Twin Pillow Block Linear Ball Bearing IPPWN08G 1/2" OpenTwin Linear Ball Bearing Pillow Block, open shelled, with a 1 2" ID (inner diameter) bearing inside a housing made of aluminum alloy. Features a low coefficient of friction for smooth operation, a heat treated alloy steel shell for durability, a resin retainer polymer cage to reduce noise, and standard double seals at no extra cost. Open shell design for use with rail supported round linear shafts Twin Double size equal to two standard bearings