elastomeric seals
- December 11, 2019
Back in the mid 70’s, an engineer named Roy Edlund of Busak & Luyken designed a high-pressure seal that had an uncommon effect of rocking in the groove. This action occurred when pressure was created on the retract side of a cylinder as the rod was being retracted into the cylinder.
The material used for the seal was generally a bronze-filled Teflon, which could resist extrusion and have a long seal life. Because the seal ring was made from a grade of filled Teflon, a small amount of oil would leak under the lip as the cylinder was being extended.
One of the most unique features of this style seal was that as the rod was being retracted back into the cylinder, the buffer ring would rock or rotate slightly to the low pressure side thereby forcing leaked oil back into the retract side of the cylinder under the buffer ring.
This seal is commonly called a Buffer Ring (for reasons we’ll explore in this blog), but this seal helped usher Teflon into most high-pressure hydraulic systems today.
Pre-Buffer Ring Sealing Problems
Manufactures of high-pressure hydraulic systems in equipment, such as back hoes or hydraulic cranes found that their products were having seal failures prior to reaching warranty. This resulted in downtime and large warranty expense to repair these cylinders in the field.
In normal operations, the standard U-Cup made from a variety of Urethanes did an excellent job of creating a “near” zero leak sealing system. The problems would occur as the “bulk” oil temperature rose due to usage, pressure spikes in the system would cause premature failure of the Urethane U-Cup.
It was the pressure spikes that usually wreaked havoc with the U-cup seal design, causing the urethane to break down and eventually crack, creating a leak, and resulting in equipment shut-downs.
The Buffer Ring Solution
The Buffer Ring turned out to be the answer. By adding another sealing element in front of the Urethane U-cup, the life of the U-cup was greatly extended, overall friction in the system was reduced, and the bulk temperature in the hydraulic system was lowered.
All these advantages came by adding a sealing element. The true savings showed up in dramatically improving equipment up time. This also reduced warranty costs of equipment to the OEM.
How Does the Buffer Ring Work?
The answer to this question was initially difficult for many manufacturers to understand. Normally, putting one seal in front of another should cause a pressure trap, sending pressure loads much higher than relief valve settings, which locks up the cylinder.
The secret was in the way the Buffer Ring performed its job.
The seal leaking is very important to its design. If oil didn’t reach the U-Cup, the U-cup would generate heat and begin to wear out prematurely. So, since the Buffer Ring allowed a small amount of fluid to seep under the lip, this fluid lubricated the U-cup and kept its friction to a minimum.
Being elastomeric in nature, the U-cup did an excellent job of wiping the rod nearly completely dry.
But what about the pressure trap?
The Buffer Ring and its unique quality allowed fluid back into the system. Testing verified that this would happen anywhere from zero to about 100 PSI.
The U-cup spent most of its life in a well-lubricated, low-pressure / low-temperature environment compared to the original design.
The Buffer Ring also made suppliers of U-Cups extremely happy, as their failed seal had been given new life compared to the holes blown through the back of their urethane product.
- November 20, 2019
Wettablility of the Sealing Lip
The optimum function of rotary shaft seals depends on many factors. One of them is the "wettability" of the sealing lip. This parameter plays a particularly important role with synthetic lubricants such as polyglycol. If wetting is too low, not only does wear on the sealing lip increase, but the contact with the rotating shaft can also damage the shaft itself. The engineers at Freudenberg Sealing Technologies (FST) and Freudenberg Technology Innovation (FTI) have developed a new coating that forms a flexible bond with the elastomer of the seal and significantly improves wetting with synthetic lubricants.
Thanks to different materials and shapes, radial shaft seals for sealing rotating shafts can be used in a wide variety of industrial applications. What they all have in common is the demand for the lowest possible friction, low wear,
- October 01, 2019
Jay Berwanger, Heisman Winner.
To football aficionados, Jay Berwanger is well-known as the winner of the first Heisman Trophy and the first player chosen during the National Football League’s first draft. Less well-known is that he achieved his athletic successes at the University of Chicago, a school now more closely associated with Nobel prizes than big-time football. Berwanger, a halfback, played for the University of Chicago Maroons at a time when Chicago was a member of the Big Ten Conference–before Robert Hutchins, the University’s president, famously abolished varsity football in 1939.
