Depending on the relative motions acting upon them, rubber seals and rubber gaskets can be classified as one of two types.
A static condition exists in a sealing application when there is little or no motion of the mating assembly parts upon the seal itself. This type of seal typically has only an initial compression (the sealing of a hose joint), or periodic compressions caused from opening and closing (door seal in a house).
These seals, when compressed, will cause pressure to act in a vertical direction relative to the seals face, thus squeezing between two opposite surfaces to eliminate any gaps between them. This is common in both o rings and face gaskets.
In undertaking a static seal application, the compression forces should be properly assessed. The recommended compression range for static rubber seals is 10-30% with an upper limit of 50%. If the compression is too high, a variety of design elements can be added to alleviate any difficulties.
- The part can be looked at to be made in sponge or foam
- A lower durometer can be pursued
- The walls can be thinned
Dynamic conditions exist when the seal experiences repeated motions against it in a circumferential, axial or angular direction. Typical seals in dynamic applications that experience repeated motions include piston ring seals and shock absorption seals.
Because of the repeated changing in stress levels, design considerations for dynamic sealing are often more complex than those of static seals. In addition to the static environment considerations, dynamic seals environments may require:
- Temperature changes or thermal cycles
- High tensile strength or tear resistance
- The ability to recover from distortions — being squeezed or stretched
- Recovery speed after compression
- Compression set parameters
- Abrasion resistance