When classic sealing materials – for instance in temperature ranges above 300°C and below -50°C – reach their limits alternative materials are required such as metal with appropriate coating/plating.

Parker offers metal seals made of stainless steel or nickel alloys in C, E and other designs characterized by high pre-loading force and significant resilience. Drawing on many years of experience in the gas turbine market, Parker has continually expanded its expertise in large diameters and developed special problem solutions that substantially increase the efficiency of the machines.

The most important manufacturing technologies used to produce metal seals from stainless steel or nickel alloys are rolling, forming, CNC machining, welding, heat treatment and coating/plating. In its more than 60-year history of producing metal seals, Parker has continually tackled the challenge of manufacturing increasingly large metal seals. Currently, spring-energized C-rings with a diameter of up to 7.6 m can be produced for which special forming machines and patented welding techniques were developed. They are supported by optimized special heat treatment and electroplating processes that make it possible to manufacture high-quality products even in such large dimensions. Additionally, Parker offers non-rotationally symmetric metal seals. These E-, O- and C-seals can be produced in lengths of up to 2.3 m on machines specifically developed for this purpose.

• C-seals: ≤ 3,000 mm
• Spring-energized C-seals: ≤ 7,600 mm
• O-rings: ≤ 1,200 mm
• E-seals:
  • Heat-treated ≤ 2,700 mm
  • Segmented ≤ 7,600 mm

The base materials used are special nickel alloys that withstand temperatures of more than 800 °C. These cobalt-nickel-chromium-tungsten alloys or heat-treatable nickel super-alloys make high demands on the welding technology used and are reliably processed at Parker due to optimized manufacturing processes and comprehensive suitability tests.
The choice of plating is primarily focused on wear protection, corrosion resistance and improvement of the sealing properties. For this purpose, the surface properties of the metal seal are modified and a formable external surface layer with adjusted hardness is created. Parker’s application engineering team will advise you in making the appropriate selection from the available plating range of gold, silver, nickel or TriCom® coating. 
 

• Power Generation – Sealing Solutions for Gas Turbines
• Chemical Processing – Sealing Solutions for Heat Exchangers
   

Website: XXL Size Seals and Molded Parts
Download Whitepaper: Large-Diameter Seals and Moldings - Material and Special Manufacturing Aspects
Download Brochure: XXL- Size Seals and Molded Parts - Powerful Solutions for Large-Scale Applications



 

Article contributed by
Thorsten Kleinert, business unit manager composite sealing systems
Engineered Materials Group Europe, Prädifa Technology Division

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As automatic transmissions become more advanced as a system, the components within must evolve as well. Performance drives the design and there are many factors that must mesh together to achieve the overall goal. Parker used its materials science and design expertise to enable an engineering breakthrough—a cost effective alternative to traditional sealing in radial seal applications where low friction is critical.

Improved efficiency is the leading factor when it comes to system improvement. The move to eight, nine, and ten-speed automotive transmissions are focused around improving fuel economy. Obviously, with the increased number of gear ratios, the number of clutches and clutch pistons has also increased. A reduction in the amount of energy needed to engage and retract a clutch piston directly reduces the parasitic loss of energy within the entire system. This has resulted in a significant focus on the impact of seal drag within each clutch piston. 

For more than a decade, D-Rings have been the preferred sealing technology for automotive transmission clutch pistons as they provide several advantages to other seal designs including:

• Bi-directional sealing capability,

• Symmetrical design simplifies assembly,

• Lower drag than other radial compression seal designs,

• Reduced variation in drag response, and

• Eliminates potential spiral failure existent with O-rings.

In the past, there was always enough fluid pressure to overcome seal drag and activate the clutches. Because these same size pumps must supply so many more clutch circuits in the transmission, this energy supplied is in greater demand. Similarly, heavy springs in the past provided enough return force to ensure immediate disengagement of the clutch. Limited availability of space, as well as lower cost targets, have resulted in smaller, lower force return springs.

Our engineers have developed a low drag D-ring which reduces the drag forces in the application by 50%. All other advantages of D-ring seals are maintained including the symmetrical profile which simplifies the assembly process. This improves the first time through capability and reduces warranty costs.

By creating volume space to which the deflected rubber can easily flow to, the reaction forces against the mating surface (normal force) is reduced while still maintaining sufficient sealing forces.

In addition, Parker’s unique manufacturing approach provides a significant cost reduction over compression and injection molded designs. The absence of mold parting lines in this new technology also eliminates molded rubber flash on the sealing surface which is inherent with injection or compression molded products.

Lower clutch seal drag provides several advantages including:

• Smoother shifts,

• Overall improvement in system efficiency, and

• Smaller return springs save space, weight, and cost.

This technology is also available for applications beyond transmission clutches. Any reciprocating piston application can benefit from low drag D-rings. Examples would include active differentials and other torque management applications.

Please contact one of Parker’s application engineers to discuss how low drag seals can improve the performance and efficiency of your applications.

To learn more about Parker's off-road machinery solutions, stop by booth #S80245 at IFPE/CONEXPO-Con/AGG in Las Vegas, March 10-14.

Article contributed by Scott A. Van Luvender, automotive applications engineering manager, Engineered Materials Group.






 

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There are many situations where an O-Ring may not last as long as one thinks that it should.  When the expectation is realistic and yet the seal fails earlier than expected, the Applications Engineering team is often asked to help discover the failure mode(s).

Seal failures are often due to a combination of failure modes, making root cause difficult to uncover.  When we begin a failure analysis, we will ask for: hardware information, how the seal is installed, application conditions (temp, fluids, and pressure exposure), and how long into the service that the seal failed.  These details help bring the overall application into focus and enable us to quickly diagnose and resolve seal failures.  In part one of our seal failure blog series, we will discuss compression set, extrusion, and spiral failure.
    

• Compression set is likely the most common failure mode for elastomer seals.  Compression can be defined, or rather quantified, by the seals ability to return to its original shape after compression is removed.  Zero percent compression set indicates that no relaxation (permanent deformation) has occurred, while 100% compression set indicates that total relaxation (seal no longer applies a force on the mating surface).  When investigating material options, note that the lower the % compression set for a given compound, the more resilient the material is.  However, it is extremely important to ensure you are making equal comparison in terms of time and temperature for the test conditions.

• There are many potential causes for compression set.  Poor material properties, improper gland, fluid incompatibility, or temperature exposures above the recommended range for the material.
• For more information on compression set including an animation to further demonstrate, visit our O-Ring eHandbook online.

• The driving force (pun intended) for this failure mode is the pressure load that the seal is exposed to.  Extrusion most often occurs when a seal material deforms into the space between the bore and the outside of the tube (commonly referred to as the extrusion gap or “E-gap”).  An approximation for the pressure rating for a seal can be determine by evaluating figure 3-2 of our O-Ring handbook.  The X-axis shows the size of the clearance gap (total gap, or diametral gap), and the Y-axis is the pressure load.  The curves on the chart correspond to the hardness of the rubber.  Extrusion can also occur due to gland overfill, when the deformation from compression of the seal fills the entire groove and lips over into the extrusion gap.
• Face seals do not usually have an extrusion gap, so this orientation can achieve much higher pressure loads than a radial seal.  Without a gap for the seal to extrude into, the risk of significant extrusion is highly diminished.
• Extrusion in radial seals can by combated by reducing the clearance gap or by adding a back up ring.

• Spiral failure can be more simply described as the O-Ring rolling in the groove.  This failure more is most common in dynamic reciprocating O-Ring applications.  However, spiral failure can also occur during installation.  An image of spiral failure is unique, and relatively easy to diagnose, but the root cause of spiral failure can sometimes be difficult to pinpoint.  Uneven surface finish, poor lubrication, side loading, eccentricity, or perhaps stroke speed can all contribute to spiral failure.  

Parker provides numerous resources to support the diagnosis of seal failures and the best sealing solutions. Check out our latest tech webinar on seal failures modes or utilize our leak troubleshooting app. Also, be sure to keep an eye out for part 2 of our seal failure mode blog series, where I will discuss the rapid gas decompression, abrasion, installation damage, and fluid incompatibility failure modes!

