Some applications will use a sealed ball type bearing that require a seal to be installed down inside the axle tube about 4 inches inside from the bearing itself. In some cased a higher capacity type cone & roller (Timken set 20) are used that have a seal that is installed while the bearing cone is being pressed onto the axle shaft. This seal (part of Timken set 20) fits between the axle retainer (retains the axle in the housing with 4 - 3/8" or 1/2" bolts) and the cone then the bearing assemly and retainer is pressed on. I am told the later does NOT use the seal that is otherwise installed into the axle tubebecause it gaines its lube from the gear
oil. The cone type bearing will accept more thrust
and is a better choice for this application in my opinion, as we
slide in four wheel drifts through the neighborhood.
The conventional seal is only used when using the sealed "ball type bearing assembly". They do not require additional lubrication. If you forget ito install the seals you WILL ultimately have gear lube leaking from the axle flanges.
As a side note the above mentioned cone type bearing by way of design will widen the track about .100 per side, compared to the ball bearing type.
Rick