A description is provided of a seal for preventing harmful material passing through a gap (3) between two machine elements (4,5) with said seal comprising two sealing parts (1,2) where at least one of the sealing parts (1) is fixed to one of the machine elements (5) and being configured for rotational operation together with said machine element (5) and where at least one of the machine elements (5) is configured for a movement back and forth in its axial direction relative to the other machine element (4). The seal is characterised in that the sealing part (1) being configured for rotational operation is provided with at least one helical thread (6) and in that one of the sealing parts (2) forms a casing (2a,2b,2c) around a portion of the other sealing part (1) and in that at least a portion of the helical thread (6) is located inside the casing (2a,2b,2c). In this way material (e.g. harmful dust) is restricted from passing through the gap between the two machine elements regardless of the fact that both machine elements perform a rotational and translatory movement relative to each other. The rotating sealing part with the helical thread ensures that most of the material coming into contact with the seal will be thrown away from the gap even when a significant relative axial displacement between the machine elements takes place.
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