This article was contributed by William Pomeroy, applications engineer, Parker O-Ring & Engineered Seals Division. 

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So, you’ve unboxed the shiny new Parker seals you ordered – now what?  Installing seals for the first time can be challenging without the right know-how and tools. In this article we’ll discuss best practices for seal installation in linear fluid power systems, and how to design your system to make seal installation fast and damage-free.

•    Closed 
•    Stepped, and 
•    Open (or two-piece)

The closed seal groove fully encapsulates the seal and is the most common style used (see Figure 1).

Closed grooves are simple to machine and offer the best support for seals. Since seals in this configuration are surrounded by solid metal, without a well-developed process, installation can be challenging. Rod seals need to be folded to fit into internal (throat) grooves and piston seals must be stretched over the outside of the piston.

Notice how both designs shown in Fig. 2 and Fig. 3 utilize static seals (turquoise colored seal) on the opposing side of the dynamic, primary seals. Therefore, installation in either instance requires techniques and tools for both rod and piston seals.

Typically utilized to ease seal installation, stepped grooves feature a reduced diameter on the low-pressure side of the seal as shown in Fig. 4 and Fig. 5.
  

As shown, the “step” is just wide enough to hold the seal in place as the rod or piston strokes back and forth. This way, seals don’t have to be folded or stretched nearly as much when installing. This design works well for single seals only holding pressure from one direction, like Parker FlexiSeals™.  

When using multiple seals stacked in series or in systems with bi-directional pressure, a closed or two-piece groove is needed for support on both sides.

Open or two-piece grooves are used when the seal is either too small to be stretched or folded into a closed groove, or if it’s made of a material that doesn’t spring back after flexing.
Figures 6 and 7 show two examples of open grooves. Figure 6 uses a washer and a snap ring to hold the seal in place. Figure 7 uses a bolt-on cap. These groove designs can be used for bi-directional seals, too. As you can see, open grooves cost more to produce but seal installation is a snap.  

Open grooves also make removing the seal much easier – useful in systems which require periodic seal replacement.

Installing seals with your bare hands is tough!  Parker has several tips and tricks you can use to ease the process. For large scale production, building a set of custom tools or even designing a fully automated setup will save time and reduce labor.

Different installation tactics are required for the diverse range of seals Parker manufactures.  Polyurethane and rubber seals are extremely resilient and won’t be damaged by stretching or folding. Cylinder components made from hard plastics are less flexible. Parker FlexiSeals incorporate a metal spring inside of a thin PTFE jacket, so stretching or folding should be avoided. If a PTFE seal must be installed in a difficult location, we offer “cap seal” designs which are energized by an O-ring and are robust enough to handle deformation during installation. Installation speed is key in these cases. Testing has shown that when PTFE rings are swiftly stretched over a piston (less than a second), they recover closest to the original diameter, whereas seals stretched slowly need a re-sizing tool passed over the seal to aid recovery. 

Hard plastic components like PEEK backups and Nylon wear bands are supplied with splits to aid installation. Split rings can be collapsed to a smaller diameter to snap inside a cylinder head, or opened to snap over a piston. Since these secondary components aren’t doing the sealing, there’s no concern with the split causing a leak.

For low volume production on rod sizes above 2”, hand-folding each rod seal into the groove can work. For smaller sizes where a hand won’t fit, do yourself a favor and order (or make) a 3-legged rod seal installation tool (see Fig. 8). 

These tools grip and fold the seal for insertion, which is especially useful in deep cylinder heads housing several seals in a row (see Fig. 9).

There are many online vendors who sell rod seal installation tools. If you need help, your local authorized Parker seal distributor may have a recommendation. Click this link to locate the authorized distributor nearest to you.

The two tools pictured in Figure 10 were made for use in our lab. Multiple sizes are needed for different rod diameters.             

The size of the rod seal will dictate the method of installation. Seals with larger cross-sections are more robust, but harder to fold into place. Small diameter cylinder heads don’t leave much room for installation. For closed grooves, this is the rule of thumb: the rod diameter needs to be 5 times larger than the cross-section. By following this rule, Parker's smallest 1/8" cross-section BD, BT, and PolyPak rod seals will require open grooves when rod diameters are smaller than 5/8".  

Piston seals are less challenging, purely because you’re not confined to a small space while installing. The simplest method for stretching seals into a groove is to hook one side over the piston, and then use a piece of non-scratching brass (see Fig. 11) or plastic bar stock to gently pry the seal over the other side.

While simple, the above method can pinch or unevenly stretch the seal, causing it to leak. For an upgrade, machine a cone and pusher tool out of aluminum and plastic, respectively (see Fig. 11). These are especially convenient for stretching large seals because an arbor press or mallet can be used for additional force. While pushing, the flexible fingers of the tool ensure the seal is being evenly stretched over the piston. PTFE cap seals in particular will be damaged by “necking” caused by uneven stretching.

Since my tools are only used for low volume testing, 3D-printed pushers work great. For production-grade tooling, cut pushers from a tough plastic billet like Nylon.
 

In addition to gently coercing seals into place, there are other elements of the assembly process to consider. Often overlooked, poor cleanliness of the work area can break an otherwise good procedure. Dirt and metal shavings present in manufacturing areas can become trapped in a cylinder. Over time, they will abrade the seal lip and scratch expensive polished metal sealing surfaces, leading to premature leaking.

Additionally, ensure hardware is deburred beforehand so the seals don’t get cut by a sharp corner. Our catalog recommends a light break (0.005” max) on groove corners to prevent cutting the seals, and – from personal experience – also the hands of the assembler. Larger breaks are fine in areas which aren’t supporting a seal (e.g. the outside of a cylinder head). If smoothing a corner or installing from another direction is not an option, like in some ultra-high-pressure systems, mask sharp surfaces with tape or a plastic sleeve before passing the seal over it. This method is also highly recommended when passing seals over threads.

Even when using an installation cone or protective sleeve, lubricating the seals and hardware beforehand with system fluid or a compatible grease prevents damage and helps the seals slide into place. For stiffer, higher durometer seal materials like Resilon® and PolyMyte®, heating seals in an oven or hot fluid bath will temporarily soften them, making them easier to fold or stretch (be sure to use system-compatible fluid). Soaking for thirty minutes at 200°F is within the max temperature rating of most seal materials and isn’t too hot to handle with bare hands.

Some installation headaches can also be overcome with clever system design. Generous chamfers on the ends of the rod and bore help gently compress seals when the piston or rod is inserted. This also keeps the seal lips from getting caught on a corner and folding, thus ruining the seal. Switching to a bi-directional seal will eliminate problems with seals being installed upside-down (nobody likes to admit it, but this happens).

Be smart and safe when installing seals. Buy or create tools to do the hard work for you. If leaks are happening right after assembly, consider the installation process suspect. For help, we at Parker are happy to answer questions or assist with installation tool design.

View our short video “Installation Techniques for Linear Fluid Power Seals”, which demonstrates various considerations discussed here.

For more installation and cylinder design information, see chapter 2 of our 5370 Fluid Power Sealing Catalog.

Recommendations on application design and material selection are based on available technical data. They are offered as suggestions only. Each user should make their own tests to determine the suitability for their own particular use. Parker offers no express or implied warranties concerning the form, fit, or function 
 

This article was contributed by Nathan Wells, application engineer, Engineered Polymer Systems Division.  

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There are many ways to improve the electrical continuity of electronic enclosures and the resultant EMC performance using a variety of electrically conductive gaskets and mechanical fasteners.  Such arrangements have the advantage of being clean and easy to use as well as allowing for the equipment to be disassembled and the components replaced.  So, ask the question: Is a compound the right solution in this particular case?

