Abstract:
A hybrid incline/horizontal screen for separating particulate material into predetermined size fractions, the screen having multiple screen decks mounted on a frame, at least one uppermost screen deck having an inclined receiving end and a horizontal discharge end. Each screen deck has a screening medium with predetermined sized openings that screens the particulate material. The screen decks are in stacked relationship, the upper screen deck having the largest screen medium openings, the openings becoming progressively smaller the lower the screen deck is mounted on the frame. The inclined receiving portion of the one or more uppermost screen decks provides for rapid material separation and conveyance through the upper screen decks to the lower screen decks while the horizontal discharge end reduces the tendency for the material to pile up (snowball) and maintaining a more uniform material bed, while providing for a shorter, more transportable screen. The screen incorporates perforated and non-perforated feed boxes at the screen deck receiving end to protect the screen decks from dropping material impact loads, to provide for an extended screening surface, and to pre-distribute the material as it falls into the feed box.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a screen for separating rock material, and more particularly to a modified inclined vibrating screen that enhances the screens ability to receive and process material to be screened.  
         BACKGROUND OF INVENTION  
         [0002]    Screens are used in the aggregate business for separating rock, crushed rock, gravel, sand, and the like (referred herein as material) into various component sizes, referred to as size fractions. Screens comprise one or more screen decks containing a perforated screening medium which acts as a sieve through which the material is separated. A charge of material is deposited on the receiving end of the screen, and as the material is conveyed to the discharge end, smaller material falls through the openings leaving the larger material behind.  
           [0003]    In a common application in the production of gravel, such as for road building, at the quarry site, a charge of material is crushed using a rock crusher. The crushed material is then conveyed to the screen for separating. In an example of the use of a three-deck screen, material is separated into four sizes: large, medium, small, and smallest. The larger material is retained on the upper screen deck and conveyed off of the screen deck at the upper discharge end, the medium-sized material is retained on the middle screen deck and conveyed off of the screen deck at the middle discharge end, the smaller size material is retained on the lower screen deck and conveyed off of the screen deck at the lower discharge end, and the smallest material is deposited below the lower screen deck. The larger material, if too large for a particular purpose, may be collected from the screen and reprocessed by the crusher and re-screened until the desired size is obtained. Screens are commonly very large machines that are capable of continuously separating large quantities of material, hundreds of tons per hour, as part of the quarry operation.  
           [0004]    There are various types of screens loosely classified by the configuration of the screen deck and the method used to pass the material through the screening medium. One common method to pass the material through the screening medium is to submit the screen deck to vibratory motion to agitate and expose the material to the screening medium surface. The screens have a front or receiving end that receives the mixed material and a back or discharge end that discharges the separated material.  
           [0005]    The screen deck generally consists of a rigid frame upon which a screening medium is laid or supported. The screening medium contains a plurality of openings of a predetermined size. Examples of screening medium include woven wire cloth and perforated plate. Material is placed upon the screening medium and material that is smaller than the predetermined size falls through the openings in the screening medium, and thus separates the smaller material from the larger material. The material that is larger than the predetermined size of the openings is subsequently removed from the screen deck, and commonly made to move across the screen deck to be discharged at a location separate from the smaller material. The capability of the screen to convey the material in combination with screening allows for continuous material processing.  
           [0006]    Screens come in two basic screen deck configurations; inclined and horizontal. Inclined screens have one or more screen decks with an elevated receiving end with respect to the discharge end. Material is placed on the higher end of the screen deck, and as the material moves down the inclined screen deck to the discharge end, the smaller material passes through the openings of the screening medium. The larger material is discharged from the screen deck at the discharge end.  
           [0007]    The movement of material down the screen deck is provided by gravity, or, more commonly, in combination with the assistance of a vibrating mechanism. The vibrating mechanism is not only used to assist gravity, but also to agitate the material to more efficiently present the smaller material to the screening medium.  
