Dynamic (secondary) seals are all secondary seals (mostly O-rings), which exert axial movements on the hydraulic diameter under the action of spring force and hydraulic power, e.g. to compensate wear of sliding face materials or to balance shaft displacements. Dynamic seals are subjected to higher wear, in particular due to the effect of axial vibrations and deposits; in O-rings this can lead to flattening on the contact surface facing the hydraulic diameter.