Sphered seat washers are used together with thrust ball bearings with sphered housing washers when the bearing is fitted into a machine component manufactured with a flat surface. The seat washers are supplied as separate components. Depending on bearing series, they have the basic designation U 2, U 3 or U 4 followed by a two-digit number, which identifies the size, e.g. sphered seat washer U 320 for bearing 53320.