           [0008]    Quarry-sized inclined screens are very tall machines. Being such tall machines, inclined screens are difficult to transport from quarry to quarry. When transportation is required, inclined screens are commonly disassembled and broken down requiring significant labor and time for both disassembly and re-assembly.  
           [0009]    Horizontal screens are configured such that the screen deck is level or horizontal. Horizontal screens are normally selected when there is a need to maintain a lower profile, such as for use in confined spaces or for transportation/mobility considerations. Horizontal screens require the use of a vibrating mechanism to agitate the material for effective separation. The vibrating mechanism is configured in its construction and operation to not only agitate the material, but also convey the material from the receiving end to the discharge end in screens having a continuous material processing capability. Horizontal screens require significantly more powerful and aggressive vibrating mechanisms to agitate and convey the material along the screen deck as compared with the inclined screen.  
           [0010]    It is common that screens utilize a plurality of screen decks in a stacked arrangement, one above the other, to separate the material into multiple sizes. In the case of a three-deck screen with an upper, middle and lower screen deck, the upper screen deck comprises the largest openings, the middle screen deck comprises smaller openings, and the lower screen deck comprises the smallest openings. As the material traverses the upper screen deck, the larger material remains on the upper screen deck while the smaller material falls to the middle screen deck. The middle screen deck with the smaller openings contains the medium sized material while allowing the passage of smaller material to the lower screen deck. The lower screen deck with the smallest openings contains the smaller material while allowing the smallest material, such as dust or fines, to pass through. As the separated material is conveyed along its respective screen deck to the discharge end, it is deposited into four separate areas for collection; large, medium, small, and smallest material size fractions. The three-deck screen, therefore, is capable of separating material into four material size fractions.  
           [0011]    In operation, the multiple-deck screen will deposit material onto the underlying screen decks at different rates and locations. For example, the material that passes through the upper screen deck will fall to the middle screen deck somewhat down-line from the receiving end of the upper screen deck. In like fashion, the material that passes through the middle screen deck will fall to the lower screen deck somewhat further down-line from the receiving end of the upper screen deck. The delay in dropping the material through the screen decks is due to the fact that the particles must transcend down through the layer of material, referred to as the material bed, on one screen deck before it can drop through to the screen deck below. Therefore, the length of the screen depends on the number of screen decks and the relative speed that the material passes through each subsequent screen deck.  
           [0012]    It is common for screens to utilize a vibrating mechanism to assist in the separation process as well as in the conveyance of the material towards the discharge end. The one or more screen decks are coupled together to a common rigid frame. The assembly comprising the multiple screen decks and the common frame is known as the screen box. The screen box is vibrated by a vibrating mechanism that is coupled to the common frame. Therefore, one vibrating mechanism vibrates all the screen decks simultaneously. The vibratory motions promote stratification in the material bed, bringing the smaller material down to the screening medium surface to be passed through the openings.  
           [0013]    The common types of vibrating mechanisms can be characterized by the form of the vibration and the number of bearings used in the mechanism. A two bearing, circle throw, inclined screen utilizes a counter weight on a shaft to vibrate the screen box, and therefore the screen decks, in a desired motion. Common vibrating mechanisms produce motions that include circular, elliptic and straight-line reciprocating movement. The motion can be directed to propel the material toward the discharge end to help convey the material in that direction. The screen box is isolated from the ground or support structure by springs or other damping apparatus.  
           [0014]    Separation efficiency is determined in part by the operating parameters of the vibrating mechanism. Those parameters include frequency, amplitude, attack angle and travel velocity imparted on the material. For a given material size distribution, weight, shape and quantity, as well as size of the openings, an optimum set of parameters can be determined for a given screen deck. Since a common vibrating mechanism is used to vibrate all of the screen decks simultaneously, the parameters set on the vibrating mechanism for multi-deck screens will be a compromise of efficiency for any one particular screen deck.  