Even in an era before football teams were divided up into offensive and defensive squads, Berwanger was renowned for his versatility.
In 23 varsity games in three seasons
- August 08, 2019
Valve Seals Help Control Oil Consumption and Valve Lubrication
Valves are an important part of regulation in any system, and their seals are designed to be used in different types of engines for controlling oil consumption, and valve lubrication.The design and manufacturing of the seal is the key to ensure seal performance and longevity.
Valves have many uses and are found in virtually every industrial process, including water & sewage processing, mining, power generation, processing of oil, gas & petroleum, food manufacturing, chemical & plastic manufacturing and many other fields.
Some examples of valve seals include: ball valve seats,
- August 06, 2019
Rubber seals are used in numerous industries to prevent the unwanted leakage of liquids and gases in various components such as pumps, valves, pipe fittings, and vacuum seals, to name only a few. However, all seals are not created equally. Rubber seal design consists of several elements to ensure that the seal delivers optimal performance in the given environment.
One of the most common types of industrial rubber seals, the O-ring, relies on mechanical compressive deformation to act as a barrier between mating surfaces, thus restricting the flow of fluid in predetermined areas. Several factors must, therefore, be taken into account in O-ring seal design to sustain the compressive force and maintain an effective seal.
Key Design Considerations
Rubber seals are available in a large number of material compositions, each with its own set of advantages and limitations. The selection of the appropriate material involves the consideration of specific factors including:
- July 23, 2019
Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.
Original content can be found on Parker’s Website and was written by members of the O-Ring & Engineered Seals Division. Jacob Ballard - research and development engineer, Jason Fairbanks - market manager, and Nathaniel Sowder - business development engineer.
- May 14, 2019
Seals are one of the most important components in many medical devices. While small in cost, seals for medical devices have a profound affect on the function of said device and the outcome of a medical procedure.
Engineered sealing solutions have advanced to meet the new medical device designs due both to new materials and to new processes for producing these seals. An understanding of the fundamentals of seal design, the tools available to assist in the manufacturing process and pitfalls to avoid will help in achieving a successful seal and medical device outcome.
Classifying the three basic seal designs
When approaching a new seal design, It is important to classify the seal based on its intended function. All seals fall into one of three distinct groups. While certain applications may combine more than one group, there is always one that is dominant. The three basic seal designs are:
Static -- seal applications where there is no movement.
Reciprocating -- seal applications where there is linear motion.
Rotary -- seal applications where there is rotation.
Static seal applications are the most common and include those that prevent fluids and drugs from escaping into or out of a medical device. The seal design can range from basic O-rings to complex shapes. Static seals can be found in the broadest range of medical devices from pumps and blood separators to oxygen concentrators.A reciprocating seal application with linear motion would include endoscopes that require trocar seals. These trocar seals are complex in design and allow the surgeon to insert and manipulate instruments to accomplish the medical procedure. These procedures range from relatively simple hernia repairs to the most difficult cardiac procedures. All of these minimally invasive surgeries employ endoscopes with seals that rely on seal stretch, durability and ability to retain shape during lengthy and arduous procedures. This particular seal application combines both reciprocating and rotary motion with the main function being linear motion.
A rotary seal application most commonly includes O-rings used to seal rotating shafts with the turning shaft passing through the inside dimension of the O-ring. Systems utilizing motors such as various types of scanning systems require rotary seals but there are many other non-motorized applications that also require rotary seals. The most important consideration in designing a rotary seal is the frictional heat buildup, with stretch, squeeze and application temperature limits also important.
Function of a particular seal design
What is the function of the seal? It is important to identify specifically if the design must seal a fluid and be impermeable to a particular fluid. Or will the seal transmit a fluid or gas, transmit energy, absorb energy and/or provide structural support of other components in device assembly. All of these factors and combinations need to be thoroughly examined and understood to arrive at successful seal design.
