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.