           [0015]    The efficiency of operation of screens is determined in part by the power required to separate a given quantity of material. The power to operate an inclined screen includes the power to lift the material to the height of the receiving end of the screen, as well as the power used to move the material across the screen decks. Inclined screens take advantage of gravity to convey the material towards the discharge end. In contrast, the horizontal screen power requirement is potentially less to load the material onto the receiving end, but is significantly more to move the material along the screen deck.  
           [0016]    The screening medium surface is the most life-limited part of a screen. The screening medium surface must be strong enough to withstand the initial impact of the bulk material onto the receiving end of the screen deck as well as the material falling on the lower screen decks. The screening medium surface must also support the weight of the material and be flexible enough to withstand the vibration. Additionally, the screening medium must provide enough open area to allow the desired throughput of material while preventing the openings from becoming clogged.  
           [0017]    The above mentioned vibrating screens have a number of drawbacks. Regarding the inclined screens, the height of the screen is a significant hindrance for moving the screen from place to place. Most particularly, the inclined screens require disassembly in order to move them along improved roadways with overhead obstructions requiring significant labor and time.  
           [0018]    Inclined screens are known to cause a “snowball” effect as the material is conveyed down the screen decks. That is, material placed on the receiving end of the screen deck is at first conveyed slowly down the screen deck but increases in speed and momentum sufficient to overcome the preceding material. This causes a piling up of material increasing the material bed depth. As the material bed depth increases, separation efficiency decreases as it takes longer for the smaller material to transcend the material bed and make contact with the screening medium surface.  
           [0019]    Horizontal screens are more readily transportable but require considerable power to operate and move the material through the machine. Further, horizontal screens are limited to the number of screen decks, commonly three, that can be used. This is due to the length of screen deck required to pass the material through each subsequent screen deck, in part caused by the delay in material dropping from the screen decks above.  
           [0020]    An improved screen is needed that incorporates the reduced height of a horizontal screen for improved transportability and reduced power requirements in lifting the material to the receiving end, with the power efficiencies of the inclined screen, while keeping the overall length of the screen to a minimum and decreasing the detrimental effects of the “snowball” effect. Improvements are also needed to increase the lifetime of the screening medium, particularly to reduce the damage caused by the initial impact loads of the material dropping onto the screen decks.  
         SUMMARY OF INVENTION  
         [0021]    In an embodiment of the screen in accordance with the present invention, a screen is provided with one or more upper screen decks having a hybrid configuration consisting of an inclined receiving end transitioning into a horizontal discharge end. Each screen deck having an inclined portion is segmented into a plurality of sections with each adjacent section being inclined at a decreasing angle with the distance away from the receiving end. The uppermost screen decks have receiving portions having a greater incline angle which progressively decreases from higher to lower screen decks. The inclined portion of the screen decks provides for rapid material separation through the screening medium and faster conveyance close to the receiving portion. The decreasing inclination angle with down-line distance from the receiving end provides a more uniform and consistent material bed depth, effectively preventing the “snowball” effect of constant inclined screen decks.  
           [0022]    In addition, the hybrid incline/horizontal screen optimizes the benefits of the inclined screens with the benefits of the horizontal screens to produce a screen providing improved energy efficiencies as well as a shorter, more easily transportable screen.  
           [0023]    The hybrid screen allows for adjustment of a number of screen deck parameters, such as: the inclination angle of any of the individual feed box and screen sections, the number of screen sections per screen deck that is inclined, the same or different inclination angles between adjacent screen sections, the number of screen decks used, the total number of screen sections used per screen deck, the length and width of the screen sections, and whether or not a feed box, with or without openings is used.  
           [0024]    Therefore, a hybrid inclined/horizontal screen is within the scope of the present invention having one or more screen decks having one or more sections of each screen deck inclined at an inclined angle from horizontal. The inclination angle of the sections of each screen deck is reduced with down-line distance from the receiving end of the screen. A hybrid screen also within the scope of the invention consists of the hybrid screen as described that utilizes a vibrating mechanism to assist in the conveyance and separation of the material being separated.  
