This set of four 6-inch by 2-inch casters pairs two rigid wheels with two locking swivel wheels, giving carts and workbenches the directional stability of straight-line rigid casters plus the maneuverability of full-swivel wheels, with a side brake on the swivel pair to hold everything in place once you've rolled it into position. The non-marking polyurethane tread rides on double ball bearings, so loads roll smoothly and quietly across concrete, tile, or wood floors without leaving black marks. Each caster is rated for up to 1000 lbs, so the set can carry roughly two tons when spread across a heavy workbench, machine base, or storage rack. The 5.5mm galvanized steel plate resists corrosion and wear even in a garage or warehouse, and each 4.5 x 4-inch top plate has four mounting holes that line up with standard bolt patterns for a straightforward swap-in installation.