When is an O-Ring used? Dynamic applications

Although the O-Ring is not very suitable for dynamic applications (it is rather suitable for static applications), there are areas where it is used for seals on moving devices. Dynamic applications are defined as those where the seals act as dividers on the surfaces of two elements that move one against the other.

We position dividing the dynamic applications of an O-Ring into alternating and rotating linear applications.

Alternating linear dynamic applications.

Dynamic motion is given by the linear sliding of one element against the other. In this type of application, the O-Ring is used as a seal, balancing it with the correct friction to create the motion. Given the pressures involved it is preferable to mount the O-Ring with side supports (called Back-up rings).

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Some considerations to bear in mind:
Up to 5 MPa, it is not necessary to use the Back-up ring. For higher pressures, it is recommended to use the Back-up. The Back-up ring is positioned behind the O-Ring in the pressure direction and avoids the “drawing” effect due to high pressure. One Back-up is required for one movement (single effect) or two for two movements (double effect). Maximum recommended speed up to 0.5 m/s.

The presence of the Back-up ring avoids the “drawing” or “extrusion” effect visible in the figure below. Excessive pressure and possible softening of the compound (for example due to a rise in temperature) can cause excessive deformation of the cross-section by pushing it into the space created between one element and another. The backup-ring creates a “barrier” that contains the deformation effect.

Causes of “extrusion/drawing” of the O-Ring:

High operating temperature (with consequent softening of the compound)
Coupling tolerances
High pressure (compared to the hardness of the compound)

To ensure tightness, the O-Ring cross-section must be compressed with an initial crushing between 6 and 27%.

The graph below shows the initial crushing percentage to be applied to the O-Ring according to the cross-section size (cord).

Dynamic rotating applications.

The rotating seal principle is based on the fact that an elongated elastomeric ring contracts when heated (known as the Joule effect).

The O-Ring is sized by increasing the diameter by 2 / 4% with respect to the shaft to be sealed and is pressed against the shaft by the outer diameter of the seat, which is correctly sized, therefore smaller than the O-Ring diameter.

The difference in size deforms the O-Ring, which will have corrugations with some contact points on the shaft (in the drawing the deformation of the O-Ring is amplified to make it easier to understand). The points of contact with the shaft will guarantee the seal while the non-contact points will allow better lubrication.

Correct compression is necessary in order to obtain and control the correct sealing force, to avoid too high friction, which could compromise the seal and cause excessive wear.

The crushing in this case has the task of: