Rotational motion part: oil seal (built-in spring). Please notice whether there is a scratch on the shaft side.
Flange mounting face, chimera: O-ring and sealant. Please notice whether the plane is skewed or the meshing of the O-ring; (for the O-ring and O- groove for reducer installation, See chart below of china harmonic reducer).
Screw hole part: use the screw locking agent with a sealing effect (Loctite 243 is recommended) or sealing tape.