Patent Publication Number: US-6988624-B2

Title: Vibrating screen with a loading pan

Description:
FIELD OF THE INVENTION 
   This invention pertains to vibrating screens for screening gravel, top soil, and the like, and more particularly, it pertains to a vibrating screen having a loading pan thereon for receiving loads of screenable material from a bucket loader and for controlling the flow of these loads to the screen box. 
   BACKGROUND OF THE INVENTION 
   Small and portable vibrating screens are used for examples, by landscape contractors, gardeners, farmers, and excavation and trucking companies. These vibrating screens are usually loaded by small Skid-Steer™ loaders or other similar front-end bucket loaders. This type of small portable vibrating screens is illustrated and described in Applicant&#39;s U.S. Pat. No. 5,899,340 issued on May 4, 1999. 
   When a load of gravel is dropped all at once in the upper end of a common vibrating screen, the upper springs become compressed, thereby collapsing the upper half of the screen box for a few seconds. During that period, the amplitude of the vibration of the screen box is reduced at the top and increased at the bottom. The screening action is correspondingly reduced at the top. The efficiency of the vibrating screen remains low until the upper springs can recover their operating shapes. This collapsing of a vibrating screen under sudden loads is typical of all common machines having coil springs set vertically under the screen box. Most small portable vibrating screens of the prior art have this type of spring arrangement and suffer from the same drawback. 
   Therefore, it is believed that there is a market need for a small portable vibrating screen which can maintain a better efficiency when a load of screenable material is dropped in the upper end of the screen box. 
   A first attempt to reduce the collapsing of the upper end of a vibrating screen has been disclosed in the U.S. Pat. No. 5,082,555, issued to James L. Read on Jan. 21, 1992. In this invention, the vibrating screen has a tilting hopper laid over and covering the screen box. The screenable material is dropped into this hopper by a front-end loader. The hopper is pivoted on the upper end of the machine&#39;s frame, and is raised and lowered by hydraulic cylinders. The hopper has a discharge end which coincides with the top end of the screen box. Once loaded, the hopper is tilted at a desired speed to control the flow of screenable material to the screen box. 
   Although this hopper feeding system has undeniable merits, it has several moving parts and is controlled by an electric timer and a photoelectric switch. These control devices and moving parts are subject to deterioration from dust and shocks associated with the environment in which a vibrating screen operates. Therefore, it is believed that there continues to be a need for a sturdy and maintenance free loading arrangement to control the flow of material in a vibrating screen. 
   SUMMARY OF THE INVENTION 
   In the vibrating screen according to the present invention, there is provided a static combination of elements which contribute cooperatively and individually to control the flow of screenable material to the screen box. 
   In a first aspect of the present invention, there is provided a vibrating screen for separating fine materials from coarse materials. The vibrating screen comprises a frame having a vertical tall end, a vertical short end and a screen box having an upper end, a lower end, a top screen therein and an inclination from the horizontal plane. A first pair of springs are affixed to the tall end of the frame for supporting the upper end of the screen box over the tall end of the frame, and a second pair of springs are affixed to the short end of the frame and to the lower end of the screen box for supporting the lower end of the screen box over the short end of the frame. The vibrating screen also has an eccentric shaft affixed to the screen box and a drive means affixed to the frame and to the eccentric shaft for rotating the eccentric shaft and for imparting a reciprocal movement to the screen box. 
   The vibrating screen according to this first aspect of the present invention is characterized by a loading pan affixed to the upper end of the screen box, and rigid structural members extending under the screen box and the loading pan for maintaining the loading pan in a same plane as the screen box. The loading pan is set substantially over the upper springs such that a flexion of the structural members in use under the loading pan is minimum. 
   In accordance with another aspect of the present invention, the loading pan is wider than the screen box. More specifically, the loading pan is about 60% wider than the screen box. The loading pan has a plated bottom surface and sloped sides forming a funnel on the upper end of the screen box. In use, the sloped sides retain about 30% or more of a load of screenable material in the loading pan until most of the central portion of the load has been moved over to the top screen. The flow of screenable material from the loading pan to the top screen is thereby more uniform. 
   In yet another aspect of the present invention, each of the first and second pairs of springs have torsion bushings therein, and a pair of arms joining the torsion bushings and forming an acute angle pointing toward the lower end of the screen box. The top arm in each spring makes an angle with the horizontal plane, which is greater than the inclination of the screen box. Because of this characteristic, the friction forces caused by a load of screenable material in the loading pan produce a torque on each spring in a direction opposite a vertical loading on each spring, to reduce a collapsing of the springs in use. 
