51000 Series: Single direction thrust ball bearing, consists of a shaft washer, a seat and a steel ball- cage assemblies
52000 Series: Double direction thrust ball bearing, consists of a shaft washer, two seats, and two balls - cage assembly. The bearings are separated, all parts can install alone
53000 Series: No aligning seat washers aligning thrust ball bearings
53000U Series: Aligning seat washers aligning thrust ball bearings