Contact seals protect, but add friction
2RS variants use contact sealing lips that retain grease and exclude contamination. They generally increase friction and reduce the permissible speed compared with open bearings.
Shields are non-contact barriers
2Z metal shields reduce large-particle ingress without a rubbing seal. They suit applications where some air exchange or lower running torque is required.
Open bearings need a system-level lubrication plan
Open designs expose the raceway and cage directly. Oil circulation, grease delivery and external sealing become part of the machine design.