WOOD CHIP SCREENING APPARATUS WITH BLADES HAVING NONLINEAR TOP EDGES

Abstract: A wood chip screening system has an infeed end, outfeed end and elongated thin blades (40, 42) mounted for reciprocating motion extending therebetween. The blades (40, 42) are spaced apart to allow chips with an acceptable thickness dimension to pass therethrough. The improvement includes configuring the top edges of the blades (40, 42) in a nonlinear design.

Description: WOOD CHIP SCREENING APPARATUS WITH BLADES HAVING NONLINEAR TOP EDGES BACKGROUND OF THE INVENTION This invention relates generally to sorting wood chips prior to a pulping operation where an incoming flow of chips has a portion with chips of acceptable thickness and a portion of unacceptable thickness. More particularly it relates to a wood chip sorting device where elongated spaced apart reciprocating blades allow chips of an acceptable thickness to pass therebetween while the overthick chips are conveyed atop the blades to the outfeed end. Chip sorting machines utilizing elongated parallel blades or bars that reciprocate are well known and are presently used in chip sizing and sorting operations throughout the world. It is known that the digesting stage of wood pulp manufacture is particularly sensitive to the thickness parameter of incoming chips. For that reason chip "screening" is an important process step upstream of the pulp mill. For example, for typical North American softwood species, it has been determined that wood chip process overall economics are most favorable when the thickness falls within a range of from 7-9 mm. In the recent past both disk screens and bar screens have been utilized by industry to sort incoming chips according to thickness. Disk screens have been installed but tend to be relatively difficult to maintain and are expensive. Blade or bar screens have become more popular in the past few years. Blade screens are designed to provide a high capacity (chips by weight or unit volume per hour per unit area of screen surface) processing system while reducing the number of acceptable chips remaining within the unacceptable portion flowing past the outfeed end. An example of the important considerations can be reviewed by referring to U.S. Patent 5,368,168 in the name of Marrs et al., which disclosure is incorporated herein by reference. This patent, among other teachings, discloses a staggered height feature for the bars or blades whereby chips are tipped up by the action of the bars so the thickness dimension tends to be presented more effectively to the openings between the bars. Such enhanced tipping causes more chips with an acceptable thickness to fall between bars thereby increasing screening efficiency. Another U.S. Patent, No. 5,305,891, also discloses a similar screening system for sizing and sorting wood chips. In this patent two sets of interleaved bars are driven at 200-250 rpm and have the bars in at least one of the sets at variable heights, i.e. non-coplaner. All of the top edges on each of the bars are, however, linear and parallel with on another. It is further stated that the individual bars should be one- half inch in thickness and one and one-half to three inches in height from top to bottom. During operation bar displacement on the order of 1 " is preferred. While the screening devices and methods disclosed in the above two referenced patents with their staggered bar heights yields an improvement in capacity, further improvement is desirable in order to further limit "accepts carry-over" without the need for more equipment. Ideally, a set of elongated reciprocating screening blades should function to cause, as quickly as possible, as many of the chips to be tipped so as to present the thickness dimension to slots between blades while simultaneously motivating the overthick chips atop the blades toward the outfeed end. As noted in the referenced patents, tipping of chips can be effectuated by blades having a staggered height thereby allowing larger relative displacements between adjacent blades than vertical reciprocation alone provides. Unfortunately, when all of the blades or bars have linear top surfaces throughout their length there is not an effective restraining force created to provide an optimum dwell time for the chips when they are loaded at the infeed end. While tilting a blade screen results in a restraining force, tilting can also result in a reverse flow which is undesirable. Similarly, flat parallel top edges, even with staggered heights will not provide an optimum agitation action to the bottom surface of the chip mat as it is formed and conveyed atop the blades. This is a particular problem as screen mass per unit screen are per unit time loading rates are increased and the semi-coherent "chip mat" is formed, wherein individual chip reorientation and tipping is restrained by adjacent chips. Thus, it becomes apparent that if a wood chip blade screening device could be provided with good chip tipping, restraint of flow rate, and good mat agitation an improved blade screen will result. Not only will "accepts carry-over" be reduced but the improved system will be highly efficient and cost effective. Such then, is the primary object of the present invention. SUMMARY OF THE INVENTION Briefly stated, the invention is a wood chip screening system having parallel spaced apart blades or bars which are mounted for reciprocating motion in a vertical and horizontal plane with the improvement comprising a portion of the elongated top edges of the blades having a nonlinear configuration. