A mechanical drive system operating by magnetic force, to be fitted on a pump, comprises a basic structure on which a drive shaft, which extends along a longitudinal axis, is supported rotatably. A driving element operatively connected to the drive shaft is provided with driving magnets arranged in a ring. A driven element provided with driven magnets arranged in a ring is also mounted on the basic structure. A bell is inserted between the driving element and the driven element and isolates the environment containing the driven element. The driving element and the driven element comprise respective cages each having seats for housing the respective driving magnets or driven magnets.