parker hannafin

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

Original content can be found on Parker’s Website and was written by Vivek Sarasam, heavy duty mobile Sr. application engineer, and Jeffrey Labonte, market manager.

Parker Hannifin Engineered Materials Group has developed a wide variety of metal seals which can be formed or machined. A metal seal is a highly engineered sealing solution which provides elastic recovery or spring back to maintain good sealing, despite separation of mating surfaces due to effects of thermal cycling, flange rotation, applied mechanical or hydrostatic loads or creep.

Why use a metal seal?

A metal seal is used when the application conditions are outside the specification limits of a polymer. For example, when:

Temperature is too hot or too cold & pressure is too high or there is a vacuum.

Metal Seals are primarily used in static applications for temperatures as high as 1000°C/1832°F and pressures as high as 6825 bar/99000 psi for select applications. At low cryogenic temperatures and low pressures, such as vacuum seal applications, metal seals are far better than polymers since they do not become brittle and lose elasticity. Metal seals also have a low leakage rate down to 1 x 10^-12 cc/sec per mm circumference which in comparison to high load O-rings is almost 100x better.

Medium is corrosive and seal longevity is needed.

Unlike elastomer seals, metal seals are very highly resilient to corrosive chemicals and even intense levels of radiation. With this resilience coupled with the right material selection/coating for an application, a metal seal can be a very durable seal performing dependably year after year.

Parker has a variety of in-house developed coatings which are used based on the application conditions and base material. The chart on page D-59 of the Metal Seal Design Guide (shown below) shows examples of some of the coatings based on the base material.

What X-sections can be made?

Metal seal x-sections can vary from a solid O to a Hollow O and from a C Ring to an E Ring depending on the application load and allowable leakage rate as shown in the figure below. Each x-section has benefits based on the application use and cost as indicated in the chart below.

Page A-10 of the Metal Seal Design Guide (shown below) shows some common applications in the industry and the type of metal seal used in those applications. These are examples of applications where the application conditions exceed beyond what an elastomer is capable of handling.

Importance of surface finish for metal seal applications

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.

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.

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. 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.

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.

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 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.

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.

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.

In 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.