a magnetic yoke iron core comprising a first mounting space and a second mounting space distributed sequentially and concentrically from inside to outside of the magnetic yoke iron core, the first mounting space penetrating through a center position of the magnetic yoke iron core in an axial direction of the magnetic yoke iron core, and an opening of the second mounting space facing a second shaft end of the magnetic yoke iron core;

a first movable plate located in the first mounting space and closer to a first shaft end of the magnetic yoke iron core than to the second shaft end, the first shaft end being an opposite end of the second shaft end;

a plurality of friction disks sequentially arranged in the first mounting space along the axial direction of the magnetic yoke iron core and located at a side of the first movable plate facing the second shaft end;

at least one second movable plate each arranged between two adjacent friction disks;

a coil arranged in the second mounting space;

an armature located at the second shaft end of the magnetic yoke iron core, the armature being connected with the first movable plate through at least one connector, the at least one connector passing through the magnetic yoke iron core and being slidably matched with the magnetic yoke iron core, and the at least one second movable plate being connected with the at least one connector; and

an elastic part arranged between the armature and the magnetic yoke iron core, the elastic part having a pre-tightening force that enables the armature to be far away from the magnetic yoke iron core.