Patent Publication Number: US-2010108579-A1

Title: Screening Machine with Segmented Components

Description:
TECHNICAL FIELD 
     This invention relates to screening machines or separators of the type used to separate or classify mixtures of solid particles of different sizes. 
     BACKGROUND 
     In screening machines or separators of the type described, a screen (which may be woven, an aperture plate or another design) is mounted and pre-tensioned in what is often called a “screen frame” or “screen deck” that includes a supporting peripheral frame around the perimeter of the screen. The terms “screening machine” and “separator” and respective derivatives thereof are herein used interchangeably. A main housing of the screening machine supports one more of the screen frames, as well as agitating structures that shake or vibrate the solids deposited on the screen frames. 
     The screen frames are often removed from the screening machines for cleaning, replacement, readjustment or installation of a screen of a different mesh size or the like. In large commercial screening machines, the screen frames are inserted through and removed from a longitudinal end of the machine. In accordance with this movement of the screen frames, sufficient space must be devoted near the end of the machine, such that the generally horizontal screen frames may be inserted into and removed from the housing of the screening machine. Moreover, screen frames in large commercial screening machines tend to be relatively large, making their insertion and removal difficult or awkward at best, especially for a sole operator in tight working conditions. 
     Many machines of this type are used to separate particulate material which, over time, often clogs or blocks the screen frame mesh. Commonly, an agitating mechanism is included in the machine to unblock the mesh by dislodging the clogging material. Often, the agitating mechanism is a number of balls captured by a ball tray and accessible to the screen mesh. In large commercial screening machines, the agitating structures may be also inserted and removed respectively through the end of the machine, as described above with respect to the screen frames, thereby presenting similar space requirements. Moreover, known agitating structures tend to be large and relatively heavy, making their insertion into and removal from the housing of the screening machine a particularly difficult and work-intensive task for the operators involved. 
     It is evident, therefore, that a screening machine and screening machine components are needed that address these and other known problems associated with conventional screening machines. 
     SUMMARY 
     In a first embodiment, a screen frame assembly is provided for use with a screening machine. The screen frame assembly includes first and second generally planar screen frames and a coupling between the first and second screen frames that includes a tab in the first screen frame and a receiving slot in the second screen frame. The receiving slot is configured to receive the tab and includes a centering feature for laterally aligning the first and second screen frames relative to one another. The centering feature may include a pair of opposed inwardly angled edges of the receiving slot. The coupling may include an overlap region between the first and second screen frames that is engageable to thereby permit lifting of the second screen frame by a lifting force exerted upon the first screen frame. The first and second screen frames may be juxtaposed to one another along respective confronting end portions thereof, and a seal member is disposed between the end portions. The seal member may be positioned between a top face of one of the screen frames and a bottom face of the other screen frame. The tab may be in the general form of a hook. Alternatively or additionally, the first and second screen frames may each include pre-tensioned screens. The receiving slot may be sized to accept a human finger there through. 
     In another embodiment, a screen frame assembly is provided for use with a screening machine. The screen frame assembly includes first and second generally planar screen frames, each having respective top and bottom faces and respective end portions adapted to overlap one another. A coupling between the screen frames is configured to permit movement of the first and second screen frames by a force that is exerted upon the first screen frame. A seal member is disposed at the end portions between the bottom face of the first screen frame and the top face of the second screen frame. The seal member may extend around substantially the entire second screen frame adjacent a perimeter thereof. The second screen may be mounted to the second screen frame via the seal member. The first and second screen frames may be substantially identical and interchangeable with one another. 
     In yet another embodiment, a screening machine includes a main housing and a screen frame assembly. The screen frame assembly has first and second generally planar screen frames and a coupling between the screen frames that is configured to permit movement of the first and second screen frames by a force that is exerted upon the first screen frame. The coupling includes a tab in one of the screen frames and a receiving slot in the other screen frame and which is configured to receive the tab. The receiving slot includes a centering feature for laterally aligning the first and second screen frames relative to one another. The screening machine also includes an agitating structure that is supported by the main housing and which is configured to hold a plurality of agitating elements from moving material that is held by the screen frame assembly. 
