Consider the route used to add the grease
A sealing arrangement limits particular routes of external contamination, but relubrication deliberately introduces material into the assembly.
Dirt carried on a fitting, delivery connection or lubricant-handling tool can therefore bypass the protection assumed from the bearing's seals. Manufacturer handling and maintenance guidance emphasises cleanliness and control of contamination. The practical question is not simply whether the bearing is sealed; it is whether the entire transfer from the lubricant container to the intended lubrication point preserves the required product and cleanliness. A well-protected bearing can still receive the wrong grease or contaminated material during an otherwise routine maintenance task.