   Still another feature of the vibrating screen of the present invention is that it is susceptible of a low cost of manufacture with regard to both materials and labour, and which accordingly is then susceptible of low prices of sale to the consumer, thereby making such vibrating screen economically available to the public. 
   Other advantages and novel features of the present invention will become apparent from the following detailed description of the preferred, embodiment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One embodiment of the present invention is illustrated in the accompanying drawings, in which like numerals denote like parts throughout the several views, and in which: 
       FIG. 1  is a perspective side, top and front view of the vibrating screen according to the preferred embodiment of the present invention; 
       FIG. 2  is a partial side view of the vibrating screen; 
       FIG. 3  is a top view of the screen box; 
       FIG. 4  is a cross-section view of the loading pan as seen along line  4 — 4  in  FIG. 3 ; 
       FIG. 5  is another partial side view of the vibrating screen with the screen box shown in a cut-away view to show a load of screenable material therein; 
       FIG. 6  is a diagram representing the flexion of the structural members under the screen box in use; 
       FIG. 7  is another side view of the vibrating screen showing one of the springs supporting the screen box; 
       FIG. 8  is another perspective side, top and front view of the vibrating screen according to the preferred embodiment of the present invention, showing various optional features therefor; 
       FIG. 9  is a cross-section view of the screen box taken across the longitudinal axis of the screen box, substantially along line  9 — 9  in  FIG. 3 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will be described in details herein one specific embodiment, with the understanding that the present disclosure is to be considered as an example of the principles of the invention and is not intended to limit the invention to the embodiment illustrated and described. 
   Referring to  FIGS. 1 and 2 , the vibrating screen  20  according to the preferred embodiment is described herein below in a general form. The preferred vibrating screen  20  has an arched frame  22  supporting a screen box  24  on four springs  26  affixed to the top of the frame  22 . An engine  28  drives an eccentric shaft  30  affixed to the screen box  24 , to impart a vibrating movement to the screen box  24 . 
   The preferred springs  26  are of the type known as oscillating mountings, manufactured by ROSTA-WERK AG, a company from Switzerland having distributors throughout the world. Each spring  26  is characterized by two pairs of torsion bushings each comprising a square stub embedded in a rubber-packed housing. A torsion bushing in each pair share a common housing. The torsion bushings are perpendicularly affixed to two arms making an acute angle having a closed end near the common housing. These springs are known in the industry as ROSTA™ springs. 
   The frame  22  of the vibrating screen has a short end and a tall end. Both ends comprise ballast  32  between the vertical frame members to stabilize the vibrating screen in use. The short end ballast has a socket  34  there through to receive a tow hitch  36 , and the tall end has brackets  38  thereon to receive an axle and wheel set  40  for transporting the vibrating screen between job sites. 
   A panel  42  extends along one side of the frame  22  to form with both ends of the frame an enclosure under the vibrating screen to retain a pile of fines under the vibrating screen. 
   The screen box  24  has a discharge chute  44  on its lower end extending to one side of the frame next to the panel  42 , to accumulate the rejects of the top screen  46  at that location. Although only the top screen  46  is visible in the drawings, a second screen may be provided under the top screen to produce a third grade of screened material. The discharge end of the second screen is next to the short end of the vibrating screen, under the chute  44 . A second screen will be described later and is illustrated in  FIG. 9 . 
   The frame  22  of the vibrating screen, the engine  28 , the eccentric shaft  30  the towing accessories  34 ,  36 ,  38  and  40 , and the chute  44  are not described further herein for not being the focus of the present invention. 
   Referring now to  FIGS. 3 and 4 , one of the features of the vibrating screen  20  will be described. The screen box  24  is made of metal plates and metal structural members enclosing the top screen  46 . The screen box  24  has a loading pan  50  on its upper end, above the upper edge  52  of the top screen  46 . The loading pan  50  is also made of metal plates and metal structural members. The preferred width ‘A’ of the loading pan is at least about 1.5 times, and preferably 1.6 times or more, the width ‘B’ of the top screen. A 48 inch-wide screen for example has a preferred loading pan width ‘A’ of about 78 inches. This dimension has been found advantageous for loading the vibrating screen with a Skid-Steer™ loader or a similar small bucket loader. 
