Natural frequency vibrating screen

A vibratory screen apparatus comprising a first upper screening basket and a second lower screening basket, each basket having a deck that comprises a screen through which particles can pass that are smaller than a predetermined size, said first upper screening basket having a greater mass than said second screening basket, a normally stationary support frame on which the screening baskets are mounted in superimposed vertically spaced apart relationship with said first screening basket above said second screening basket and with their decks substantially parallel and relative to which each screening basket is oscillatingly movable for inducing material on its deck to advance thereacross in one direction, and exciter means for imparting oscillatory motion to both of said screening baskets, said apparatus being characterized by:

• A. resilient supporting means at all times providing an elastic connection between said support frame and said first upper screening basket, said resilient supporting means being substantially symmetrically disposed in relation to the center of mass of said first upper screening basket and being arranged for
• (1) supporting said first upper screening basket from said support frame,
• (2) substantially confining said first upper screening basket to oscillatory motion relative to said support frame, and
• (3) constantly exchanging energy with said first upper screening basket during oscillatory motion thereof and thereby tending to maintain such motion;
• B. coupling means at all times providing an elastic connection between said first upper screening basket and said second lower screening basket whereby said second lower screening basket is supported from and located below said first upper screening basket and is thus indirectly supported from said support frame and whereby said second lower screening basket is substantially confined to oscillatory motion relative to said support frame that is in phase opposition to the oscillation of said first upper screening basket,
• (1) said coupling means being disposed in substantially symmetrical relation to the center of mass of each of said screening baskets and
• (2) said coupling means comprising
• (a) a plurality of coiled coupling springs, each having an axis,
• (i) said coupling springs being connected between said screening baskets and
• (ii) all of said coupling springs having their axes inclined in one direction to planes in which said decks extend and at an angle thereto that is the same for all of the coupling spring axes, and
• (b) a plurality of resilient bars, each having at opposite ends thereof respective connections to said first upper screening basket and to said second lower screening basket, for constraining the screening baskets to oscillate counter to one another
• so that said coupling springs constantly exchange energy with both of said screening baskets during their oscillatory motions and thereby cause a system comprising the screening baskets, the resilient supporting means, said coupling means and the resilient bars to have a natural frequency of vibration;
• C. said exciter means comprising
• (1) driver which defines a rotation axis and
• (2) a gyratory mass which is rotated eccentrically about said rotation axis at a rate to effect vibration of said system at a frequency which is near to but below said natural frequency of vibration;
• and
• D. said exciter means being rigidly fixed to said second lower screening basket in substantially symmetrical relation to the center of mass thereof.