S61903-2RS Sealed Ball Bearing Premium ABEC-5 Quality with Si3N4 Ceramic Balls (Silicon Nitride) 17mm x 30mm x 7mm S61903-2RS Sealed Ball Bearing is double sealed, radial and deep groove ball bearing,bearing is made of Stainless Steel with Ceramic Balls (Si3N4) and Stainless steel cage,S61903-2RS bearing is closed with rubber seals to protect the bearing from dust or any possible contamination,bearing is sealed and self lubricated,the quality is set to EMQ (Premium ABEC-5 quality) to ensure high speed,S61903-2RS bearing is lubricated with grease.