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Why an Extrusion Gap is Important

“How much pressure can this seal handle?”

The answer to this question depends on a number of parameters and conditions. But the principle limiting factor in the pressure handling of any seal system is the extrusion gap.

Commonly referred to as the “E-Gap,” the extrusion gap is one of the most critical design aspects in any high-pressure application. Seal design, type, and material are all influenced by the extrusion gap and the desired pressure handling capability.

What exactly is an extrusion gap, and why is it so important in the successful design of a sealing system? Let’s find out.

The Basics: What is an Extrusion Gap?

In terms of sealing systems, the extrusion gap is defined as the clearance between the hardware components.

In a piston configuration, this would be the clearance between the piston and bore. In a rod configuration, this is the clearance between the rod and housing it’s passing through.

The extrusion gap can be expressed in terms of radial or diametral clearance, which can lead to some confusion. Our partners at Eclipse define the E-Gap by stating it as the radial clearance. The radial clearance is equal to the diametral clearance divided by two.

 

It’s important to note that while hardware components might be machined to have a specified clearance, this gap might not be perfectly realized or maintained. Continue reading Why an Extrusion Gap is Important

Designing the Perfect Rotary Shaft Seal

When it comes to maintaining a high-functioning rotary shaft, you need to select the appropriate lip seal.

The shaft seal protects the rotary shaft from contaminants such as dust and dirt, and it keeps water out and lubricant in.

A rotary seal, also known as a radial shaft seal, typically sits between a rotary shaft and a fixed housing — such as a cylinder wall — to stop fluid leaking along the shaft. The rotary seal’s outside surface is fixed to the housing, while the seal’s inner lip presses against the rotating shaft.

Common applications for shaft seals include motors, gear boxes, pumps and axles. They’re also increasingly used for food and chemical processing, as well in pressurized gas applications.

Three of the most important considerations when the choosing the best lip seal for a rotary shaft are:

  1. The material the seal is made of,
  2. the hardness of the shaft’s surface, and
  3. the roughness of the shaft’s surface.

Here’s your quick go-to guide on how to achieve optimum performance and longevity for your seals and shafts, ultimately minimizing the risk of seal failure. Presented by our partners at Eclipse Engineering: Continue reading Designing the Perfect Rotary Shaft Seal

Making Seals That Keep Dust Out Of Equipment

Dust is typically a minor annoyance that haunts the surfaces of our home. But in the world of engineering, machinery, and mechanical systems, it can be the difference between a reliable piece of equipment and disaster.

Dust can cause major damage to cylinder walls, rods, seals and other components inside of machinery. And if you’re not careful, dirt, mud, debris, and water can all cause damage as well.

These foreign contaminants are real problems for mechanical systems, especially as they build up in small quantities over time. A single particle of dust today may be no big deal. But a mote of dust a day will eventually become enough of a presence to cause serious issues, like friction, surface wear, and imperfect seal contact between surfaces.

These issues could compound until the mechanical system experiences a complete failure. It may seem like perfect is impossible, and that eventually some contaminants will get into your system no matter what you do.

But in some applications, like in automobiles and aircraft, failure is simply not an option.

Beyond those industries, many types of equipment need to stay clean on the inside, even when things get extremely messy on the outside. Examples include earth movers, hydraulic cylinders in steel mills, snow plows, and metal foundries, and in seals in logging equipment.

picture of wipers

How to Keep a Mechanical System 100% Dust-Free

Just as seals keep pressurized fluids and gases in piston and cylinder systems, there are components that are designed to do the exact opposite — keep contaminants out.

In the sealing industry, the three main types of components used to keep dust at bay are wipers, excluders, and scrapers. While each are a bit different, they all serve the same basic purpose, and are fitted on the exterior side of the main seals in a system.

The exact type of dust-prevention mechanism you need depends on what exactly you’re trying to protect against. Here are three different types below:  Continue reading Making Seals That Keep Dust Out Of Equipment

The Pros And Cons Of PEEK Back-Up Rings

peek back up ring picture

Back-up rings serve an important role in world of seals. While the design principle and construction are incredibly simple, they greatly extend the usefulness of the most common and prolific sealing device in the world: the O-ring.

Back-up rings are aptly named as they do just that: they back-up an O-ring.

Back-up rings are commonly nothing more than a ring of polymer meant to space the O-ring away from the extrusion gap in hardware. By blocking off the extrusion gap, the pressure-handling ability of an ordinary O-ring is greatly increased.

Solid or split back-up rings out of virgin PTFE can usually be found on the shelf, and are largely considered commodity items.

While the design and functionality of a back-up ring rarely changes, the material selected can greatly complicate this simple device. Some applications require specific material properties and/or special material certifications.

Back-up rings can be made for a variety of unique applications;: Military-spec back-up rings out of fully certified AMS 3678/1 virgin PTFE; Certified “MS” style back-up ring (MS27595 or MS28774); PTFE blends; Thermoplastic elastomers; Urethanes. But the most common custom material for back-up rings is usually PEEK (polyether ether ketone).

In certain applications, PEEK has some distinct advantages as a back-up ring material. But with these advantages comes some potential issues.

