During operation, single mechanical seals are closed hydraulically by the pressure to be sealed. At standstill (and in vacuum mode), the closing force must be generated by mechanical means. This job is performed by a spring backing. The sliding faces are spring-loaded either directly or indirectly. With direct spring loading, the spring force acts directly on the seal face, with indirect spring loading, the spring force is directed to the seal face by way of a thrust ring and the secondary seal. The choice of spring loading must give due consideration to the static friction of the dynamic secondary seal, the wearing height of the sliding faces, and the permissible axial shaft displacement. It is customary to quote the spring loading p_F in N spring force per cm² sliding face. The values for p_F lie between 10 N/cm² (big mechanical seals with large sealing widths) and 50 N/cm² (small mechanical seals with narrow sealing widths).