Things That Need To Be Considered When Making Sealing Decision
While small in cost, seals are often one of the most important components in a product. Seals must be carefully designed and produced to ensure superior product performance. This section provides a review of the issues that need to be considered when making sealing decisions.
All sealing applications fall into one of three categories:
1. those in which there is no movement
2. those in which there is linear motion or relatively slow rotation, or
3. those involving high speed rotation.
A sealing application in which there is no movement is termed a static seal. Examples include the face seal in an end cap, seals in a split connector, and enclosure cover seals.
A sealing application in which there is linear motion (reciprocation) or relatively slow rotation or oscillation, is termed a dynamic seal. Applications involving slow rotation or oscillation are classified as a dynamic application if the surface speed is less than 50 fpm (15 meters/min).
Finally, a sealing application in which there is high speed rotation, is termed a rotary seal. Applications are classified as a rotary application if the surface speed is greater than 50 fpm (15 meters/min). It should be noted that both the seals and grooves used for dynamic and rotary applications are different in design and specification. These differences are explained in the following sections.
Internal Expanding Sealing Components (Type NM)
Shorter than expected seal life is usually the result of too fine a finish on either the rod or the cylinder bore. A highly polished (non-porous) metal surface does not retain the lubricant necessary to control friction, whereas a rough or jagged surface will abrade the seal and lead to early seal failure.
To avoid these problems, we recommend an ideal surface finish of 20-24 µin (0.5-0.6 µm) Ra, with an acceptable range of 20-32 µin (0.5-0.81 µm) Ra. The surface finish should never be finer than 16 µin (0.4 µm) Ra.
Pressure Energized Seals
It is more difficult to seal at low pressures than at high pressures. As pressure acts against a seal, the rubber material is deformed. With proper seal design, deformation can improve the seal. This concept is used in many seal designs. By adding seal beads or pressure intensification details to the seal, sealing improvements can be made to custom designs. For very low pressure or vacuum applications we recommend using a Quad-Ring® Brand seal over an O-Ring.
The functional life of a seal is affected by the level of friction to which it is exposed. Factors contributing to friction include seal design, lubrication, rubber hardness (the standard rubber hardness for most sealing applications is 70 durometer Shore A), surface finish, temperature extremes, high pressure and the amount of squeeze placed on the seal.
The use of “slippery rubber” compounds can help lessen friction and improve seal life. Surface coatings and seal treatments such as PTFE and molybdenum disulfide are also used to reduce seal friction.
It is difficult to accurately predict the seal friction which will be present in an application, given the many variables involved. When designing an application which will be sensitive to seal friction, testing will probably be required to determine the effect of seal friction.
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