Read on if switching from a PTFE to a PEEK back-up ring sounds like an enticing proposition to see what you need to consider before making the change. Continue reading The Pros And Cons Of PEEK Back-Up Rings

Eclipse Announces MicroLip™ Prototype Program

picture of microlip rotary

Eclipse has been working hard during the Covid-19 downtime on finding solutions to issues that customers have brought to the table over the past few years.

Many new designs have been sent into testing while focusing on processes that will help improve productivity and lower costs.

The MicroLip™ is an example of a viable solution to rotary seal issues that many customers have struggled with. This is especially true when the order volumes are relatively low or the shaft diameters are small, such as with encoders or chemical-processing facilities.

The Eclipse MicroLip™ Prototype Program

When moving from rubber to Teflon lip seals, Eclipse has found that the cost to bring the product to market is often a hindrance. The high cost is due to tooling and the number of pieces that must be manufactured to make the product viable in the prototype phase.

Because of this, many customers sneak by using inappropriately-applied rubber lip seals to solve rotary seal problems.

MicroLip™ seals have proven to be a powerful component in rotary services. Since the MicroLip’s inception, it has been applied to a variety of applications including mobile hydraulics, robotics, surgical drills, and semiconductor processing and encoders.

Over the last 3 years, Eclipse has designed and manufactured various styles of MicroLips in diameter sizes of under 1/8 inch (5mm) and over an inch. Since the components of the MicroLip™ can be machined, Eclipse has made the seal in quantities of less than 10, and batches in the thousands. Continue reading Eclipse Announces MicroLip™ Prototype Program

Designing Cryogenic Seals for High and Low Temperature Sealing

cryogenic

When designing for low temperature sealing, the first step is to define the temperature range that the seal will be operating in.

Typically, cryogenic as seals are those that are operating below -65 Fahrenheit. Gallagher’s partner, Eclipse, chooses this benchmark because they currently have elastomers that have a usable TR10 value at this temperature.

When designing at this level — with high temperatures around 300 Fahrenheit — an understanding of what level of leakage control is required on the low temp end. Seals that operate in aircrafts must function within this range.

However, there may be an allowable leakage rate which allows for reduced drag. When requiring zero leak, the drag in the system is often increased to support some elastomeric contact with a dynamic surface. In the case of static seals, elastomers span this range although increased squeeze may be necessary.

Eclipse Engineering routinely designs in the range indicated above.

While -65 Fahrenheit is extreme cold, it’s not considered cryogenic. Liquid nitrogen at -320° Fahrenheit (-195°Celsius) requires special hardware and seal material consideration.

To begin, many projects and applications don’t utilize lubricant in dynamic applications. To improve sealability, a better-than-average surface finish is required.

Surface finish often holds lubricity. But without this, a smooth finish reduces friction, improves life, lowers drag, and improves sealability.

Static seals are often required to have leak rates approaching zero; meaning hardware considerations and surface can be even more important. This may mean polishing the groove, which in some applications can be very challenging.

Cryogenic Seal Materials

The next criteria are the seal materials. Elastomeric materials lose their flexibility at these extreme temperatures, so Eclipse relies on polymer-type materials to bridge the gap. When we experience temperatures below -180° Fahrenheit ( -195° Celsius), that’s when it becomes wise to move away from basic PTFE to modified fluoropolymers such as PCTFE, known for operating down to -460 Fahrenheit. Continue reading Designing Cryogenic Seals for High and Low Temperature Sealing

The Role of Seals in the Quest for Medical Cures

The coronavirus has prompted all of us to do everything we can to protect ourselves from catching and spreading the virus. We are all taking important safety measures to maintain a clean and uncontaminated home environment, and limiting our exposure to a potentially hazardous outdoor environment.

picture of coronavirus medicalIn this blog, our partners at Eclipse will be examining the role that seals play throughout a pandemic. The very role of seals is to keep a certain environment in, and certain environment out, similar to how we are living these days.

In Eclipse’s last blog, they wrote about boundary seals in aircraft and how seals allow the aircraft to be pressurized. In the research lab, a different style of boundary seal is required to keep the outside environment out.

Labs all over the world are working toward preventing the spread of coronavirus. Scientists are working with test equipment to find a cure and a vaccine to prevent not just the spread of this virus, but other viruses which we’ve not yet seen.

When we design seals, we must consider keeping something as small as a single cell from entering a test chamber. Last week, Eclipse received a call directly from a customer building a prototype ventilator to be built in volume to help support patients suffering from coronavirus.

The client requested that Eclipse’s engineering and manufacturing team turn an 8-inch (203mm) seal around from concept, design, and finally produced and shipped in less than 4 hours — and they made it happen.

Keep reading to explore the important role that seals play in research equipment as scientists seek to find the cure for coronavirus and beyond. Continue reading The Role of Seals in the Quest for Medical Cures

The Advantages and Disadvantages of the Channel Seal

The Channel Seal (or Cap Seal, as it’s often referred to), was one of the earliest forms of Polymer or Teflon sealing in the seal industry.

The product is easily applied. It didn’t replace the O-ring, but instead offered improved life while reducing drag.

