Mass balance for a screen oscillating drive is provided mounting the eccentric or crankshaft assembly on extensions of the links supporting the working frame. The axis of the suspended mass m2 is spaced by a distance a2 from the pivot of the links to the base frame such that the product m2.a2, is substantially equal to the product m1.a1 where a1 is the distance between the pivot of the link to the base frame and the pivot of the links to the working frame and m1 is the mass of the working frame. The axis of the suspended mass m2 intersects the centerline of the links in the median position thereof at a normal to this centerline connecting the latter axis and the center of mass of the working frame as seen in side view.