Finding the right electrically conductive compound

Assuming you have eliminated the use of EMI shielding gaskets and mechanical fixings as being appropriate or possible for your design, the next step is to choose the most suitable electrically conductive compound for your application. These typical properties should be considered in order of importance as part of the selection process:

• Binder or base resin system (silicone, epoxy etc.)
• Electrically conductive filler type (carbon, silver, etc.)
• Single part or two-part 
• Compound type (adhesive, caulk, sealant)
• Consistency (thick pastes to nearly liquid)
• Lap shear strength
• DC resistivity
• Use temperature range
• Cure temperature and time
• Working life
• Shelf life
• Coverage
• Recommended thickness
• Coefficient of thermal expansion
• Galvanic potential
• Surface preparation

This list above does not cover all the properties of the material that may be of interest to the engineer.  In each application, the engineer needs to assess the requirements of the design problem to see how well the material properties match up to the most critical parameters.

1. Surface preparation
   
   Having chosen your compound carefully, make sure to read the data sheet and any information relating to the surface preparation.  All vendors generally either give information on the product datasheet or like Parker Chomerics on a separate surface preparation application note.
   
   Failure to prepare the surface properly will result in either a poor quality joint or one that fails prematurely, if not immediately. Preparation can be as simple as wiping down with isopropyl alcohol or involve more complex procedures such as a corona discharge treatment, etching or sandblasting and the use of primers. 
2. Correct compound type 
   
   Conductive compounds are classed as adhesives, caulks and sealants. Typically, you will find that electrically conductive adhesives have finer conductive particles and have maximum bondline thicknesses, while electrically conductive sealants or caulks have larger particles and have minimum bondline thicknesses.  
   
   Therefore, choosing a sealant and trying to use it as an adhesive with a thinner bondline than specified may result in unexpected results such as crushing of the conductive particles, damage to the mating surfaces, or unsatisfactory bond strength.  
3. Curing the compound
   
   Conductive epoxies have the option of curing at elevated temperatures but normally this may be limited by the material of the window or the housing if it is not metal.  Some two-part silicones also have the option, whereas the most commonly encountered one-part RTV silicones do not.  
   
   Normally if cured at room temperature silicone materials take 24 hours before being suitable for handling, and seven days before they are fully cured. This also depends on the level of moisture on the surfaces and in the air being adequate.  In low humidity conditions or where the bondline is thick, the cure times may need to be extended. 
4. Galvanic potential
   
   This is especially important if the joint is to be exposed to harsh or marine environments. In such situations, the joint should be galvanically matched to the substrate as closely as possible (within 0.25v) and if necessary, a secondary protective non-conductive caulk or sealant should be applied to protect the conductive material.  
5. Working life
   
   The working life of some of the one-part RTV compounds can be a little as 15 min. especially in medium to high humidity conditions.  The two-part compounds tend to have longer lives ranging from half an hour to several hours. The two-part compounds however do require thorough mixing. 
6. Thickness
   
   Adhering to the recommended thicknesses for the compound will generally ensure bonding strength and resistivity measurements that will be within the manufacturer’s specifications. While it is possible to use compounds outside the recommended thicknesses in some cases, generally, it results in one or more of the typical properties of the bond not being as expected.  
   
   For example, using an adhesive with too thick a layer may result in higher than expected resistance and lower bond strength.

Now that you're proficient in selecting an EMI shielding compound for your application, see our selector guide below to get started now. 

This blog post was contributed by Gerry Young, applications engineering team leader, Chomerics Division Europe.

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As mentioned in part one of our seal failure blog series, O-ring and seal failures are often due to a combination of failure modes, making root cause difficult to uncover. It's important to gather hardware information, how the seal is installed, application conditions, and how long a seal was in service before starting the failure analysis process. In part 1 of our blog series, we discussed compression set, extrusion and nibbling, and spiral failure. In part 2 of our series, we will review four other common failure modes to familiarize yourself with before diagnosing a potential seal failure in your application. 

Rapid gas decompression (commonly called RGD, or sometimes explosive decompression (ED)) is a failure mode that is the result of gas that has permeated into a seal that quickly exits the seal cross section, causing damage.

Detection of this failure mode can be difficult, as the damage does not always show on the exterior.  When the damage is visible, it can look like air bubbles on out the outside, or perhaps a fissure that has propagated to the surface.  The damage may also be hidden under the surface.  If the seal is cut for a cross section inspection, RGD damage will look like fissures in the seal that may or may not propagate all the way to the surface.

Parker’s guidance as to how to avoid this failure mode is: 1) Keep the depressurization rate lower than 200 psi per minute.  If this cannot be achieved, we would suggest 2) RGD resistant materials.  We offer these RGD resistant options from the HNBR, FKM, EPDM, and FFKM polymer families.

Abrasion damage is the result of the seal rubbing against a bore or shaft, resulting in a reduction of cross sectional thickness due to wear.  As the seal wears, it has the potential to lose compression on the mating surface.  This wear is compounded by the fact that dynamic applications already have lower compression recommendations.

To reduce risk for this failure mode, it requires consideration during design and seal selection.  The surface finish and concentricity of the hardware will be very important considerations.  A smooth surface results in less friction (suggest 8 to 16 RMS), which in turn results in less wear.  Increasing the durometer of the seal material helps resist wear, and there are also internally lubricated materials that could be employed.  If the application is high temperature, one should consider the impacts of thermal expansion on the elastomer being used.  The thermal expansion increases contact pressure, which would increase friction / wear.

Installation damage can occur in both face seals and radial seals but is much more common in radial applications. Installation damage can be the result of excessive installation stretch, cuts or nicks from installation tools, the installation of a seal over a burr or sharp corner or threads, improper size selection, or the insertion of a piston into a bore that does not employ an appropriate lead in chamfer.

Our guidance for avoiding this failure mode is to:
•    Break all sharp corners.  
•    Provide a 20 degree lead in chamfer for radial seal applications.  
•    Cover threads that the seal will travel over during installation.  
•    Use lubrication during installation (this is the main benefit of an externally applied lubricant).  

A failure due to fluid compatibility issues will usually be evident upon disassembly of hardware after the seal has been in service.  Fluid incompatibility can look like many things but will be the result of some sort of chemical attack on the seal itself.  The seal could be excessively swollen, from absorbing service fluid(s), softened or gummy which would likely lead to another failure mode such as extrusion or compression set, cracking / embrittlement, or a simple case of rapid compression set.

This failure mode is one that can be completely avoided by selecting an appropriate material for a given application.  Parker offers several tools to help guide to proper material family selection in our O-Ring handbook (chapter 7), as well as our e-handbook and mobile InPhorm.  We also have our live chat feature, where you can run a specific application by the Applications Engineering team.

In addition to the materials mentioned above, Parker provides numerous other resources to support the diagnosis of seal failures and the best sealing solutions. Check out our latest tech webinar on seal failures modes or utilize our leak troubleshooting app.  You can also visit our website for resources under our Solutions area for more tools and assistance. 

This article was contributed by William Pomeroy, applications engineer, Parker O-Ring & Engineered Seals Division. 

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As mentioned in part one of our seal failure blog series, O-ring and seal failures are often due to a combination of failure modes, making root cause difficult to uncover. It's important to gather hardware information, how the seal is installed, application conditions, and how long a seal was in service before starting the failure analysis process. In part 1 of our blog series, we discussed compression set, extrusion and nibbling, and spiral failure. In part 2 of our series, we will review four other common failure modes to familiarize yourself with before diagnosing a potential seal failure in your application. 

Rapid gas decompression (commonly called RGD, or sometimes explosive decompression (ED)) is a failure mode that is the result of gas that has permeated into a seal that quickly exits the seal cross section, causing damage.

Detection of this failure mode can be difficult, as the damage does not always show on the exterior.  When the damage is visible, it can look like air bubbles on out the outside, or perhaps a fissure that has propagated to the surface.  The damage may also be hidden under the surface.  If the seal is cut for a cross section inspection, RGD damage will look like fissures in the seal that may or may not propagate all the way to the surface.

Parker’s guidance as to how to avoid this failure mode is: 1) Keep the depressurization rate lower than 200 psi per minute.  If this cannot be achieved, we would suggest 2) RGD resistant materials.  We offer these RGD resistant options from the HNBR, FKM, EPDM, and FFKM polymer families.

