The physics of creating a face seal is relatively simple. In most sealing systems, the objective is to prevent fluid from leaking from a high-pressure location to a lower pressure location through a sealing gap. Sealing fundamentals should be followed for trouble free application.
Creating a face seal
Static seals are typically subject to a preload to help attain sealability. Preload occurs when the seal height is designed to be greater than the sealing gap. Under compression, the seal is elastically deformed, producing internal stresses and generating a reaction force on both the top and the bottom of the sealing gap. When fluid pressure is applied, it causes additional internal stress, which supplements the preload and consequently prevents leakage.
An example of this phenomenon is illustrated in Figure 1 below by a Finite Element Analysis (FEA) simulation performed on an o-ring.
Figure 1 - Finite Element Analysis (FEA) simulation performed on an O-ring
In order for a face seal to function, the contact pressure between the gasket and the sealing interfaces at some location on both sides of the sealing gap and must be greater than the fluid pressure being sealed. In designing a face seal for a static application, there are three important factors to consider in addition to material selection:
The most critical design parameter is the axial squeeze, or the preload of the seal. The percentage of axial squeeze is calculated using the formula below:
Below is an example of FEA simulations for a rectangular cross section. Figure 2a on the left illustrates the uncompressed seal profile in the groove; Figure 2b on the right shows the seal characteristic when compressed in the groove. These simulations show that the seal height needs to be higher than the groove wall in order to achieve desired sealability. They confirm the importance of axial squeeze (or preload) in sealing.
Figure 2a - Uncompressed seal profile in groove Figure 2b - Seal compressed in groove
Typically, the seal's axial thickness should be made such that it is deflected 25% of its original height when two mating surfaces that form the sealing gap are in their final positions at nominal conditions. It is important to consider the manufacturing tolerances of both the seal and the seal gap when designing. The suggested range of axial squeeze considering tolerances is 10 to 40%.
Gland fill is another important parameter to consider. Gland fill percentage is defined as the percentage of volume of the seal groove that the seal occupies. This percentage is represented by the following formula:
It is imperative not to overfill the gland. An excessive seal volume in a groove could cause damage to the seal and / or the sealing surfaces. The maximum gland fill recommended considering manufacturing tolerances is 95%.
The last characteristic that allows for the complete geometrical definition of a seal is the desired fit. Typically, if the higher pressure is located on the inside of the seal, an outside diameter (O.D.) interference with the gland is recommended. Conversely, if the higher pressure is located on the outside of the seal, an inside diameter (I.D.) interference with the gland is recommended. The interference should be designed in the range of 1 to 5% considering manufacturing tolerances.
It should be noted that sometimes it is not practical to design in an interference fit because of automated assembly or some other special consideration. In these cases, it is recommended to design in as little clearance as possible.
Subscribe to this Parker's Sealing and Shielding Blog RSS for more sealing fundamentals. Parker's Application Engineers can assist you with the material selection as well as the design process to ensure the optimal sealing solution for your applications. Please refer to ParFab Design Guide and O-ring Handbook for more information.
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