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.

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.

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

PTFE Rotary Shaft Seals versus PTFE Spring-Energized Seals

PTFE is an excellent material for seals: it has extremely low friction, can operate in extreme temperatures, is dimensionally stable, available in FDA approved grades, and is compatible with a wide range of chemicals. When it comes to seals for rotating shafts, PTFE is often used with both standard rotary shaft seals as well as spring-energized seals.

Are rotary shaft seals the same as spring-energized seals? The short answer is no, they are not. While they serve many of the same purposes, the application for which you are using the seal will determine which type of PTFE seal you select.

Dynamic Seals

Dynamic seals, including those used in connection with rotating shafts, face far more challenges than static seals. If the shaft is misaligned, the seal lip can lose contact with the shaft surface during rotating, compromising the integrity of the seal. Shaft surfaces must be extremely smooth when rotation is taking place to minimize the wear on the seal lip and to make the seal more effective — and this is especially true when high speeds are involved. 

PTFE Rotary Shaft Seals

Rotary shaft seals are designed to provide a seal (and in some cases a wiping functionality) for circular shafts that are rotating or swiveling. Their job is to keep lubricants (either for the bearing or for the shaft itself) from leaking out while preventing the ingression of contaminants, which is why they are often called oil seals or grease seals. Many different designs are well adapted for specific sealing applications, such as mechanical pump seals, cryogenic temperatures, and harsh environments.

When designed and specified correctly, PTFE rotary shaft seals can provide excellent performance at relatively high speeds as long as the pressures are low (there are high-speed shaft seals that can handle higher speed levels, but again the pressure must remain low). In addition, the shaft must meet strict tolerances for surface finish and must be straight and correctly aligned. They are a cost-effective sealing solution for many designs

Spring Energized Teflon Seals

PTFE Spring-Energized Seals

PTFE spring-energized seals serve the same purpose as rotary shaft seals and can achieve all that rotary shaft seals can, with a few additions made possible by the spring energizer. For example, the presence of a spring-energizer enables the sealing lip to remain in contact with the surface of the shaft even when there is a significant pressure difference or the shaft is eccentric. The springer energizer also maintains seal integrity when there are significant changes in temperature that can affect the dimensions of the shaft size or the PTFE seal lip. The drawback for this enhanced performance, however, lies in the price: PTFE spring-energized seals are going to be more expensive than PTFE rotary shaft seals.

Polymer Spring Energized Teflon Seals

Conclusion

PTFE is an excellent material choice for many dynamic sealing applications, including those that involve high speeds, dry running capabilities, FDA approved materials, extremely high temperatures, or cryogenic temperatures. There are some dynamic applications where a PTFE rotary shaft seal simply cannot provide the necessary performance, and in those instances then a PTFE spring-energized seal should be considered.

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Cheat Sheet on Surface Finishes and PTFE Spring-Energized Seals


PTFE Spring-Energized Seal Design

When designing a component that includes a spring-energized PTFE seal, it’s not enough just to specify all the seal parameters; another key part of the design is the surface finish of the mating surface.  In this article, we are going to review what surface finish is, discuss the effects of surface finish on seal life, and then go over recommended surface finishes for spring-energized PTFE seals.

PTFE spring energized seals

 

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