     The agitating structure may include a stopping element for restricting movement of the screen frame assembly relative to the agitating structure. At least one of the first and second screen frames may include a limiting tab that is engageable with the stopping element for selectively restricting movement of the first and second screen frames relative to the agitating structure. The agitating elements may include balls. The agitating structure may include first and second generally co-planar trays and a tray coupling between the first and second trays that is configured to permit movement of the first and second trays by a force that is exerted upon the first tray. The tray coupling may include a hook in one of the first and second trays that is engageable with a tray receiving slot of the other of the first and second trays. Alternatively or additionally, the first tray may include a tongue that is engageable with the first frame to restrict movement of the first frame relative to the first tray. 
     In another embodiment, a screening machine includes a main housing, a screen frame assembly, and an agitating structure that is supported by the main housing. The agitating structure is configured to hold a plurality of agitating elements for moving materials held by the screen frame assembly. The agitating structure includes first and second generally co-planar trays and a tray coupling between the first and second trays that is configured to permit movement of the first and second trays by a force exerted upon the first tray. 
     In another embodiment, a method is provided for moving screening components relative to a main housing of a screening machine. The method includes applying a force on a first of two generally planar screen frames, with such force moving the two screen frames relative to the main housing. A seal is positioned relative to overlapping portions of the two screen frames, and the first screen frame is moved relative to the other screen frame. 
     In embodiments having a screen frame assembly that is made up of two or more generally planar screen frames, insertion and removal of the screen frame assembly respectively into and out of the main housing of the screening machine is greatly facilitated. Similarly, in embodiments having an agitating structure that includes segmented co-planar trays, insertion and removal of the agitating structure is likewise facilitated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a partially broken-away perspective view of a separator or screening machine in accordance with one embodiment of the invention; 
         FIG. 2  is a perspective view of a screen frame assembly and an agitating structure of the separator of  FIG. 1 ; 
         FIG. 3  is a partially broken-away view of the screen frame assembly and the agitating structure of  FIGS. 1 and 2  showing separation between two screen frames of the screen frame assembly; 
         FIG. 3A  is an enlarged view of the encircled portion  3 A of  FIG. 3 ; 
         FIG. 3B  is a view similar to  FIG. 3A  showing the screen frame therein in a position different from that shown in  FIG. 3A ; 
         FIG. 4  is a partially broken-away perspective view showing partial disengagement between the agitating structure and the screen frame assembly of  FIGS. 2-3 ; 
         FIG. 4A  is an enlarged perspective view of the encircled portion  4 A of  FIG. 4 ; 
         FIG. 4B  is a cross-sectional perspective view taken generally along line  4 B- 4 B of  FIG. 4A ; 
         FIG. 4C  is a view similar to  FIG. 4B  showing disengaging motion between two screen frames of the screen frame assembly of  FIGS. 2-3 ; 
         FIG. 4D  is a view similar to  FIGS. 4B and 4C  showing the two screen frames being fully disengaged from one another; 
         FIG. 5  is a partially broken-away perspective view of the screen frame assembly of  FIGS. 2-3  in inserting motion into the housing of the separator; 
         FIG. 5A  is an enlarged view of the encircled portion  5 A of  FIG. 5 ; 
         FIG. 5B  is a cross-sectional perspective view taken generally along line  5 B- 5 B of  FIG. 5A ; 
         FIG. 5C  is a perspective view similar to  FIG. 5B  showing the two screen frames of the screen frame assembly in relative positions different from those shown in  FIG. 5B ; 
         FIG. 6A  is an enlarged perspective view of a stopping element and limiting tab of the separator of  FIG. 1 ; 
         FIG. 6B  is a view similar to  FIG. 6A  showing the limiting tab in a position different from that shown in  FIG. 6A ; 
         FIG. 7  is a partially broken-away perspective view similar to  FIGS. 4 and 5  showing the two screen frames in an overlapping position; 
         FIG. 8  is a partially broken-away perspective view of the agitating structure of the separator of  FIGS. 2-3 ; 
         FIG. 9  is a view similar to  FIG. 8  showing two trays and a tray coupling of the agitating structure of  FIG. 8 ; 
