Rubber deterioration can happen due to a variety of factors, including heat, light, and other environmental factors. Its greatest impact is typically with rubber seals and o-rings. This blog will discuss common deterioration causes for rubber seals and steps to help prevent underlying issues.
Elastomers are a natural or synthetic polymer that has elastic properties. Rubber is an example of an elastomer as rubber is stretchy. Rubber can deteriorate over time due to to the nature of the material. The most common causes of rubber deterioration include heat, exposure to light, and exposure to chemicals or even oxygen. Things like rubber seals and O-rings are susceptible to deterioration due to the molecular changes these factors cause. They can significantly impact the mechanical properties of the rubber seal, which results in shortened lifespan.
Mobile cranes perform a wide variety of tasks, typically of the heavy-duty kind. The work they do and the locations at which they operate are frequently exposed to harsh climatic conditions in places with insufficient infrastructure. This means that the sites at which the cranes are positioned and the environment in which they move is often not entirely suitable for this kind of heavy construction equipment.
Accordingly, there are high loads acting on the components, which often wear out prematurely as a result. A new sealing solution for swivel joints in cranes subjected to high loads, which combines a polyurethane O-ring with a nobrox® backup ring, has effectively remedied this issue.
In a typical oil refinery or chemical processing plant, 60% of fugitive emissions are attributable to leaking valves. Of these, nearly 80% are released at valve stems. Also contributing to valve failures, are leaks at the bonnet, flanges and seats. As operating conditions in these and other industries continue to subject valves to ever more extreme temperatures and pressures, sealing them effectively poses a challenge that cannot be met with traditional materials and methods. Soft, compressible elastomers provide good sealing performance, but they are porous and cannot withstand temperatures in excess of 482°F (250°C). They also become hard and brittle in cryogenic service. Metal seals have much greater temperature capabilities, high mechanical properties, lack of porosity and long shelf life. Ductility and elasticity are typically the limiting factors.
Today more resilient, metal-to-metal seals are available and being used in valves operating under extreme conditions. Spring- and pressure-energized metal seals function much like a gasket between two flanges with little or no relative motion between them.
The sealing principle of spring-energized seals is based on plastic deformation of a jacket that is more ductile than the mating surface. This deformation occurs between the sealing face and an elastic core composed of a closely wound helical spring. The spring provides specific resistance to compression, during which the resulting pressure forces the jacket to yield and fill flange imperfections. Each coil of the helical spring acts independently, allowing the seal to conform to any surface irregularities. This combination of elasticity and plasticity provides an extremely effective seal even under the decompression cycles.
Dust is typically a minor annoyance that haunts the surfaces of our home. But in the world of engineering, machinery, and mechanical systems, it can be the difference between a reliable piece of equipment and disaster.
Dust can cause major damage to cylinder walls, rods, seals and other components inside of machinery. And if you’re not careful, dirt, mud, debris, and water can all cause damage as well.
These foreign contaminants are real problems for mechanical systems, especially as they build up in small quantities over time. A single particle of dust today may be no big deal. But a mote of dust a day will eventually become enough of a presence to cause serious issues, like friction, surface wear, and imperfect seal contact between surfaces.
These issues could compound until the mechanical system experiences a complete failure. It may seem like perfect is impossible, and that eventually some contaminants will get into your system no matter what you do.
But in some applications, like in automobiles and aircraft, failure is simply not an option.
Beyond those industries, many types of equipment need to stay clean on the inside, even when things get extremely messy on the outside. Examples include earth movers, hydraulic cylinders in steel mills, snow plows, and metal foundries, and in seals in logging equipment.
Just as seals keep pressurized fluids and gases in piston and cylinder systems, there are components that are designed to do the exact opposite — keep contaminants out.
In the sealing industry, the three main types of components used to keep dust at bay are wipers, excluders, and scrapers. While each are a bit different, they all serve the same basic purpose, and are fitted on the exterior side of the main seals in a system.
The exact type of dust-prevention mechanism you need depends on what exactly you’re trying to protect against. Here are three different types below:
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.
Boundary seals that help keep a certain environment sealed in while keeping the world out are everywhere.
If you look around your home, you may be surprised to see there are seals surrounding every door — and not just at the bottom. Your oven, microwave, and of course refrigerator door all have seals around them.
All these seals are different, yet they perform the same function. Your microwave is especially interesting, as its primary purpose is to keep microwaves from escaping the chamber that’s cooking your food. Your refrigerator seal has a magnet built into it, which keeps the door sealed shut.
Boundary seals are also found in many cell phones and electronic devices, keeping them water-resistant or water-proof (depending on the manufacturer). And in the industrial world, we have seals to create explosion-proof boxes in hazardous environments. The simple O-ring is found at the end of every cylinder cap to keep fluids in and the environment out.
Bonded door seals for gate valves and slit valve door seal applications provide improved sealing performance versus conventional O-rings by reducing particle generation, extending seal life, and minimizing replacement time during preventive maintenance.
DuPont™ Kalrez® bonded door seals are designed for easy installation and low particle generation. They combine a custom seal design and proprietary adhesion technology along with the excellent plasma resistance of Kalrez® perfluoroelastomer seal materials developed for semiconductor applications. The seal is held
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 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.
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.
The Channel Seal (or Cap Seal, as it’s often referred to), was one of the earliest forms of Polymer or Teflon sealing in the seal industry.
The product is easily applied. It didn’t replace the O-ring, but instead offered improved life while reducing drag.
In doing so, hydraulic and pneumatic systems operated cooler and quieter, while improving overall performance of the product.
Before the Channel Seal, the Backup ring was established. The first Backup rings started out as leather, as this material was readily available and could be easily formed into any shape with simple dies to stamp the Backup ring out.
Back up rings provided support for the O-ring, allowing the O-ring to operate at higher pressures, while closing off the Extrusion or “E” gap. This stopped the O-ring from being nibbled in the extrusion gap, therefore extending the life of the O-ring.
Teflon Backup rings were a big improvement, as they would better fill the gap and would stay put (as opposed to leather, which tended to shift in the groove). With the use of two Backup rings, an O-ring was well supported from pressure in both directions.
It was a simple matter to connect the two Backup rings with a thin membrane of Teflon, which removed the O-ring from the sealing surface. This change reduced drag and improved performance, while still maintaining an excellent mechanism for extrusion resistance.
This design was relatively simple to machine out of Teflon, but installation was a challenge, as the Backup rings were full depth. This caused the seal to become distorted during the install process. Today, we almost never see this type of design.
With CNC machining, the ability to nestle, and an O-ring design in a complex Teflon shape, it gave rise to what is referred to today as the Channel Seal, or Cap Seal.
This style seal offers an abundance of advantages over standard back-up rings and the early version of the Channel Seal, which was simply a Backup ring with the membrane of Teflon in-between.
The function of a V-Ring seal, or V-Ring, is to act as a centrifugal seal acting against the bearing face, pushing dirt and contaminants away from the bearing area. V-Rings are not designed to seal against fluids or pressure differentials. However, as stated above, they are excellent at excluding all sorts of contaminants. They provide effective protection against loss and maintenance, reduce wear, increase the life of the retainer and bearings, and also work well in dry running applications.