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.
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.
A question we are frequently asked by our customers is “Can I use a metal hose for food-related applications?” It’s a simple question, but the answer isn’t always so simple.
The quick answer is no, metal hose generally cannot be used for the transfer of food-grade materials. This has nothing to do with the capabilities of the manufacturer or the quality of materials used in the hose. On the contrary, while the steel used may indeed be “food-grade,” the corrugations in the hose can potentially trap food media and make it difficult to clean the hose to the proper standard. Instead, you typically see PTFE hoses in these types of applications. The only exception to this would be if the media is at a temperature high enough to kill off any bacteria, which may allow for a metal hose to be used in some cases.
However, just because metal hose generally cannot be used to transfer food-grade materials does not mean that there are not applications for metal hose elsewhere in production! You just have to know where to look for them…
A key component of food production that requires the use of metal hose is clean-in-place (CIP) systems. Clean-in-place systems serve as a way to clean and sanitize the internal surface of a piping system as part of the routine cleaning of the production line or when production is being changed over to a different product. These systems utilize steam and chemicals (such as sodium hydroxide) to clean the piping system, which can be hard on any non-metal hoses that are used in the system. Instead, non-metal hoses are removed to be cleaned separately and metal hoses are installed in their place during the cleaning process. These metal hoses are better suited to handle the high heat and caustic cleaning solution running through the system, making metal hose an optimal solution for this application.
Seals are small components that have a big function. At their most essential purpose, they keep fluid in and debris out. Failure to do this can lead to costly machine destruction, and even endanger the lives of people working around these machines. Preventing seal failure is very important on many different levels.
Many factors can cause a seal failure. Being aware of these factors will help you in preventing an avoidable failure. Seals are under constant pressure because of the liquid or gas being contained. Typically, the seals in pipes or machinery have some type of matter within transferred at high velocities. This is a huge part of what they were designed to do, but too much or too little pressure could wear down the seals.
In addition, extreme temperatures can cause the seal to become brittle. A mixture of extreme temperatures and pressure is a recipe for cracking and leaking. Also, debris can contaminate the integrity of the
Eclipse has been working hard during the Covid-19 downtime on finding solutions to issues that customers have brought to the table over the past few years.
Many new designs have been sent into testing while focusing on processes that will help improve productivity and lower costs.
The MicroLip™ is an example of a viable solution to rotary seal issues that many customers have struggled with. This is especially true when the order volumes are relatively low or the shaft diameters are small, such as with encoders or chemical-processing facilities.
When moving from rubber to Teflon lip seals, Eclipse has found that the cost to bring the product to market is often a hindrance. The high cost is due to tooling and the number of pieces that must be manufactured to make the product viable in the prototype phase.
Because of this, many customers sneak by using inappropriately-applied rubber lip seals to solve rotary seal problems.
MicroLip™ seals have proven to be a powerful component in rotary services. Since the MicroLip’s inception, it has been applied to a variety of applications including mobile hydraulics, robotics, surgical drills, and semiconductor processing and encoders.
Over the last 3 years, Eclipse has designed and manufactured various styles of MicroLips in diameter sizes of under 1/8 inch (5mm) and over an inch. Since the components of the MicroLip™ can be machined, Eclipse has made the seal in quantities of less than 10, and batches in the thousands.
The Garlock Family of Companies has launched a new fully-coated isolation gasket known as EVOLUTION.
The next generation of isolation gaskets, EVOLUTION®, features easier installation, tight sealing, high-temperature operation, no permeation, hydrotesting isolation, fire-safety and chemical-resistance.
Featuring a thinner, 1/8-inch design, EVOLUTION minimizes the difficulties encountered when attempting to install thicker isolating gaskets. The full-coating encapsulation allows the gasket to be hydrotested and left in the pipeline with the same isolation properties as before it was tested.
EVOLUTION's coating is highly resistant to abrasion and impact while providing chemical resistance to hydrogen sulphide (H2S), steam, carbon monoxide, carbon dioxide and other chemicals often found in oil and gas pipelines. This fully encapsulated coating also prevents the need for expensive exotic cores, as it eliminates contact to exposed metal.
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.
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.
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.
Freudenberg Sealing Technologies has launched series production of a modular sealing unit that combines a classic radial shaft seal with a plastic outer case. The design promotes better long-term seal performance and longevity, is easier to assemble, and significantly lowers manufacturing costs in comparison with traditional metal-encased radial shaft seal units. Freudenberg has developed the innovative sealing concept for use in general industry applications that are especially focused on small, electric household appliances.
Whether it’s to knead bread dough, mix a cake batter, puree soup ingredients or blend a smoothie, most people reach for an electric kitchen appliance to get the job done. The durability of the appliance depends largely on how well the seal at the outlet point of the drive shaft protects the interior from ingress of food residue or liquids. Seals made of high-quality elastomers or the polymer polytetrafluoroethylene (PTFE) combine low wear with excellent long-term resistance against leakage. In the past, a metal case was the best option available to maintain the integrity of the seal’s performance over a long period of time. Freudenberg Sealing Technologies has now succeeded in developing a modular sealing concept with a plastic case that meets the specific requirements for long-term performance as well as those made of metal. There are three major advantages to the new design: Significantly, in the price-sensitive, small appliance industry, the lower production costs associated with forming enclosures from plastic is an important consideration. In addition, Freudenberg's modular sealing unit concept accommodates the integration of additional components, such as shaft bearings. Finally, because small appliance housings are typically made from plastic, fastening the seal case to the appliance housing is easier to achieve.