Retention and exclusion
In many applications, the exclusion of contaminants is just as important as lubricant retention. Seals with an auxiliary lip, like the HMSA10 seals (fig 1), are appropriate for these applications.
Another option is to use two seals installed in opposite directions (fig 2 and fig 3) or two opposing V-rings (fig 4) with a spacing washer.
Under extremely tough operating conditions, SKF recommends using HDDF metal face seals (fig 5) provided that the sliding velocity of the mating surfaces lies within the permissible range.