Abrasion damage is the result of the seal rubbing against a bore or shaft, resulting in a reduction of cross sectional thickness due to wear.  As the seal wears, it has the potential to lose compression on the mating surface.  This wear is compounded by the fact that dynamic applications already have lower compression recommendations.

To reduce risk for this failure mode, it requires consideration during design and seal selection.  The surface finish and concentricity of the hardware will be very important considerations.  A smooth surface results in less friction (suggest 8 to 16 RMS), which in turn results in less wear.  Increasing the durometer of the seal material helps resist wear, and there are also internally lubricated materials that could be employed.  If the application is high temperature, one should consider the impacts of thermal expansion on the elastomer being used.  The thermal expansion increases contact pressure, which would increase friction / wear.

Installation damage can occur in both face seals and radial seals but is much more common in radial applications. Installation damage can be the result of excessive installation stretch, cuts or nicks from installation tools, the installation of a seal over a burr or sharp corner or threads, improper size selection, or the insertion of a piston into a bore that does not employ an appropriate lead in chamfer.

Our guidance for avoiding this failure mode is to:
•    Break all sharp corners.  
•    Provide a 20 degree lead in chamfer for radial seal applications.  
•    Cover threads that the seal will travel over during installation.  
•    Use lubrication during installation (this is the main benefit of an externally applied lubricant).  

A failure due to fluid compatibility issues will usually be evident upon disassembly of hardware after the seal has been in service.  Fluid incompatibility can look like many things but will be the result of some sort of chemical attack on the seal itself.  The seal could be excessively swollen, from absorbing service fluid(s), softened or gummy which would likely lead to another failure mode such as extrusion or compression set, cracking / embrittlement, or a simple case of rapid compression set.

This failure mode is one that can be completely avoided by selecting an appropriate material for a given application.  Parker offers several tools to help guide to proper material family selection in our O-Ring handbook (chapter 7), as well as our e-handbook and mobile InPhorm.  We also have our live chat feature, where you can run a specific application by the Applications Engineering team.

In addition to the materials mentioned above, Parker provides numerous other resources to support the diagnosis of seal failures and the best sealing solutions. Check out our latest tech webinar on seal failures modes or utilize our leak troubleshooting app.  You can also visit our website for resources under our Solutions area for more tools and assistance. 

This article was contributed by William Pomeroy, applications engineer, Parker O-Ring & Engineered Seals Division. 

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Compressive Stress Relaxation (CSR) is a means of estimating the service life of a rubber seal over an extended period of time. As such, it can be thought of as the big brother of compression set testing. Rather than measuring the permanent loss of thickness of a compressed rubber specimen as is done in compression set, CSR testing directly measures the load force generated by a compressed specimen and how it drops over time. In part 1 of our blog series, we will explore the theory of CSR testing, common test methods, and how CSR differs from compression set testing.

To understand the value of CSR testing and how it differs from compression set testing, it is helpful to return to the basic theory of how a rubber seal functions. In a standard compressed seal design, a rubber seal is deformed between two parallel surfaces to roughly 75% of its original thickness. Because the material is elastic in nature, the seal pushes back against the mating surfaces, and this contact force prevents fluid flow past the seal, thus achieving a leak-free joint. Over time, the material will slowly (or perhaps not so slowly) relax. The amount of force with which the seal pushes against the mating surfaces will drop, and the seal will become permanently deformed into the compressed shape. In compression set testing, the residual thickness of the specimen is measured, and it is assumed that this residual thickness is valid proxy for the amount of residual load force generated by the compressed seal. In CSR testing, the residual load force is measured directly.

In practice, CSR results are typically presented very differently from compression set results. In CSR testing, it is common to see multiple time intervals over a long period of time (3,000 hours or more of testing), thus allowing a curve to be created (see Figure 1). In practice, however, specifications are written such that only the final data point has pass/fail limits. In compression set testing, it is common to see a single data point requirement with a single pass/fail limit. Multiple compression set tests can be performed to create a curve, but this is almost always down for research purposes rather than for specification requirements. In most cases, compounds that excel in compression set resistance also demonstrate good retention of compressive load force over time. However, there are exceptions.

Figure 1: Typical CSR curve.
These results display a fluorocarbon seal material immersed in engine oil at 150°C.

CSR testing can be quite complicated, and caution is needed when comparing reports to ensure a valid apples-to-apples comparison can be made. As with compression set, CSR testing can be performed in air or immersed in a fluid. Because most seal materials will oxidize in the presence of hot air, the results for CSR in air can be strikingly different (worse) than the results for CSR in a fluid at the same temperature. In addition to time and temperature, the sample size (usually a button 12.7 mm in diameter and 6.35 mm thick) and amount of compression (typically 25%) must be the same to make a valid comparison.

There are multiple test procedures and fixture designs within the CSR test world that also have a significant impact on results. Numerous fixture designs exist, all of which produce different results. Regardless of Hornig’s1 conclusions regarding his preferred fixture, in practice, most CSR data for elastomer seal materials is gathered with a “Dyneon-modified Wykeham-Farrance” jig (see image).

Finally, CSR force measurements can be made intermittently at discreet time points using a standalone compressive load cell or continuously using a dedicated CSR testing device that incorporates a load cell for each test fixture and one or more integrated environmental chambers.  With the intermittent test method, fixtures are removed from the oven and/or oil bath, allowed to cool to room temperature (23°C), manually tested on a compressive load cell, and returned to the oven for additional aging. Continuous testing does not involve this repeated thermal cycling, which contributes to accelerated relaxation and worse results. As a result, intermittent CSR generally appears worse than continuous CSR for the same material under the same conditions. In addition, continuous CSR data points are gathered at the test temperature rather than at room temperature. Gathering load force data at elevated temperatures results in a higher measured load force; when compared to an initial load force data point taken at room temperature, as some procedures require. This can result in a counterintuitive situation whereby it appears that a material initially improves with thermal aging.  (See Figure 2.)  If this is observed, it should be considered an artifact of the difference in test temperature. When comparing CSR results, it is absolutely essential to confirm that the same test method and fixture were used for both tests. 

Watch for part 2 of this series, where we will explore some of the powerful insights that can be gleaned through CSR curve interpretation. For more information or assistance with your sealing challenges, contact our applications team at oesmailbox@parker.com or chat with us online at the Parker O-Ring & Engineered Seals Division website. 

References:

1. Hornig, R.  Comparison of various CSR methods regarding the static long-term sealing behaviour of AEM, ACM and HNBR compounds, International Polymer Science and Technology, 37, No. 4, 2009.

This article contributed by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring Division.

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The first and most basic point of understanding is the end point. Does the material continue to maintain contact pressure throughout the test, or does it fall to zero (below the detectable limit of the load cell) before the end of the test? While there is no definitive correlation from residual load force to onset of leakage, it should be intuitive that a material that completely relaxes and loses all contact force is likely to leak in application. Anecdotally, multiple customers have reported that the load force must drop to very close to zero for leakage to occur in their particular test apparatus. While this is good guidance, these anecdotal reports should not be taken as a definitive answer that applies in all circumstances.

Specifications are often written such that a minimum of 10% of the initial contact load force must remain for a passing result. In practice, there is nothing special about 10%. This is a semi-arbitrary value that ensures a material continues to apply some non-zero load force to the mating surfaces, with some safety factor to ensure that it does so even after all normal test variations are considered. In practice, this appears to be a conservative limit, there is nothing magical about the 10% number.

The loss of compressive load force can be broken down into three different types of phenomena, each with its own time frame. All rubber materials relax viscoelastically when initially compressed, and this loss stabilizes within the first 24 hours. That initial drop seldom has much direct impact on real-world applications. However, in the specific case of an assembly having neither a compression limiter nor solid-to-solid contact, meaning the assembly torque of the fasteners is controlled solely by compression of the seal, this will be observed as “torque fade” if the fastener torque is rechecked a day or two after assembly. In such a case, Parker recommends against retorquing the fasteners unless leakage is observed as this retorquing can easily result in damage to the seal from excessive compression.