   The preferred length of the loading pan ‘C’ is about 24 inches, such that the loading pan  50  can receive the entire load of a small bucket loader. The loading pan  50  has a central plated surface  52  defined by inclined side surfaces  54 . The loading pan  50  also has inclined sloped surfaces  56  defining a funnel between the inclined side surfaces  54  and the sides  58  of the screen box  24 . Each sloped surface  56  forms an angle ‘D’ between 120° and 150°, and preferably about 135° with a respective inclined side surface  54 , or with a respective side  58  of the screen box. The depth ‘E’ of the loading pan  50  is about the same as the depth of the screen box  24 . 
   The central region  60  of the loading pan  50  preferably lies upon the axis  62  of the upper springs  26 , although there are also advantageous results to be obtained with the central region  60  of the loading pan  50  lying on the screen side of this axis, within the span ‘F’ between the axis  62  of the upper springs and the axis  64  of the lower springs. These advantageous results will be explained later when making reference to  FIG. 6 , in particular. 
   It is to be noted that the shape of the loading pan  50  causes a load of screenable material to be partially and temporarily retained inside the loading pan, and to be released therefrom in a controlled manner. The projections ‘G’ of the sloped surfaces  56  across the loading pan  50  constitute at least one third, and more precisely, about 38% of the total width of the loading pan. Therefore, a similar proportion of a load of screenable material dumped into the loading pan is temporarily retained against these sloped surfaces  56  until a central portion of the load has been moved over to the top screen  46 . 
   It will be appreciated that a load of screenable material inside the loading pan is also partially and temporarily retained therein by friction forces against the bottom surface  52  of the loading pan  50 . It has been found that the shape of the loading pan causes a load of screenable material to flow in sequence from the top to the bottom of the central portion and then from the centre to the sides thereof, with the side portions flowing last. It has been found that this flow sequence helps to control the amount of screenable material moving to the top screen  46 , and contributes to maintaining the efficiency of the vibrating screen from the start to the end of each load. 
   The centring of the load upon the axis  62  of the upper springs also contributes to improving the flow of material over the screen surface. As can be appreciated from the illustrations in  FIGS. 5 and 6 , the screen box  24  and the loading pan  50  are on a same pair of structural members  70 , with the loading pan  50  centred on the axis  62  of the upper springs  26 , as mentioned before. In use, the structural members  70  flex up and down in reaction to the rotation of the eccentric shaft  30 , as illustrated in  FIG. 6 . 
   It will be appreciated that the amplitude  72  of the vibration shown in an exaggerated manner in  FIG. 6  is maximum at a mid-span of the structural members and is minimum at the springs  26 . This flexion amplitude added to the displacements  74  of the springs causes the vibration of the screen box to be maximum at the mid-span of the screen box and minimum at the upper and lower axes  62 ,  64 . This minimum vibration at the central region  60  of the loading pan  50  also contributes to improving the uniformity of a flow of screenable material from the loading pan to the screen box. 
   It will also be appreciated that the position of the loading pan in-line with the axis of the upper springs or within the span ‘F’ of the springs contributes to reducing any cantilevered loading on the structural members  70 . It is known that such cantilevered loading would occur if the loading pan would be centred well above the upper springs. It is also known that such cantilevered loading can cause a deflection in the structure of a screen box which is out-of-phase with the rotation of the eccentric shaft, and damage the vibrating screen. 
   Another feature of the present invention will be described while making reference to  FIG. 7  in particular. The structural members  70  under the screen box  24  and the loading pan  50  are preferably set at an inclination ‘H’ of about 18° from the horizontal plane for screening loam, peat moss and the like, and at 22° for screening sand and gravel. 
   As mentioned herein before, each spring  26  has two arms  80 ,  82  joining two pairs of torsion bushings. The lower mounting housing  84  is affixed to the frame  22  of the vibrating screen, and the upper housing  86  is affixed to the screen box  24 . The other two torsion bushings are mounted in the common housing  88 . 
   The springs  26  are selected to maintain in use, and angle ‘J’ of about 45° to 90° between the arms  80 ,  82  with the closed end of this acute angle near the common housing  88 . The mounting surfaces of the housings  84 ,  86  are set horizontally, and the closed end of the acute angle ‘J’ is pointing toward the lower end of the screen box  24 . 
