Axial movement of the shaft relative to the machine housing in the area of the shaft opening through the housing wall, i. e. in the area of the (mechanical) seal. Shaft displacement is either required for functional reasons (cf. picture) or is

Single, balanced standard mechanical seal type NB in conformance with DIN 24 960

caused by unequal expansion of machine parts under alternating thermal loads. By making design modification to the mechanical seal itself or to the mechanical seal's connections, it is possible to compensate the shaft displacement in the area of the mechanical seal. Shaft displacements up to +/-4 mm can be compensated by NU and NB type mechanical seals in conformance with DIN 24 960 (figure) if the proper steps are taken. Special mechanical seal designs (multiple spring-backing with spring guide pins) can compensate shaft displacements of up to approximately 60 mm; generator seal. Bigger shaft displacements (currently up to around 450 mm) are compensated by metal expansion joints that link the shaft sleeve with the shaft (figure). The mechanical seal itself is unaffected by the shaft displacement. In the case of large-capacity horizontal vessels, in which – for thermal reasons – the shaft displacement coincides with angular and radial distortion of the vessel's face wall, the mechanical seal fixed on the shaft is provided in the seal housing with an own bearing that follows the shaft movements (figure). The axial, radial and angular movements are compensated by a metal bellows positioned between the seal housing and the vessel's face wall.