The second set of phenomena to resolve are solvation effects that occur when an elastomer is immersed in a liquid test media. The rubber material will absorb some amount of test fluid, causing some swelling of the rubber and a small increase in load force. When this happens, the test fluid may extract liquid constituents from the rubber, resulting in shrinkage of the seal material and a loss of load force.  These processes occur simultaneously, and both typically reach equilibrium within the first 72 hours. However, the net impact on the measured load force will only be noticeable if the volume change is significant.

The third set of phenomena are degradative effects caused by high temperatures and chemical reactions. These reactions are ongoing and cumulative.  If the test temperature remains constant, the rate of degradation will remain constant, as well. Unfortunately, there is no way to distinguish what percent of any degradation is due to thermal effects versus chemical effects from a CSR curve. Comparing a CSR output curve to one generated from testing in an inert control fluid at the same time and temperature may allow a user to isolate thermal effects from chemical ones, but it must be known that the control fluid does not also produce chemical degradation of the rubber material.

Ultimately, it is more important to consider the slope of the curve after the initial drop than the initial drop itself. The material should stabilize to a relatively flat line, and the slope of that line reflects how the material responds to thermal and chemical aging effects. A curve with very little slope (Figure 1) is extremely stable long-term, whereas as a CSR curve that shows a steeper negative slope (Figure 2) means the material is continuing to degrade due to chemical and/or thermal effects in that fluid and at that temperature. This does not mean the material is incompatible with that environment, but the continuing losses mean the end of service life point (onset of leakage) would be expected relatively soon after the end of the test.

Figure 1: A fluorocarbon in engine oil at 150°C shows very little change after the initial relaxation response.

Figure 2: An HNBR in engine oil at 150° shows ongoing degradation after the initial relaxation response. 

In summary, much knowledge about how a material will respond in an application can be gleaned from Compressive Stress Relaxation testing if one knows what to look for.  Watch for part 3 of this series, where we will focus on how to use the understanding gained from CSR testing and how to incorporate it into a material specification.

For more information or assistance with your application, contact our applications team at oesmailbox@parker.com or chat online by visiting Parker O-Ring & Engineered Seals Division website. 

This article was contributed by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring & Engineered Seals Division.

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Electrically conductive elastomers are elastomeric polymers filled with metal particles. They can be grouped by filler type and elastomer type. Then within each of these classes, there are standard materials and specialty materials.

Parker Chomerics manufacturers electrically conductive elastomer gaskets, also known as EMI elastomer gaskets, under the CHO-SEAL brand. We won't get so much into gasket configurations and dimensions here, we'll just stick to classes of materials. So what is available? Let’s find out.

Conductive elastomers are metallic particle filled elastomeric polymers, the particles giving the shielding performance and the polymer making them “rubber." There are many materials within this generic material type, but we'll focus on the below.
 

Setting up the grades of conductive elastomers by filler types involves six different particles:

• Silicone
  A polymer that has a large temperature range especially on the low end down to -55F. It is a very soft material with a low compression set. 
   
• Fluorosilicone
  Close to silicone, but will not swell and degrade when exposed to solvents, fuels hydraulic fluids and other organic fluids. Although slightly harder than silicone, it is still relatively soft with low compression set properties.
   
• Ethylene propylene diene monomer (EPDM)
  Does not have the temperature range nor the softness of silicone, but is resistant to highly chlorinated solvents used for compliance with NBC decontamination and is only used for applications with those needs.

All of these materials are cured or cross linked when the gasket is made. The cure either happens with heat or atmospheric moisture.
 

• Compression molding - uncured material is placed into a mold that has a cavity machined into it. The mold is closed, put into a press, then with heat and pressure the material is formed into the gasket shape mirroring the cavity and cured with heat. This process is used for sheet stock also.
   
• Injection molding - instead of hand loading the mold with approximately the proper amount of material, the mold is closed, then the proper amount of material is injected into the cavity. The process is much faster and because the right amount of material is injected tighter size tolerances can be obtained. Injection molding tooling is more expensive than compression molding.
   
• Extrusion - raw material is pushed through a die with the cross section machined into it to create extruded cord stock. Heat is applied on the die and a finished continuous gasket strip is made. These gasket can be supplied in lengths or cut to size and then corners or intersections glued or spliced together to make a ready to use gasket.

In general, for small cross sectional electrically conductive elastomer shapes, the smaller the particle the more producible the part will be. Particle fillers have little to do with mechanical properties of the gasket except that they simply increase hardness.

Now that you have more information about how to select particle and polymers for electrically conductive gaskets, download or Electrically Conductive Elastomer Handbook and get started today!

This blog contributed by Jarrod Cohen, marketing communications manager, Chomerics Divison.

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Parts 1 and 2 of this series discussed the theory behind CSR testing and what to look for in a CSR result curve.  This 3rd and final section will focus on how to use CSR data and apply it to real world applications and how to incorporate it into a material specification.

For the reasons discussed previously, it is important to view a full CSR curve, rather than a single data point, and to resist the urge to draw conclusions from incomplete data.  For example, Figure 1 compares a FKM to an HNBR material.  Because the fluorocarbon material has a larger viscoelastic loss within the first 24 hours of the test, it appears to be worse (less retained seal load) than the HNBR for most of the test duration.  However, the slope of the HNBR curve is steeper than that of the fluorocarbon, and the curves of retained load force cross at about the 2,300 hour point.  If these curves are extrapolated, the HNBR is predicted to reach the point of zero residual load force at 4,262 hours, whereas the fluorocarbon is not expected to reach the same point until 8,996 hours have elapsed.  Had the HNBR material been selected for this application based solely on the higher percent retained load force observed at 1,008 hours, the end user would have achieved roughly half of the service life they could have enjoyed had they selected the FKM compound instead.

Figure 1: An HNBR and a fluorocarbon in engine oil.

Several caveats remain in attempting to use CSR curve data to predict real-world performance.  First, a CSR procedure cannot mimic real-world application conditions.  In fact, it is not intended to.  CSR testing is meant to be an accelerated aging test, where specimens are exposed to constant temperatures hotter than anticipated in the actual application.  While some continuous CSR test machines are capable of temperature cycling, no CSR test can realistically replicate the actual temperature conditions, pressure fluctuations, or vibration seen in a real-world application.  Second, CSR testing cannot replicate actual fluid exposure conditions seen in typical seal applications.  CSR testing completely immerses a rubber specimen in a small amount of fluid that is kept at a high temperature – typically hot enough and with enough air exposure to cause the fluid to degrade.  In a real-world application, a seal is usually installed in a groove with fluid exposure on one side and air on the other.  In many applications, the fluid is contained within a sealed system that minimizes oxidative degradation of the fluid.  Third, there is as yet no definitive correlation between the onset of leakage and residual load force, regardless of whether that force is expressed in absolute terms (Newtons or pounds-force, for example) or as a percentage of the initial developed load force.  Finally, because low temperatures are more challenging for a seal, cycling the temperature to include load force measurements at -40°C would be considered a worst-case scenario.  Continuous measurement CSR devices are now commercially available with the capacity for running such a temperature cycle, but their use to date has been extremely limited due to the cost of the equipment.

Several factors should be considered before incorporating CSR testing into a material specification.  CSR is a more expensive procedure than compression set, and it requires dedicated and expensive test equipment.  As a result, it is not always appropriate to replace compression set testing in a material specification with CSR testing.  Second, establishing limits for “what good looks like” should never be done arbitrarily.  It is extremely difficult to convert a real world application service life into a number of test hours at a set temperature.  Therefore, it is more practical to start with a material that is known to maintain a satisfactory service life in the application, test it for CSR at a relevant temperature, and establish meaningful limits based on the performance of that material.   Finally, resist the temptation to get too exotic with specification limits.  For example, it may seem valuable to impose specification limits related to the slope of the CSR curve or to the extrapolated “time to zero load force”.  These can be valuable tools for gathering engineering insight, but they are far too complicated to incorporate as specification limits.  CSR is a more complicated test than compression set, and it requires more “homework” to set proper limits and test conditions.