A seal's operating environment
In what environment will a seal operate? Water, chemicals and solvents can cause shrinkage and deformation of a seal. It is important therefore to identify the short and long term effects of all environmental factors including oxygen, ozone, sunlight and alternating effects of wet/dry situations. Equally important are the effects of constant pressure or changing pressure cycle and dynamic stress causing potential seal deformation.
There are temperature limits in which a seal will function properly. Depending on the seal material and design, a rotary shaft seal generally would be limited to an operating temperature range between -30° F and +225°F. To further generalize, the ideal operating temperature for most seals is at room temperature.
Expected seal life - How long must the seal perform correctly?
- May 07, 2019
Dynamic Sealing Applications
This article will discuss how we understand and control friction in dynamic sealing applications.
It’s easy to stop a leak in a system by just welding it shut. But when you create a dynamic application, you generally have a limited amount of power to move the device you’re sealing.
Friction is a force that must be overcome in all moving pieces. Controlling friction allows us to make efficient equipment that can have a long wear life and move with a limited amount of force.
There are many factors that drive friction up or down in a dynamic application. Although this blog will focus on shaft seals, the same considerations apply to piston or face seals.
Below we’ll cover the following factors and how they affect the friction calculation in our seals:
- Shaft material, hardness, and finish.
- If the system will operate when lubricated or dry.
- The system pressure or vacuum.
- System operating temperature
- Seal material and the types of fillers.
Seal Substrate
As a seal supplier, we usually like shaft materials to be hardened steel with surface finishes that are highly effective. Hardness above 50 Rc usually gives long wear life.
Having a good finish of 8 Ra. will insure long seal life and carry lubrication. However, depending on the application, there are times when a super finish of 2 or 3 Ra is justified.
Depending on shaft loading, there are many choices of surface finish that can reduce friction and improve the life of the seal. Understanding the bearing load under the seal helps to understand what finish is required to withstand the operating conditions.
There are some finishes that are detrimental to seal life. An example is a heavy chrome surface that looks sturdy, but usually can’t be ground smooth and is left with large peaks or valleys. Thin, dense chrome is often the opposite, giving good seal life if applied correctly. The engineers at Eclipse Engineering are prepared to make recommendations on hardness and finish.
- February 07, 2019
Article re-posted with permission from Parker Hannifin Sealing & Shielding Team.
Original content can be found on Parker’s Blog.
You’ve probably heard a bit about microwave absorbers and how they are used to reduce or absorb the energy that is present in a microwave. But what are they exactly? And how do they work? Go ahead, read on.
What are microwave absorbers?
Simply put, microwave absorbers are special materials, often elastomer or rubber based, which are designed to offer a user-friendly approach to the reduction of unwanted electromagnetic radiation from electronic equipment. They also work well to minimize cavity to cavity cross-coupling, and microwave cavity resonances. When comprised of a silicone elastomer matrix with ferrous filler material, microwave absorbers provide RF absorption performance over a broadband frequency range from 500 MHz to 18 GHz.
- January 31, 2019
Elastomer Technology in Mechanical Seals
Evaluate properties of rubber during installation and seal life.
Elastomers (or rubbers) are a ubiquitous family of materials whose use stretches across nearly the entire range of mechanical seal designs. From plant-sourced natural rubber, so named by John Priestly in 1770 for its utility in rubbing away pencil graphite, to petroleum-sourced synthetic rubber first developed around the turn of the 20th century, the "elastomer" and their properties are familiar but should not be overlooked—especially when dealing with mechanical seals.
