Tag Archives: Parker Hannafin

How Much Do You Know About Compressive Stress Relaxation? CSR Part 3

Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.

Original content can be found on Parker’s Website and was written by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring & Engineered Seals Division.


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.

CSR Curve

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 (below) 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.

picture of HNBR and FKM graph
Figure 1: An HNBR and a fluorocarbon in engine oil.

Continue reading How Much Do You Know About Compressive Stress Relaxation? CSR Part 3

How Much Do You Know About Compressive Stress Relaxation? CSR Part 2

Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.

Original content can be found on Parker’s Website and was written by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring & Engineered Seals Division.


picture of compressive stress relaxationIn Part 1 of this series, the theory behind Compressive Stress Relaxation (CSR) testing was discussed, as well as a brief discussion of the fixtures used to measure it. In Part 2, we will explore what to look for in a CSR result. A significant understanding of how a rubber seal material responds to a particular environment can be gleaned if one knows what to look for in a compressive stress relaxation curve.

The Endpoint

The first and most basic point of understanding is the endpoint. 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 the onset of leakage, it should be intuitive that a material that completely relaxes and loses all contact force is likely to leak in the 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, Gallagher’s partner, Parker, recommends against retorquing the fasteners unless leakage is observed as this retorquing can easily result in damage to the seal from excessive compression. Continue reading How Much Do You Know About Compressive Stress Relaxation? CSR Part 2

How Much Do You Know About Compressive Stress Relaxation? CSR Part 1

Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.

Original content can be found on Parker’s Website and was written by Dan Ewing, senior chemical engineer, Parker Hannifin O-Ring & Engineered Seals Division.


black o-ringsCompressive 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 the 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.

Theory of Compressive Stress Relaxation 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, compressive stress relaxation 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 done 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.

Continue reading How Much Do You Know About Compressive Stress Relaxation? CSR Part 1

How to Properly Choose Commercially Available O-Ring Cross Sections

Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.

Original content can be found on Parker’s website and was written by Dorothy Kern, applications engineering lead, Parker O-Ring & Engineered Seals Division.


There are 400+ standard O-ring sizes, so which is the right one for an application? Or maybe you are wondering if one O-ring thickness is better than another. This short article will walk through some of the design considerations for selecting a standard, commercially available O-ring for an application.

Design Considerations

Hardware geometry and limitations are the first consideration. A traditional O-ring groove shape is rectangular and wider 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.

AS568 Sizes

From a sourcing perspective, selecting a commercially available O-ring size is the easiest option.  AS568 sizes are the most common options available both through Parker and from catalog websites.  A list of those sizes is found in a couple of Parker resources including the O-Ring Handbook and the O-Ring Material Offering Guide. They are also listed here.  The sizes are sorted into five groups of differing cross-sectional thicknesses, as thin as 0.070” and as thick as 0.275”, shown in Table 1 below.
Continue reading How to Properly Choose Commercially Available O-Ring Cross Sections

What’s the Difference Between EPR and EPDM?

EPR vs EPDM – How do they differ?

What’s the difference between EPR and EPDM? What do the different abbreviations (EPR, EPM, EPDM, EPT, etc.) mean? These questions pop up from time to time in the seal industry, and here are basic answers to these questions.

EPDM MaterialsIn the range of ethylene-propylene (EP) rubber there are two lightly different branches: EPR (EP copolymer) and EPDM (EP terpolymer.) The differences are subtle, and a basic knowledge of polymers and rubber compounding is necessary to grasp the differences.

First of all, polymers (derived from the Greek for “many units”) are long  chemical chains that can be thought of as behaving like long pieces of cooked spaghetti. Each chain is made of one or more monomers (Greek “single unit”) linked together end-to-end. A copolymer (Greek “two units”) is composed of two monomers, while a terpolymer (Greek “three units”) is composed of three monomers. EPR (aka EPM, EP copolymer) contains only ethylene and propylene monomers. EPDM (aka EPT, EP terpolymer) is composed of ethylene, propylene, and a third monomer called a diene (three different dienes are in common use today, but discussing their differences gets extremely dry and technical.)

To make a rubber material rubbery, we essentially have to “glue” the polymer chains together. We do this through a process called vulcanization or curing. This is where the subtle difference between EPR and EPDM is found. Because of the chemistry of the polymer chain, EPR can only be vulcanized with a peroxide-based cure system. On EPDM, the additional diene monomer provides a specialized cure site that allows the polymer to be vulcanized with peroxide- or sulfur-based chemistries. Because of this added flexibility, most EP compounds in the seal industry today use EPDM terpolymer instead of EPR copolymer. In other industries (hose, roofing products, etc.) EPRs may still be the material of choice.

From a functional standpoint, there are very few performance differences between EPR and EPDM. Both swell dramatically in petroleum products, and both are excellent in water, steam, and polar solvents like MEK and acetone. There are some notable performance differences in extremely demanding applications: EPRs or very tightly cured EPDMs are suited for the nuclear industry (E0740-75 is recommend), and for applications involving concentrated acetic acid, some EPDM compounds (like E0692-75) show superior performance to most EPRs. In other applications, the performance difference is difficult (if not impossible) to identify.


