Monthly Archives: July 2017

Dynamic Seal Applications

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When there is motion between a hardware component and a sealing element, you have a dynamic seal. Dynamic seal applications vary greatly. The motion can be oscillating, rotary, or reciprocating. A valve stem seal may have a combination of more than one type of motion. The most common single component elastomeric seals are below.

Reciprocating Seals

Applications which involve a moving piston and/or rod will use a reciprocating seal. These seals constitute the predominant dynamic application for O-rings. For the best performance of reciprocating seals, you need to consider compound selection for thermal cycling, pressure shocks, squeeze, and stretch.

O-rings may not provide a leak proof seal if they have to take compression at elevated temperatures and then fail to rebound at a low temperature. Thermal cycling from high to low may causes havoc. In low pressure, reciprocating applications O-ring leaks may occur. When it is expected that there will be extreme operating thermal cycles, you will want a seal compound that will exceed the temperature range, resilience needs, and compression set.

Hydraulic components can create system pressures that exceed seal extrusion capabilities when there is sudden stopping and holding of high loads. Pressure shocks should be addressed and dealt with in order to prevent extrusion and the eventual O-ring failure. If it is required there may have to be accumulators or pressure relief valves built into the hydraulic system. You can also use back up rings or increase the seal durometer to prevent O-ring extrusion.

You can refer to the table for general guidelines. If there if lower squeeze that what is shown in Table A, friction is reduced but that could possibly cause leaking in conditions with low pressure. If there is greater squeeze, friction is increased providing a greater ability to seal but it makes assembly difficult. The seal will also wear more quickly and the potential for spiral failure is amplified.

An O-ring’s cross section is reduced when the I.D. of the O-ring is stretched. Make you consider the O-ring’s smaller cross section so that it maintains the correct squeeze percentage. The stretch, in most cases, should not go over 5%.

Rotary Seals

Rotary seals need to be designed to specifications for the intended application. You need to consider application temperatures, seal stretch, frictional heat buildup, squeeze, and shaft and glandular machining. With a rotary seal application, there is a turning shaft that protrudes through the I.D. of a seal component.

Applications that require operating temperatures lower than -40F or higher than +250F should not use rotary shaft seals. The O-ring will effectively seal when the application is close to room temperature.

Frictional heat will be generated so it is a good idea to have a seal composed of compounds that have the maximum heat resistance and minimal friction generating properties. Typically, internally lubricated compounds will be used for rotary applications.

I.D stretch has to be eliminated in rotary seal applications. The shat diameters need to be designed so that they are not bigger than the free state I.D. of the seal. Rotary or oscillating applications should have shaft seals with no stretch over the shaft. Seals can fail if an elastomer is stressed and there are higher temperatures. The seal will contract instead of expanding and this will increase the heat until the seal fails.
O-ring squeeze should be 0.002” by using an O-ring with an O.D. of 5% larger than the gland. Peripheral compressing will put the O-rings I.D. in contact with the turning shaft which will minimize friction heat buildup. This will prolong the life of the seal.

The experts at Real Seal can design the seal you need for any application. Contact them to find out how more about dynamic seal applications. Visit Real Seal online to see the products that they have available and contact them to discuss your needs.

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The Evolution of Plastic

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What comes to mind when you think of the uses of plastic? Many people think of bags and packaging, plastic cutlery, containers, or other kitchenware, and toys. Not many of us think of sweaters, food, or wood furniture. Unbeknownst to many, all of these things are made from polymers; some are called plastics and some are not and the distinction can be arbitrary.

Plastics consist primarily of polymer molecules made of carbon. Similar to a bicycle chain, the links of the polymer are attached together. Polymers are a broad category that include plastics and silicones. Silicones are based on silicon rather than carbon. Polymers also include DNA.

The shape of polymers gives plastics their elasticity which allows them to be molded into shape. We have used naturally derived plastics for many years. In Mexico, the Olmecs played with balls made of rubber which is a natural polymer. They did this a millennium and a half before Christ. Uses of plastic go back even before the use by the Olmecs though. Wood contains cellulose which is a polymer that give plants tough cell walls. The cellulose gives wood its stiffness and durability. The long strands of cellulose are separated at pulping mills and that is what gives paper its strength.

Cellulose also provided the raw material for another breakthrough in plastic. Parkesine is a material that has been used in many products including cutlery, buttons, and combs. Two Americans took Parkesine and added camphor to it which made it more malleable and they renamed it celluloid. This became a raw material used in the film industry.

In the 18th Century, French explorer Charles-Marie de la Condamine discovered that there was a rubber tree in the Amazon basin. It was not until the 1840s that American Charles Goodyear and British born Thomas Hancock took out patents for vulcanized rubber – rubber which was treated with Sulphur to make it more durable.

Rubber also made electrification possible as it was used to insulate electrical switches. The big breakthrough which is considered the birth of modern plastics came in 1907 when Leo Baekeland invented Bakelite. Bakelite is one of the first plastics that is made from synthetic components; it was made from fossil fuels.

More synthetic plastics followed with polystyrene in 1929, polyester in 1930, polyvinylchloride and polythene in 1933, and nylon in 1935. The plastic industry grew during the war effort when they were used in everything including military vehicles and radar insulation.

Companies were building plants where they could turn crude oil into plastics. In 1948 new products such as Tupperware were becoming available.
Polyethylene terephthalate (PET) was a versatile product that was invented in 1941. It can be used to make many different things such as drink bottles, winter gloves, and plastic for packaging flowers. The only difference is the way the material is cast.

Properties and uses of plastics can be changed by altering their structure just a little bit. The plastic that a milk bottle is made of can be changed from polyethylene to polypropylene (a stronger material) simply by adding one carbon to it. The material made by adding a carbon can strengthen the plastic enough to make things like sippy cups for toddlers.

Some polymers are compostable and over months or years they will be broken down by microbes. Some plastics however do no break down and cause an environmental concern.

It may become more difficult to get plastics. Most currently come from oil and gas but these sources could run out and we will have to go back to using biological sources. Some plastics that have been derived from crude oil are now being produced from sugar cane.

Oil may become too expensive to use. At that point things may turn to industrial bio-technology for the manufacture of plastics.

Real Seal offers seal products and mechanical components. They are known for expertise in polyurethane materials but also have many performance oriented plastic and rubber solutions for sealing and mechanical applications. For more information on their products visit Real Seal on the internet today.