         FIG. 9A  is an enlarged view of the encircled portion  9 A of  FIG. 9 ; 
         FIG. 9B  is an elevational view of the tray coupling of  FIG. 9A ; 
         FIG. 9C  is a view similar to  FIG. 9B  showing partial disengagement between two trays of the agitating structure of  FIG. 9 ; 
         FIG. 9D  is a view similar to  FIG. 9C  showing another relative position between the two trays different from that shown in  FIG. 9C ; 
         FIG. 9E  is a view similar to  FIGS. 9B-9D  showing the two trays fully disengaged from one another; 
         FIG. 10A  is an enlarged perspective view of a different embodiment of a tray coupling between two trays of an agitating structure; 
         FIG. 10B  is an elevational view of the coupling structure shown in  FIG. 10A ; 
         FIG. 10C  is a view similar to  FIG. 10B  showing two trays in partial disengagement from one another; 
         FIG. 10D  is a view similar to  FIGS. 10B and 10C  showing the two trays in relative positions different from those shown in  FIGS. 10B and 10C ; and 
         FIG. 10E  is a view similar to  FIGS. 10B-10D  showing the two trays fully disengaged from one another. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the figures, and more particularly to  FIG. 1 , a screening machine or separator  10  includes a main housing  14  having an access end  18  located at a longitudinal end of the housing  14  for inserting and removing screening components such as a screen frame assembly  20  and an agitating structure  30  cooperating with the screen frame assembly  20  to agitate or vibrate solids (not shown) deposited on the screen frame assembly  20 . The main housing  14  includes a locking feature in the form of a pair of brackets  32  at the access end  18  and configured to receive a bolt or similar (not shown) through a hole  34  to secure the screen frame assembly  20  and agitating structure  30  within the main housing  14  and relative to one another. 
     With reference to  FIGS. 1-3 , the screen frame assembly  20  of this exemplary embodiment is in the form of a segmented assembly that includes first, second and third generally planar screen frames  40 ,  42 ,  44  that are substantially identical to and interchangeable with one another. It is contemplated that the screen frame assembly  20  may instead have any number of screen frames other than three including, without limitation, a single screen frame, and still fall within the scope of the present disclosure. Each of the screen frames  40 ,  42 ,  44  includes several pre-tensioned screens  40   a ,  42   a ,  44   a  supported about the respective perimeters of the screen frames  40 ,  42 ,  44  and further includes a top face  40   b ,  42   b ,  44   b  and an opposite bottom face  40   c ,  42   c ,  44   c . The screen frames  40 ,  42 ,  44  are engageable with one another to define overlap regions  58  between respective confronting ends of adjacent pairs of the screen frames  40 ,  42 ,  44 . The overlap regions  58  prevent or at least minimize the likelihood of solids deposited on the screen frames  40 ,  42 ,  44  falling onto the underlying agitating structure  30 . Each of the screen frames  40 ,  42 ,  44 , moreover, includes a pair of holes  46  at respective front corners thereof, and which are designed to be in registration with the holes  34  of the brackets  32  of the main housing  14 . These holes  46  are similarly configured to receive a bolt or similar element (not shown) there through to secure the screen frame assembly  20  relative to the main housing  14 . Each of the screen frames  40 ,  42 ,  44  further includes a series of spaced apart grooves  48  ( FIG. 3 ) along their respective front edges for securing the position of the screen frame assembly  20  relative to the underlying agitating structure  30 , as explained in further detail below. 
     In this exemplary embodiment, the agitating structure  30  is in the form of a segmented tray assembly having first and second generally coplanar trays  50 ,  52  that are also engageable with one another through a tray coupling generally designated with the numeral  60 . Each of the first and second trays  50 ,  52 , in turn, in this exemplary embodiment, is partitioned into several compartments  53  ( FIG. 4 ). Longitudinal lips  40   d ,  42   d ,  44   d  of each of the screen frames  40 ,  42 ,  44  engage the side walls  50   e ,  52   e  of the first and second trays  50 ,  52  and thereby determine a vertical position of the screen frames  40 ,  42 ,  44  relative to the trays  50 ,  52  when these are engaged with one another within the main housing  14  (shown in phantom in  FIG. 2 ). Each of the longitudinal lips  40   d ,  42   d ,  44   d  extends only partially along the respective length of the screen frame  40 ,  42 ,  44  to define a gap  62  between each of the lips  40   d ,  42   d ,  44   d  and a limiting tab  66 . 
