CA2093342A1 - Bar screen having a reciprocating action - Google Patents

Abstract

A bar screen (10) for sizing wood chips which includes a plurality of parallel bars (12-12) supported at their respective ends (14, 16) in such a manner and driven in such a manner that each bar moves both longitudinally and vertically. Two alternating sets of bars are included, one set being 180· removed in position relative to the other set. Each set of bars alternate between relatively high and low vertical positions. When each set is in its relatively high position, the bars are moving longitudinally toward the infeed end of the bar screen (10). The bar screen (10) is inclined downwardly from the infeed end to outfeed end at an angle of approximately 15·.

Description

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i BAR SCREEN HAVING A REClPROCATING ACTION

This is a continuation-in-par~ of U.S. patent application Serial 5 No. 390,620, filed on August 7, 19~9, in the name of Gevan R. Marrs.

Technical Field This invention relates generally to the particle sizing art, such as for wood chips, and more specifically concerns a bar screen type particle sizing 10 apparatus.

Background of the Invention In the sizing of wood chips for various purposes, such as for pulping, it is well known to use a combination of gyratory and disc screens in order to 15 accomplish the desired separation of wood chips into dimensionally acceptableand non-acceptable (typically over-thick) portions. The gyratory screen typicalJy comprises a flat plate or sheet having holes of a selected size and dimension punched therein, while the disc screen comprises a plurality of closely spaced thin discs vertically mounted on successive hori~onal rods. Each row of discs is 20 interleaved to an extent with the discs from adjacent rows. The relative spacing of the discs in a single row and the spacing between successive rows of discs are selected relative to the sizing function being accomplished, i.e the desired dimensions of the particles. An example of such a combined gyratory and disc screen system is shown and described in U.S. Patent No. 4,376,042 to Brown.
Neither the gyratory screen alone, nor the disc screen alone are very efficient, and even the combination of the gyratory screen and the disc screen~
with a combined efficiency of approximately 90%, still leaves significant room for improvement, with resulting cost savings. The gyratory screen is relatively inexpensive to manufacture and maintain, but as stated above, is so inefficient 30 that it normally cannot be used by itself The disc screen, while more efficient than the gyratory screen, is expensive to manufacture, construct and maintain.

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Approxirnately 75% of all current screer~ systems comprise only a disc screen Further, while the gyratory screen can be readily and inexpensively replaced or altèred to change the screen surface openings after installation, disc screens are very difficult and expensive to alter once installed. In particular, a change in the size of the disc screen openings basically requires replacement of the entire screen, which as mentioned above, is quite expensive. In addition, the disc screen elements will physically wear over time and will require replacement, which is expensive.
The combination of a gyratory screen and a disc screen, which is now fairly commonplace in the industry, minimizes many of the disadvantages of disc screens while maintaining the respective advantages of both the gyratory screen and the disc screen, but the combination still has some significant disadvantages both in efficiency and overall expanse.
It would be most desirable to have a single sizing screen which is both efficient and inexpensive to manufacture, install and change if necessary.
Single screening devices known as bar screens are an example of such a single sizing screen. Bar screens typically comprise a plurality of longitudinal bars which are spaced apart a selected distance, depending upon the size of the particles to be processed. In certain applications, including chip sizing, it is known that relative movement of the bars is important for proper operation of bar screens, especially to avoid plugging, and further that the bar movement be such that alternate bars are always at different points in their indiYidual cycles of movement so that there is always a relative difference in position of adjacent bars. Examples of bar screens having these capabilities are shown in U.S. PatentNo. 4,660,726 to Woode and Swedish Patent No. 88,615 to Granquist.
However, it has been well established that bar screens in general have significant pluggin~ problems, i.e. the chips or other particles being processed by the screen become caught between the bars, preventing acceptable chops from falling through. As a result, bar screens are not widely used for particle sizing, including sizing of wood chips. Bar screens are also typica]ly characterized by an inability to tip up the particles being processed on edge so .. . . . . .
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that SiZillg may be accomplished on the basis of thickness. Since thickness is currently a very significant parameter in sizing wood chips, bar screens are not~ypically used in new wood chip sizing installations.
Hence, there is at the present tîme no inexpensive, easy to mainta~n, S yet highly efficient screening apparatus for sizing wood chips and the like.
Disclosure of the Invention Accordingly, the present invention is a bar screen which is adapted to separate wood chips and the like in accordance with preselected dimensional criteria, comprising a plurality of elongated bars which are connected together so as to define a screening member, wherein the screening member has an infeed portion and an opposing outfeed portion and wherein the screening member is arranged and supported such that said screening member inclines downwa~ly from the infeed portion to the outfeed portion. The apparatus further incll~des means for moving the elongated bars both longitudinaily and vertically, wher~in the movement of the elongated bars tends to inhibit the flow of wood chips ~rom moving from the infeed to the outfeed to the screening member.