           [0025]    It is further anticipated that the screen is configurable to accommodate for changing the screen deck parameters without undue modification to the screen frame. For example, in an embodiment of the invention, the frame will accommodate attachment apparatus to adjust the inclination angle of any given screen deck section, without replacement of frame parts or rework of the frame itself.  
           [0026]    A feed box is presented for coupling with the receiving end of the uppermost and lower screen decks, that provides for impact protection for the screening medium as well as an extended screening surface. The feed box is provided with a base plate having openings of a predetermined size to correspond with the openings of the corresponding screen deck medium. The feed box plate can be configured to be more capable of resisting the impact loads of the dropped material, while assisting in the distribution of the material bed prior to conveyance onto the screen deck medium.  
           [0027]    It is appreciated that the invention of the screen may be practiced without the inclusion of the upper and middle feed boxes, and, as such, is within the scope of the invention. It is further appreciated that the upper and lower feed boxes are advantageously used on conventional horizontal and inclined screens to protect the screen medium, to provide for an extended screening surface, and to pre-distribute the material as it falls into the feed box, and as such, is within the scope of the invention.  
           [0028]    The invention and its advantages will be further appreciated upon reference to the following detailed description and the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0029]    [0029]FIG. 1 is a partial cut-away side view of a screen in accordance with an embodiment of the present invention;  
         [0030]    [0030]FIG. 2 is an end view of the screen of FIG. 1;  
         [0031]    [0031]FIG. 3 is a perspective view of a screen section in accordance with the embodiment of FIG. 1;  
         [0032]    [0032]FIG. 4 is a perspective view of a feed box in accordance with the embodiment of FIG. 1;  
         [0033]    [0033]FIG. 5 is a side partial cut-away view of the screen in accordance with the embodiment of FIG. 1; and  
         [0034]    [0034]FIG. 6 is a simplified cut-away view of the screen in accordance with the embodiment of FIG. 1 illustrating a method for using the screen. 
     
    
     DESCRIPTION  
       [0035]    In the following detailed description, reference is made to the accompanying drawings which form a part hereof wherein like numerals designate like parts throughout, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. An embodiment of a screen having three screen decks is presented. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention; including, but not limited to, a screen having one or more screen decks and having either no vibrating mechanism or any of a variety of vibrating mechanisms known in the art. Therefore, the following detailed description is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims and their equivalents.  
         [0036]    [0036]FIGS. 1 and 2 illustrate a screen  10  arranged to separate particulate material, such as crushed rock, gravel, sand, and the like (referred herein as material) into various component sizes, referred to as size fractions. The screen  10  comprises a frame  12  having a first side wall  12   a  and a second side wall  12   b  spaced apart from the first side wall  12   a.  An upper screen deck  20  extends between the first and second side walls  12   a,b.  In addition, a bottom screen deck  40  extends between the first and second side walls  12   a,b  of the frame  12 , below the upper screen deck  20 . Between the upper screen deck  20  and the bottom screen deck  40  is a middle screen deck  30  also extending between the first and second side walls  12   a,b.  The screen  10  has an inlet end  14  for receiving mixed material and a discharge end  16  for discharging separated material.  
         [0037]    The upper screen deck  20  comprises an upper feed box  21  and three upper screen sections  22   a - c,  each proximate to or coupled end to end. The upper feed box  21  is arranged strategic to the screen inlet end  14 . Each upper screen section  22   a - c  comprises screening medium  24   a  supported by a frame  23   a,  as shown in FIG. 3. The screening medium  24   a  is formed to include a plurality of openings  25   a  therein to permit pieces of particulate matter smaller than the openings  25   a  to fall through the screening medium  24   a.  The screening medium  24   a  can be of a variety of configurations, including, but not limited to, woven wire cloth and perforated plate.  
