Technical field BAR SCREEN FOR WOOD CHIPS

The present invention relates to bar screens for wood chips and similar products. Characteristic of bar screens is that they generally comprise two screen elements, each of which consists of a large number of parallel, spaced apart tubes or bars, which are inserted in the interspaces between the bars of the other screen element. Between the bars of the screen elements gaps are defined through which smaller fractions of chips may pass, while larger fractions remain on top of the bars. To achieve this, the screen elements are subjected to vibration and/or other movements relative to each other.

Prior art

Bar screens of the above described kind are pre¬ viously known. The movements to which the screen elements are subjected are generated by one or more driving devices, which are connected to both screen elements adjacent the two ends thereof. Examples of such bar screens are found in US-A 1 552 397 and US-A 5 117 983.

In the bar screen according to US-A 1 552 397 the driving device for the screen elements is constituted by a crankshaft which via arms is connected to both ends of the screen elements in order to impart uniform movements thereto along their entire length.

In the bar screen according to US-A 5 117 983 the driving device for the screen elements is constituted by a motor-driven eccentric shaft located at the ends of the screen elements and connected via a chain to a further eccentric shaft at the other ends of the screen elements so as to impart to the screen elements the same movements along their entire length.

The above described known bar screens have several disadvantages. One disadvantage is that the motion trans¬ mission devices of the same require great precision during manufacture and assembly so that no tensions will arise

which could jeopardize the mode of operation of the bar screen. Another disadvantage is that the movements of the screen elements are uniform along the entire length thereof, which reduces screening efficiency. Yet another disadvantage is that external additional equipment is required to spread the material evenly across the width of the screen surface before screening and/or to collect screened material under the bar screen.

Description of the invention

It is an object of the present invention to elimi¬ nate at least partly the disadvantages of previously known bar screens and to provide a bar screen which requires driving only at one end of the bar screen, whereby the structure of the bar screen is simplified and made more reliable and also is statically defined so that the risk of built-in, migrating tensions arising therein is reduced, and which subjects the material screened to non-uniform movement along the length of the screen, thereby increasing the efficiency of the screen.

This object is achieved in that the bar screen according to the invention presents the features stated in the characterizing portion of claim 1.

Further, by each screen element being attached to a frame, distributors for the material fed out to the screen surface may be disposed across the entire width of the screen and in a simple manner may be integrated in the structure, and also fixed screens for the material delivered from the bar screen as well as guide means therefor may be arranged under the bar screen and may be integrated in the structure in a simple manner.

These and other features are evident from the subclaims and the following description of an embodiment of the invention with reference to the drawings attached.

Description of the figures

Fig. 1 is a schematical, partly cut perspective

view of a bar screen according to the invention,

Fig. 2 is a partly cut side view of the bar screen of Fig. 1 ,

Fig. 3 is a sectional view along the line III-III of Fig. 2,

Fig. 4 is a sectional view along the line IV-IV of Fig. 2,

Fig. 5 is a sectional view along the line V-V of Fig. 2, and Figs. 6a - 6e are schematical representations of various configurations of the movement pattern of the bar screen according to the invention.

Preferred embodiment The bar screen base 1 consists of a horizontal frame 1 on which the other parts of the bar screen are disposed. The base 1 supports two frames 2 and 3 via a driving device 4 and a pendulum device 5. The base 1 also supports four chips feeding out chutes 6a - d. The frame 2 forms part of a first screen element which also consists of a plurality of bars 7 or strips disposed on the frame, which bars suitably have a substan¬ tially rectangular cross section. The bars 7 are parallel and equally spaced apart. The frame 3 forms part of a second screen element which also consists of a plurality of bars 8 or strips disposed on the frame, which bars are of the same kind as the bars 7 and are parallel to each other and equally spaced apart. The bars 8 project into the inter¬ spaces between the bars 7 so that adjacent bars will be equally spaced apart in the horizontal plane such that gaps are formed between them.

Adjacent their right-hand ends according to Figs. 1 and 2, both frames 2 and 3 are provided with two pairs of bearings 9 and 10 respectively, for example, ball bearings. A shaft 11 supports, with its eccentric races 12 and 13 respectively, the frames 2, 3 at the right-hand portions of the same via the bearings 9, 10. Both ends of the shaft 11

are rotatably mounted in bearing brackets 14 on the base 1. The shaft 11 is connected with a driving device, such as an electric motor 15.

Adjacent their left-hand ends according to Figs. 1 and 2, the frames 2, 3 are supported by two pairs of pendulum elements 17 and 16 respectively. As is evident from Figs. 1, 2 and 5, the pendulum elements 16 are constituted by arms the lower ends of which are pivotally mounted on a shaft 18 attached to the base 1. At the upper end of each arm 16 two linkage arms 19 are pivotally mounted. The upper ends of the linkage arms 19 are fixedly connected with the frame 3. Each pendulum element 17 consists of an arm the upper end of which is pivotally mounted on a shaft 20 which is attached to a clamp 21 mounted to the base 1. The lower end of the arm 17 is pivotally connected with a pair of linkage arms 22 which are attached to the frame 2.

On the underside of the inner frame 3 there are attached two perforated screens 23 and 23a located one under the other and a plate 24 located thereunder, forming an angle with the horizontal plane. The outlet end of each perforated screen 23, 23a and of the plate 24 open above a chute 6a - 6d, as best can be seen from Fig. 2.

