A device for preventing penetration of dust from the outside inwardly into an annular gap between a cutter arm of a cutting machine and a rotatable cutting head carried on said arm, said cutting head having an interior space containing lubricating oil, said device comprising a first seal closing off said interior space and located in said gap, a second seal located in said gap outwardly of said first seal, the grease flow resistance of said second seal in the outward direction being less than the grease flow resistance of said first seal in an inward direction, and a pressurized grease supply line arranged within said cutter arm opening into said gap at a location between said first and second seals whereby during operation of the cutting head grease flows outwardly through said gap and prevents entry of dust into said gap.