Brief Description of the Drawings Fig~re 1 is a schematic view of a particle (wood chip) s~zing apparatus incorporating the principles of the present invention.
Figure 2 is a schematic view showing an end portion of several bars in the bar screen of Figure 1 and the means for supporting and moving the bars.
Figures 3A-31) are side elevational views showing the relat~ve movement at various points in time of the end portions of two adjacent bars in the bar screen of Figure 1.

Best Mode for Ca~he Invention Figure 1 shows the particle bar screen of the present invention, arranged to separate a wood chip flow into those chips which are within an acceptable pre-established size range and those chips where are oversize in the .... , : - .
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thickness dimension, i.e. over-thick. The bar screen is shown generally at 10 and comprises a plurality, i.e. 25-30, of parallel rig~d metal bars 12-12. The details of the geometry of the bar screen 10 and the bars 12, i.e. length, width, thickness, number, material and relative spacing will vary significantly from apparatus to 5 apparatus. A preferred embodiment is dis,closed in det~il hereinafter.
The s~lccessive individual bars 12-12 are supported by a cam and camshaft arrangement positioned at both the forward end 14 and rear end 16 of the bar screen 10. In the embodiment shown, each bar 12 has a can~ element 18 (Figure 2) positioned in each end thereof. A camshaft 20 extends through an 10 opening 21 in each cam element, thereby connecting the bars 12-12 together attheir forward ends. A sirnilar rear camshaft 20a connects the rear ends of the bars 12 in similar fashion. The opening 21 in the respective cam elements 1~18 is offset from the center of the cam element and is configured tO receive the camshaft 20, which is square in cross section, so that rotation of the camshaft 20 15 will result in movement of the bars 12-12. In the embodiment shown, ~he arrangement of the camshaft 20, the cam element 18 and the bars 12 is such that each point on the bar will move in a circle approximately one inch in diameter.
Thus, for instance, the upper front corner of bar 12 will move vertically a total of one inch and horizontally a total of one inch during one complete revolution 20 of camshaft 20.
In the embodiment shown, the individual bars 12-12 are separated by spacers 22-22, with the camshaft 20 extending through an opening 23 in the spacers 22-22. The details of the cam elements 18, shaft 20, and spacers 22 are discussed hereinafter. A drive motor 24 is conneGted to the camshaft 20. The 25 free end of camshaft 20, which is relatively away from the drive motor 24, includes a sprocket 26. The rear camshaft 20a also includes a sprocket 28 on oneend thereof, on the same side of the bar screen 10 as sprocket 26, so that sprockets 26 and 28 are in the same plane. Sprockets 26 and 28 are connected by a chain 30 so that when camshaft 20 is driven by drive motor 24, the rear 30 camshaft 20a is driven as well.