     With reference to  FIGS. 3 ,  3 A, and  3 B, the functionality of the limiting tabs  66  is described in further detail.  FIG. 3  shows an exemplary removing motion of the screen frame assembly  20  from main housing  14  ( FIGS. 1-2 ) and, more particularly, the removing motion of the first and second screen frames  40 ,  42 . As the screen frame assembly  20  is pulled in the general direction of arrows  70 , the screen frame assembly  20  is moved relative to the underlying agitating structure  30 . This motion of the screen frame assembly  20  is such that the lip  40   d  of the first screen frame  40  rides over the side wall  50   e  of the underlying tray  50 . This motion is stopped by engagement of the limiting tab  66  of the first screen frame  40  with a front end wall  72  of the tray  50 . Any further removing motion of the screen frame  40  requires lifting of the first screen frame  40  in the general direction of arrow  80  ( FIG. 3A ) and subsequent continuation of the pulling motion (i.e., removing motion) of the first screen frame  40  in the general direction of arrows  70  ( FIGS. 3 and 3B ). Notably, the restriction of the removing motion of the first screen frame  40  provided by the engagement of the limiting tab  66  with the front end wall  72  provides a natural stopping point at which the operator is signaled to disengage the first screen frame  40  from the second screen frame  42  ( FIG. 3 ), one manner of which is described in further detail below. 
     With reference to  FIGS. 4 ,  4 A,  4 B,  4 C, and  4 D, an exemplary removing motion of the screen frame assembly  20  is depicted. In  FIG. 4 , the screen frame assembly  20  is illustrated in removing motion (arrow  70 ), moving away from the main housing  14  ( FIGS. 1 and 2 ) and from the agitating structure  30 . A coupling  100  between the second and third screen frames  42 ,  44  permits movement of the third screen frame  44  with the pulling force exerted on the second screen frame  42 . More specifically, and with particular reference to  FIG. 4A , the coupling  100  includes a generally hook-shaped tab  112  extending from the leading edge  42   f  of the second screen frame  42  that is received in and engages a generally trapezoidal-shaped receiving slot  116  in the trailing portion  44   g  of the third screen frame  44 . As the second screen frame  42  is pulled in the general direction of arrow  70  ( FIG. 4 ), the tab  112  engages a front edge  116   a  of the receiving slot  116  such that a pulling force applied on the second screen frame  42  results in pulling (i.e., removing motion) of both of the second and third screen frames  42 ,  44 . Pulling of the second screen frame  42  is facilitated by the size and shape of the receiving slot  116 , which is configured to receive at least one human finger there through to enable engagement of the finger with the second screen frame  42  and pulling thereof. 
     In operation, when engagement of the limiting tab  66  with the front end wall  72  of the agitating structure  30  occurs, the second screen frame  42  is disengaged from the third screen frame  44  as shown in the exemplary sequence illustrated by  FIGS. 4B-4D . More specifically, the second screen frame  42  is first moved toward the main housing  14  ( FIGS. 1-2 ) in the general direction of arrow  122  until the tab  112  reaches the general position shown in  FIG. 4C . The operator then proceeds to tilt the second screen frame  42  relative to the third screen frame  44  in the general direction of arrow  118  ( FIG. 4C ), and then to remove the second screen frame  42  away from the third screen frame  44  in the general direction of arrow  70  ( FIG. 4D ). 
     With reference to  FIGS. 5 ,  5 A,  5 B,  5 C,  6 A,  6 B, and  7 , an exemplary insertion of the screen frame assembly  20  into the main housing  14  is illustrated. Inserting motion of the screen frame assembly  20  is effected in the general direction of arrow  130  and is such that an inserting force exerted onto the second screen frame  42  results in inserting movement of the second and third frames  42  and  44 . With particular reference to  FIGS. 5A-5B , the tab  112  of the second screen frame  42  is inserted within the receiving slot  116  of the third screen frame  44  and permitted to contact and engage the trailing edge  44   h  of the third screen frame  44 , to thereby push the third screen frame  44  in the general direction of arrow  130 . Notably, inserting movement (arrow  130 ) of the tab  112  within the receiving slot  116  is guided by a pair of inwardly angled lateral edges  116   b  of the receiving slot  116  such that lateral movement of the second screen frame  42  (arrow  144 ) is restricted. The guiding of the tab  112  within the receiving slot  116  thus defines a centering feature of the coupling  100  that aligns the second and third screen frames  42 ,  44  relative to one another. Moreover, the lateral edges  116   b  are also oriented downwardly to define an angle with the general plane of the third screen frame  44 , as best appreciated in  FIG. 5B . 
