In addition, the Kingpinless design features 3/8” diameter HRC45 steel ball bearings which absorb thrust and load in severe environments. The rounded face “donut” style soft tread is made of thermoplastic rubber that is non-marking, non-marring and provides easier mobility over standard flat tread wheels, resulting in a long lasting, high capacity wheel capable of easy rolling under heavy loads. 4 Inch Kingpinless Thermoplastic Rubber Wheel Swivel Caster with Swivel Lock SCC Lastly, Service Caster’s Series KP30 casters are finely crafted with grease fittings, and 4 position notches to accept 4-position locks.Even drywall carts, welding carts, and wall panel carts will be ready to roll on the Series KP30, keeping you moving forward.