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

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.


black o-ringsCompressive 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.

Figure 1: Typical CSR curve. These results display a fluorocarbon seal material immersed in engine oil at 150°C.

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

How to Properly Choose Commercially Available O-Ring Cross Sections

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.
Continue reading How to Properly Choose Commercially Available O-Ring Cross Sections

Metal Detectable O-Rings, A Must Have in the Food & Beverage Industry to Avoid Product Recalls

Metal Detectable O-Rings are a Must-Have in the Food & Beverage Industry

Metal Detectable O-Rings are used in equipment that produce and package food. During service, o-rings are exposed to temperature variations and corrosive clean-in-place (CIP) chemicals used during equipment sterilization.

picture of food and beverageOver time, o-ring materials can break down and fail during operation causing material fragments from the o-ring to break off and enter the food stream. Without a way to find material fragments quickly, machine operators must shut down equipment to locate the contaminate source resulting in lost production time and loss of revenue.

Metal detectable o-rings from DICHTOMATIK are formulated with the specific rubber and metal combination that allows for easy detection using metal detection machines that locate and remove contaminated product quickly or by using metal separators. Having an efficient inspection process to monitor product consistency and quality allows for worn o-rings to be found and replaced quickly to avoid product recalls further downstream once the product is placed on the shelf.

DICHTOMATIK O-Rings & Sealing Solutions That Meet FDA Standards

The DICHTOMATIK complete catalog of food grade rubber seals meet FDA and food safety standards outlined under the Food Safety Modernization Act (2011).

Available Materials

  • Blue – FKM
    • [Best Use Running Condition] *Where  high acid and temperature resistance is critical
  • Yellow – EPDM
    • [Best Use Running Condition] *Offers very good water and steam resistance
  • Green – NBR
    • [Best Use Running Condition] *Use when working with oil and animal fats
  • Orange – Silicone
    • [Best Use Running Condition] *Operate within a wide temperature range

The original blog article was written by the marketing team at DICHTOMATIK and can be found here.

Gallagher Fluid seals is an authorized distributor of DICHTOMATIK seals. For more information about their products or to find a solution that works for you, contact our engineering team.

Vesconite Steps Up to Help During Coronavirus Pandemic

A message from one of Gallagher’s valued suppliers:


Vesconite Willing to Manufacture Ventilators

The Coronavirus is likely to result in a need for hundreds of thousands of ventilators worldwide: data shows 5-10 % of people infected need to be supported on ventilators for two to three weeks.

Vesconite has extensive machining capabilities. With 70 CNC lathes and machining centers, they can manufacture a wide range of mechanical components.

Though they are experts in bearings and bushings, they know nothing about making ventilators… but they stand by to help those in need.

If you have ventilator expertise or know a company that does have the expertise and would like to have Vesconite become involved, please fill out this form. Join with them so that they can help to manufacture the thousands of ventilators that are needed.


From the CEO of Vesconite, Dr Jean-Patrick Leger:

South Africa, where Vesconite is headquartered, has a population of 58 million. If 10% of the population is infected (5.8 million), an estimate (based on Italy with 7% of cases critical) is there may be a need for 400,000 ventilators.

To listen to a report from “the front line”, here is a New York Times interview with the head of the respiratory unit of an Italian hospital:


Gallagher is an authorized distributor for Vesconite products. For more information, contact Gallagher Fluid Seals today.

Metal Detectable & X-Ray Detectable Rubber Materials

Food, Beverage, and Pharmaceutical Regulations

picture of metal detectable o-ringStringent government regulations mandate that food, beverage, and pharmaceutical manufacturers keep foreign material out of ingredients to ensure food and drug safety for consumers. Preventing foreign material from entering the processing stream is of the utmost concern but there must also be measures in place to detect contaminated product and quarantine it before distribution.

Component parts that are used in food and drug processing equipment can become damaged by improper installation and/or excessive shear experienced during operation that causes fragments of rubber, plastic, and metal to contaminate ingredients. Chemicals used for cleaning and sterilization of equipment can cause rubber seals to degrade, increasing the probability of particles breaking off and entering the consumable products. Part failures causing product contamination can lead to machine down time, scrap product, product recalls and result in legal problems and negative media attention. All of which have a significant financial impact and can compromise brand loyalty within the market.

