轴承上必须有密封件，防止外部的灰尘和水分进入，也防止轴承内部的润滑油脂外漏。
The bearing must be equipped with seals that are able to keep dust and water out and the grease inside the bearing.
含氟塑料作为自润滑和密封材料将被广泛用在汽车零部件上。
As a self lubrication and self sealing material, fluoroplastics is widely used in auto parts.
钢制零件通常涂上润滑脂，以防生锈。(为了防锈，钢制零件通常需涂上润滑脂。)
Steel parts are usually covered with grease for fear that they should rust.
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