       FIG. 5C  illustrates engagement of the tab  112  with the bottom face  44   c  of the third screen frame  44 . This engagement of the tab  112  with the bottom face  44   c  permits a lifting force exerted upon the second screen frame  42  to similarly lift the third screen frame  44 , as explained in further detail below. The engagement of the tab  112  with the bottom face  44   c  also permits the formation of the overlapping portion  58  between the second and third screen frames  42 ,  44  which, as discussed above, prevents or at least minimizes the passage of solid deposited on the screen frame assembly  20  to the underlying agitating structure  30  ( FIGS. 1-2 ). 
     With particular reference to  FIGS. 6A and 6B , the functionality of the limiting tab  66  and the gap  62  during the inserting motion of the screen frame assembly  20  (arrow  130 ) is illustrated in further detail with reference to the exemplary insertion of the third screen frame  44 , though the same explanation is applicable to insertion of the other screen frames  40 ,  42  as well. The inserting motion is stopped when the limiting tab  66  of the third screen frame  44  engages the front end wall  72  of the tray  50  of agitating structure  30 , as shown in  FIG. 6A . In operation, the operator proceeds to exert a lifting force upon the third screen frame  44  in the general direction of arrow  166 . This lifting force is applied onto the third screen frame  44  by an exertion of a lifting force onto the second screen frame  42  ( FIGS. 5A-5C ) and through the engagement of the tab  112  of the second screen frame  42  with the bottom face  44   c  of the third screen frame  44 , as discussed above. When the limiting tab  66  of the third screen frame  44  clears the front end wall  72 , as shown in  FIG. 6B , the operator proceeds to continue with inserting motion of the second and third screen frames  42 ,  44  in the general direction of arrow  130 , by exerting the pushing force onto the second screen frame  42 , as also discussed above. 
     Referring now to  FIG. 7 , the second and third screen frames  42 ,  44  of the screen frame assembly  20  are shown in a fully engaged position. In this regard, the screen frames  42 ,  44  are shown having an overlapping portion  58 . The overlapping portion  58  is defined, in this embodiment, by confronting end portions of the adjacent screen frames  42 ,  44 . Thus, the overlapping portion  58  shown in  FIG. 7  is formed from the juxtaposition of the bottom face  44   c  of the third screen frame  44  and the top face  42   b  of the second screen frame  42  where the leading edge  42   f  of the second screen frame  42  and the trailing edge portion  44   g  of the third screen frame  44  meet. 
     With continued reference to  FIG. 7  and referring again to  FIGS. 6A and 6B , each overlapping portion  58  includes a seal member  212  disposed between the respective top and bottom faces of the screen frames  40 ,  42 ,  44  at the confronting portions thereof. This position of the seal member  212  further prevents the passage of any solids deposited on the screen frame assembly  20  and onto the underlying agitating structure  30 . Moreover, in this exemplary embodiment, the seal member  212  is disposed adjacent substantially the entire perimeter of each of the screen frames  40 ,  42 ,  44  and is enmeshed at least partially with the screens  40   a ,  42   a , and  44   a . In this regard, the exemplary seal member  212  of this embodiment also facilitates mounting of the screens  40   a ,  42   a ,  44   a  to their respective screen frames  40 ,  42 ,  44 . In operation, the seal member  212  may be applied to a bottom face  40   c ,  42   c ,  44   c  of a screen frame  40 ,  42 ,  44  either substantially adjacent the entire perimeter of each of the screen frames  40 ,  42 ,  44  or just in the overlapping portion  58  between two adjacent screen frames  40 ,  42 ,  44  before the two adjacent screen frames  40 ,  42 ,  44  are engaged with one another. Similarly, when a screen frame  40 ,  42 ,  44  is removed from the main housing  14  ( FIGS. 1-2 ), the seal member  212  is removed between each pair of adjacent screen frames  40 ,  42 ,  44  as part of the disengagement of adjacent screen frames  40 ,  42 ,  44  from one another. In one embodiment, the seal member  212  is a silicone or similar compound and described in U.S. Patent Application Publication No. US 2007/0125688, which is incorporated by reference in its entirety. 
