Most Common Questions about Canted Coil Springs } Advanced EMC Technologies
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

What You Need to Know about Spring-Energized Seals and Backup Rings

One of the common problems in high-pressure applications is extrusion of the seal into the extrusion gap. This will lead to a damaged seal, which could quickly prove both dangerous and expensive depending on the application. And this problem is not limited just to more traditional seal designs.

What do you need to know about spring-energized seals and backup rings? Similar to traditional seal designs, spring-energized seals can also face problems with extrusion–which is why backup rings are sometimes needed in conjunction with a spring-energized seal.

Seal Extrusion

Extrusion becomes an issue when the pressure acting on the seal is greater than the extrusion resistance limitation of the seal or when there are operating conditions that reduce the seal’s extrusion resistance, such as high temperatures or moisture absorption. Extrusion is also a problem when guide rings or wear rings have increased the extrusion gap. BURs fit between the seal and the extrusion gap to prevent the seal from deforming and becoming caught in the gap.

And this issue is not limited to standard rotary shaft seals: it can also be a serious issue for spring-energized seals. If energized seals are failing and the edge of the sealing lip looks as if it has been “nibbled” on, there is a strong probability that the problem is seal extrusion–and a backup ring may be all that is needed to solve the problem.

Backup Rings

Backup rings (often abbreviated BUR or referred to as anti-extrusion rings) are used to prevent mission-critical seals from being damaged when they are continually exposed to high pressures and run the risk of extrusion. Backup rings prevent the seal from extruding or deforming by reducing the extrusion gap on the low-pressure side of the seal but do not serve as a seal themselves. 

Oklahoma Spring Energized Seal Company

Backup Rings with Spring-Energized seals

Materials used for BURs must be extrusion resistant themselves, which means they need to be hard. Like the seal lip, they also need to be resistant to the chemicals and environment to which they will be exposed. Ideally, backup rings should also have low friction and excellent wear characteristics. There are several different materials used for backup rings, including polymers and elastomers. The most common polymers used include PTFE, PEEK, Nylon, and UHMW PE. For spring-energized seals, both PTFE and PEEK are often used. 

Filled PTFE (e.g., glass, carbon, etc.) can operate with pressures up to 5,800 psi. However virgin PTFE is limited to about 3,600 psi which means that for backup rings filled PTFE is preferred. PTFE has a maximum operating temperature of around 575°F and it is very chemically resistant and has extremely low friction. It is also dry running and available in sufficient hardness for backup ring applications.

PEEK performs well at pressures up to 20,000 psi and has a maximum temperature operating temperature of 500°F.  It is also available will fillers to provide additional strength and hardness. Like PTFE,  PEEK is compatible with a range of chemicals, including many aggressive cleaning solutions. PEEK is often used with spring-energized PTFE seals.

Conclusion

If extrusion is the problem with a spring-energized seal, or you know that a seal design will run the risk of extrusion, then backup rings can be used to prevent extrusion. The use of backup rings not only extends the life of the spring-energized seal but reduces the probability of failure due to extrusion.

PTFE Rotary Shaft Seals
by Jackie Johnson Jackie Johnson No Comments

PTFE Rotary Shaft Seals in High Speed Applications

The design and specification of rotary shaft seals is challenging enough, but things get even more complicated for high-speed seals. High-speed rotary shaft seals pose their own set of wear and heat generation problems that can make it difficult to select an appropriate lip material, but PTFE is up to the challenge.

Rotary Shaft Seals in High-Speed Applications

In the context of rotary shaft seals, high speeds are often defined as those above 3,600 rpms. Such seals can be found in industries such as pulp and paper, wind energy, pumps, gearboxes, steel and aluminum processing, electric motors, medical devices, etc.

High-speed applications, such as those found in turbomachinery, can cause a seal to wear out faster and generate more heat because speed and friction do not get along well together. If the heat generated is sufficient, it can result in higher operating temperatures and changes to the geometry of the seal. And not all high-speed applications are compatible with lubricants, so in some cases, the seal may need to be capable of dry running. It is also key that these seals do not exhibit stick and slip behavior at startup.

Requirements for High-Speed Rotary Shaft Seals

PTFE Rotary Shaft Seals

PTFE Rotary Shaft Seals

High speed rotary shaft seal materials, in addition to the normal requirements for seals, must be …

  • abrasion and wear-resistant (to reduce wear)
  • Dimensionally stable (to prevent changes in geometry due to high temperatures)
  • Thermally conductive (to dissipate heat generation)
  • High operating temperature (to account for heat generated during use)
  • Possess an extremely low coefficient of friction (to reduce heat generation and wear)
  • Reduced stick-slip and breakout friction
  • Self-lubricating (for when lubricants cannot be used)

While there are several options available for seals that meet these requirements, one in particular stands-out: PTFE, or polytetrafluoroethylene.

PTFE High-Speed Rotary Shaft Seals

PTFE exhibits several key qualities necessary for high-speed rotary shaft seals. It has good abrasion and wear-resistant properties, is dimensionally stable, and has good thermal conductivity. PTFE also has an operating temperature of up to 500°F and a melting point of almost 650°F. It also has the lowest coefficient of friction of any solid currently known to mankind, exhibits reduced stick-slip, has extremely low breakout friction, is self-lubricating, and can continuously operate as a dry-running material.

PTFE also comes in various grades beyond virgin PTFE. It is available fillers such as Molybdenum Disulfide (MoS2) for increasing wear resistance, carbon for increasing wear resistance while keeping friction low, glass for better hardness and wear resistance, or various combinations of these. Keep in mind that there are also FDA approved seals for use in connection with pharmaceuticals and medical applications as well as food and beverage production.

PTFE rotary shaft seals are available in hydrodynamic, plain and multi-lip configuration and for situations where the production volume is low, they can be constructed from machined shells so there are no tooling charges.  At the same time, high production volumes can be manufactured from pressed shells to reduce unit costs.

Conclusion

For applications that demand reliable, long-lasting high-speed rotary shaft seals, PTFE is the engineer’s choice for reliable performance. It combines low friction, high operating temperatures, good wear properties, and dry running capabilities that can handle the rigors of high-speed applications.