How Elastomers Work in Mechanical Seals
Rubber seals come in a variety of profiles—O-rings, cup gaskets, bellows diaphragms, sealing/wiper lips and many others. They are classified as either static or dynamic and create positive pressure
against surfaces to eliminate or control the leakage of liquids and/or gases while preventing the entrance of external contaminants such as dust and dirt. Static sealing occurs between adjacent surfaces with no relative motion, such as between the pump casing and cover. Due to frictional wear and heat generation, dynamic sealing is less straightforward, occurring between adjacent surfaces that are continuously or intermittently moving relative to another, such as between the pump casing and shaft.In mechanical face seals, elastomers tend to take second chair because the primary seal—the dynamic seal between the housing and rotating shaft—is achieved by sliding contact between the pair of stiffer, lapped-flat sealing faces, one stationary in the housing and one rotating with the shaft. In many designs, rubber provides the secondary seal between each seal face and adjacent surface. One seal face is fixed and sealed statically using an O-ring or cup gasket. The other is spring-loaded and requires a semi-dynamic seal to accommodate some axial play, such as a dynamic O-ring in pusher-type mechanical face seals or elastomeric bellows in nonpusher ones. These semi-dynamic applications (involving flexing and sliding of the elastomer) can be critical for maintaining proper contact between the faces through face wear, shaft movement, etc.
Although the seal face pair tends to be the most critical design feature, mechanical face seals are often used in the most demanding applications.
Rubber technology features prominently in radial lip seals, where typical applications have lower pressurevelocity (PV) values relative to those involving mechanical face seals. Still, the flexible elastomer lip must handle considerable relative motion in the form of shaft/bore rotation, reciprocation or a combination of both. In addition to standard designs and sizes, numerous customizations and proprietary approaches exist. The simplest designs rely on a single rubber lip’s inherent resiliency, although common enhancements include multiple sealing lips, a circumferential garter spring installed in a groove over the sealing lip to maintain contact with the shaft, and an auxiliary wiper lip or “excluder” to prevent abrasive dust or debris from compromising the primary sealing surface. For improving service life and performance in rotary applications, unidirectional or bidirectional hydrodynamic pumping aids can be added in the form of custom-shaped extrusions on the backside of the sealing lip to return leaked fluid to the sealing interface, increase lip lubrication and lower operating temperatures.
Benefits of Rubber
The definition of an elastomer provides initial insight into where rubber gets its resilient sealing quality: “a macromolecular material which, in the vulcanized state and at room temperature, can be stretched repeatedly to at least twice its original length and which, upon release of the stress, will immediately return to approximately its original length.”
When the rubber is squeezed by the adjacent surfaces of the clearance gap to be sealed, it has the characteristic
properties of malleably deforming and taking the shape of each surface in response to the stress and applying a force back against the surfaces in its attempt to return to its original dimensions. Elastomers consist of large molecules called polymers (from the Greek “poly” meaning “many” and “meros” meaning “parts”), which are long chains of the same or different repeating units, called monomers, usually linked together by carbon-carbon bonds (the
most notable exception being silicone elastomers, which are linked by silicon-oxygen bonds). Soft and hard plastics are also composed of polymers. However, the regularity of the monomers in their polymer chains allows neighboring segments to align and form crystals, making the macromolecular plastic material rigid and inelastic.One can prevent this crystallization by breaking up the regularity of the polymer chain, resulting usually in a viscous “gum” that is readily shaped into molds. At the molecular level, the polymer chains are similar to spaghetti-like strands flowing past each other.
During the process of vulcanization, or curing, the viscous liquid is heated with sulfur or peroxides and other vulcanizing agents, and crosslinks form between polymer chains, tying them together with chemical bonds, converting the gum into an elastic, thermoset solid rubber that retains its shape after moderate deformation.
In addition to the selection and preparation of base polymer(s) and cure system ingredients, formulating the final rubber product, also known as compounding, involves five other broad categories of ingredients, which have percentage compositions expressed in parts per hundred rubber (phr). Fillers include various powders that thicken the polymer mixture, improve strength and resistance to abrasives, and reduce final cost. Plasticizers are oils and other liquid hydrocarbons that lower viscosity to ease processing, soften the final compound and in some cases improve low temperature performance. Process aids are specialized chemicals added in low concentrations to improve mixing, flow properties and final appearance.
Antidegradants protect the rubber from environmental attack. Finally, various miscellaneous ingredients may be added for special purposes, including foaming agents, dyes, fungicides, flame
retardants, abrasives, lubricants and electrically conductive particles. A simplified description of processing these ingredients includes mixing via tangential or intermeshing mixers, forming into desired shapes and vulcanizing into the final product.