The caster pivots on the Stander ZK, X, and Intensity are a trailer axle type design with taper roller bearings. Taper roller bearings require consistent tension, and so we use spring washers on top of the bearings to avoid the need to continuously re-torque the top nut. The sealed caster pivot has a high quality shaft seal and the flat cap is recessed below the top of the housing so it doesn't get knocked off easily. This premium setup only requires maintenance if the yoke begins to wobble or leak grease (often 1000+ hrs.).