The present invention concerns a method for screening of wooden chips and the similar stuff and a device forming a screen for carrying out the invention. A known screening device includes a chute arran¬ ged upon a bottom frame, by which the chute is provided with an upper bottom which includes a screening grid con¬ sisting of mutual parallel screening rods, and is brought to vibrate relative to the frame bY help of a vibrating device. The bottom frame is provided with longish car¬ riers, which are arranged projecting up between the screening rods so that gaps are obtained between the rods. When the chute is vibrating, e.g. oscillates in a plane in its longitudinal direction, the screening rods performs a to-and-fro movement relative to the carriers, by which stuff with a piece size less or equal to the width of the gaps are falling through the screening grid and will be picked up on a lower botton arranged on the chute.

It has appeared that the known device has a range of application limited to certain kind of stuff and a limited capacity. Thus it is a wish partly to widen the range of application of the device and partly to encrease the capacity inside given limitations.

The purpose with the present invention is to satisfy the wishes mentioned above. The purpose has been achieved by an invention which is constituted by a method for screening with a screening device which partly shows the same members as the known screening device and which is characterized in that the carriers is brought to vib¬ rate partly relative to the bottom frame and partly rela¬ tive the chute with the screening rods, by which the chute preferably is vibrating in the same plane as the carriers. In particular the relative movement between the screening rods and the carriers is controlled by a mutual phase displacement of these respectively movement of

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vibration.

The invention also includes a screening device which is characterized in that the carriers are movably arranged on the frame. Preferably the carriers are con- nected to the frame by struts, which include a spring device. The vibrating movement of the carriers thereby is effected by a vibration device which is attached to any of the struts. The vibration movement of the chute and the screening rods is effected by a vibration device attached on the frame and which is connected with the chute. The vibration device thereby preferably is consti¬ tuted by a crank device which is driven by an electrical motor.

In an alternative performance of the invention, the carriers and the chute are vibrated by crank devices attached to the frame, whose crank motion is controllable for adjusting of a mutual phase displacement of the two vibration movements.

The carriers are provided with a mass, which is considerably less than the one of the chute, by which the carriers easier can be brought to vibrate with a higher frequency than the chute. By this an intencive movement in the stuff to be screened will be achieved at the gap between the carriers and the screening rods, by which an effective screening will be achieved. The higher frequen¬ cy of oscillation of the carriers also has proved to pre¬ vent clogging of the screening grid.

The screening method in accordance with the in¬ vention is not limited to a division of a stuff into two fractions. By providing the chute with several bottoms a division in several fractions can be achieved. The method is in particular active by screening wooden chips, but is not limited to this kind of stuff. The method also can be used e.g. to separate gravel from wooden chips and the like.

The invention is described below in an performan¬ ce concerning a screen for wooden chips according to

accompanying drawings.

Figure 1 shows a screen for transporting and screening of wooden chips as seen fron one side.

Figure 2 shows the screen in accordance with figure 1 in a cross-section.

The screen in accordance with the example inclu¬ des a chute 1 with an U-formed cross-section, which is carried by a frame 2 by suspension bars 3. The chute 1 is provided with an upper bottom 4 and a lower bottom 5. A screening aperture is arranged in the upper bottom - of the chute, which is covered by a screening grid consis¬ ting of mutually parallel screening rods 6 made of round iron.

The frame 2 futher carries two loop-formed struts 7 whose attachements in the frame 2 consist of plate springs 8 which can be bent in the longitudinal direction of the chute 1. The loop struts 7 are twistable connected mutually by longish carriers 9 made of flat iron which are arranged edgeways so that one carrier 9 projects up between two screening rods 6 so that gaps are obtained between the screening rods 6 and the carriers 9. At one of the loop struts 7 a vibrator 10 is arranged, which oscillations coincides with the bending plane of the plate springs 8. The chute 1 futher is connected to the frame 2 by a crank vibrator 11, which stroke length and number of revolutions can be adjusted relative to the carrier vib¬ rator 10 which oscillations also is adjustable.

The wooden chips which have to be screened is feeded into the chute 1 at the arrow against the upper bottom 4 and is moved by the shaking movement of the chute 1 effected by the crank arm vibrator 11 down to the screening grid where chips having a piece size less than the gap between the screening rods 6 and the carriers 9 is brought to fall through down to the lower bottom 5 in order to be moved on by the shaking movement of the chute 1 and away form it at the arrow from the lower bottom 5.

The coarse fraction which pass over the screening grid is moved on by the shaking movement of the chute 1 and out from this one at the arrow from the upper bottom 4. The two fractions are transported ahead separately by help of following conveyors.

In an alternative performance of the example above, the chute 1 and the screening rods 6 is controlled relative to the loop struts 7 and the carriers 9 by help of control devices as e.g. guides or other in itself known devices which are not shown in the figures. To that one secures that the gap between the screening rods 6 and the carriers 9 has a constant width at the relative move¬ ment between the screening rods 6 and the carriers 9, which in its turn gives a moore exact limit between the two fractions that are separated in the screen. Such an exact-fractionating has not earlier been possible to achieve for instance by help of the known screening de¬ vice mentioned above, where the carriers are firmly con¬ nected to the base of the device and where the chute with the screening grid is not provided with any guide to the base, e.g. indirectly the carriers which projects in the screening grid. Eventually play which arise by wearing of the suspension attache ents brings the chute and the screening grid to move also sideways causing a varying width of the gaps between the screening grid and the carriers.

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