An apparatus includes a rotatable common shaft configured to be rotatable about a longitudinal shaft axis extending along the rotatable common shaft. Rotatable electric machines are arranged on the rotatable common shaft. This is done in such a way that disk assemblies of the rotatable electric machines and the rotatable common shaft are concurrently rotatable. The rotatable electric machines are arranged on the rotatable common shaft in such a way that the net cogging force, which is generated by the rotatable electric machines, and which is imparted to the rotatable common shaft, is reduced.