What we notice is the base oil becomes involved with various chemical reactions causing the NLGI rating to thicken until such a time where as the bearing is running on the thickener soap with very little base oil and failure will soon follow, relubing prevents this from occurring.

If the bearing housing is well designed, it must have a grease niple and an oulet, I think it is called a "down commer" (excuse me if I am wrong) so the old grease is displaced.

By design, the best situation is to have a purge plug opposite the grease fitting (zirk). When regreasing at regular intervals, remove the purge plug and re-grease while the bearing is rotating. Continue greasing until fresh grease is seen emerging from the purge hole. Leave the purge plug out while running the bearing for 10 minutes or until the bearing reaches its normal speed and operating temperature. The bearing will purge any excess grease to prevent undue pressure on any sealing material. Replace the purge plug.