         [0038]    The upper feed box  21  and the first upper screen section  22   a  is coupled to the frame  12  at a first angle  26  to the horizontal. The first angle  26  is determined in consideration of the material to be separated, as will be discussed below. In the embodiment shown in FIG. 1, the first angle  26  is 20 degrees. The second upper screen section  22   b  is proximate to or coupled to the first upper screen section  22   a  and coupled to the frame  12  at a second angle  27 . Second angle  27  is more shallow than the first angle  26  of the first upper screen section  22   a.  In the embodiment shown in FIG. 1, the second angle  27  is 10 degrees. The third upper screen section  22   c  is coupled to the frame  12  in a horizontal orientation, with the third upper screen section  22   c  proximate to or coupled to the second upper screen section  22   b.  An upper discharge chute  28  is coupled to the frame  12  proximal to the third upper screen section  22   c  such that the separated material is conveyed from the third upper screen section  22   c  to the upper discharge chute  28 .  
         [0039]    The middle screen deck  30  is arranged in similar fashion as the upper screen deck  20 . The middle screen deck  30  comprises a middle feed box  31  and three middle screen sections  32   a - c,  each proximate to or coupled end to end. The middle feed box  31  is coupled to the frame  12  at a third angle  36   a  to the horizontal. The third angle  36   a  is greater than the first angle  26  of the upper feed box  21  and the first upper screen section  22   a.  The first middle screen section  32   a  is coupled to the frame  12  at a fourth angle  36   b  to the horizontal, the fourth angle  36   b  being more shallow than the first angle  26  of the upper feed box  21  and the first upper screen section  22   a.  In the embodiment shown in FIG. 1, the third angle  36   a  is 45 degrees and the fourth angle  36   b  is 15 degrees. The second middle screen section  32   b  is proximate to or coupled to the first middle screen section  32   a  and coupled to the frame  12  at a fifth angle  37 . The fifth angle  37  is more shallow than the fourth angle  36   b  of the first screen section  32   a,  and likewise, at a more shallow angle than the second upper screen section  22   b.  In the embodiment shown in FIG. 1, the fifth angle  37  is 7.5 degrees. The third middle screen section  32   c  is coupled to the frame  12  in a horizontal orientation, with the third middle screen section  32   c  proximate to or coupled to the second middle screen section  32   b.  A middle discharge chute  38  is coupled to the frame  12  proximal the third middle screen section  32   c  such that the separated material is conveyed from the third middle screen section  32   c  to the middle discharge chute  38 .  
         [0040]    The lower screen deck  40  comprises one lower screen section  42 . The bottom screen deck  40  is arranged in a horizontal orientation and parallel with the third upper and middle screen sections  22   c,   32   c  of the upper and middle screen decks  20 , 30 , respectively. A lower discharge chute  48  is coupled to the frame  12  below the lower screen section  42  such that the separated material is conveyed from the lower screen section  42  to the lower discharge chute  48 .  
         [0041]    The upper and middle feed boxes  21 , 31  are arranged in substantial vertical alignment with each other. Likewise, the first upper and middle screen sections  22   a,   32   a  are in substantial vertical alignment with each other, the second upper and middle screen sections  22   b,   32   b  are in substantial vertical alignment, and the third upper and middle screen sections  22   c,   32   c  are in substantial vertical alignment. The upper, middle, and lower screen decks  20 , 30 , 40  are in substantial vertical alignment with each other.  
         [0042]    The upper, middle and lower screening medium  24   a - c  of the upper, middle and lower screen sections  22   a - c,   32   a - c,   42 , respectively, is formed to include a plurality of openings  25   a - c  therein to permit particulate matter smaller than the openings  25   a - c  to fall through the respective screen decks  22 , 32 , 42 . In one embodiment in accordance with the invention, the upper screening medium  24   a  in each of the upper screen sections  22   a - c  comprise the same size openings  25   a.  Likewise, the middle screening medium  24   b  in each of the middle screen sections  32   a - c  comprise the same size openings  25   b,  and the lower screening medium  24   c  in the bottom screen section  42  comprises the same size openings  25   c.  The upper screen openings  25   a  are larger than the middle screen openings  25   b.  And likewise, the middle screen openings  25   b  are larger than the lower screen openings  25   c.    