In conclusion, Compressive Stress Relaxation is a powerful tool for comparing the performance of two or more materials in a particular application, provided it is done on an apples-to-apples basis.  It also serves as a reasonable means of (roughly) estimating the long-term service life of a seal material in a given application.  There are too many assumptions for it to be used to guarantee a specific level of performance, and there remains much work to be done in correlating results to real-world observations, but it currently offers the best laboratory-scale means of evaluating a seal material for long-term use in a given application.

This article was contributed by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring & Engineered Seals Division.

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We’re thrilled to announce the Chomerics Division of Parker Hannifin Corporation, the global leader in motion and control technologies, has been awarded the prestigious 2019 GM Global Supplier Quality Excellence Award for its Engineered Plastic Solutions business unit in Fairport, NY.

This award recognizes the suppliers who have demonstrated consistent quality performance throughout the year and is only given to the top performing supplier manufacturing locations. GM suppliers who receive this award recognition have met or exceeded a very stringent set of quality performance criteria and have achieved the cross-functional support of the entire GM organization.

“I am proud of all of our Chomerics team members at Fairport for their contributions to earn this award three years running,” said John Beswick, global business unit manager, Chomerics Division.

"We know that maintaining a focus on the details to deliver defect-free product[s] to our assembly plants consistently is not an easy task, so much that only a fraction of our suppliers has earned this prestigious recognition," noted Richard Demuynck, General Motors' executive director, global supplier quality and development. "You are a critical part of the team who helps ensure the customer is delighted with their product purchase and we want to recognize and thank you for that."

“Congratulations to the Fairport team,” said Dave Hill, global general manager, Chomerics Division, “It’s a direct reflection of GM's confidence in our operation, and of the hard work and dedication of our remarkable team members.”

Parker Chomerics Engineered Plastic Solutions business unit in Fairport, NY, located 10 miles outside of Rochester, NY, heavily utilizes robotics and automation to achieve high quality and delivery standards across many industries.

For more information on Parker Chomerics products, visit our website or download our Engineered Custom Injection Molded Plastics Solutions brochure. 
 

This blog was contributed by Jarrod Cohen, marketing communications manager, Parker Chomerics Division.

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Design factors for satellite, high altitude, and space-based applications vary dramatically from those of land or ship-based programs. These factors include: low payload capacity, low operating temperature, and meeting material limits.

Outgassing standards, established by NASA, set a limit on the release of gasses that can possibly interfere with sensitive technology within vacuum environments such as Low Earth Orbit (LEO). This is especially important for sensitive optical systems or camera lenses where the smallest bit of vapor or gaseous components can dramatically reduce performance. 

Test and metrics

NASA outgassing requirements are often used interchangeably with ASTM E595 which establishes the test method for determining outgassing levels. During the E595 test, small samples of material are kept under vacuum and heated to 125°C for a 24-hour period. While the samples are heated, all gasses are channeled through a single release port where a chromium-plated disk is used to collect the volatile materials.

After the test, there are two key metrics that are collected and used in certifying a material to NASA Outgassing Standards. 

While NASA does keep an extensive database of all materials that they have tested in house, certified labs often run ASTM E595 testing as well. For a complete list of Parker Chomerics products that pass traditional outgassing requirements (and associated NASA Data Reference Numbers), please see: Parker Chomerics NASA Outgassing Information. 

Outgassing in EMI shielding solutions

The most common materials to release stored vapors and gasses are sealants, adhesives, and less heavily crosslinked elastomers and polymers. Conversely, metals and glasses with few impurities tend to have a very low level of outgassing. 

While often true, many conductive elastomers made of silicone and fluorosilicone can meet these standards due to high quality raw materials and efficient processing. 

General trends for outgassing in EMI Shielding:

• Many conductive elastomers, with a variety of conductive fillers, such as: silver, silver-plated copper, silver-plated aluminum, and nickel-plated aluminum pass NASA outgassing requirements (Parker Chomerics Conductive Elastomer Handbook for more information)
• Nearly all metal mesh EMI gaskets including those made of Monel®, aluminum, and Ferrex pass outgassing
• Copper foil tapes with conductive and non-conductive adhesive meet NASA requirements
• EMI shielding heat shrinkable tubing meets outgassing levels 

Steps can be taken to reduce the amount of vapor or gasses that are released by materials. One such example is known as post-baking, sometimes referred to bake-off or bake-out. This process involves baking materials at elevated temperatures (and sometimes in vacuum environments) after they have been manufactured in order to lower vapor and/or volatile compounds.

It is important to note that some materials that pass NASA outgassing standards are only able to do so after post-baking for some amount of time. It is possible that some NASA post-baking occurred at temperatures above the maximum recommended operating temperature of these materials. This elevated temperature exposure can change the physical, thermal, or electrical properties of tested materials.

Parker Chomerics has a long history of supplying manufacturers with outgassing-compliant solutions to EMI Shielding problems for vacuum and space-based. For a complete list of Parker Chomerics products that pass NASA outgassing requirements (and associated NASA Data Reference Numbers) and more information, please see: Parker Chomerics NASA Outgassing Information.

This blog contributed by Ben Nudelman, market development engineer, Chomerics Division.

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Over the years, we've taken many questions from customers just like you about the measurement and testing of our thermal interface materials. How are they used? How are measurements determined? What's the best way to characterize performance? 

So we asked our engineers for help answering these questions. Read on to find out their answers.

Parker Chomerics’ standard test method of characterizing TIM performance is by ASTM D5470.
 

ASTM D5470 measures thermal impedance (resistance) of a flat disk-shaped specimen or controlled volume of a liquid TIM between two flat polished calorimeter surfaces under controlled load.
 

Apparent thermal conductivity is a calculated value that uses the thermal impedance (resistance) measured from ASTM D5470 and the sample thickness to calculate a thermal conductivity value.  This value is influenced by how effectively the sample contacts (or “wets out”, if a dispensable) the calorimeter surfaces.  

The thermal resistance at the interface between the sample and the probes is called contact resistance. Contact resistance adds to the overall thermal impedance (resistance) and may produce a lower measurement than bulk thermal conductivity.
 

Bulk thermal conductivity is an intrinsic property of any homogenous material.  To measure bulk thermal conductivity, we must subtract the contact resistance from the individual ASTM D5470 thermal resistance measurements.  

This is achieved by measuring thermal impedance (resistance) of the material at multiple thicknesses (at least three) and generating a straight-line plot. The y-intercept of that plot is the total contact resistance and the slope can be converted to bulk thermal conductivity.
 

A material can have a very high intrinsic bulk thermal conductivity but be outperformed by a material of lower bulk conductivity that is softer and conformable. Measuring apparent (effective) thermal conductivity can help better identify real world performance of a thermal interface material in many cases.
 

Generally, there is no “go-to” correction factor or simple equation to “convert” from apparent to bulk conductivity. The contact resistance can vary widely across different thermal interface materials and there are also many other factors to consider including pressure during test, flatness and thickness uniformity of sample, contact area, etc.

Both apparent and bulk conductivity are useful values for fully understanding a thermal interface material’s performance and expected behavior in application. It is useful to consider the bulk thermal conductivity as the maximum attainable thermal transfer efficiency parameter while apparent thermal conductivity values can offer an indication of how well the material performs in real world application where contact resistance cannot be ignored.
 

It is always difficult to compare values since it is unlikely that reported values from varying sources were generated using the same test method and parameters. There are many test instruments and methods used in the marketplace. Parker Chomerics relies on ASTM D5470 for accuracy and reliability.

The selector must be sure to consider test method used as well as any parameters used in the test that would influence outcomes (temperature, pressure, etc.). In addition, it is important to be aware of any “modified” methods reported. Without knowing the nature of the modifications, one can fall victim to overstatements of product performance.
 

