ALBION Heavy-Duty Casters support both manual and power-drawn loads. Casters include: large diameter ball bearings, double ball hardened raceways, hardened roller bearings running on hardened, ground bushings, zinc-plated finish with zerk grease fittings; 3/4" king pin, and 1/4" thick top plate with slotted bolt holes.Wheel type codes are:(SS) semi-steel(PH) phenolic(PU) polyurethane tread on steel core(DF) drop-forged steel(MR) mold-on rubber on steel core