         [0043]    In another embodiment in accordance with the invention, the screening medium  24   a - c  on each screen deck  22 , 32 , 42  has screen openings  24   a - c  that either increase or decrease in size along the length of each screen deck  22 , 32 , 42 . For particular applications, screening operation efficiencies can be improved wherein each screen deck  22 , 32 , 42  has screen openings  24   a - c  of decreasing size from the first screen section  22   a,   32   a,   42   a  to the third screen section  22   c,   32   c,   42   c.  For example, the screening medium  24   a  of the first upper screen section  22   a  comprises larger openings  25   a  than the screening medium  24   a  of the second upper screen section  22   b,  which, in turn, has larger screen openings  25   a  than the screening medium  24   a  of the third upper screen section  22   c.  Such a configuration is used in situations wherein a more rapid screening of material is desired with the use of oversized screen openings  25   a - c  in one or more of the first screen sections  22   a,   32   a,   42   a,  and it is acceptable that some oversized material passes to the screen deck below.  
         [0044]    Similarly, for particular applications, screening operation efficiencies can be improved wherein each screen deck  22 , 32 , 42  has screen openings  24   a - c  of increasing size from the first screen section  22   a,   32   a,   42   a  to the third screen section  22   c,   32   c,   42   c.  For example, the screening medium  24   a  of the first upper screen section  22   a  comprises smaller openings  25   a  than the screening medium  24   a  of the second upper screen section  22   b,  which, in turn, has smaller screen openings  25   a  than the screening medium  24   a  of the third upper screen section  22   c.  Such a configuration is used in situations wherein it is desired to produce a more even material bed depth by delaying the screening of material of a certain size until the material reaches a desired location down-line.  
         [0045]    Screening medium having different size openings on each of the screen decks, in any combination, is within the scope of the invention.  
         [0046]    The upper and middle feed boxes  21 , 31  are used primarily to protect the screen decks  20 , 30  from the impact of the received material. The upper feed box  21  comprises a upper feed box frame  52   a  lined with a rigid perforated plate  54   a.  The perforated plate  54   a  comprises a plurality of openings  56  sized to correspond with the upper screen openings  25   a  of the upper screen sections  22   a - c.  The upper feed box  21  serves as the receiving end  14  of the screen  10 . Material is dropped into the upper feed box  21  rather than directly onto the first upper screen section  22   a,  therefore protecting and prolonging the life of the screening medium  24   a  in the first upper screen section  22   a.  The upper feed box  21  also helps in distributing the material widthwise prior to entering the first upper screen section  22   a  for more effective screening, as the material bed will be more uniform across the width of the upper screen sections  22   a - c  and the screen decks  30 , 40  below.  
         [0047]    The upper feed box  21  additionally provides an extension to the effective screening area of the upper screen deck  20 . Material small enough to pass through the openings  56 , will fall into the middle feed box  31 . Middle feed box  31  comprises a middle feed section frame  52   b  with a solid bottom plate  54   b.  The middle feed box  31  helps to protect the first middle screen section  32   a  from the impact of the material dropping down from the upper feed box  21 , serving to prolong the life of the screening medium  24   b.    
         [0048]    [0048]FIG. 5 is a side partial cut-away view of the screen in accordance with the embodiment of FIG. 1. The frame  12  is coupled to a stationary chassis  60  by spring mount assemblies  62 . The spring mount assemblies  62  isolate the frame  12  from the chassis  60 .  