Parker Chomerics reports bulk thermal conductivity for most TIM products on technical data sheets. Thin bond line products (such as phase change materials and thermal greases) data sheets will show thermal impedance at fixed pressure instead of bulk thermal conductivity as this is more practical and useful to the designer.

For lot-to-lot conformance testing, Parker Chomerics measures and retains apparent thermal conductivity and thermal impedance for every manufacturing lot of product.
 

This blog was contributed by Dana Drew, quality manager, Parker Chomerics Division.

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In the rush to massively increase the number of ventilators available to treat patients with severe cases of Covid-19, using the correct seal materials for those ventilators should never take a back seat to expediency.

Patient ventilators are mechanical devices that essentially breathe for a patient with damaged lungs. They force air into the lungs and draw it out, augmenting or even replacing the natural functions provided by the movement of the diaphragm and the inflation/deflation of the lungs themselves. These devices can supply room air, pure oxygen, or nearly any ratio of the two to the patient, depending on health needs.

First, seals within the device must be compatible with air and pure oxygen. They should not harden or crack, nor should they contain a significant amount of volatile matter that can evaporate out of the seal where it could be inhaled by the patient or potentially catch fire in a concentrated oxygen environment. Further, it should be assumed that any air that contacts the seals will likely end up in the patient’s lungs. As a result, Parker strongly recommends using seal materials that have passed USP <87> Class VI testing for any seals used in a ventilator.

Parker O-Ring & Engineered Seals Division has already helped several customers ramp up production of critical medical equipment with supplying the right materials and O-rings for the application. 

These application requirements limit the recommended compounds to only a small handful.

Parker’s silicone compound S1138-70 is an excellent first choice for many ventilator seal applications. It has low outgassing, it has been tested to and passes USP Class VI, and it’s completely compatible with air and pure oxygen. In addition, silicone is also naturally non-flammable – even self-extinguishing – so it offers no fire concerns. However, abrasion resistance is poor, so it should not be used in a dynamic application. In addition, permeation resistance is also poor, but pressures in a ventilator are low enough that this small amount of air or oxygen loss is usually negligible.

Parker’s EPDM compound E3609-70 offers similar compatibility and outgassing with much better abrasion resistance and improved permeation resistance when compared to silicone. It is also less prone to pinching and tearing upon assembly than silicone. E3609-70 has also passed USP Class VI and performs well in air and oxygen.  As a result, it is recommended for dynamic applications such as seals on moving shafts. EPDM polymer will burn under the wrong conditions, but this is not usually a real-world concern for ventilator applications.

Caution should be exercised with Parker’s E1244-70 compound. While the combination of passing USP Class VI and internal lubrication may seem attractive for reducing friction in dynamic applications, these internal lubricants are organic molecules which can be flammable.  When deposited in a thin film on mating surfaces as in a dynamic application, they could ignite when exposed to oxygen. This should be tested prior to use to ensure it does not cause a problem in application.  In general, it is safer to use a non-flammable liquid lubricant such as silicone or perfluorinated polyether (PFPE) oil or grease to reduce friction in dynamic applications. Parker’s Super O-Lube is a pure silicone lubricant, but it has not been tested or approved to any medical device specifications.

Finally, Parker’s fluorocarbon compound V0680-70 offers an interesting balance of properties. While it is not as good for abrasion resistance as an EPDM, it is still much better than any silicone. Like silicone, it is self-extinguishing and fully compatible with all combinations of room air and oxygen. Permeation resistance and outgassing properties are outstanding. V0680-70 has been tested and passed USP Class VI and is the same rust color as most silicone rubber compounds. 

During this time of crisis, Parker is prioritizing production of seals needed for critical medical equipment, and these four compounds in particular are strongly recommended for use in ventilators. They offer excellent long-term compatibility with room air and oxygen, have low outgassing, and have all passed USP Class VI testing. Learn more about the solutions available. 

This article was contributed by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring & Engineered Seals Division

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Hardware geometry and limitations are the first consideration. A traditional O-ring groove shape is rectangular and more wide than deep. This allows space for the seal to be compressed, about 25% (for static sealing), and still have some excess room for the seal to expand slightly from thermal expansion or swell from the fluid.  Reference Figure 1 as an example. Once the available real estate on the hardware is established, then we look at options for the O-ring inner diameter and cross section.

Hardware geometry and limitations are the first consideration. A traditional O-ring groove shape is rectangular and more wide than deep. This allows space for the seal to be compressed, about 25% (for static sealing), and still have some excess room for the seal to expand slightly from thermal expansion or swell from the fluid.  Reference Figure 1 as an example. Once the available real estate on the hardware is established, then we look at options for the O-ring inner diameter and cross-section.

Heavy duty equipment moves industry forward in all climates, from the sunny Caribbean to icy Greenland. Effective, reliable sealing is what allows hydraulic systems in heavy duty equipment to do work, no matter the temperature. Reliable sealing solutions allow cylinders on dump trucks and excavators to move icy, frozen tundra, and allow actuators on subsea valves to operate 5,000 - 20,000 feet below the surface of the ocean. We depend on these seals for our safety and productivity, so a little chilly weather is no reason to call it quits.

Most objects shrink as they get cold, with few exceptions (water, I’m looking at you). This applies to all matter in the universe. Materials shrink at different rates, and this is a measurable property called the Coefficient of Thermal Expansion (CoTE). Thermoset elastomers and thermoplastics shrink roughly 5 times more than metals1 for a given temperature change. This means at cold temperatures, seals shrink more than their housings, and thus have less “squeeze” to make a tight seal.

To make matters worse, elastomers also harden as the temperature drops. At some temperatures, known for each material as its Glass Transition Temperature (abbreviated ‘Tg’), seals become rock hard and brittle … like glass. We don’t make seals out of glass for a reason; they wouldn’t work. In order to keep seals springy and resilient we need to specify materials with a Tg below the coldest temperature a system will see.

In very high pressure, low temperature applications, there is one additional concern. Applying pressure to seals effectively raises the Tg of the material by about +1°C per 750 PSI. This is called Pressure-Induced Glass Transition and is the reason high pressure seals fail slightly above their measured Tg.

So, there’s low temp, and then there’s looooow temp. I work at Parker Hannifin Engineered Polymer System (“EPS”) Division’s headquarters in Salt Lake City, Utah. Winter low temperatures in downtown Salt Lake are typically just below freezing. In this climate, most seal materials function just fine.

Siberia purportedly has the coldest inhabited villages in the world, with temperatures down to -60°C (-76°F). Northern Canada isn’t much warmer. In these regions, an operating hydraulic system can generate enough heat from friction to keep the seals sufficiently warm. However, if hydraulic equipment is unused or stored overnight in such frigid environments, the use of cold-rated seals with a low Tg is critical to preventing leaks in equipment. 

For rotating equipment that is idled or shut off in extreme cold temperatures, the lip on a rotary shaft seal can freeze to the shaft when moisture is present at the seal lip/shaft interface. When the shaft starts, the tip of the lip can be ripped or sheared off, leaving a small band of rubber on the shaft (cold temperature seal fracture).  

And then there are cryogenic systems, which are entirely different beasts. Liquid nitrogen tanks require seals that can handle the -320°F fluid. At this temperature, all elastomers will be rock solid. So what seal material can handle that? Polytetrafluoroethylene (PTFE).

Pure (unfilled) PTFE, while not considered an elastomer, remains flexible down to -425°F. That’s 35 degrees above Absolute Zero – the coldest possible temperature. The chart in Figure 1 shows the effective ranges for seal materials offered by Parker 2, 3.

After looking at this chart, you might think, “Why doesn’t Parker make all seals out of PTFE and dump the rest?” PTFE is a great seal material, but it comes with its own set of tradeoffs. Hardware manufacturing and seal installation tend to be more complicated with PTFE than with elastomer seals.

Fluorocarbon rubber (FKM), and more recently perfluorinated rubber (FFKM), have traditionally been selected to seal hot temperatures and nasty chemicals. However, they perform poorly in cold. Parker offers special low-temp blends, spanning -40 to 400°F and -40 to 600°F respectively. “Do-everything” materials such as these tend to be more costly than traditional FKM rubber.