     With reference to  FIGS. 8 ,  9 ,  9 A,  9 B,  9 C,  9 D, and  9 E, the agitating structure  30  is described in further detail. As discussed above, the agitating structure  30  in this exemplary embodiment is in the form of a segmented structure made up of individual trays  50 ,  52  engageable with one another through a tray coupling  60 . It is contemplated, however, that an alternative agitating structure may have other shapes and/or include any number of trays other than the two trays of this exemplary embodiment. Such alternatives include, without limitation, an agitating structure having a single tray rather than a segmented tray assembly. Each of the individual trays of the illustrated embodiment includes several compartments  53  that hold agitating elements that move and thereby vibrate the solids deposited on the screen frame assembly  20 . The agitating elements, although not shown, may for example and without limitation include balls made of metal, plastic or other suitable material and are held within the compartments  53 . The agitating elements are supported by a support screen  218  and confined in the compartments  53  by partition walls  224 , the side walls  50   e ,  52   e , and longitudinally extending rails  228  defining the compartments  53 . 
     A pair of securing brackets  228  extend from the front end wall  72  of the tray  50 , adjacent the respective ends thereof, and cooperate with the brackets  32  of the main housing  14  ( FIGS. 1-2 ) to permit securement of the agitating structure  30  within the main housing  14 . To this end, each of the securing brackets  228  includes a hole  230  that is in registration with a corresponding hole  34  of each bracket  32  and which is similarly configured to receive a bolt or similar element (not shown) there through to thereby secure the agitating structure  30  relative to the main housing  14  ( FIGS. 1-2 ). Moreover, because of the registration of the holes  34  of the brackets  32  with the holes  230  of the securing brackets  228  and the holes  46  of the screen frame assembly  20 , the screen frame assembly  20  and the agitating structure  30  are both secured from movement relative to the main housing  14  ( FIGS. 1-2 ). 
     With continued reference to  FIGS. 8 ,  9 , and  9 A- 9 E, and further referring again to  FIGS. 1 and 2 , a series of tongues  246  extend from the front end wall  72  of the tray  50  and are spaced and sized to be received within the corresponding grooves  48  of the screen frame assembly  20 . Accordingly, engagement of the tongues  246  with the grooves  48  permits further securing the position of the screen frame assembly  20  and agitating structure  30  relative to one another. This engagement is best illustrated in  FIGS. 1 and 2 . 
     With particular reference to FIGS.  9  and  9 A-gE, details of the tray coupling  60  are further explained. The tray coupling  60 , as discussed above, is configured to permit movement of the second tray  52  by a force exerted upon the tray  50 . Accordingly, for example, an operator is able to pull both of the trays  50 ,  52  by exerting a pulling force on the first tray  50  and is further able to push both trays  50 ,  52  into the main housing  14  by exerting a pushing force on the first tray  50 . The tray coupling  60  of this exemplary embodiment includes a finger in the general form of a hook  276  extending from a leading edge portion  280  of the first tray  50  that is engageable with a pin  284  adjacent a trailing end wall  288  of the second tray  52 . More particularly, the pin  284  is inboard of the trailing end wall  288  and is supported at both ends thereof by a support plate  292  of the second tray  52 . The trailing end wall  288  includes an aperture  300  that permits insertion of the hook  276  there through such that the hook  276  may reach the pin  284 . In operation, accordingly, an operator pushes the first tray  50  relative to the second tray  52 , permitting the hook  276  to travel through the aperture  300  and under the pin  284 . To this end, and although not shown, the operator would tilt the first tray  50  relative to the second tray  52  to allow a generally U-shaped portion  276   a  of the hook  276  to wrap around the pin  284 , as illustrated in  FIG. 9B . When the hook  276  is thus engaged with the pin  284 , any pulling force exerted on the first tray  50  results in a corresponding pulling force exerted by the generally U-shaped portion  276   a  on the pin  284 , thereby resulting in movement of the second tray  52  as well. In this embodiment, moreover, the generally U-shaped portion  276   a  is sized to closely surround the pin  284  such that relative vertical movement between the first and second trays  50 ,  52  is at least minimized. The tray coupling  60 , moreover, permits pushing motion (into the main housing  14 ) of both trays  50 ,  52  by a pushing force exerted onto the first tray  50 . More particularly, the insertion of the hook  276  through the aperture  300  and under the pin  284  is configured to permit the abutment of the confronting end walls  280 ,  288  of the first and second trays  50 ,  52 , respectively, as shown in  FIG. 9 , such that any pushing force exerted onto the first tray  50  is transmitted through contact between the end walls  280 ,  288  onto the second tray  52 . 
