Dynamic sealing

In seals made from polymeric material there is a natural ability to form films, when moving in all combinations of motion direction and pressure drop direction. In the Metseal seal, this spontaneous ability does not exist. For example the double acting seal, type D, will form a film in only one combination of motion and pressure drop direction. This gives different degrees of friction in the two moving directions. The explanation for this characteristic is that the Metseal seal has a small compact cross-section of metal that does not allow deformation of the cross - section and contact surface. If, on the other hand the contact pressure over the seal's total width is almost constant, the whole cross section can be twisted and film formation is obtained. The double acting asymmetrical piston seal, type DA, as well as the single acting counter balanced piston type SB, are examples of seals where the cross section has been designed to obtain optimum characteristics regarding friction and leakage. These will then adapt to various applications and their specific demands and difficulties. Formation of films and low friction at the same time involves the transportation of pressure media across the seal. Leakage is normally the result of transport of medium in two motion directions. To get low resulting dynamic leakage one requirement is that transportation in the dangerous leakage generating direction is smaller or equal to transportation in the harmless leakage generating direction.

Fig 2 Type D
PCR = Contact pressure
PCA = Axial contact pressure
Pp = Pre-stress pressure
Δp = Pressure difference

Fig 3 Type D
after wearing in