         [0049]    A vibrating mechanism  70  is used to impart vibratory motion to the frame  12  and thus to the upper, middle and lower screen decks  20 , 30 , 40 . The assembly of the frame  12  and the screen decks  20 , 30 , 40  is referred to as the screen box  11 . Vibrations from the vibrating mechanism  70  is used to agitate and convey the material along the upper, middle and lower screen decks  20 , 30 , 40  towards the discharge chutes  28 , 38 , 48 . Any of a variety of types of vibrating mechanisms  70  can be employed to impart motion to the screen box  11 . One type of vibrating mechanism  70 , shown in FIGS. 2 and 5, comprises a drive shaft  72  located within a housing  71  of the vibrating mechanism  70 . A drive wheel  74  is coupled to the drive shaft  72 . A motor drive shaft  75  of a motor  76  is coupled to the drive wheel  74  by a drive belt  77  to rotate the drive shaft  72 . Counterweights (not shown) are coupled to the drive shaft  72 . Rotation of drive shaft  72  causes rotation of counterweights (not shown) which vibrates the screen box  11 .  
         [0050]    [0050]FIG. 6 is a simplified cut-away view of the screen in accordance with the embodiment of FIG. 1 illustrating a method for using the screen. The screen  10  separates mixed material  80  into four size fractions: large  82 , medium  84 , small  86 , and smallest  88 . As the vibrating mechanism  70  is engaged, a charge of mixed material  80  is deposited in the receiving end  14  which is coincident with the upper feed box  21 . The mixed material  80  passes over the perforated plate  54   a  of the upper feed box  21  and onto the upper screen sections  22   a - c,  becoming distributed over the width of the screen deck  20  as a material bed  81   a.  As the material bed  81   a  passes over the upper screen deck  20 , medium, smaller and smallest material  84 , 86 , 88  passes through the upper screen sections  22   a - c.  The medium, smaller and smallest material  84 , 86 , 88  is deposited onto the middle screen deck  30  forming middle material bed  81   b.  The remaining larger material  82  is conveyed to the discharge end  16  and out of the upper discharge chute  28  and conveyed away by collection chute  92 . In similar fashion, the middle and small material bed  81   b,c  traverses the middle and lower screen decks  30 , 40 , respectively, while the separated middle and small material  84 , 86  is discharged out of the middle and lower discharge chutes  38 , 48 , respectively and conveyed away by collection chutes  94 , 96 . The smallest material  88  falls through the lower screen section  40  and is taken away on a conveyor  98 .  
         [0051]    The screen  10  of FIG. 1 combines the benefits of an inclined screen with the benefits of a horizontal screen. The upper and middle feed boxes  21 , 31  and the upper and middle first and second screen sections  22   a - b,   32   a - b  are inclined from the horizontal which provides a number of benefits. The material will be conveyed down the inclined surfaces faster than if the surfaces were horizontal which helps to prevent material pileup near the receiving end of the screen. Additionally, smaller material will pass more quickly through the upper, middle and lower screen decks  20 , 30 , 40  due to the tumbling action of the material down the inclined surfaces. The “snowball” effect is substantially reduced due to the combination of faster material separation and the decreasing inclination down-line from the receiving end. The combination of faster conveyance and faster separation provides a more uniform material bed along each of the screen decks  20 , 30 , 40 .  
         [0052]    The upper screen deck  20  is required to process all of the material  80  and therefore handle the greatest amount of material  80  as compared with the middle and lower screen decks  30 , 40 . As the material  80  is received onto the upper screen deck  20 , the steeper inclination angle  26  of the upper feed box  21  and first screen section  22   a  causes the material to rapidly move forward towards the third screen section  22   c  which is the horizontal portion of the upper screen deck  20 . This rapid movement of the material prevents the material from accumulating or piling up at the receiving end  14 . The inclination angle  27  of the second upper screen section  22   b  as compared with first upper screen section  22   a  is less to account for the decreased quantity of material being conveyed as the smaller material falls to the lower screen decks  30 , 40 , as well as to decrease the speed of conveyance to prevent the “snowball” effect.  
