DR Series castors; Pressed Steel Bracket With Black Rubber Wheel.      

 Rubber tyred castor wheels give a smoother ride and are quieter than hard tread castors.  Black rubber castor wheels can mark floors, in this case choose Grey castor tyres or black /Blue elastic rubber castors.   Load capacities of these castors are not very high.   Also, standard rubber castors,  if they are not used over a period of time these castors can develop flats.