outer surface of the collar 40 contiguous the gearbox case 20 in the most inboard part of the seat 26 immediately outboard of the double lip seal 50 is a single lip seal 54 the radially inner surface of which engages the outer surface of the wear sleeve 42 inboard of the step 43 and the radially outer surfaces of which engage the inner surface of the seal 26 adjacent the rib 28 a labyrinth seal 57 is mounted in the outer seat section 31 of the gland with its inner surface engaging the outer surface of the wear sleeve 42 and its outer surface engaging the inner surface of the outer seat section 31 the outboard radial surface of the inner single lip seal the inboard radial surface of the outer labyrinth seal the radially outer surface of the wear sleeve and the radially inner surface of the gland in this embodiment of the rib 28 define a chamber 59 with which the passage 33 and nipple 35 communicate a source of air pressure 60 such as an air compressor and tank is operably connected to provide a continuous flow of air at a pressure within the chamber 59 of for example 15 lbs psig it will be evident.