U circle under the condition of low pressure, the deformation produced by the interference of labial ministry seal only, due to the contact area is lesser, friction is relatively low, with increasing pressure, lip elastic deformation increases, the tensile, compression and bending stress increases, the u-shaped coil radial pressure automatically, increasing the length of contact with the sealing face until the u-shaped axial length and sealing surface contact, has good sealing to ensure high pressure condition.
UN seals

PU seals