Abstract: The invention involves devices for moving two masses (1, 2) with respect to a central axis of a given center (3) in a direction of movement (4). The device is basically distinguished by the fact that it comprises two sets of slide rails (5, 6) permitting movement in the direction (4), mechanisms for mounting each of the two masses on the slide rails so that they can move symmetrically with respect to the point (3), two traction bands (16, 17) that can move in any direction making an angle other than a right angle with the direction of movement (4), mechanisms for applying at least one force on each of the two traction bands, mechanisms to link each mass with each of the traction bands, respectively, such that a force applied to one of them permits a force with a component parallel to the direction of movement to be exerted on the corresponding mass. First and second mechanisms (30, 40) transmit in opposite directions the forces applied to a first traction band to a second traction band and vice versa.