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a blade screen apparatus in a commercial environment. Fig. 2 is a perspective view of a portion of the blade screen of the present invention, showing a particular nonlinear blade configuration. Figs. 3-7 illustrate alternative nonlinear configurations for the top edges in a blade screen of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to Fig. 1, the general structure and environment utilizing the present invention will be described. Fig. 1 illustrates a typical bar or blade screen apparatus for sorting wood chips. The screen apparatus referred to generally at 10, is positioned on a base support assembly 12 which in Fig. 1 supports the screen apparatus 10. The screen apparatus could be positioned at various angles, either inclining downwardly or upwardly in the direction of the outfeed end, or it could be flat. In a typical embodiment, the screen itself will be approximately ten feet long, with a selected width. In commercial embodiments, the widths typically will be 6V-., 8V_ or 10 feet. The screen apparatus will typically include two sets of interleaved blades, with each set being driven separately but such that the blades in one set have a selected spatial relationship with the blades in the other set. An example of such a screen with two sets of interleaved blades is shown in U.S. Patent Nos. 5,117,983 to Marrs and 5,368,168 to Marrs et al., assigned to the same assignee as the present invention, the contents of which are hereby incorporated by reference. In the particular embodiment shown in Fig. 1, one set of blades is supported at the respective ends thereof by end support assemblies 14 and 16, while the other set of blades is supported by end support assemblies 19 and 20. The end support assemblies shown in Fig. 1 are adapted specifically to maintain each individual blade under tension. Such a system is necessary, for instance, where the elements are relatively thin and would otherwise twist or bend significantly in operation. Such a tension type bar screen and its supporting elements are shown in more detail in U.S. Patent No. 5,284,251 to Marrs, assigned to the same assignee as the present invention, the contents of which are hereby incorporated by reference. The two sets of interleaved blades in the apparatus of Figure 1 are driven by a drive mechanism 22, which is positioned near the infeed end 24 of the screen apparatus. The drive mechanism 22 operates from a motor via gears, drive shaft and eccentrics to flange elements which connect to the support frames in which the respective two sets of blades are mounted. The driving action is also transmitted to the outfeed end of the support frames by means of a belt assembly and pulley arrangement shown generally at 26. In operation, the two sets of interleaved blades are operated at a relatively high speed, within the range of 200-350 rpm, with the movement of one set of blades being approximately 180° behind the blades in the other set. This is explained in more detail in the '983 patent. Wood chips from a loaded hopper or other container (not shown) are loaded onto screen 10 at infeed end 24, shown diagrammatically in Fig. 1. The wood chips are bounced around by the action of the interleaved screens, with the individual chips being tipped up so that their thickness dimension is presented to the openings between the blades. Those chips having a thickness dimension less than the opening size between adjacent blades will fall through the screen to a conveyor 32 which moves those acceptable size chips to a predetermined location for storage or processing into pulp or other use. Those chips which have a thickness dimension larger than the screen openings or which for some reason where not tipped properly so as to present their thickness dimension to the openings eventually move off the outfeed end 28 of the screen to another conveyor 34 which moves those chips to another station for further size processing, such as to an apparatus known as a chip slicer, or similar device, which reduces the thickness of the chips. The chips from the chip slicer are then typically added directly to the acceptable chip flow but could be returned to the screen. The above process continues as long as chips are loaded onto the screen. As indicated above, screen capacity is a significant issue in screen operation. Those screens which have a fairly large percentage of open area offer what would appear to be an inherent capability for a large loading capacity. The invention described in the '168 patent is designed to significantly improve bar screen loading capacity, particularly for those screens which use relatively thin blades. The invention includes two sets of interleaved blades, similar to a conventional bar screen. In the '168 patent description, however, selected ones of the blades/bars in each set are higher than the remainder of the blades, i.e., the blades present a "staggered" appearance over at least a portion, if not all, of their length. In the staggered configuration not only is thickness sorting carried out but sorting capacity with an acceptable carry-over rate is effectively increased with a smaller bar screen size. The present invention, incorporating a nonlinear configuration for the top edges of the blades, has been found to increase capacity even further, thus allowing a smaller overall size for the screening apparatus. The present invention can be incorporated into a staggered height blade screen or into one which does not utilize any height differential. Turning now to Fig. 2, a particular embodiment of the present invention will be described. Each set of blades has a plurality of spaced apart individual thin blades, indicated at 40 and 42 respectively in each sent, where each set is mounted in the typical reciprocatable frame. In the typical screening apparatus, as described in the '983, '168 and '251 patents, there will be at least two independent reciprocatable sets of blades or bars interleaved with one another. In Fig. 2 blades 40 and 42 have nonlinear top edges. A preferred configuration is a cyclic, smooth repeating pattern such as a "sinusoidal wave" having peaks and valleys with sloping edges therebetween. In one set with blades 40, the blades are substantially in lateral alignment where the peaks of adjacent blades are aligned In the other set with blades 42 they, too, are substantially in lateral alignment; however, the peaks in this set are longitudinally spaced from the peaks in the first set. It has been determined that a satisfactory amplitude ("peak to