Materials

There are three leading materials used for most back-up ring (BUR) applications: PTFE, Nylon, and PEEK. Each of these materials has specific benefits that it can bring to your application, starting with their stiffness and compressive strength.

Why Back-up Rings Are Important

Seal extrusion is one of the most common causes of polymer seal failure. Whether the cause of extrusion is a large gap between the mating surfaces, high temperatures, or extreme pressures, back-up rings can help. The right choice of a BUR can prevent seal extrusion, lengthen the useful life of the seal, and reduce the chances of a catastrophic failure.

PTFE Back-Up Rings

PTFE is well-known for its extremely low friction, dry running capabilities, and outstanding chemical resistance. Filled PTFE (either glass, carbon, graphite, or bronze filled) can handle operating pressures up to 5,800 psi, making it an excellent choice for both medium and high-pressure applications. Virgin PTFE has a much lower maximum operating temperature (around 3,600 psi) and is limited to low-pressure situations. PTFE also has a maximum operating temperature of 575°F, and that combined with chemical compatibility and the high-pressure capabilities of filled PTFE mean that it is an excellent option for harsh condition environments.

Nylon Back-Up Rings

Nylon 6,6 (sometimes written Nylon 6/6 or Nylon 66) is a polyamide material commonly used for back-up rings. It can handle high pressure very well but is limited to temperatures below 186°F. It possesses excellent rigidity, good compressive strength, and thermal stability, all of which are key to effective backup rings.

When used for back-up rings, Nylon is typically filled with Molybdenum Disulfide (MoS2) to achieve an even lower coefficient of friction. It is not recommended for use in wet or humid environments because it does absorb water unless fillers such as glass are added to offset the absorption effects. 

PEEK Back-Up Rings

Another commonly used back-up ring material is PEEK, which can handle temperatures of up to 500°F and pressures up to 20,000 psi. Like PTFE, it is low friction, dry running, and resistant to a wide variety of aggressive chemicals. It is also available with fillers to enhance properties such as compressive strength and stiffness. However, PEEK is much harder than PTFE: PTFE has a Shore hardness of D50 while PEEK has a significantly greater hardness of D85. For these reasons, PEEK back-up rings are often used in aggressive environments, such as those found in the oil and gas industry. 

Conclusion

If you are having issues with extrusion-related seal failure, polymer back-up rings are a cost-effective solution that can extend the life of your seals. When it comes to polymer back-up ring materials, the top three choices are PTFE, Nylon, and PEEK. While each has its own pros and cons, they are excellent options for solving the problem of seal extrusion. PEEK works best for high pressure, high temperature environments that can involve exposure to corrosive materials. PTFE can also handle high temperatures and corrosive environments, but its maximum operating pressure is lower than that of PEEK. Nylon is also an excellent choice with excellent hardness and thermal stability, with its main limitations being high temperatures and exposure to humidity and moisture. 

Polymer bearings can be found in almost any industry and environment, and this includes the clean rooms of electronics to the harsh conditions of the oil and gas industry. And this is no surprise considering the host of benefits that polymer bearings provide, including their resistance to corrosive chemicals, low maintenance, lightweight, and low friction.

So, what exactly are the top five materials used in polymer bearings? The top five polymer bearing materials include Torlon PAI, Bearing Grade PEEK, Bearing Grade PPS, Lubricated PET, and Lubricated Nylon.

1. Torlon PAI

PAI stands for Polyamide-imide and it is the highest performing polymer that is melt-processable. It offers excellent wear resistance, has an extremely low coefficient of friction,  and can handle operating temperatures up to 500°F. The primary drawback of Torlon PAI lies in its relatively high level of moisture absorption. On the other hand, it has a low coefficient of thermal expansion and a high level of creep resistance, both of which are key characteristics for an effective bearing. Torlon PAI is often used in bushings, bearings, and wear rings.

2. Bearing Grade PEEK

Bearing grade PEEK is known for its excellent wear characteristics, good abrasion resistance, extremely low coefficient of friction, and outstanding chemical resistance. It can handle environmental operating temperatures up to 500°F and performs well even when continuously exposed to hot water and steam. Bearing grade PEEK is also easy to machine, has low moisture absorption, and possesses a high PV rating. 

3. Bearing Grade PPS

PPS (polyphenylene sulfide), like the other bearing grade polymers discussed so far, has excellent wear resistance and a low coefficient of friction. However, it also offers very good wear resistance and dimensional stability even at elevated temperatures. Bearing grade PPS has a rated operating temperature of 425°F and offers outstanding chemical resistance. In addition, bearings can be made to extremely high tolerances when PPS is used.

4. Lubricated PET

Lubricated PET combines the stiffness, wear resistance, and dimensional stability of PET with the low friction demands of bearing applications. It offers extremely low water absorption, good abrasion resistance, and can be machined to very tight tolerances. It is internally lubricated using a dispersed solid and is dry running (needing no additional lubrication). The internal lubrication is released during operation, further reducing the naturally low coefficient of friction that PET possesses.

The primary drawback of PET lies in its limitations with regard to temperature: its continuous service temperature is 210°F, which makes it unsuitable for extreme temperature service conditions. 

5. Lubricated Nylon

Nylon does an excellent job of balancing toughness and strength while combining good abrasion resistance with the ability to be extruded, cast, or machined. Lubricated Nylon, much like lubricated PET, includes a solid dispersal of lubricants that greatly reduces the standard coefficient of friction of virgin Nylon and allows it to be used in dry running applications. One of the more common lubricants used is MDS or Molybdenum Disulfide. 

The primary issue with Nylon is its ability to absorb up to 7% of its weight water, which can affect its dimensions. However, it does have an extremely high limiting PV rating and excellent wear characteristics. 

Conclusion

The top five polymer bearing materials–Torlon PAI, bearing grade PEEK, bearing grade PPS, lubricated PET, and lubricated Nylon–are commonly used to replace metal bearings in a variety of applications. They offer the wear resistance, high PV ratings, low friction, and chemical resistance that are required. If you are in the market for new or replacement bearings, be sure to consider polymer bearings and bushings, also.