Highly saturated nitriles (HNBR) offer higher temperature capability with better wear resistance compared to standard nitrile (NBR). To improve low temperature resilience, Parker has developed HNBR compounds that perform better at low temperatures than most general HNBR compounds.  

Special grades of silicone can handle colder temperatures, but their wear properties are so poor Parker EPS does not recommend these for dynamic sealing. Ethylene propylene rubber (EPDM) compounds are also capable of remaining quite flexible at low temperatures, but care must be taken to ensure that the application is compatible. EPDM often has the look and feel of nitrile rubber but reacts to fluids much differently.   

Parker polyurethanes (compounds start with a ‘P’ in the table in Fig. 1) are popular because they offer the best all-around balance between low and high temperature sealing, wear resistance, pressure rating, and cost. Parker’s compound P5065A88 is compounded specifically to be more resilient at low temperature than most other polyurethane compounds.  

In all sealing applications where temperature ratings are a concern, it is important to know that sealing compounds perform their best when they stay well within their temperature range. Applications that push seal compounds to the end of their temperature range may only perform for a short period of time before damage to the seal occurs, or they become too stiff to effectively control leakage. A good rule of thumb for long lasting seals is to remain within 80% of the compound’s temperature range.  

Now that you’ve selected a seal material with a Tg low enough for the cold environment it will be used in, are you done? Make sure other properties such as pressure rating, wear resistance, fluid compatibility, and high temp capability are also adequate for the system. Be aware of tradeoffs when switching materials to avoid causing a problem in another area. If in doubt, send us an e-mail and we will be happy to help.

Stay cool! Much more information about seals can be found in our Fluid Power Seal Design Guide, Catalog EPS 5370.

Recommendations on application design and material selection are based on available technical data. They are offered as suggestions only. Each user should make their own tests to determine the suitability for their own specific use. Parker offers no express or implied warranties concerning the form, fit, or function of a product in any application.

This article was contributed by Nathan Wells, application engineer, Engineered Polymer Systems Division.  

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The demand for dispensing thermal interface material (TIM) has increased exponentially because dispensables offer engineers flexibility in the designs and patterns that can be applied to electronics packaging.

Single-component, dispensable TIMs such as THERM-A-GAPTM GELs, can cover a variety of gaps and form complex geometries which provide reduced thermal contact resistance, thus reducing the temperature and increasing the efficiency of the application. 
 

In applications that require precise dispensing, like automotive vision and controller modules found on Advanced Driver Assistance Systems (ADAS) systems, high volume dispensing machines can dispense large volume cartridges or pails of TIM on thousands of parts per hour.

• These dispense machines offer the greatest control and yield
• As well as the fastest cycle times with generally the lowest material cost per piece
• The proper equipment choice will be a function of geometry, throughput requirements, material type, and package 

However, issues tend to arise when these large cartridges and pails must be changed out when material is used up. Today, many customers deploying high volume dispensing on their lines are running into throughput issues due downtime and re-calibration required to change out an empty cartridge or pail.  
 

In some instances, our engineers have seen customer downtime go from minutes to hours or even days when processing and other systems need to be adjusted between changeovers. 

It's clear users of high volume dispense material in applications such as automotive ADAS cannot afford this severe interruption in their supply chains.  

This is where THERM-A-GAP GEL 37, the next generation of single component, no cure required, thermal interface materials from Parker Chomerics is so useful. 

With a higher thermal conductivity and better flow control than its predecessor, we consider THERM-A-GAP GEL 37 to be the dispensable TIM that will help to lower downtime and re-calibration between vessels.

THERM-A-GAP GEL 37 significantly improves dispense variability and limits downtime due to material and processing adjustments. Other benefits include:

• Pigmented blue for ease of use by vision systems
• Offers a 50% higher flow rate compared to THERM-A-GAP GEL 30
• Ideal for the automotive market segment where a dispensed thermal material with a tighter flow rate and an easier dispense material help to solve those critical changeover interruptions

THERM-A-GAP GEL 37 will be manufactured in Hudson, NH, and is available today for immediate sampling and prototyping. 

This blog was contributed by Jarrod Cohen, marketing communications manager, Parker Chomerics Division.

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Clipper® Oil Seals are one of the Parker Engineered Polymer Systems (EPS) Division's most widely used rotary seal products. They are an effective solution – especially when used as direct replacements for traditional metal case seals. This is a testament to their precision-molded rubber/aramid fiber heel construction which eliminates the metal case (see image above). In this blog we will review the benefits of using Clipper® seal profiles as direct replacements for metal case seals:

The composite rubber/aramid fiber heel provides a gasket-like seal for improved sealing against the bore. The surface conditions of bore housings are frequently riddled with imperfections due to damage during improper seal installation and removal, or simply due to cost sensitivity in their original manufacture. Metal can seals lack the ability to conform to such imperfections, frequently necessitating the use of supplemental gaskets or bore sealants during installation to prevent bore leakage.

The outside diameter of the flexible, composite elastomer/aramid fiber heel is slightly oversized to create a tight interference press fit. The tight fit and compression-set-resistant heel construction eliminate the necessity of compression plates for bore retention1. It’s essential to note that bore plates (shown in green) can cost as much as $100 per inch of shaft diameter because of additional part cost and added assembly time. 

Clipper seals have a composite elastomer/aramid fiber heel and rubber elastomeric lip so there is no concern for rust or corrosion. The only metal component is a 302 stainless steel garter spring. The stainless spring handles higher operating temperatures and resists rust/corrosion better than carbon steel springs used in other rotary shaft seals.

The Clipper Oil Seal features one-piece molded construction across its entire available size range (0.25” to 92”+ shaft diameters). This molding process is more robust as compared to metal case rotary seals constructed from multiple components that are glued and/or crimped together during assembly.

Metal case seals are prone to sealing complications brought on by thermal expansion. As an example, consider what happens when interfacing a carbon steel metal case seal with aluminum housing in applications where large rapid temperature swings occur. Aluminum housings will expand at a different rate than the carbon steel case of the rotary lip seal; leaving you vulnerable to bore retention issues and/or failure due to leakage from the housing/seal OD interface. 

Clipper’s flexible, rubber heel prevents seal damage and component damage during installation. Seal installation, aided by proper chamfering, requires only finger pressure to start the seal into the housing, followed by light tapping with a soft-faced mallet until flush. Issues associated with installing rigid metal cans, such as damage due to cocking, are avoided. To further ease installation and save time, because the outer diameter conforms to bore imperfections as mentioned earlier, there’s no need to add -- or wait for drying time of -- bore sealants.

Due to the Clipper Oil Seals non-metallic construction, Parker can produce certain profiles that have been precision split. Downtime due to seal replacement is minimized. There’s no need to uncouple or disassemble equipment since Split Clipper seals can be installed in place.  For metal case variants, this is not an option. View the short video demonstrating the ease of installation here: Split Clipper Oil Seals Installation

The rubber molding process allows for greater customization to fit specific design envelopes. The addition of flanges, buttons, mounting and bolt holes, etc. are all possible. 

Non-stock items can be produced and delivered faster than metal case seals. Seal production of Clipper’s composite rubber design is not hindered by long lead times commonly experienced with the fabrication or procurement of metal components. Clipper Oil Seals 14” in diameter and smaller typically ship within 10 days.  

Since all Clipper Oil Seals are made in the USA, the supply chain is simplified. Procurement difficulties due to global import or tariff variables are non-existent.

Learn more about Clipper® Oil Seals and other rotary shaft seal solutions from Parker. Included on the page is a link for you to contact us and initiate discussions on your next project. In addition, your local Authorized Parker EPS Distributor can provide assistance with off-the-shelf or customized Clipper Oil Seal solutions from Parker EPS Division. To locate your nearest representative, follow the instructions provided on Parker's website under"Where to Buy".

1 Exceptions exist where regulatory or unique application conditions govern bore plate use. 

This article was contributed by Alan Wiedmeyer, application engineer, Parker Engineered Polymer Systems Division.  

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