and cons of their use.

Where Using Metal Seals is a Good Approach

Metal seals do their best work in extreme environments where polymer and elastic seals typically fail, including extremely high temperatures (above 200°C) and pressures. 

For example, in the oil and gas industry, they are used with wellhead equipment, subsea Christmas trees, valve bonnets, and high-pressure pipework, which require metal ring gaskets (such as API ring joint gaskets) because they can withstand extreme downhole pressures and resist corrosive fluids.

In aerospace and defense, metal seals are found in rocket engine nozzles, spacecraft propulsion systems, and missile guidance housings. Such applications rely heavily on metal seals to maintain integrity through thermal cycling, vibration, and vacuum conditions.

The nuclear industry uses metal seals in reactor pressure vessels, containment flanges, and coolant piping. Metal seals (often manufactured from Inconel or stainless steel) perform extremely well because they resist radiation embrittlement and provide reliable long-term sealing without the problems associated with outgassing.

Semiconductor and vacuum systems employ metal seals in ultra-high-vacuum (UHV) equipment such as particle accelerators, electron microscopes, and chip fabrication chambers. These applications often use knife-edge metal seals (CF flanges) to achieve near-zero leak rates without risking contamination in clean environments.

Metal seals are also used in cryogenic applications, including liquid nitrogen or liquid hydrogen storage and transfer systems. Polymers become brittle at cryogenic temperatures, while soft metals like aluminum or copper remain ductile, making them an ideal solution for use at cryogenic temperatures.

Engineers also use metal seals extensively in chemical processing applications, such as reactors and heat exchangers, where aggressive acids, solvents, or oxidizers (which use metal spiral wound or ring gaskets)   are used. Metal seals work well where the chemical compatibility is a significant concern.

Benefits of Using Metal Seals

Metal seals used for several reasons. First, metal seals do not experience creep or relaxation when exposed to heavy, sustained loads, which can be a major issue with some elastomers and polymers. Depending on the material chosen, metal seals can also be engineered to be chemically inert to most process fluids, including strong acids, solvents,  alkalis, and oxidizers. In addition, they are not subject to permeation or outgassing issues that can occur with some elastomers. They are also inherently fire safe because of their high melting point, and can handle some of the most aggressive cleaning and sterilization processes.

Metal seals can also survive extreme thermal cycling without experiencing permanent set. In fact, metal seals exhibit excellent temperature resistance, with some metals capable of withstanding temperatures from -270°C to over 1000°C, which cannot be achieved with a polymer or elastomer. They can also handle extreme pressures, from ultra-high vacuum pressures to tens of thousands of psi. Finally, they experience a long service life in static applications and do not have a limited shelf life or aging concerns.

Things to Keep in Mind when Using Metal Seals

Metal seals do require stricter surface finish requirements than polymer and elastomeric solutions, and a high seating stress is required to achieve a reliable seal. Most metal seals are single-use, and the cost of their replacement can add up quickly over the equipment’s life cycle. Metal seals are also more sensitive to misalignment and vibration, which makes them unsuitable for applications involving thermal distortion across joints of movement. 

The raw material costs of metal seals (e.g., Inconel, stainless steel, titanium) and the precision machining required to manufacture them make them more expensive than their polymer and elastomeric options.  In addition, soft metals (e.g., copper, aluminum, soft iron) can gall or seize against the flange during installation, which can lead to expensive rework. Metal seals can also be difficult to disassemble and are not suitable for low-pressure applications. Finally, metal seals have limited self-healing properties.

Conclusion

Metal seals have applications where they significantly outperform their elastomer and polymer counterparts, including those involving temperature and pressure extremes. If you are in the process of selecting a metal seal for your design, contact the seal experts at Advanced EMC today.