         [0053]    The smaller material passes through the upper screen deck  20  onto the middle screen deck  30 . For similar reasons stated above, the middle feed box  31  and the first middle screen section  32   a  has a higher inclination angle than the second middle screen section  32   b.  Further, the middle feed box  31  and first middle screen section  32   a  has a lower inclination angle than the upper feed box  21  and first upper screen section  22   a  as there is less material being deposited on the middle screen deck  30  as compared with the upper screen deck  20 .  
         [0054]    The lower screen deck  40  has a horizontal orientation owing to the fact that the lower screen deck  40  processes an even lesser amount of material as the upper and middle screen decks  20 , 31  and therefore does not require the inclination for rapid material separation.  
         [0055]    Therefore, large particles  82  are carried by the upper screen deck  20  and are discharged at the upper discharge chute  28 . Medium size particles  84  fall through the upper screen deck  20  and are carried by the middle screen deck  30  and are discharged at the middle discharge chute  38 . Small particles  86  pass through both the upper and middle screen decks  20 , 30  and are carried by the lower screen deck  40  and are discharged at the lower discharge chute  48 . Very small size particles  88  fall through the lower screen deck  40  to be deposited below the screen  10 .  
         [0056]    Since the inclination of the screen decks  20 , 30  for the most part dictates the overall height of the screen  10 , a trade-off of the benefits of the inclination with the benefits of the reduced height must be considered. Further, for a given specific need for the use of the screen  10 , it may be beneficial to tailor and adjust the screen decks  20 , 30 , 40  for efficient material separation.  
         [0057]    A number of screen deck parameters may be adjusted, such as: the inclination angle of any of the individual feed box and screen sections, the number of screen sections per screen deck that is inclined, the same or different inclination angles between adjacent screen sections, the number of screen decks used, the total number of screen sections used per screen deck, the length and width of the screen sections, and whether or not a feed box, with or without openings, is used.  
         [0058]    Therefore, a hybrid inclined/horizontal screen is within the scope of the present invention having one or more screen decks having one or more sections of each screen deck inclined at an inclined angle from horizontal. The inclination angle of the sections of each screen deck will generally decline with the down-line distance from the receiving end of the screen. A hybrid screen also within the scope of the invention consists of a hybrid screen as described that utilizes a vibrating mechanism to assist in the conveyance and separation of the material being separated.  
         [0059]    It is further anticipated that a frame  10  can be configured to accommodate for changing the screen deck parameters without undue modification to the frame  10 . For example, in an embodiment of the invention, the frame  10  will accommodate attachment apparatus (not shown) to adjust the inclination angle of any given screen deck section, without replacement of frame parts or rework of the frame  10  itself. In one embodiment, each screen section comprises spring-loaded pins (not shown) which are inserted into corresponding apertures (not shown) of the frame  10 , removably coupling the screen section to the frame  10 . A series of apertures for each spring-loaded pin is provided on the frame  10 , such that the inclination angle of the screen section can be adjusted by removing the pin from one aperture and receiving it within another aperture corresponding to the desired inclination angle.  
         [0060]    It is appreciated that the invention of a screen may be practiced without the inclusion of the upper and middle feed boxes, and as such, is within the scope of the invention.  
         [0061]    It is further appreciated that the upper and middle feed box is advantageously used on conventional horizontal and inclined screens to protect the screen medium, to provide for an extended screening surface, and to pre-distribute the material as it falls into the feed box, and as such, is within the scope of the invention.  
         [0062]    Although specific embodiments have been illustrated and described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations calculated to achieve the same purposes may be substituted for the specific embodiment shown and described without departing from the scope of the present invention. Those with skill in the art will readily appreciate that the present invention may be implemented in a very wide variety of embodiments. This application is intended to cover any adaptations or variations of the embodiments discussed herein. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.