Yellow zinc chromate finish on these E.R. WAGNER Casters lasts three times longer than clear zinc finish. Rubber grease seals keep contaminants out while holding lubricants in. Raceways are heat-hardened and have grease zerk fitting in the swivel for easy maintenance. Wheels have roller bearings and grease zerk axles. Total-Lock brake simultaneously locks wheel and swivel.Wheel type codes are:MR—Mold-On Rubber. Soft rubber tread permanently bonded to an aluminum core. Recommended for light to medium loads on hard-surface floors. Quiet rolling, provides excellent floor protection with a cushioned ride.MU—Mold-On Urethane. Polyurethane tread molded and mechanically locked to an aluminum core. Recommended for medium to heavy loads on all floors. Quiet rolling, provides good floor protection, and is non-marking.PH—Phenolic. Black phenolic resin impregnated and reinforced with macerated fabric for high load capacity. Recommended for medium to heavy loads on all floors. Good floor protection and excellent rolling ease. Resistant to most chemicals and acids.