Apparatus for clamping two rotatable relative superimposed portions

Apparatus (10) for clamping two rotatable against each other bearing parts (17, 19), each having a surface (22, 24) which can be acted upon which to form a frictional engagement between the two surfaces (22, 24) with a force, wherein said is force for pressing together or loosening of the two surfaces (22, 24) is applied by a deformation of a component (28) having a reversibly deformable material (30), and the material deformation by means of supplying electric power to control.