Seals can be static seals that provide sealing between non-moving parts, or dynamic seals such as on an actuator piston or switch plunger. Historically, seal and groove geometry and overall seal requirements have been specified in MIL-G-5514. However, this document is no longer used for new military designs but is still available as a reference. MIL-G-5514 has been replaced by a SAE document
Today, the best source of data on types and seals and their applications are seal manufacturers. Seal material (specifically material type, hardness and friction characteristics) has a big impact on seal performance over time.
O-rings are the most common type of seal and have been used in both static and dynamic applications. However, in dynamic operations o-ring seals can roll, tear and may have reliability issues. Other types of seal configurations include T-seals, H-seals (Hat Seals), L-seals, wedge or triangular type seals, plus others. A T-seal cross-section is shown in Figure 1.
Figure 1 T-Seal Cross Sectional View (Dynamic Seal)
Associated with most seal installations are backup rings. Backup rings provide support for the seal and do not usually provide any sealing. The also function to keep the surfaces clean so that small debris (hydraulic contamination) does not damage the seal.
When evaluating a seal for a particular application, the main design variables are fluid compatibility, operating pressures, expected impulse pressures (levels and cycles), design life goals, external environment, desired friction levels and leakage. Service history data for a particular seal configuration in a similar environment can assist evaluations. Seal manufacturer data and experience is also very helpful. Selected seal arrangements should be tested in a thorough qualification test program, which includes endurance testing in an environmental chamber.
In applications where seal reliability is critical, dual seals can be used in selected locations. The dual seals can be different designs, for example, a T-seal with a backup wedge seal. Seals should be qualified individually to establish the robustness of both seal designs in the application. The disadvantage of dual seals is increased friction (heat generation), increased assembly time and cost.