A valve assembly has a valve opening which can be sealed and un-sealed by movement of a membrane 6, a force generator, such as an electromagnetic force generator having a solenoid and armature arrangement, and a shaft that is operably connected to the membrane and which is movable along a movement axis under the influence of the generated force to move the membrane and thereby regulate the degree of opening of the valve. Bearing assemblies are provided to procure a magnetic suspension of the shaft, permit its movement along the axis, and also to optionally produce a magnetic biasing of the shaft in a direction along the axis, particularly in the absence of a generated force.