The K Series Needle Roller Cages (also known as needle roller and cage assemblies) are a standardized type of needle bearing component comprising a cage that precisely retains needle rollers, without including an inner or outer ring. The "K" designation typically refers to a metric series conforming to international standards (like ISO 3031). This design represents the most basic and radially compact form of a needle roller bearing. The assembly is designed to be used with a hardened and ground shaft (which acts as the inner raceway) and a hardened housing bore (which acts as the outer raceway). Its primary advantage is enabling extremely high load capacity within a minimal radial space.
