Fixed Castors or Rigid Castors
A basic, Fixed/Rigid castor is made up of a wheel mounted into a stationary fork i.e. there is no swivel action.
A fixed/rigid castor is used to help with maneuverability of a trolley. The fixed castor helps to keep one end of the trolley moving in one direction and thus helps the operator steer the trolley in the desired direction.
Likewise if more movement is needed or the amount of space to move in is restricted it it advised not to fit fixed castor, use either swivel castors or braked castors.