On the upper side of the outer frame 2, at its right-hand end according to Fig. 1, a distributor is attached, which distributor is in the form of a box 25 the base of which is inclined. The box 25 has no wall in its left-hand part and extends across the entire width of the frame 2. The material (chips) to be screened is tipped into the box 25 and is distributed over the screen elements 7, 8, in operation.

The function of the above described bar screen will be described below.

The chips poured into the distributor 25 and the transportation of which through the bar screen is indicated by arrows provided with dash-dotted lines in Fig. 2 leave the distributor and are evenly spread across the screen elements on starting of the motor 15, and an orbiting eccen-

trie movement is imparted to the frame 2 in the right-hand portion thereof and an essentially reciprocating movement is imparted to the frame in the left-hand portion thereof. The corresponding movements are also imparted to the frame 3, but these movements are displaced 180° relative to the movements of the frame 2. Owing to the movements being non- uniform along the length of the frames, and therefore also along the bars 7 and 8, and owing to the distance of move¬ ment becoming shorter in the vertical direction from the right-hand ends of the screen elements to the left-hand ends thereof, the chips will be transported to the left on said elements. During the transport, the largest chip fractions will remain on top of the screen elements and will there¬ after fall into the chute 6a at the left-hand end of the bar screen. Smaller chip fractions fall through the bar screen and collect on the perforated screen 23, whose movement pattern is the same as that of the frame 3. Larger chip fractions falling onto the screen 23 are fed out through the chute 6b while smaller chip fractions fall through the holes of the screen and collect on the perforated screen 23a. After screening by means of the screen 23a, larger chip fractions will be fed out to the chute 6c and smaller frac¬ tions will fall onto the plate 24, which feeds out the smallest fractions to the chute 6d. Figs. 6a-e are schematic views of various configu¬ rations of the structure and location of the pendulum device 5. The arrows on the left in said figures indicate the move¬ ments made by the left-hand ends of the frames 2, 3 and their screen elements 7, 8 during operation of the bar screen according to the invention. The different end posi¬ tions of the pendulum arms are indicated in dashed lines. Fig. 6a schematically shows the configuration of Figs. 1 - 5 with the shaft 11, the bearings 9, 10 connected with the frames 2 and 3, the frames 2, 3 with the screen elements 7, 8, the pendulum arms 16 and 17 and the shafts 18 and 20, which form two supports.

As will be seen from Fig. 6a, on rotation of the

shaft 11 the right-hand ends of the frames 2 and 3 will describe orbiting movements having both horizontal and vertical movement components. The orbiting movements are circular and counter-directed, implying that the right-hand ends of the frames 2, 3 will alternately get nearer to and retreat from one another both vertically and horizontally. The pendulum arms 16 and 17 pivotally connected with the frames 2, 3 will turn round the supports 18 and 20, respec¬ tively. The left-hand ends of the arms 16, 17 according to Fig. 6a will therefore describe comparatively large -counter- directed movements in the horizontal plane and small counter-directed movements in the vertical plane.

From Figs. 6b and 6c may be seen that two supports 26, 27 corresponding to the supports 18 and 20 in Fig. 6a are located adjacent each other. On rotation of the shaft

11, the right-hand ends of the frames 2, 3 will describe the same movements as they do in Fig. 6a while their left-hand ends will pivot in almost parallel curved travel paths. Fig. 6b differs from Fig. 6c in that the supports 26, 27 in Fig. 6b are located under the frames 2 and the corresponding supports 28 and 29 in Fig. 6c are located over the frames 2, 3. In Fig. 6c the ends of the arms 16, 17, which ends are pivotally connected with the frames 3, 2, pivot in almost parallel paths with centres of curvature located above said arms.

Fig. 6d shows supports 30 and 31 located in a plane between the frames 2 , 3. In this embodiment the left-hand ends of the frames 2, 3 pivot in counter-directed paths with centres of curvature located close to each other. Fig. 6e shows three frames 32, 33 and 34 which are intended to correspond to three frames and associated sets of screen elements instead of two, as illustrated in the other figures. The right-hand ends of the frames 32 - 34 are attached to three bearings and describe orbiting (circular) movements, while their left-hand ends are pivotally attached to pendulum arras 35, 36 and 37, which in turn are pivotally attached to supports 38, 39 and 40 and describe essentially

parallel, reciprocating movements according to the arrows with the supports 38 - 40 as centres of curvature.

As will be evident from Figs. 6a - e, the structure of the pendulum device 5 may be varied in various ways by altering the location of the pendulum arms and of the supports. Further variations may include alteration of the length of the pendulum arms. However, the structure of the pendulum device 5 is selected such that effective screening of different materials and material sizes will take place and a uniform transportation of material fed out onto the screening elements will take place towards the discharge end of the screen elements, i.e. at the left-hand end of the bar screen in Fig. 1.

Also the structure of the driving device 4 may be varied in order to provide effective screening. Thus, for example, the eccentric movement at the right-hand ends, in the figures, of the screen may be replaced by some other movement; however, it is to advantage if the vertical move¬ ments of the screen elements are counter-directed. While only a few embodiments have been described above and shown in the drawings it will be understood that the invention is not limited to said embodiments but only by what is stated in the claims.