Lubbed Dust caps are an essential but often overlooked part of any load bearing system. The main purpose of the Dust caps is in the name, to keep dust from the bearings which will extend the life of the bearings/axle. Bearing require constant lubrication to remain effective, hence why there are two options that we have available, those with a rubber cap and those that do not have a rubber cap and are completely enclosed at the top. Depending on the use and the area of use the usual recommendation is to check and replace yearly and two times yearly if you are around saltwater and heavy dust areas. Be sure to measure your bearing size to get the correct size dust cap, also keep in mind regarding installation and re-greasing your dust caps to use the correct tools, you may find that using a screw driver or some type of pry could possibly damage the dust caps rendering them useless and causing you some downtime. At M-Parts we pride ourselfs on offering our customers everything they need for trailer parts and accessories in a one stop shop!