Sealing and Shielding Blog

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 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.

  4 Limitations of CSR data

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.

  Other considerations

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.

 

 

 

 

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

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

How to Read a Rubber Test Report: The 4 Most Common Misunderstandings

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

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.
 

Particle fillers

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

 

 

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.
 

Three main processes used to process the material into a gasket

• 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.

 

 

 

 

Related content

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In 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. 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 CSR curve.

 

  The end point

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.

  Solvation effects

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.

  Degradation

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.

 

 

 

 

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

How to Read a Rubber Test Report: The 4 Most Common Misunderstandings

5 Factors to Consider When Determining Compressive Load of a Seal

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

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

 

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.

 

Theory of CSR 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 Test Methods

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. 

 

  Figure 2: Continuous method CSR testing above shows an initial increase in load force due and overall a higher percent retained load force than observed with the intermittent method.

 

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.

 

How to Read a Rubber Test Report: The 4 Most Common Misunderstandings

5 Factors to Consider When Determining Compressive Load of a Seal

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

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

 

 

 

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

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

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

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).  

 

Fluid incompatibility 

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. 

 

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

3 Guidelines to Ensure Proper Seal Installation

A Simple Guide to Radial Seals | Sealing Fundamentals

 

 

 

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

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

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

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).  

 

Fluid incompatibility 

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. 

 

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

3 Guidelines to Ensure Proper Seal Installation

A Simple Guide to Radial Seals | Sealing Fundamentals

 

 

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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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 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.

 

   

 

 

 

 

 

 

 

 

 

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.
  

      

 

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.  

 

 

 

 

 

 

 

 

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

  INSTALLATION METHODS AND TOOLING

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.

Seal material considerations

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.

  Installing Rod Seals

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".  

    Installing piston seals  

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.
 

 

 

 

 

 

 

 

 

 

 

  

        OTHER CONSIDERATIONS

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).

  CONCLUSION

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 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

• 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.

 

Extrusion and nibbling

• 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

• 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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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.

 

Metal seal types and sizes

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.

Products

• 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

 

Materials and coatings

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. 
 

Case studies

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

More information:

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

 

 

 

 

Additional articles:

Continuous Molding Enables Production of Large-Size Elastomer Seals in Precision Quality

CAD Library for Seals Makes Work Easier for Design Engineers