valley") for the wave like nonlinear pattern is 1.0 inch, which can vary with satisfactory results from 0.50"-2.0". A satisfactory frequency (linear distance between repeating cyclic patterns) can be four cycles/foot of blade length with an acceptable range being from one cycle to about six cycles per foot on blade length. It is also possible to utilize a non-uniform pattern, i.e. for example, one which has a variable frequency and/or a variable amplitude over the length of the blades within the chip screen. Also, only a portion of the length of the top edges of the blades could be configured with a nonlinear pattern while the other portion could have a linear flat edge. In this manner the chip agitation can be altered according to the relative proportion of the chip feed that remains on the screen surface at a particular location down the length of the screen. In addition, the linear top edges could have staggered heights as disclosed in the '168 patent. In another embodiment the blades 40, 42 within their respective sets could have their peaks and valleys offset longitudinally so each nonlinear pattern would begin at a different point in the cycle. When arranged with the other set of blades, which would likewise have offset peaks and valleys, a very large number of relatively large vertical difference in heights of adjacent blades result in many locations on the screening surface. During the reciprocation of the blade sets these locations change, causing not only the desired "tipping" action due to the cycling height differences between adjacent and near adjacent blades but the nonlinear configuration results in a certain "uphill down-hill" agitation beneath the mat of chips. Such an action will induce differences in the length-wise flow rate for adjacent portions of the chip mat. This action has the effect of upsetting the chip mat coherence which leads directly to a higher level of chip thickness exposure to the blade openings and passage of acceptable chips therethrough over a shorter overall length of screening apparatus. Other alternative nonlinear configurations for at least a portion of the top edges within the blade sets are illustrated in Figs. 3-7. Fig. 3 illustrates a "sinusoidal-like" pattern but one with a variable frequency and amplitude. The blade sets with such a pattern would have their peaks and valleys offset in order to produce vertical displacements between adjacent blades larger than the amplitude of reciprocation of the screen apparatus; for example, for a 1.0" difference in height (peak to valley) and with a 1.0" vertical height of reciprocation gives a maximum 2.0" relative displacement at extremes of travel. Alternatives could hold one or the other characteristic constant while varying the other. Fig. 4 illustrates a triangular shaped wave pattern where again the blades in each blade set would be offset. Similar variations would be workable alternate embodiments. Fig. 5 illustrates a rectangular tooth configuration, Fig. 6 illustrates a saw tooth configuration, and Fig. 7 illustrates a trapezoidal configuration; all having the workable alternates, either varying frequency, amplitude or both. All such alternative patterns, where a large relative height difference occurs frequently throughout reciprocation over at least a portion of the length of at least some of the laterally spaced apart blades in the blade sets will accomplish the desired result. Thus, a detailed description of the present improvement for sorting wood chips has been given as have several alternative embodiments. Those with ordinary skill in the wood chip sizing and sorting art will readily appreciate its benefits and will be able to carry out practice of the invention. Modifications may occur to those skilled in the art and all such modifications are intended to be included within the scope of the claims that follow.

Claims: CLAIMS 1. A wood chip screening system for sorting a supply of incoming chips into an acceptable portion based on the chip thickness and an unacceptable portion of the type comprised of a plurality of substantially parallel spaced apart elongated blades mounted in a frame structure for longitudinal and vertical reciprocating movement with selected adjacent blades moving out of phase with respect to each other whereby wood chips that are loaded onto the infeed end of the system are tipped, in part, by the vertical reciprocating action of the blades causing vertical relative displacement between top edges of the blades so the thickness dimension thereof tends to become aligned with the openings between adjacent blades allowing the acceptable portion to pass through the spaces and the unacceptable portion to be conveyed atop the blades toward the outfeed end; the improvement comprising: a preselected portion of the elongated top edges of the blades being nonlinear, whereby vertical displacements between selected portions of top edges of adjacent blades during a portion of the reciprocating cycle will be greater than the amplitude of reciprocation. 2. The improvement as in claim 1 in which the nonlinear top edges form a plurality of peaks and valleys. 3. The improvement as in claim 2 in which the peaks will be approximately opposite the valleys in adjacent blades as blades reciprocate. 4. The improvement as in claim 2 in which the nonlinear edges are configured with a cyclic repeating pattern. 5. The improvement as in claim 2 in which the nonlinear edges are configured with a variable frequency pattern. 6. The improvement as in claim 2 in which the nonlinear edges are configured with a variable amplitude pattern. 7. The improvement as in claim 2 in which the nonlinear edges are configured with a variable frequency and amplitude. 8. The improvement as in claim 1 in which the blades are uniformly spaced apart to provide spaces therebetween falling within a range of from about 7 mm to about 9 mm.

Inventors: MARRS GEVAN R, MARRS, GEVAN, R.
Applicants: WEYERHAEUSER CO, WEYERHAEUSER COMPANY
Application year: 1996
Main classification: B07B1
IPC codes: B07B1, D21B1, D21D5