S61904-2RS Sealed Ball Bearing Premium ABEC-5 Quality with Si3N4 Ceramic Balls (Silicon Nitride) 20mm x 37mm x 9mm S61904-2RS is double sealed, radial, deep groove ball bearing,S61904-2RS bearing is made of Stainless Steel with Ceramic Balls (Si3N4) and Nylon cage,S61904-2RS bearing is closed with rubber seals to protect the bearing from dust or any possible contamination,S61904-2RS bearing is sealed and self lubricated,the quality is set to EMQ (Premium ABEC-5 quality) to ensure high speeds,S61904-2RS bearing is lubricated with grease.