   For the purpose of understanding the following discussion, it should be noted that the upper arm  82  in each spring  26  is always inclined from the horizontal plane, at an angle larger than the inclination ‘H’ of the screen box  24 . 
   The weight ‘W’ of a load of screenable material  76  generates a cosine force  90  perpendicular to the surface  52  of the loading pan  50 , and a sine force  92  tangent to, or in-line with the structural members  70  under the screen box  24 . The sine force  92  between a load of screenable material and the surface  52  of the loading pan  50  is composed of surface friction forces as illustrated by arrows  94  in  FIG. 3 , and holding forces applied by the sloped surfaces  56 , as illustrated by arrows  96 . A complete analysis of the magnitude of these forces is not necessary to understand the principle of the present invention. Generally, the sum of these forces  94 ,  96  is always related the total weight of a load  76  in a proportion corresponding to the sine  92  of the inclination ‘H’ of the screen box. 
   With a screen box inclined at an angle ‘H’ of between 18° to 22°, the friction forces  94 ,  96 , and consequently the sine force  92  at each spring  26  corresponds to the sine of that angle times the weight of the load ‘W’. In other words, the sine force  92  on each spring  26  corresponds to between 30% to 37% of the total load ‘W’ supported by that spring. 
   Because each spring  26  is mounted with the angle ‘J’ of the arms  80 ,  82  pointing toward the short end of the screen box, and the top arm  82  is angled downward from the structural members  70 , the sine force  92  translated to the upper housing  86  applies a torque  100  on the spring  26  in a direction causing the spring to extend. This torque  100  is opposite from the torque  102  caused by the cosine component  90  of the load ‘W’. While the cosine component  90  of a load tends to collapse the spring  26 , the sine component  92  tends to extend the spring. For this reason, the total deflection of each spring  26  is not as much as in same size vibrating screen having coil springs for example. The initial collapsing of the upper springs when a load is dumped all at once in the screen box is thereby not as severe as compared to vibrating screens of the prior art. 
   Referring now to  FIGS. 8 and 9 , there are illustrated therein four optional features that are advantageous to accommodate different situations. 
   Firstly, a small, short-arm loader with a shallow bucket may have difficulty reaching under the vibrating screen  20  to handle all the fine material therefrom. In these situations, the panel  42  is preferably mounted inside the frame  22  directly under the top screen  46 . In this arrangement, a deflector  120  joins the top edge of the panel  42  to the side framing member  122 , to deflect the fines to the far side of the vibrating screen  20  relative to the view illustrated in  FIG. 8 . 
   In a second option, the rear edge of the loading pan is preferably enclosed by a plate  124  as illustrated in  FIG. 8 , when working with non-adhering material in a vibrating screen that is set at the lower preferred inclination. The plate  124  prevents runout of screenable material toward the rear end of the machine. The plate  124  also facilitates the loading of the loading pan using a small bucket loader having limited horizontal reach with the arms in a raised position. 
   When production is more important than material retention inside the loading pan, the bottom surface of the loading pan, as shown by dotted line  126  in  FIG. 8 , is preferably inclined more than of the top screen  46  by an angle of about 4°–5°. This slope promotes a faster delivery of material to the top screen  46 . 
   Lastly, the screening of moist and sticking materials can represent a challenge to manufacturers of vibrating screens. A good solution to this problem has been obtained by providing a crown of about 1″ over 48″ across both the top screen  46  and the bottom screen  130  as illustrated in  FIG. 9 . It has been found that these curvatures promote an even distribution of materials over the screen surfaces. 
   In the screen of the present invention, the top screen  46  is supported by transversely curved flat bars  132 . The bottom screen  130  is supported by a rectangular insert  134  having longitudinal flat bars  136 ,  138  of different widths, mounted on their edges. The rectangular insert  134  is preferably fastened to the structural members  70  of the screen box by bolts  140 , such that it is easily removable for replacement with a flat screen when necessary. 
   As to other manner of usage and operation of the present invention, the same should be apparent from the above description and accompanying drawings, and accordingly further discussion relative to the manner of usage and operation of the vibrating screen would be considered repetitious and is not provided. 
   While one embodiment of the present invention has been illustrated and described herein above, it will be appreciated by those skilled in the art that various modifications, alternate constructions and equivalents may be employed without departing from the true spirit and scope of the invention. Therefore, the above description and the illustrations should not be construed as limiting the scope of the invention which is defined by the appended claims.