Hazard Analysis Critical Control Point (HACCP)

picture of precision metal detectable o-ringsMany processing operations now employ HACCP (Hazard Analysis Critical Control Point) programs which stipulate that all parts have to be metal detectable and X-ray detectable. This made it necessary to develop special rubber materials that would allow food processors to conduct routine inspections for this type of contamination utilizing in-line metal detectors and X-ray machines. Rubber must be compounded with special additives to make detection possible. However, certain foods have phase angles similar to metal detectable rubber so a complete understanding of the rubber product’s application is necessary for proper compound selection.

Metal Detectable O-Rings | X-Ray Detectable O-Rings

Precision Associates has developed four Metal and X-Ray detectable materials made with ingredients sanctioned under FDA Title 21 CFR 177.2600.

All four materials are 3A Sanitary 18-03 approved and are available in Silicone, Nitrile, EPDM, and FKM. Each is 70 durometer and blue in color. (The industry standard color is blue but materials can be colored for specific customer requirements and any polymer can be made metal detectable).

All compounds were tested by an independent laboratory and found to have magnetic properties that exceed industry standards.

picture of compound table precision o-rings


The original article was written by Precision Associates, Inc. and can be found here.

For more information about what Gallagher can offer through Precision Associates, or to talk to a technical sales expert about these materials, contact us today.

Heisman Winner… and Rubber Salesman?

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.

Berwanger PosingIn 23 varsity games in three seasons with the Maroons, he scored 22 touchdowns and kicked 20 extra points. He gained 1,839 yards on 439 rushes for a 4.2 average. As a sophomore, he played 60 minutes of every Big Ten conference game and was voted the team’s MVP. During his collegiate career he returned 54 kickoffs and punts for a 31.8-yard average, completed 50 of 146 passes for 921 yards and caught 12 passes himself for 189 yards. He averaged 38 yards on 233 punts and 46.3 yards on 34 kickoffs. He once recorded 14 tackles playing linebacker in one half.

In 1935, the Chicago Tribune awarded Berwanger the Silver Football for Most Valuable Player in the Big Ten. His coach, Clark Shaughnessy, called him “every football coach’s dream player. You can say anything superlative about him, and I’ll double it.” Of the 107 opposing team players he faced during his senior year, 104 said the six-foot, 195-pound Berwanger was the best halfback they had ever seen.

In November of 1935, Berwanger received a telegram from Manhattan’s Downtown Athletic Club, informing him that he had won a trophy for being the “most valuable football player east of the Mississippi,” as well as a trip for two to New York. “It wasn’t really a big deal when I got it,” Berwanger recalled in 1985. “No one at school said anything to me about winning it other than a few congratulations. I was more excited about the trip than the trophy because it was my first flight.” The prize was later renamed the John W. Heisman Memorial Trophy, after the club’s athletic director, the following year.

He was born John Jay Berwanger in 1914 in Dubuque, Iowa. In high school, he excelled at wrestling and track as well as football, winning renown as an all-state halfback. After graduation, Iowa, Michigan, Minnesota, and Purdue all tried to recruit him, but he opted for Chicago, which had offered only a basic tuition scholarship of $300 a year. To meet his expenses, Berwanger waited tables, cleaned the gymnasium, fixed leaky toilets, and operated elevators. “Times were tough then,” he said. “I wanted to attend a school that would give me a first-rate education in business, without special treatment, so that I would be prepared when opportunities were certain to return.”

During his freshman year, Berwanger was coached by the legendary Amos Alonzo Stagg, who helped define the game of football as we see it today. Berwanger’s first year at Chicago was Stagg’s last. As well as captain of the football team, Berwanger was captain of the track team, senior class president, and head of his fraternity, Psi Upsilon.

Berwanger was the only Heisman recipient who was ever tackled by a future president of the United States–Gerald Ford, during a 1934 game between Chicago and Michigan. “When I tackled Jay in the second quarter, I ended up with a bloody cut and I still have the scar to prove it,” President Ford recalled. “Jay was most deserving of his Heisman Trophy. He could do it all. He was an outstanding runner as well a passer, he could kick, punt, and make field goals–and in those days the ball was round so it was much harder to throw. He and I had met several times in the years since that game and I remember him fondly as one of the greatest athletes I’ve known.”

