Anchorage device for fastening a counterweight to a washing machine tub comprising: an hollow shell (1) in which a concrete ballast mass (2) is adapted to be poured, said shell (1) being provided with fastening housings (3), and threaded fastening means (4) adapted to cooperate with the fastening housings (3) in order to secure the shell (1) to a washing machine tub (5). The fastening housings (3) comprise expansion cavities (6) shaped so as to define an equivalent diameter (7), the threaded fastening means (4) comprise a head (8) integrally provided with a flange adapted to engage the tub (5) and a shank adapted to be screwed into the expansion cavities (6), said shank comprising a self-tapping thread portion (9) having a crest diameter (10) and a root diameter (11) and a not-threaded pin portion (12) having a pin portion diameter (13). The ratio of the root diameter (11) to the equivalent diameter (7), the ratio of the crest diameter (10) to the equivalent diameter (7) and he ratio of the pin portion diameter (13) to the equivalent diameter of said expansion cavities (6) being in predetermined range.
