The iron-tipped wheel is the instrument of the saint's torture, as she was tied to it to be drawn and quartered. The wheel miraculously broke, leading to Catherine's final beheading.
"Then a prefect advised the emperor to have four wheels fitted with iron spikes prepared, and to use them to tear Catherine's flesh, so as to frighten other Christians by such an example [...]. [...] But the saint prayed to God [...] that he would destroy this dreadful machine". - Jacques de Voragine, La Légende Dorée.