Comprising one outer ring and two inner rings, the bearing is assembled as a unit by an internal locking structure. The width of the outer ring is less than total width of two inner rings by 1 mm, two snap rings are fitted on the outer ring, oil grooves and holes are made at contact face of two inner rings, therefore the bearing application design is simplified. On both sides of the bearing mounted are steel seals, or optional rubber seal or nylon seals. All outside surfaces of the bearing components are phosphate to ensure the bearing able to work in the open air.