Rod seals feature integral pressure relief

Edited by the Engineeringtalk editorial team May 6, 2005

Omega OMS-MR PR and Omega OMS-S PR rod seals feature an integral pressure relief function that automatically prevents such a dangerous pressure increase from occurring in the sealing system.

Rod sealing systems in hydraulic cylinders are generally made up of one primary and one secondary seal.

In certain operating conditions, the pressure between the sealing system's two seals can rise.

If the pressure in the gap between the two seals exceeds the pressure in the main hydraulic chamber, the primary seal will shift in the groove, become worn, and eventually break down.

The recently developed and patented Omega OMS-MR PR and Omega OMS-S PR rod seals feature an integral pressure relief function that automatically prevents such a dangerous pressure increase from occurring in the sealing system.

With the outward motion of the hydraulic cylinder rod, a film of oil is always dragged out beneath the sealing edge of the primary seal.

The secondary seal then wipes this film away.

With the inward motion of the rod, the remaining oil is drawn back completely into the main hydraulic chamber by the return capability of the primary seal.

If the pressure in the main chamber drops quickly, or in the event of long strokes, unfavourable speed ratios between the inward and outward motion of the rod, or high stroke speeds, the volume of oil transported back into the chamber can be smaller than the volume of oil dragged out.

This results in an increase in pressure in the gap between the primary and secondary seals that can quickly exceed the pressure in the main hydraulic chamber.

This critical difference in pressure affects the back of the primary seal in such a way that the primary seal can shift in its groove or extrude in the direction of the main chamber.

If this happens several times, the sealing edge is damaged, the sealing effect reduced, the amount of expelled hydraulic medium increased, and the return capability minimised.

The rapid increase in pressure in the gap between the two seals eventually leads to further gap extrusion and, consequently, complete breakdown.

Freudenberg has finally succeeded in coming up with a solution to this problem in the form of a patented pressure relief bore hole with a diameter of less than 1mm, which is bored at an angle through the PTFE sealing element of the primary seal.

Generally speaking, when the pressure in the main chamber exceeds the pressure in the gap between the seals, the primary seal is positioned in such a way that the pressure relief bore hole is covered by the contact between the PTFE sealing element and the wall of the groove.

If the pressure in the gap exceeds the pressure in the main chamber, even a minimal difference in pressure is enough to lift the seal slightly away from the groove wall and uncover the pressure relief bore hole.

Pressure compensation is then automatic.

This patented sealing system is available in two versions: as the Omega OMS-MR PR for hydraulic systems that are subjected to normal pressure levels, and as the Omega OMS-S PR for heavy-duty hydraulic applications.

The Omega OMS-MR PR fits into OMS-MR series installation areas, whereas the Omega OMS-S PR is designed for use in the heavy-duty OMS-S series.

The heavy-duty hydraulic primary seal Omega OMS-S PR also features a shaped elastomer prestress element and a supporting edge with an axial groove.

The square shape of the prestress element increases the anti-rotation effect both during installation and operation, and ensures that the prestress is evenly distributed, thereby guaranteeing optimum sealing edge contact pressure.

This heavy-duty hydraulic primary seal is mainly used in large presses, hydraulic steelworks, special mechanical engineering applications, and the offshore sector.