Berwanger was also the first player chosen for the National Football League during its first-ever draft in 1936. After the Philadelphia Eagles signed him, Coach George Halas of the Chicago Bears acquired the signing rights. But when Berwanger asked for $25,000 over two years, Halas decided that was too much money, so Berwanger took a job as a foam-rubber salesman.

Jay Berwanger, Rubber Salesman and Lieutenant Commander

Shortly after starting his job as a rubber salesman, Berwanger enrolled in the Navy’s flight training program and became a Naval officer during World War II. He eventually earned the rank of lieutenant commander.

After the war, Jay Berwanger founded Jay Berwanger, Inc., a manufacturers’ sales agency specializing in rubber, plastic, urethane and other elastomeric materials for car doors, trunks, and farm machinery. His company established a guiding philosophy to create superior value for all customers and principals through dedicated service and by providing integrated solutions to customers’ applications.  Jay sold his company in the early 1990’s when its annual revenue was $30 million.

Post-Heisman and Rubber

Berwanger Heisman and FootballBerwanger was never sure what to do with his Heisman Trophy, which was too wide for a mantelpiece and too large for a coffee table. For years, his Aunt Gussie used it as a doorstop. Berwanger eventually gave the trophy to the University of Chicago where it is on display.

In 1954, Berwanger was inducted into the College Football Hall of Fame. In 1989, he was included on Sports Illustrated’s 25-year anniversary All-America team, which honored players whose accomplishments extended beyond the football field.

Berwanger died in the summer of 2002 and is survived by three children, three step-children, 20 grandchildren, and 13 great-grandchildren.


Information for this article was gathered from Sports Illustrated’s vault, the University of Chicago, and New York Times.

Gallagher has no relationship or partnership with Jay Berwanger Inc. If you liked this article or have an idea about another topic, please let us know!

The Proper Use of Lubrication and its Success with Bolted Flange Connections

Extra energy transmitted can produce larger sealing stress on the gasket.

The bolt as a screw is one of the six simple machines. A simple machine magnifies or changes the direction of an input force. By means of mechanical advantage, a bolt can dramatically increase its input force. Take for example an 8-bolt flange with 3/4-inch diameter bolts. By manual effort alone, a person can easily develop a total bolt load of over 110 tons. This article explains the mechanics by which the mechanical advantage is possible and then draws attention to how friction can deteriorate the end effect of a bolt’s mechanical advantage.

The means by which a bolt can greatly magnify input load is by leverage. One source of this leverage is afforded by the geometry of the bolt. The other is the leverage from a tightening tool—in this example, a torque wrench. To illustrate the mechanics of a screw thread, consider a 3/4-inch diameter, Unified Course (UNC), A193 B7 bolt with a yield strength of 105,000 pounds per square inch (Image 2 identifies the geometrical characteristics of a bolt that create leverage).

The pitch circle is defined as the circle that passes through the pitch line of the threads. The pitch line is the theoretical point of contact between the male and female threads.

Threads of a bolt
Image 1. The threads of a bolt essentially form an inclined plane

The pitch is the distance between thread crests, and is the axial distance the bolt travels in a single 360-degree turn. The threads of a bolt essentially form an inclined plane wrapped around the minor diameter of the bolt (Image 1).

The horizontal distance (pitch circle) traveled divided by the vertical distance traveled (pitch) is the mechanical advantage of the bolt geometry.

For the 3/4-inch bolt, the values for the pitch (P), pitch circle diameter (Pd) and pitch circle circumference (Pcc) take on the following values: 0.10 inch, 0.6850 inch and 2.152 inch, respectively. The helix angle becomes Tan-1 (P/Pcc) = 0.266 degrees.

Image 2. Geometrical characteristics of a bolt that creates leverage

The pure mechanical advantage of the threads, designated as MAt, is the horizontal distance traveled divided by the vertical distance traveled, in Image 2. Its value becomes MAt = 2.152 inch/0.10 inch = 21.52:1. Relative to the pitch circle, 1 pound of input force would create almost 22 pounds of output force.

Now evaluate the additional mechanical advantage of a torque wrench. Presume the use of a 3/4-inch drive, 48-inch-long torque wrench with an effective length (based on the pull-point of a person’s hand) of 44 inches. The mechanical advantage of the wrench is its effective length (Le) divided by radius of the pitch circle.

Now calculate the pure mechanical advantage of the wrench (MAw) as Mw= Le/(Pd/2) = 44 inches/(0.6850/2) = 128:1. A single pound of input force on the torque wrench results in 128 pounds of output force.

To get the total pure mechanical advantage of this system, combine MAt and MAw. This is done by taking the product of the two values.

The total pure mechanical advantage (Mtot) of the two effects then becomes Mtot = Mt x Mw = 21.5 x 128 = 2,752:1. If the mechanical advantage actually attained this value, 15 pounds of input force would yield the bolt.

Obviously, it does not. The reason is, pure mechanical advantage of a system can never be attained. There are always (energy) losses.

In the instance of a bolt being torque tightened, those losses can be extraordinarily high. The most important energy loss is friction. To better understand how dramatically friction can negatively impact a bolt’s clamping force, evaluate its affect using a common, long-form version of the torque equation. Tlbf is the calculated torque to attain a bolt load, FB.

The first term in Equation 1 is the useful input energy that goes to stretching the bolt. The second and third terms account for the energy being lost to friction.


T = [(pt/2 π) + (utdt/2 cos(a)) + (un dn/2)] FB/12
Where:
pt = pitch of the bolt thread = 0.10 inch
dt = mean contact diameter of thread = 0.685 inch
dn = mean contact diameter on nut spot face = 0.8738 inch
a = half thread angle = 30 degrees
ut = friction coefficient on threads = 0.13
un = friction coefficient on bearing surface (spot face) = 0.08
FB = value of (single) bolt load = 17,561 lbf.
T = torque, in. lbf.
Equation 1


Specifically, the second term is the energy lost to friction between mating threads, while the third term accounts for the energy lost to bearing surfaces during the tightening process. Now evaluate these terms for the example bolt.

In addition to bolting geometry values, consider the published values of friction coefficients for a commonly used paste, lubricant. The definition and values for each variable are noted in Equation 1.

Substituting these into the respective terms they are evaluated and compared in Image 3. Term 1 is the useful work in creating the clamping force.

Image 3. Input energy going into stretching the bolt, and energy lost to friction

The sum of the other two terms is the energy loss to friction.

In this particular instance, with the presumed coefficients of friction, only ~16 percent of the applied energy is converted to the useful work of creating bolt stretch. A total of 84 percent of the tightening load is lost to friction. This explains why only a small portion of the system’s mechanical advantage is realized. The effect of friction should not be underestimated. A torque wrench measures the value of torque being input to the bolt system. It does not reveal how much useful energy is actually being delivered to the bolt.

Clearly, the proper use of lubrication can have a dramatic effect on the success of a bolted flange connection. The liberal use of lubrication is one of the easiest, least expensive, quickest and most effective ways to ensure the targeted clamping load is realized. In the case of gasketed, bolted flange joints, the extra energy transmitted produces a larger sealing stress on the gasket and ultimately results in few emissions and a cleaner, safer environment.


For more information about the right sealing solution for your specific application, contact Gallagher Fluid Seals.

This original article was features on Pumps & Systems website and was written by Randy Wacker, P.E., consultant for Inertech Inc.

Symptoms of Bad Valve Seals

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.

bad valve seals symptomsValves 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, globe valve discs, stem packing, stem seals, valve discs, valve packing, valve seals, and valve stem packing.

Having a proper valve seal can save you thousands of dollars in repairs at the end of the day, so it’s important to check them semi-regularly. For example purposes, we’ll focus on cars, but this can be translated across a variety of systems and industries. Here are some symptoms of a bad valve seal that may need to be replaced:

Performing the Cold Engine Test

One sure-fire way to tell if you have a faulty valve seal is to perform a cold engine test. When your vehicle has been sitting overnight or for a longer period of time, the top of the head of the valve cover will have some oil left over from the last time you drove. When you start the engine, the oil ends up getting sucked down through the bad seal into the combustion area, producing a blueish smoke out of the tailpipe. This may indicate that your valve is not securely sealed and that it’s time to get a new one.

Idling

Another way to test a bad valve seal is to be aware of what happens while your vehicle is idling. When your vehicle is stopped for a significant amount of time, high vacuum levels will cause the oil to build up around the valve system while it is closed. In a faulty valve seal situation, when you begin to accelerate again, this oil can end up getting sucked past the seal an into the valve guide. This causes more of this blueish smoke, due to the burning of oil, to come out the tailpipe.

