Abstract: An electric precipitator has a plurality of electrodes arranged in a housing, and a knocking device for cleaning the electrodes and including a plurality of anvils connected with the electrodes, a plurality of hammers arranged on a rotary knocking shaft, a drive unit with a driving shaft, and an insulating coupling for transmitting rotation of the driving shaft to the knocking shaft, wherein the insulating coupling includes engaging members mounted on the shaft, an intermediate coupling member, and drive bushes connecting the intermediate member with the engaging members so that each drive bush has a polygonal receiving opening with rounded corners and concave connecting lines, and each engaging member has a shape corresponding to the shape of the receiving opening with a small play and engages in the receiving opening.