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''' 20933~2 In the embodiment shown, the bar screen 10 is supported on a base assembly shown generally at 32 which is open at its top, where the bar screen lais located. A conventional feed conveyor :34 moves particles to be sorted on~c~
a downwardly inclined feed tray 36, which c onnects the feed conveyor 34 to the rear end 16 of the bar screen 10. At the bolttom of the base element 32, beneaththe bar screen 10, is an "accepts" conveyor :38 which moves those particles which have fallen through the bar screen 10 in operation thereof, ancl hence have dimensions in the acceptable range, to a desired destination, such as the input ~o a pulping station or to storage. At the forward end 14 of bar screen 1() is an outlet tray 38 which guides the chips remaining on top of the bar screen 1~
during operation thereof, when they reach the forward end 14 of the bar screen 10, to an o~er-thick conveyor 40. The conveyor 40 moves the over-thick chips to a slicer or other device (not shown) for reducing the si~e of the chips or to some other destination.
Typically, but not necessarily, bar screen 10 is supported so that it is flat in the embodiment shown. However, the bar screen 10 could be inclined downwardly, which would assist in the conveying, i.e. movement, of the chips frorn the rear end to the forward end of the bar screen 10, or the bar screen 10 cou~
be inclined in the other direction, i.e. upwardly, in the particular application. 1~2e precise amount of the incline, of course, can be selected for a particular application.
The details of the bar screen 10, in particular an embodiment of the structure for supporting and moving the bars 12-12, is shown in Figures 2 and 3.A portion of the plurality of bars 12-12 comprising the bar screen 10 is shown in Figure 2; Each bar 12 includes a cam element 18 near the front end thereof. In the embodiment shown, the cams 18 are circular and fit into a mating cutout portion 43 in a bar 12. In the embodiment shown, the bars 12~12 are rectangular and relatively thin, approximately 1/~ inch thick. Generally, the thickness of the bars 12-12 is the minimum thickness which will result in an acceptable amount of deflection at the midspan point of the bars.

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In the embodiment shown, the thickness dimension of the bars remains constant from the tOp ed8e 42 of the bar ~o the bottom 44 thereof. This appears to be the most effective cross-sectional shape, since it is easy lo manufacture and inexpensive tO replace. Other cross-sectional shapes could be S used, including elliptical, or round, or the bars could be inversely wedge-shaped with the large dimension at the top, or still ~urther, the upper portion of the bar could be peaked so as to tip up the chips more effectively.
In the embodiment shown, the top surface 42 of each bar is smooth and flat. However, it could be roughened to some extent, or knurled or even 10 have teeth of a selected size, to assist in the movement and agitation of thematerial being screened. The teeth could be relatively small, i.e. one-quarter inch, up to tow inches high or more. The side surfaces 52 of the each bar 12 mayalso have some contour or be abraded to some extent. In the embodiment shown, the sides of adjacent bars are parallel, such that successive slots, defined 15 by adjacent bars, have the same width from top to bottom. The slots, however;may also have other shapes, including a configuration in which the space betweenbars increases *om tOp to bottom, which could be accomplished by using tapered bars.
Referring still specifically tO Figures 2 and 3, camshaft 20 ex~ends 20 through each one of the cams 18 positioned in the bars 12. In the embodiment shown, the camshaft 20 is square in cross-section, but could also have other shapes. The cam 1B is positioned midway between the top and bottom ed~es of each bar 12. As mentioned above, the camshaft is offset from the center line of the bar so as to give the amount of bar movement desired. Positioned between 25 the successive bars are spacers 22 which in the embodiment shown are circular, having a diameter slightly greater than the width of bars 12-12 and approxima~ely the same thickness. The thickness of the spacer is independent of the thickness of the bar and is typically chosen to produce a slot width which will produce ~he desired particle separation. The slot width is slightly smaller than the desired30 particle size, i.e. thickness, due to midspan deflection of the bars. The camshaft 20 extends through a matching hole 23 in each spacer 22. In the embodiment ., ,.. ,~. .,~., .,, ~ . .

