The RNAF183012 is a Light Duty Open Type . Single Row Machined Needle Roller Bearing. This Bearing has no Inner Ring . meaning the Needle Rollers are mounted directly onto the shaft. As such . the Bearing has better running accuracy as it isn't affected by assembling an Inner Ring with the shaft. In caged Needle Roller Bearings like this one . the Needles are precisely guided by the Cage to achieve ideal load distribution and the Cage is separable from the Outer Ring. These Bearings are designed for high load carrying capacity in radially compact arrangements and are used in most general purpose applications. The Bearing's dimensions are 18x30x12mm.