Designed for industrial and commercial equipment, Medium-Duty Casters feature: hardened roller bearings and double-ball raceway swivels; grease fittings and zinc-plated finish; 4x4-1/2", 1/4" thick heat-treated carbon steel top plate with slotted bolt holes; 1/4" steel legs; 1/2" dia. axle; and 5/8" king pin. Wheel type codes are (PH) phenolic(SS) semi-steel(PU) polyurethane(RS) rubber on steel(PO) polyolefin(PPU) polyurethane/polyolefin