To make elastic seals actually seal they are deformed on installation such that radial forces, the so-called preload, are produced that make it impossible for the hydraulic fluid to flow around the seal even in the unpressurised state. With increasing pressure increasingly large parts of the seal are pressed against the mating surface with increasing force. As a result the sealing gap becomes smaller in proportion to the pressure and at the same time the seal length increases and therefore also the sealing action (Figure A 28). Automatic sealing is only possible with elastic materials.
Figure A 28: Automatic sealing