High Levels of Oil Consumption

High levels of oil consumption is another indicator that you have a bad valve seal. This is because oil is being leaked out or burned excessively and causing oil to decrease at a higher rate than normal. You can detect this loss of oil with a basic oil dipstick and keeping a regular log of oil levels. If no oil leaks can be found around the vehicle, you may still have a bad valve seal, as the oil will likely be burned up causing excessive smoke.

High Levels of Smoke

Another indicator of a faulty valve seal, as mentioned above, is the high presence of smoke. It’s common for some exhaust smoke to be present when you first start your vehicle, but if it begins to last longer than normal, your valve seal may be deteriorating. In addition, if you have a bad valve seal, the excessive smoke will tend to come in waves as an indicator of oil burning.

Engine Braking Test

Engine braking is when other ways besides external braking are used to slow down your vehicle within an engine. When you have a bad valve seal, the oil that collects at the front cover of the head will end up burning when you push on the accelerator after coasting for a while. This is apparent especially when going downhill and again will be indicated by the excessive smoke that leaves the tailpipe. The oil here burns longer than in normal cases.

Acceleration Power is Compromised

The final indicator of a poor valve seal is a lack of acceleration power. You can also perform a compression test to see if this is the case. A higher level of compression will indicate that it’s a valve seal problem, while a low level of compression will indicate a piston ring problem. These two areas can be very similar in their faulty symptoms so it’s best to be informed on their differences.

A badly designed seal can result in engine oil flooding, which can eventually cause a breakdown. Gallagher Fluid Seals understands the importance of a well-designed industrial seal and can help design a custom solution for you, or supply you with standard off-the-shelf seals from the world’s top suppliers.


For more information about valve seals what why they fail, or to find solutions, contact Gallagher’s engineering department.

The original article can be found on Real Seals’ website.

The Complete Guide for Mechanical Seals & API 682 4th Edition Piping Plans

Mechanical Seals & API 682 4th Edition

A sealing system, consisting of a mechanical seal and an associated supply system that is balanced by individual applications, is the utmost guarantee for a reliable sealing point and uninterrupted pump service. The performance of the seal is greatly influenced by the environment around the seal faces, making the provision of suitable, clean fluids as well as a moderate temperature an essential topic.

This guiding booklet provides a condensed overview of all piping plans established by the API 682 4th edition guidelines. Each illustrated piping plan is briefly described, and a recommendation that considers the media characteristics in terms of the relevant application and corresponding configurations is given to help you reliably select your sealing system. Furthermore, the content of this booklet has been enriched by providing clues – so-called ‘remarks and checkpoints’ – where EagleBurgmann shares the experiences gained from multiple equipped plants.

Sealing solutions to meet any requirement

Several factors play a major role when choosing the product, the product type, the materials used and how it is operated: process conditions at the sealing location, operating conditions and the medium to be sealed.

No matter what requirements our customers have, EagleBurgmann understands how these factors affect functionality and economic viability, and they translate this expertise into outstanding long-term, reliable sealing solutions. EagleBurgmann has all the expertise needed to manage and support the entire development, life and service cycle of its sealing solutions.

Plan 75 Piping Plan Example

EagleBurgmann and API 682

EagleBurgmann offers customers the widest product portfolio of seals and seal supply systems according to API 682 4th edition. The configurations listed for each individual piping plan are to be understood as recommendations including possible utilizations which may also be applied.

EagleBurgmann Profile

EagleBurgmann is one of the internationally leading companies for industrial sealing technology. Their products are used wherever safety and reliability are important: in the oil and gas industry, refining technology, the petrochemical, chemical and pharmaceutical industries, food processing, power, water, mining, pulp & paper and many others. More than 6,000 employees contribute their ideas, solutions and commitment towards ensuring that customers all over the world can rely on their seals and services. More than 21,000 EagleBurgmann API-seals and systems are installed world-wide.

What to Know, Avoid, and Consider When Planning Seals for Medical Devices

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.

trocar design
New advances in trocar designs incorporating specialized seals allow multiple instruments to be inserted in the single trocar.

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?

Continue reading What to Know, Avoid, and Consider When Planning Seals for Medical Devices