HAMILTON Workhorse Heavy-Service Casters offer superior performance and reliable service. CNC-machined 3-1/4" dia. ball raceways offer smooth swivel. Casters include: 5/16" thick drop forged steel swivel top plate and yoke base, 1/4x2" plate steel legs robotically welded inside and out for rigidity, and 3/4" dia. integrally forged steel kingpin that is guaranteed for life. Slotted bolt holes. Wheel type codes are:(PU) polyurethane on iron hub(SS) semi-steel(PH) phenolic(SP) soft polyurethane on iron hub(FS) forged steel