pancake slip ring Through bore pancake slip ring Through bore slip rings (also known as «through hole slip ring») are slip rings with a hollow shaft in the center. This series is designed for being installed with a flange on a sleeve bracket and it is offering a free inner space for either connecting the rotor to a rotating shaft of the customer machine or for leading through pipes or cables not affected from rotation. The hollow shaft slip rings are available with inner diameters from 3mm up to 300mm