by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Spring-Energized Seals for the Medical Industry

Spring-energized seals, when designed correctly, provide a highly-reliable sealing solution for medical applications where failure can be fatal. Selecting the right seal jacket material and energizer is critical, but also complex. In this week’s blog post, we will discuss spring-energized seals in the medical industry, the best materials, how they are used, and more!

Spring-Energized Seals in the Medical Industry

Spring-energized seals are regularly used in equipment that involves rotary motion, including a variety of surgical instruments such as high- and low-speed handpieces, surgical saws, bone shavers, oscillating saws, and bone drills.  They are also seen in rotary catheter systems, centrifuges, and both small motors and small pumps. 

Reciprocating equipment also may require spring-energized energized seals, with typical applications including respirators, oxygen compressors, dialysis machines, syringe pumps, and blood analysis equipment. In addition, spring-energized seals work exceptionally well in cryogenic applications.

How Spring-Energized Seals Work

A spring-energized seal makes use of an energizer, most often in the form of a spring, to enable the seal lip to stay in contact with the mating surface. For high pressure applications, the pressure of the media is usually able to keep the seal lip in contact with the mating surface and the springer-energizer then takes over for high pressures. On the other hand, when used in applications involving low pressures, the spring-energizer is responsible for keeping the seal lip in contact with the sealing surface at all times. 

Seal Jacket Materials

There is a wide variety of seal jacket materials that can be used, but the choices are significantly limited for medical applications. The materials used must be FDA, USP Class VI, and ISO 10993-5 compliant. This significantly limits what materials can be used for the polymer jacket. The three most common choices are these: UHMW PE, PTFE, and PEEK.

UHMW PE

UHMW PE (Ultra High Molecular Weight Polyethylene) is a high-performance engineering polymer that possess the following characteristics:

  • Low coefficient of friction
  • Good chemical compatibility
  • Low moisture absorption
  • Good dimensional stability
  • Good operating temperature range up to 180°F
  • Can withstand extended exposure to hot water and steam
  • Self-lubricating

In addition, it exhibits excellent wear and abrasion resistance. UHMW PE is also known for its high purity and is also commonly used in orthopedic implants, in part because of its strength and toughness.

PTFE

PTFE (Polytetrafluoroethylene), often referred to as Teflon, has the following properties:

  • Extremely low coefficient of friction
  • Excellent chemical compatibility
  • Good dimensional stability
  • No moisture absorption
  • Excellent operating temperature range up to 450°F
  • Self-lubricating
  • Can withstand extended exposure to hot water and steam
  • Low cost

The primary weakness of virgin PTFE is its wear resistance, making it best adapted to light-service duty applications. However, wear resistance can be significantly improved through mineral additives.

PEEK

PEEK (polyether ether ketone) offers these properties:

  • Extremely low coefficient of friction
  • Excellent chemical compatibility
  • Moderate moisture absorption
  • Good dimensional stability
  • Wide operating temperature range up to 480°F
  • Self-lubricating
  • Can withstand extended exposure to hot water and steam

It is also extremely tough, abrasion resistant, and known for its outstanding heat resistance. In addition, PEEK is also used quite often in medical implants because of its biocompatibility, stiffness, strength, and toughness. 

Spring Energizers

Spring energizer materials are either metal or elastomeric, with metal being the most common. For medical applications, the three most commonly used metals for the spring energizers are stainless steel, Elgiloy, and Hastelloy.

Metal energizers are ideal for several reasons, with the first being their natural stiffness. They also work well in applications that involve autoclaving because of their thermal characteristics and contribution to maintaining the shape of the seal jacket. In addition, a metal spring helps to dissipate heat, reducing the potential effects of thermal deformation in the seal jacket. 

In the medical industry, the energizing spring will fall into one of these categories: canted coil, helical, or cantilever. Because spring loads can be customized, canted coil springs (also known as slanted coil springs) can serve in a variety of operating conditions. Heavy force canted coil springs perform extremely well in high pressure applications but may experience more wear than other configurations. Springs designed to provide more of a light force are ideal for high-speed applications but should not be used in cryogenic environments or with vacuum pressures.

In general, helical springs for medical applications involving cryogenic temperatures, vacuum pressures, and high pressures with only moderate wear, but are not recommended for high speed applications. Helical springs work extremely well on seals interacting with lightweight gases or fluids.

Cantilever springs offer excellent performance in vacuum pressure conditions but, as with helical springs, should not be used in high speed applications. However, a high degree of wear is to be expected. And, because the energizing force will be concentrated at the very front of the seal, they work extremely well for scraping and exclusion applications. 

Choosing the Right Spring-Energized Seal 

Dynamic sealing solutions for medical applications can prove extremely tricky for several reasons. For example, seals can be exposed to a variety of fluids and media, which can include bodily fluids and materials such as adipose, hemoglobin, proteins, carbohydrates, and general bioburden. For such applications, PTFE with its natural hydrophobic properties is an excellent option.

For seals which may be exposed to abrasive materials such as bone shavings, wear and abrasion resistant polymers such as UHMW PE and PEEK work very well, although there are FDA-approved fillers for PTFE that can enhance its wear properties. 

Cleaning, sterilization, and disinfection are critical factors in deciding on an appropriate sealing solution. Sterilization in particular is the most aggressive of the tree, and may involve the use of autoclaves (also known as steam sterilizers) that require elevated temperatures and pressures. There are other methods, such as dry heat, plasma gas, VHP (Vaporized Hydrogen Peroxide), and chemical sterilization. 

Chemical sterilization may use bleach, ozone, hydrogen peroxide, or EtO (ethylene oxide). While all three materials have good chemical neutrality and handle heat quite well, warpage may occur due to residual stresses and should be considered when developing the seal jacket molding process. 

There may also be issues with the use of lubricants, therefore many medical sealing applications require the use of a self-lubricating material of which PTFE, UHMW PE, and PEEK all qualify. However, for the lowest coefficient of friction and least slip-stick behavior or startup torque, PTFE is optimal.

Conclusion

Many different factors go into choosing the right spring-energized seal for a mission-critical medical application, and engineers must consider factors such as pressure, sterilization methods, lubricants, chemical compatibility, wear, and other. There are, however, proven sealing solutions for medical industry applications.

If you need a reliable seal for  the complex environment of a medical application, contact the sealing group at Advanced EMC. We can put our years of experience in polymers, spring energizers, and mission critical sealing solutions to work for you. Contact us today!

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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