Open Design Open spindle bearings make optimum use of the internal space by allowing a higher number of balls and a window cage. This results in maximum load carrying capacities and therefore maximum bearing life. This open design is recommended for oil lubrication, as it allows relubrication using spacers. No dirt must be permitted to enter the bearing and continuous relubrication should be used. Special Design ACI As a general rule, spindle bearings have the open shoulder on the outer ring (ACO). However, for some special applications the open shoulder can be positioned on the inner ring (ACI) (e.g. dismountable bearings).