The original article was written by our partners at Parker and can be found on their website here.

For more information about which material(s) might be the best fit for your specific application, contact Gallagher Fluid Seals’ engineering department.

How Gallagher’s Suppliers are Helping Support the Fight Against Covid-19

Gore, Precision Associates, and Parker Join the Fight Against Covid-19

picture of gore logoWhat Gore is doing to help

As the COVID-19 pandemic continues to impact communities around the world, Gore is working hard to identify ways in which they can apply their materials science expertise and production capabilities to help during this time of need.

Several initiatives are underway that bring together the knowledge, skills and capabilities from across Gore.

As an immediate and initial response to the personal protective equipment (PPE) shortage, Gore rapidly engineered prototype reusable mask covers to supplement clinicians’ primary face masks.

The effort went from a product concept to prototypes in less than one week. Gore currently has prototypes being evaluated at a limited number of U.S. facilities in COVID-19 outbreak hot spots.

Additionally, by providing their customers and other manufacturing companies with Gore’s highly specialized component materials, they can use them to produce a variety of finished PPE items, such as:

  • Protective medical gowns, using fabrics laminates from Gore’s current inventory
  • Universal filter cartridge prototypes for fixed masks that incorporate Gore’s filtration materials intended to provide N95 particulate protection,
  • Disposable N95 respirators, using Gore’s filtration laminates

It is through these collaborations with those who have the technical capabilities and production capacity needed to produce the finished goods in volume that together, Gore and its partners truly are improving life.

Gore has even donated medical supplies and protective gear to healthcare workers in the communities in which their facilities are stationed. They’ve also extended a hand to provide engineering and prototyping support to address other urgent equipment needs at local hospitals.


picture of precision logoWhat Precision Associates is doing to help

Precision Associates, Inc. has ramped up its production of several essential rubber components for ventilator manufacturer Ventec Life Systems. Ventec recently announced plans to partner with GM to increase the output of these crucial units in response to the COVID-19 pandemic.

Critical patients suffering from the virus have difficulty breathing and require ventilator assistance for life support. GM is now transforming one of its factories to begin assembling ventilators for Ventec in early April. Continue reading How Gallagher’s Suppliers are Helping Support the Fight Against Covid-19

What You Should Know About Electrically Conductive Elastomers

Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.

Original content can be found on Parker’s Website and was written by Jarrod Cohen, marketing communications manager, Chomerics Divison.


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 elastomers in gasket form, 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.

Particle fillers

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

picture of 6 essential fillers

Three types of elastomer material

  • 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. Continue reading What You Should Know About Electrically Conductive Elastomers

Reduce Downtime and Costly Seal Replacements: Seal Failure Diagnosis Part 2

Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.

Original content can be found on Parker’s Website and was written by William Pomeroy, applications engineer, Parker O-Ring & Engineered Seals Division.


As mentioned in part one of Parker’s 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, compression set, extrusion and nibbling, and spiral failure were discussed. In part 2 of Parker’s series, they will review four other common failure modes to familiarize yourself with before diagnosing a potential seal failure in your application.

Rapid gas decompressionRapid Gas Decompression

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, they would suggest 2) RGD resistant materials.  Parker offers these RGD resistant options from the HNBR, FKM, EPDM, and FFKM polymer families.

AbrasionAbrasion

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. Continue reading Reduce Downtime and Costly Seal Replacements: Seal Failure Diagnosis Part 2

Installation of Linear Fluid Power Seals

Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.

Original content can be found on Parker’s Website and was written by Nathan Wells, application engineer, Engineered Polymer Systems Division.


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.

SEAL GROOVE STYLES

picture of closed rod seal groove

First, let’s look at three common groove styles:

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

Closed groove

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.

picture of cylinder head and piston seals

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.

Stepped groove

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.

picture of rod stepped groove

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 and two-piece grooves

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.

picture of open rod groove

Open grooves also make removing the seal much easier – useful in systems which require periodic seal replacement. Continue reading Installation of Linear Fluid Power Seals

Reduce Downtime and Costly Seal Replacements: Seal Failure Diagnosis Part 1

Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.

Original content can be found on Parker’s Website and was written by William Pomeroy, applications engineer, Parker O-Ring & Engineered Seals Division.


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, Applications Engineering teams are often asked to help discover the seal failure mode(s).

Seal failure is often due to a combination of failure modes, making root cause difficult to uncover. When beginning a failure analysis, items usually asked for include: 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 a quick diagnosis to help resolve seal failures. In part one of the seal failure blog series, we will discuss compression set, extrusion, and spiral failure.

Compression set

  • picture of compression setCompression 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
    • Temperature exposures above the recommended range for the material.

Extrusion and Nibbling

  • picture of extrusionThe 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 the Parker 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

  • picture of spiral failureSpiral 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.

Check out Parker’s neat video about Seal Failure Modes:

Parker and Gallagher Fluid Seals can help diagnose seal failures and the best sealing solutions for your application.

Stay tuned for Part 2 in this series.


For more information about how Gallagher Fluid Seals can help you, contact our engineering department today.