     Disengagement of the first and second trays  50 ,  52  from one another is best explained with particular reference to the exemplary sequence illustrated in  FIGS. 9C-9E . A step in the disengagement sequence includes the operator moving the first tray  50  forward (i.e., into the main housing  14 ) relative to the second tray  52 , as shown in  FIG. 9C , in the general direction of arrow  310 . This relative forward movement of the first tray  50  permits, as shown, partial disengagement of the hook  276  from the pin  284 . A subsequent step in the disengagement process includes tilting the first tray  50  upward relative to the second tray  52 , as shown in  FIG. 9D , in the general direction of arrow  314 , to permit the generally U-shaped portion  276   a  of the hook  276  to clear the pin  284 . More specifically, the portion  276   a  is partially received within a recess  320  at the base of the support plate  292  to permit clearing movement of the portion  276   a  relative to the pin  284 . An operator then exerts a pulling force on the first tray  50 , in the general direction of arrow  322 , to complete the disengagement process of the first and second trays  50 ,  52 , as illustrated in  FIG. 9E . 
     With reference to  FIGS. 10A ,  10 B,  10 C,  10 D, and  10 E, an alternative embodiment of a tray coupling  350  is illustrated. For ease of understanding, like reference numerals in  FIGS. 10A-10E  refer to like features of the preceding figures. The tray coupling  350  includes a finger  356  that has a distal portion shaped differently from the portion  276   a  of the hook  276  ( FIGS. 9A-9E ) and which is received through an aperture  360  of a plate  382  situated similarly to the support plate  292  of the tray coupling  60  of  FIGS. 9A-9E . When the finger  356  is received through the aperture  360 , an upwardly oriented portion  356   a  of the finger  356  engages the back surface  382   a  of the plate  382 , thereby preventing removing motion (arrow  394 ) of the first tray  50  relative to the second tray  52 . Accordingly, a pulling force in the general direction of arrow  394  applied onto the first tray  50  results in a corresponding removing motion of both, the first and second trays  50 ,  52 . 
     Disengagement of the first and second trays  50 ,  52  having the alternative tray coupling  350  is best explained with particular reference to the exemplary sequence illustrated in  FIGS. 10C-10E . A step in such disengagement sequence includes the operator moving the first tray  50  forward (i.e., into the main housing  14 ) relative to the second tray  52 , as shown in  FIG. 10C , in the general direction of arrow  310 . This relative forward movement of the first tray  50  permits, as shown, partial disengagement of the finger  356  from the back surface  382   a  of the plate  382 . A subsequent step in the disengagement process includes tilting the first tray  50  upward relative to the second tray  52 , as shown in  FIG. 10D , in the general direction of arrow  314 , to permit the upwardly oriented portion  356   a  of the finger  356  to clear the plate  382  through the aperture  360 . An operator then exerts a pulling force on the first tray  50 , in the general direction of arrow  322  to complete the disengagement process of the first and second trays  50 ,  52 , as illustrated in  FIG. 10E . 
     From the above disclosure of the general principles of the present invention and the preceding detailed description of at least one preferred embodiment, those skilled in the art will readily comprehend the various modifications to which this invention is susceptible. Therefore, we desire to be limited only by the scope of the following claims and equivalents thereof.