HAMILTON Champion Extra Heavy-Duty Casters offer maximum capacity up to 3500 lbs. CNC-machined 3-3/4" dia. ball raceways offer smooth swivel. Casters include: 1/2" thick drop forged steel top plate, 3/8x3" plate steel legs robotically welded inside and out for rigidity, 1" dia. integrally forged steel kingpin guaranteed for life. Slotted bolt holes. Wheel type codes are:(PU) polyurethane tread on iron hub(FS) forged steel(SS) semi-steel(PH) phenolic(SP) soft polyurethane tread on iron hub