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The position of the cams 18 and in particular the openings ~I
therein in the bars 12 determine the magnitude of movement of individual bars relative to the camshaft 20. The relative positioning of the cams 18 in successive 10 bars then determines the movement oE the bars relative to each other. For instance, in the embodiment shown, there are in effect two sets of alternating bars with each set comprising half the bars in the screen 10. Both sets of bars follow the same path of rnovement; however, the opening 21 in the cams 18 o~
one set of bars in the embodiment shown is 180 removed from the openings in 15 the other set of cams so that the position of one set of bars will be exactly 180 removed from the position from the other set of bars at any one time. In the embodiment shown, the bars from the two sets are arranged in alternating succession. As shown in Figure 2, the cams 18 in one set of bars are at their highest position, so that the bars of that set are all high, while the cams in the 20 other set (alternating with first set) are all low so that the bars of the other set are all low. This resul~s in a high, low, high, low, etc. bar pattern, as shown in Figure 2. While the embodiment shown comprises two sets of alternating bars at 180, the screen 10 could comprise 3 or even more sets of alternating bars, although the sets should be balanced relative to each other (3 sets at 120, 4 sets 25 at 90, etc.), so as to minimize vibration and wear on bearing surfaces.
Figures 3A-3D show the concept of relative movement of the h~o sets of bars of the present embodiment in a more detailed matter. A first bar 66is shown with its cam 68, as well as a second adjacent bar 70 with its cam 72. Aspacer 74 separates the 2 bars 66 and 70. Camshaft 76 extends through cams 68 and 72 and the spacer 74. Camshaft 76 is fixed in position, other than rotating . ~ , .

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counter-clockwise. Hence, movement ot the two bars 66, 70 is about the camshaft 76.
Figure 3A shows camshaft 76 at a first rotational position, in which the first bar 66 is in a relatively folward anrl relatively high position while bar 70 S is in a relatively low and rear position. The two bars are exactly 180 out of phase, because the cams are positioned in the bars such that the respective openings therein are 180 apart.
Figure 3B shows the relative position of the two bars 66 and 70 following movernent of the camshaft 76 90 in a counter-clockwise direction. In this position, the first bar 66 is in a still relatively forward but now relatively low position in its path of movernent while bar 70 is still relatively to the rear b~t also now relatively high.
Figure 3C shows the relative position of the two bars 66 and 70 when the camshaft 76 has moved another 90 counter-clockwise. In this position of the camshaft, the first bar 66 is now relatively to the rear and still relatively low, while the bar 70 has now moved to a forward position and is still relatively high.
Figure 3D shows the relative position of the bars ~6 and 70 upon further rotation of the camshaft another 90. In this position, the first bar 66 is still relatively to the rear but is now relatively high, while bar 70 is still relatively forward but now is relatively low as well. Another 90 rotation of the camshaft will bring the two bars 66, 70 back to the posi~ion of Figure 3A.
As indicated above, it is important that there be both vertical and horizontal relative motion between the bars comprising the bar screen. This helps to tip the chips upwardly on edge so as to present the thickness dimensionof the chips to the slots and keeps the screen from plugging. The range of motion, in particular the vertical cornponent of the movement, could be anywherefrom 1/16th inch to 3 inches, although a range of 3/4 inches to 1-1/2 inches would be typical for wood chips.
In the embodiment shown there are, as indicated above, two separate sets of bars which comprise the bar screen 10, with bars of one set ',:: ' ' . ' '~: " ' wO 92/05~ 9 3 3 '1 2 pC ~'/ l !.'il,~ l /0'7'2 ~ ~