In doing so, hydraulic and pneumatic systems operated cooler and quieter, while improving overall performance of the product.

picture of channel seal

Evolution of the Channel Seal

Before the Channel Seal, the Backup ring was established. The first Backup rings started out as leather, as this material was readily available and could be easily formed into any shape with simple dies to stamp the Backup ring out.

Back up rings provided support for the O-ring, allowing the O-ring to operate at higher pressures, while closing off the Extrusion or “E” gap. This stopped the O-ring from being nibbled in the extrusion gap, therefore extending the life of the O-ring.

Teflon Backup rings were a big improvement, as they would better fill the gap and would stay put (as opposed to leather, which tended to shift in the groove). With the use of two Backup rings, an O-ring was well supported from pressure in both directions.

It was a simple matter to connect the two Backup rings with a thin membrane of Teflon, which removed the O-ring from the sealing surface. This change reduced drag and improved performance, while still maintaining an excellent mechanism for extrusion resistance.

This design was relatively simple to machine out of Teflon, but installation was a challenge, as the Backup rings were full depth. This caused the seal to become distorted during the install process. Today, we almost never see this type of design.

With CNC machining, the ability to nestle, and an O-ring design in a complex Teflon shape, it gave rise to what is referred to today as the Channel Seal, or Cap Seal.

This style seal offers an abundance of advantages over standard back-up rings and the early version of the Channel Seal, which was simply a Backup ring with the membrane of Teflon in-between. Continue reading The Advantages and Disadvantages of the Channel Seal

Evolving from Plastic to Teflon Seals

The term “plastics” is generic way of describing a synthetic material made from a wide range of organic polymers. Organic polymers describes a man-made substance that is formulated using polymer chains to create what we commonly refer to as…(you guessed it), plastics.

Before plastic, leather had been used to create Backup ring devices behind O-rings. Leather allows fluids to be retained, providing lubrication for the O-ring when the system was running dry.

picture of teflonThe problem with leather was that it could become dry and shrink away from the sealing service, exposing the elastomer to same pressure it was intended to protect against.

With the advent of polymers, a piece of plastic could be cut or formed into the exact shape to allow for zero extrusion gap, and for continued protection for the O-ring.

Some polymers were very brittle. Since they needed to be deformed to allow for installation into solid glands, the cut of the plastic could nibble at the O-ring, causing premature failure of the element it was supposed to be protecting.

The Revolution of PTFE

When PTFE moved out of the lab and into industrial use, it quickly found itself adjacent to the O-ring. PTFE offers extrusion resistance and, at the same time, doesn’t erode or nibble at the O-ring due to the “softness” of the polymer.(Hardness between 55 and 65 Shore D)

Given the composition of PTFE, or Teflon, it could be utilized as a sealing element to protect Backup rings and conform to the shaft. The bonus was it was generally easy on shafts (depending on the filler added to the PTFE).

There are some negative aspects to Teflon that needed to be overcome by early engineers. First, it has a fairly high rate of Thermal expansion which, by its own nature, could often times lose contact with the sealing surface. This meant some kind of loading was necessary to ensure contact.

PTFE is as tough as other polymers, so the fact that it could seal on a shaft made it vulnerable during installation for tears or nicks on sealing surface.

Second, if it were stretched during installation, the material had to be sized back to its original shape due to its poor elastic properties. Continue reading Evolving from Plastic to Teflon Seals

The Manufacturing Challenges of Tiny Spring Energized Seals

tiny spring energized sealEclipse Engineering has in-house capabilities to manufacture seals up to 55 inches in diameter, and over 100 inches through production partners.

While seals with huge diameters certainly grant their own significant levels of intricacy, here we’ll look at the other end of the spectrum: the micro-sized seals.

We won’t just look at a simple seal ring, but an inherently more complicated and geometrically detailed spring energized seal. As we’ll see, very small diameters make multiple manufacturing aspects more involved and challenging.

The Client’s Issue

A sealing solution in a customer’s epoxy dispensing equipment. They needed an effective seal for the reciprocating rod responsible for the flow-control and metering of the epoxy while being dispensed.

Operating Conditions:

  • Reciprocating Rod Seal
  • Epoxy Dispensing Head
  • Rod Diameter: 1.2mm [0.047”]
  • Stroke Length: 6mm [0.236”]
  • Cycle Rate: 15 per min
  • Media: Epoxy
  • Operating Pressure: 1,500 PSI
  • Temperature: 70° to 150°F

In general terms, most viscous media sealing solutions have three things in common:

  1. A variant of UHMW for the seal jacket,
  2. heavy spring loading, and
  3. multiple point contacts with increased interference.

In most cases, multiple nested V-Springs are incorporated to provide optimal load and energize the compound contact points on the seal. With this formula, we’ve had great success sealing media like epoxy, urethane, silicones and acrylics.

The heavy loading is necessary to effectively wipe the reciprocating rod. This is balanced with the correct material and design geometry to provide long wear life of the seal, which has the potential to be compromised under such loading.

The challenge in this case was to incorporate these same proven principles in a micro-sized seal.

The Eclipse Solution

Continue reading The Manufacturing Challenges of Tiny Spring Energized Seals