alternating with bars o~ the other set. Referring to Figures 3A-3D, bar 66 is representative o~ the movement of all the bars in one set, while bar 70 is representative of the movement of all the bars in the other set. However, it should be understood that other bar movements patterns could be used, including S non-alternating sets, such as two consecutive bars up, to consecutive bars down, or other patterns. In addition, as mentione:d above, there could be more th~n two sets of bars, positioned at balanced angular intervals.
It should be noted that in the embodiment shown, the motion of the bars 12 is such that both ends of the bars 12 are always in the same relative vertical position, i.e. the respective ends of each bar always move in tandem, i.e.
unison, as opposed to the relative position of the bar ends changing on a regular basis. Patterns of bar motion can be produced in which the incline or slope of the bar will vary on a cyclical basis. Still further, different types of bar drive devices could be utili~ed such that the ends of the bar will follow a different path than the circular path of the embodiment shown.
Also, different driving systems could be used for the bars. For instance, the two sets of bars could be supported on two separate frames, with the frames then driven by separate cams. A single cam could also be used with a connecting arm to the frame.
The inventors have discovered that both the relative movement of the bars and the speed of the bars are important to achieve effective screening action, keep the screen clear and prevent plugging. Movement of the bars alone is not sufficient to prevent plugging. In the embodiment shown, the speed of thebars as well as the horizontal and vertical movement of the bars are such in combination that sufficient acceleration is imparted to the wood chips Iying on top of the screen that the chips move about on the bar screen and encounter a slnt between two adjacent bars. Usually, the acceleration will be approximately lG (32 feet per second per second) or slightly greater, such that the chips willjust break contact with the bars at some point (overcome gravity) and will tend to become vertically oriented as they encounter a slot. In the embodiment shown, a camshaEt speed of 228 RPM is sufficient to achieve the desired results . .
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by imparting sufficient acceleration to the wood chips tha~ the chips move abouton the bar screen, the chips tending to be tipped up on edge and fall between adjacent bars, thereby encountering a slot in the screen. It is the combination of both vertical and horizontal motion of the bars and sufficient speed thereof which 5 has been found to be effective.
In the operation of a bar screen such as shown and described above, it has been observed that there are three basic conveying variables. The first variable is the inclination of the screen. The inclination may be zero, i.e. thescreen is flat, or the screen may be inclined either downwardly from the infeed 10 end to the outfeed end, or vice versa, upwardly from the infeed end to the outfeed end. When the screen is inclined downwardly, conveying of the input material is promoted, while when the screen is inclined upwardly, conveying of the input material is inhibited. The amount of the inclination of course will directly affect the extent to which the material is either promoted or inhibited.
The second variable is the rotational direction of the individual bars.
Rotating the bars in a direction corresponding to the flow of material, e.g. from the infeed end to the outfeed end, such as described in the above embodiment, will tend tO promote the conveying of material from the infeed end toward the outfeed end, while rotating the bars in the opposing direction will tend to inhibit 20 the conveying of material toward the outfeed end. The speed of rotation will affect the extent of the promotion/inhibition of the conveying of the input material.
Thirdly, the flow of the input chip mass itself will have a significant effect on the conveying of the input rnaterial. The greater the amount of input 25 material which is placed on the bar screen, referred to generally as the "loading"
of the screen, the greater the flow pressure there is on input material already on the screen. Any increase iri flow pressure tends to promote the conveying of theinput material already on the screen. When the screen is lightly loaded, there is little flow pressure created by the input material itself and there is accordingly 30 little impact on the conveying of the input material, ". .-, . . .
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In one particular embodiment, the screen 10 is inclined ~lownwardly from the infeed end to the outfeed end. The slope of the screen in this particular embodiment is fairly steep, approximately 15, but this could be altered to an extend, within the general range of 5' to 3(P. The bars 12 are rotated in S a reverse direction, i.e. against the flOw of input material, and opposite to ~.hat described above. In such an arrangement, the directional arrows of Figures 3A-3D are reversed from that shown. Such an embodiment thus combines a screen inclination which tends to promote the conveying of the input material from the infeed end of the screen to the outfeed end and a screen bar rotation 10 direction which tends to inhibit the conveying of the input material. In ~he embodiment shown, the speed of rotation of the bars is approximately 22û
revolutions per minute. This could be varied, within a range of 150 to 450 revolutions per minute, depending upon the amount of loading, the characteristics of the input material and the particular inclination of the bar screen.
The loading of the bar screen may be varied depending upon lthe type of input material used, as well as other characteristics of the system. With respect to all three variables discussed above, good results have been obtained with an input of wood chips having approximately 1~o over-thick chips grea~er than 8 mm~ with a loading of 0.15-0.22 units per hour per square foot of scr~en 20 area, when the inclination of the bar screen is downward at 15 and the bars are rota$ed in a reverse direction at 220 rpm. Such a system provides a good retention time of wood chip mass on the screen, and produces a very high percerltage removal of chips within the acceptable size range (85%-95%). The significant advantage of this embodiment is a performance level which 25 approaches or even surpasses that of a conventional disc screen, while providing significant improvements relative to the disc screen in terms of overall cost ancl maintenance.
Hence, an apparatus has been described which is capable of performing particle sizing, such as for wood chips, in a practical and efficient way.
30 The bar screen of the present invention is inexpensive to manufacture and maintain, particularly compared to convention disc screens, and ~urther more is , , . , . . ,. '.' ,: : ' .
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Although embodiments of the invention have been disclosed herein S for illustration, it should be understood that various changes, modifications and substitutions may be incorporated in such embodiments without departing from the spirit of the invention which is defined by the claims which follow:

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Claims (11)

1. A bar screen adapted to separate wood chips and the like in accordance with pre-selected dimensional criteria, comprising:
a plurality of elongated bars, connected together so as to define a screening member, wherein the screening member has an infeed portion and an opposing outfeed portion, and wherein the screening member is arranged and supported such that said screening member inclines downwardly from the infeed to the outfeed thereof;
means for moving said elongated bars both longitudinally and vertically, wherein the movement of the elongated bars is such as to tend to inhibit the flow of wood chips from moving from the infeed to the outfeed of thescreening member.

2. An apparatus of Claim 1, wherein the moving means includes means for moving a first plurality of bars in a longitudinal direction toward the infeed portion and simultaneously moving a second plurality of bars toward the outfeed portion during the time that the first plurality of bars is higher vertically than the second plurality of bars, and for moving the first plurality of bars toward the outfeed and the second plurality of bars toward the infeed when the second plurality of bars is higher vertically than the first plurality of bars.

3. An apparatus of Claim 2, wherein the longitudinal and vertical movement of the bars imparts sufficient momentum to the chips that substantially all the chips tend to a vertical orientation, thereby encountering a slot between two adjacent bars in the thickness dimension.

4. An apparatus of Claim 3, wherein the momentum imparted to the chips is such that substantially all the chips break contact with the bars at some point in time.

5. An apparatus of Claim 2, wherein the screen is angled downwardly at an angle within the range of 5°-30°.

6. An apparatus of Claim 5, wherein the angle of the screen is approximately 15°.

7. An apparatus of Claim 2, wherein the speed of rotation of the bars is within the range of 150-450 revolutions per minute.

8. An apparatus of Claim 7, wherein the speed of rotation is approximately 220 revolutions per minute.

9. An apparatus of Claim 6, wherein the speed of rotation is approximately 220 revolutions per minute.

10. An apparatus of Claim 4, wherein the bars are elongated, generally rectangular and plate-like in configuration, having substantially the same thickness from upper edge to lower edge thereof, and further including spacer elements which are positioned between successive bars.

11. An apparatus of Claim 2, wherein the bars area supported such that the first plurality of bars move in unison and the second plurality ofbars move in unison, wherein the bars comprising the first plurality of bars alternate with the bars in said second plurality of bars, and wherein the position of the bars in the first plurality is always approximately 180° removed from the bars in the second plurality.